Sage Journals: Discover world-class research

Abstract

This study investigated trans-cerebral internal jugular venous-arterial bicarbonate ([HCO₃⁻]) and carbon dioxide tension (PCO₂) exchange utilizing two separate interventions to induce acidosis: 1) acute respiratory acidosis via elevations in arterial PCO₂ (PaCO₂) (n = 39); and 2) metabolic acidosis via incremental cycling exercise to exhaustion (n = 24). During respiratory acidosis, arterial [HCO₃⁻] increased by 0.15 ± 0.05 mmol ⋅ l⁻¹ per mmHg elevation in PaCO₂ across a wide physiological range (35 to 60 mmHg PaCO₂; P < 0.001). The narrowing of the venous-arterial [HCO₃⁻] and PCO₂ differences with respiratory acidosis were both related to the hypercapnia-induced elevations in cerebral blood flow (CBF) (both P < 0.001; subset n = 27); thus, trans-cerebral [HCO₃⁻] exchange (CBF × venous-arterial [HCO₃⁻] difference) was reduced indicating a shift from net release toward net uptake of [HCO₃⁻] (P = 0.004). Arterial [HCO₃⁻] was reduced by −0.48 ± 0.15 mmol ⋅ l⁻¹ per nmol ⋅ l⁻¹ increase in arterial [H⁺] with exercise-induced acidosis (P < 0.001). There was no relationship between the venous-arterial [HCO₃⁻] difference and arterial [H⁺] with exercise-induced acidosis or CBF; therefore, trans-cerebral [HCO₃⁻] exchange was unaltered throughout exercise when indexed against arterial [H⁺] or pH (P = 0.933 and P = 0.896, respectively). These results indicate that increases and decreases in systemic [HCO₃⁻] – during acute respiratory/exercise-induced metabolic acidosis, respectively – differentially affect cerebrovascular acid-base balance (via trans-cerebral [HCO₃⁻] exchange).

Keywords

Acidosis bicarbonate carbon dioxide exercise trans-cerebral exchange

Introduction

The regulation of extracellular pH – which directly affects cells via local changes in intravascular or extravascular/interstitial conditions – is affected by rapid chemical buffering reactions, involving phosphate, glycolysis, and carbon dioxide tension (PCO₂). The cerebral vasculature is exceptionally sensitive to changes in arterial PCO₂ (PaCO₂),^1–3 such that cerebral blood flow (CBF) rapidly increases by 6–8% per mmHg increase in PaCO₂ (reviewed in: Hoiland et al.⁴) This CBF responsiveness to PaCO₂/[H⁺] acts to stabilize CO₂ gradients and thus regulate pH across the blood-brain barrier (BBB).^4–6 As CO₂ travels rapidly across the BBB – indicated by the swift changes in CBF (<15–30 seconds) following stepwise changes in PaCO₂^7–10–perivascular/interstitial fluid (ISF) and intracellular brain tissue pH are tightly related to PaCO₂. The Fick principle explains that elevated PaCO₂ will be related to reductions in the trans-cerebral venous-arterial PCO₂ difference as: 1) CBF varies directly with PaCO₂,^1,3,11 and 2) the cerebral metabolic production of CO₂ ( $\dot{V}$ CO₂) is unaltered in the physiological range; e.g., CBF = $\dot{V}$ CO₂/PvCO₂–PaCO₂.^12,13 Experimental results in several acid-base pathologies indicate the trans-cerebral venous-arterial PCO₂ difference is reduced and increased with acute respiratory acidosis/alkalosis, respectively.^{3,5,12,14–18} These regulatory changes in the venous-arterial PCO₂ difference contribute to tight regulation of a narrow range of cerebral interstitial pH^19,20 and are influenced by the responsiveness of CBF to acute alterations in PaCO₂; i.e., cerebrovascular CO₂ reactivity.⁶

Exercise-induced metabolic acidosis results when the rate of ATP hydrolysis (H⁺ release) eventually exceeds the maximal buffering capacity; the excess CO₂ is then removed via hyperventilation^21,22 and these PaCO₂ changes in part explain the CBF kinetics response with exercise.^23–25 With exercise-induced acidosis, arterial [HCO₃⁻] is markedly reduced due to excessive buffering of [H⁺]^21,22 – however, whether this change in arterial [HCO₃⁻] is related to alterations in trans-cerebral [HCO₃⁻] and PCO₂ exchange as well as in vivo buffering capacity has not been experimentally addressed in humans. The cerebral metabolic rate of oxygen (e.g., CMRO₂ = CBF × arterial-venous oxygen content difference) increases linearly by up to 30% at maximal exercise intensities to support substrate utilization in the face of reductions in CBF.^26–33 At maximal exercise, there is a higher relative contribution of cerebral anaerobic glycolysis and lactate oxidation.^34–36 There is evidence to support that maximal exercise-induced acidosis facilitates trans-cerebral lactate exchange and related increases in lactate oxidation mediated via pH-sensitive transcellular [H⁺] gradients^37–39 and increases in systemic lactate availability.^{28,32,40–42} Additionally, metabolic acidosis achieved with exhaustive exercise reportedly increases BBB permeability with direct relevance for transcellular CO₂ transport.^38,43 Overall, cerebrovascular acid-base regulation is likely interrelated with cerebral substrate prioritization during exercise via pH-sensitive utilization of lactate.^44,45

The Henderson-Hasselbalch relationship (equation (2)) explains that acute elevations in arterial [HCO₃⁻] would increase arterial buffering capacity; for example, with increases in arterial [HCO₃⁻] any given change in PaCO₂ will result in a lesser change in arterial [H⁺]/pH.^13,46 This relationship would theoretically apply with reductions in arterial [HCO₃⁻] and therefore be reflective of related decreases in arterial buffering capacity. Pre-clinical experiments in anesthetized and artificially ventilated cats show that rapid/transient [HCO₃⁻]/Cl⁻ exchange occurs within 15 seconds between intravascular and extracellular fluid in response to elevated systemic arterial [HCO₃⁻] at maintained PaCO₂ ⁴⁷ – these results indicate that extracellular [HCO₃⁻] is partially and transiently altered with changes in arterial [HCO₃⁻], however, such changes in extracellular [HCO₃⁻] are restored (within 1 hour) and these [HCO₃⁻]/Cl^- exchange kinetics are appreciably less influential than persistent and freely diffusible CO₂ transport.^48,49 At rest, cerebrospinal fluid (CSF) PCO₂ – typically indexed via internal jugular venous sampling – is 6 to 11 mmHg higher and pH is 0.05 to 0.08 units lower than that of the arterial blood.^13,50,51 As such, resting trans-cerebral net exchange of [HCO₃⁻] and PCO₂ (e.g., CBF × venous-arterial difference) is a positive value reflective of net release. A narrowing of the venous-arterial difference would indicate a shift toward net uptake and can be achieved, for example by: 1) larger increase in arterial relative to venous value; and 2) reduction in venous with a corresponding increase in arterial values. No study to date has investigated whether alterations in the trans-cerebral venous-arterial [HCO₃⁻] and PCO₂ differences during acute respiratory/exercise-induced metabolic acidosis are related to CBF and regulatory systemic changes in arterial [HCO₃⁻] in vivo in humans.

The objective of this study was to investigate acid-base balance via alterations in trans-cerebral internal jugular venous-arterial [HCO₃⁻] and PCO₂ exchange utilizing two separate experimental interventions to induce acidosis: Study 1) acute respiratory acidosis via elevations in PaCO₂ (range: 35 to 60 mmHg); and Study 2) metabolic acidosis via incremental cycling exercise to exhaustion (range: 7.45 to 7.20). We hypothesized that: 1) acute hypercapnic acidosis would reduce the venous-arterial [HCO₃⁻] and PCO₂ differences and this reduction would be related to higher CBF, less trans-cerebral [HCO₃⁻] exchange, and elevated arterial [HCO₃⁻]; and 2) arterial [HCO₃⁻] would progressively decrease with maximal exercise-induced acidosis and – as explained by the non-linear CBF response with incremental exercise – this reduction in systemic [HCO₃⁻] would be unrelated to any change in CBF or trans-cerebral venous-arterial [HCO₃⁻] and PCO₂ exchange.

Methods

Ethical approval

All participants provided informed written consent before participating in these studies. The original research studies were approved by the University of British Columbia Clinical Review Ethical Board (CREB: H16-01028, H18-01755, H15-00166, H11-03287) and the Comité d’éthique de la recherche de l’Institut universitaire de cardiologie et de pneumologie de Québec (CER: 21557) and according to the principles established by the Declaration of Helsinki (except for registration in a database). As the current study included a secondary analysis from previous institutional review board approved studies and the use of de-identified data sharing, this study did not require additional institutional review board review.

Participants

These data were obtained during five separate experimental studies previously conducted at the University of British Columbia, Kelowna, British Columbia, Canada and the Université Laval, Québec City, Québec, Canada. Thirty-nine healthy adults completed the CO₂ trials (n = 34 males and n = 5 females) and twenty-four healthy adults completed the exercise trials (n = 19 males and n = 5 females). Participants had no history of cerebrovascular, cardiovascular, or respiratory disease and were not taking any prescription medication at their time of participation. Participants refrained from alcohol and caffeine consumption as well as vigorous exercise or activity for at least 12 hours prior to testing.

Experimental overview

The experimental questions addressed in this study involved post-hoc data analysis; therefore, few of the arterial and venous blood gas data have been reported separately in various context for previously published works from these studies; e.g.,.^36,52,53 Importantly, the current experimental questions involved new data analyses that have not been previously reported.

Study 1: Respiratory acidosis protocols

Participants completed one of the following protocols to elicit progressive stepwise steady-state elevations in PaCO₂ via dynamic end-tidal forcing: 1) +3, +6, +9 mmHg PaCO₂ (n = 11 males); 2) +4.5 and +9 mmHg PaCO₂ (n = 12 males); 3) +8 mmHg PaCO₂ (n = 5 females; n = 7 males); or 4) +10 and +20 mmHg PaCO₂ (n = 4 males). The alterations in PaCO₂ were calculated from the resting eupneic breathing end-tidal values. All measurements were taken after at least 2–3 minutes of steady-state. Previous investigations from our group have shown that cerebrovascular CO₂ reactivity (utilizing the same dynamic end-tidal forcing system) is highly linear in the hypercapnic range up to +20 mmHg.^4,54 Additionally, recent retrospective analysis by our research group¹⁰⁹ has shown that following a rapid stepwise change in P_ETCO₂ (via the exact same experimental methods/techniques in the current studies), relevant cerebrovascular and cardiorespiratory variables achieve steady-state within 2-minutes of exposure duration to elevated inspired PCO₂. As such, by applying a linear mixed model analysis, there is no statistical or physiological rationale to expect these results to be different whether participants completed the exact same stepwise steady-state elevations in PaCO₂ compared to the varied stages utilized in this post-hoc analysis.

Study 2: Exercise protocols

Participants completed supine incremental cycling exercise to exhaustion at various relative (0, 20, 40, 60, 80, 100% maximal workload; n = 12 males) and fixed exercise intensities (0, 50, 75, 100 watts; n = 5 females and 0, 75, 100, 125 watts; n = 7 males). Each workload was 3–5 minutes in duration and steady-state blood samples were drawn within the last 20 s of each exercise stage.

Blood sampling

Approximately 1.0 ml of radial arterial and internal jugular venous blood were drawn at the same time into pre-heparinized syringes (SafePICO, Radiometer, Copenhagen, Denmark) and analyzed immediately using a commercial blood gas analyzer (ABL90 FLEX, Radiometer) at each experimental stage (n = 39). This analysis included measurements of PaCO₂ and oxygen tension (PaO₂), arterial oxygen saturation (SaO₂), arterial oxygen content (CaO₂), base excess, [H⁺], hemoglobin concentration ([Hb]), and pH. Data collected at the Université Laval were analyzed within 10−15 minutes for n = 12 (ABL800 FLEX, ABL825, Radiometer).

Arterial oxygen content (CaO₂) was calculated as: ${CaO}_{2} (mL \cdot {dL}^{- 1}) = [Hb] \times 1.34 \times [{SaO}_{2} (%) / 100] + 0.00 3 \times {PaO}_{2}$ (1)Where 1.34 is the O₂ binding capacity of hemoglobin and 0.003 is the solubility of O₂ dissolved in blood.^55,56

Acid-base balance data analysis

Blood gas analyzers do not typically have the capacity to directly measure [HCO₃⁻]; instead, it is calculated from measured PaCO₂ and pH, by rearranging the Henderson-Hasselbalch equation (equations (2) and (3)). $pH = 6.1 + log \frac{{[HCO}_{3}^{-}]}{0.031 \times {PCO}_{2}}$ (2) ${[HCO}_{3}^{-}] = 0.031 \times {PCO}_{2} \times 10^{(pH-6.1)}$ (3)Where 6.1 is the pKa (i.e., −log of the acid dissociation constant) at 37.0°C⁵⁷ and 0.031 mEq⋅l⁻¹ per mmHg PCO₂ is the solubility factor for dissolved CO₂ plus carbonic acid (H₂CO₃) at 37.0°C in plasma.

The following calculations were used to quantify an index of in vivo buffering capacity termed “extracellular pH defense”.^58,59 All in vivo buffering capacity calculations were analyzed with the average venous-arterial values of [HCO₃⁻], [La], and pH as an estimate of cerebral tissue values. These results were consistent when expressed as arterial or internal jugular venous buffering capacity.

Exercise: $Δ [{HCO}_{3}^{-}] \times Δ {pH}^{-1}$ (4)

The reduction in [HCO₃⁻] per unit pH is reportedly attributable to hyperventilation and HCO₃⁻ buffering.⁵⁸ $- Δ [La] \times {pH}^{-1}$ (5)

This index includes lactate ([La]) and is considered total pH defense by hyperventilation, as well as HCO₃⁻ and non-bicarbonate buffers.⁵⁸

Respiratory acidosis: $- Δ [{HCO}_{3}^{-}] \times Δ {pH}^{-1}$ (6)

Equation (4) above has been adapted as [HCO₃⁻] increases with respiratory acidosis.

Cardiorespiratory

Detailed cardiorespiratory experimental methods and results have been previously reported elsewhere; e.g., literature.^36,52,53 Briefly, the partial pressures of end-tidal CO₂ and O₂ (i.e., P_ETCO₂ and P_ETO₂, respectively) were controlled during acute respiratory acidosis using a custom-designed dynamic end-tidal forcing system to effectively regulate end-tidal gases across wide ranges of P_ETCO₂ and P_ETO₂,^60,61 independent of ventilation ( $\dot{V}$ _E).⁶² Beat-by-beat arterial blood pressure was continuously acquired via the radial artery pressure transducer positioned at the height of the right atrium (Edwards Lifesciences, TruWave VAMP, CA, USA) and the arterial blood pressure waveform was averaged to calculate MAP.

Cerebrovascular

Detailed cerebrovascular experimental methods and results have been previously reported elsewhere; e.g., literature.^36,52,53 Briefly, extra-cranial blood velocity and diameter of the internal carotid artery (ICA) and vertebral artery (VA) were measured using a 10-MHz multifrequency linear array Duplex ultrasound (Terason t3000; Teratech, Burlington, MA, USA). Pulse-wave mode was used to measure beat-to-beat peak blood velocity and arterial diameter was instantaneously measured via B-mode imaging; data were analyzed using custom edge-detection and wall tracking software (BloodFlow Analysis, version 5.1). The vessel location was decided on an individual basis to allow for reliable image acquisition, with the same location and consistent insonation angle (60°) repeated within participants and between trials.

Blood flow was calculated as: $Q (mL·min - 1) = \frac{peak envelope blood velocity}{2} \times (π (0.5 \times diameter)^{2}) \times 60$ (7)

Total cerebral blood flow (CBF) was calculated as: $CBF (mL · min - 1) = 2 \times (Q_{ICA} + Q_{VA})$ (8)

Net [HCO₃⁻] exchange was calculated as: $Net {[HCO}_{3}^{-}] exchange (mmol × {min}^{−1}) = ({[HCO}_{3}^{-}]_{v} - {[HCO}_{3}^{-}]_{a}) \times CBF$ (9)

Net PCO₂ exchange was calculated as: $Net {PCO}_{2} exchange (mmHg· min - 1) = ({PvCO}_{2} - {PaCO}_{2}) \times CBF$ (10)Where a negative value indicates a net uptake and a positive value indicates a net release.⁶³

Statistical analyses

All data are presented in-text as mean ± SD and as individual values in figures. Statistical analyses were performed using SPSS software (IBM statistics, Version 23.0) and Prism (GraphPad Software, Version 9.1.0). Normality was assessed using Shapiro-Wilk tests and by visual inspection of Q-Q plots. Statistical significance was set at P < 0.05. Relationships between select variables were analyzed using linear regression. A linear mixed-model analysis with fixed effects of either arterial [H⁺], pH, or PaCO₂ was used to compare PCO₂, [HCO₃⁻], [La], in vivo buffering capacity, trans-cerebral exchange, and CBF separately for the respiratory acidosis and exercise interventions. Subjects were included as a random effect.

Results

Study 1: Respiratory acidosis

Total CBF increased by 4 ± 1 ml ⋅ 100 g⁻¹ ⋅ min⁻¹ per mmHg elevation in PaCO₂ across a wide range from 35 to 60 mmHg PaCO₂ (P < 0.001; Figure 1(a)); this hypercapnia-mediated increase in CBF was related to narrowing of the venous-arterial PCO₂ difference (P < 0.001; Figure 1(b)) such that trans-cerebral PCO₂ exchange was unaltered with stepwise increases in PaCO₂ (P = 0.057; Figure 1(c)). Arterial [HCO₃⁻] increased by 0.15 ± 0.05 mmol ⋅ l⁻¹ per mmHg elevation in PaCO₂ (P < 0.001; Figure 1(d)). There was a relationship between respiratory acidosis and narrowing of the venous-arterial [HCO₃⁻] and PCO₂ differences; e.g., −0.16 ± 0.11 mmol ⋅ l⁻¹ and −0.52 ± 0.22 mmHg reduction in venous-arterial [HCO₃⁻] and PCO₂ differences per nmol ⋅ l⁻¹ increase in arterial [H⁺], respectively (both P < 0.001; Figure 1(e) and (f)). The venous-arterial [HCO₃⁻] and PCO₂ differences were each related to hypercapnia-induced increases in CBF (both P < 0.001; Figure 1(g)). As such, [HCO₃⁻] exchange was reduced by -0.05 ± 0.08 mmol ⋅ min⁻¹ per nmol ⋅ l⁻¹ increase in arterial [H⁺]; i.e., less venous [HCO₃⁻] efflux/release with respiratory acidosis (P = 0.004; Figure 1(h)).

Figure 1.

Venous-arterial [HCO₃⁻] and PCO₂ exchange with hypercapnic acidosis in humans. Total cerebral blood flow (CBF) increases with stepwise elevations in arterial PCO₂ (PaCO₂) (a); this hypercapnia-mediated increase in CBF is related to narrowing of the venous-arterial PCO₂ difference (b) such that – explained via the Fick principle – trans-cerebral PCO₂ exchange is unaltered with respiratory acidosis (c). Arterial [HCO₃⁻] increases with progressive elevations in PaCO₂ (d); this response may theoretically contribute to localized changes in CBF with severe respiratory acidosis – via increases in extravascular [HCO₃⁻] – and thus, regulatory changes in [HCO₃⁻] may help explain the maximal cerebrovascular vasodilatory reserve to PaCO₂. There is a relationship between reductions in arterial pH (e.g., respiratory acidosis) and narrowing of the venous-arterial [HCO₃⁻] (e) and PCO₂ (f) differences. The reduction in venous-arterial [HCO₃⁻] difference is achieved by a decrease in venous [HCO₃⁻] and increase in arterial [HCO₃⁻]; whereas, PaCO₂ increases to a larger extent relative to PvCO₂ with respiratory acidosis (illustrated by the coloured inlay figures). This narrowing is reflective of less [HCO₃⁻] exchange (h) in part attributable to increases in [HCO₃⁻] and CO₂ ‘wash-out’ due to higher CBF with hypercapnia (g); i.e., the venous-arterial [HCO₃⁻] and PCO₂ differences were each related to increases in CBF (both P < 0.001; subset n=27). There was no relationship between the average venous-arterial in vivo buffering capacity (−Δ [HCO₃⁻] × Δ pH⁻¹) with stepwise respiratory acidosis when indexed against average venous-arterial pH (i). Data are individual values across stepwise progressive increases in PaCO₂ for n = 27 (a, b, c, g, h, i) and n=39 (d, e, f) participants via dynamic end-tidal forcing.

Study 2: Exercise-induced metabolic acidosis

Total CBF was related to PaCO₂ throughout progressive cycling exercise to exhaustion corresponding to 1 ± 1 ml ⋅ 100 g⁻¹ ⋅ min⁻¹ per mmHg change in PaCO₂ (P = 0.004; Figure 2(a)). There was no relationship between CBF and the venous-arterial PCO₂ difference (P = 0.177; Figure 2(b)); therefore, trans-cerebral PCO₂ exchange was unrelated to PaCO₂ during exercise (P = 0.155; Figure 2(c)). Arterial [HCO₃⁻] was reduced by −0.48 ± 0.15 mmol ⋅ l⁻¹ per nmol ⋅ l⁻¹ increase in arterial [H⁺] with exercise-induced acidosis at maximal cycling exercise (P < 0.001; Figure 2(d)). There was no relationship between the venous-arterial [HCO₃⁻] difference and arterial [H⁺] with exercise-induced acidosis (P = 0.801; Figure 2(e)). There was, however, a relationship between widening of the venous-arterial PCO₂ difference by 0.12 ± 0.20 mmHg per nmol ⋅ l⁻¹ increase in arterial [H⁺] during incremental cycling exercise to exhaustion (P = 0.006; Figure 2(f)); i.e., increased PCO₂ uptake reflective of acidosis. The exercise-induced CBF response was unrelated to the venous-arterial [HCO₃⁻] difference (P = 0.682; Figure 2(g)); as such, there were no changes in trans-cerebral [HCO₃⁻] exchange during exercise when indexed against arterial [H⁺] or pH (P = 0.933 and P = 0.896, respectively; Figure 2(h)).

Figure 2.

Venous-arterial [HCO₃⁻] and PCO₂ exchange with progressive submaximal to maximal cycling exercise. Total cerebral blood flow (CBF) is positively related to arterial PCO₂ (PaCO₂) throughout progressive cycling exercise to exhaustion (a); however, there is no relationship between CBF and the venous-arterial PCO₂ difference (b), therefore, trans-cerebral PCO₂ exchange is unrelated to PaCO₂ during exercise (c). Arterial [HCO₃⁻] is reduced with exercise-induced acidosis at maximal exercise (d). There is no relationship between the venous-arterial [HCO₃⁻] difference and arterial pH (e) – as indicated by the equivalent reduction in arterial and venous [HCO₃⁻] – however, there is widening of the venous-arterial PCO₂ difference with exercise-induced acidosis (f) as indicated by a larger relative reduction in PaCO₂ versus PvCO₂ (illustrated by the coloured inlay figures). The exercise-induced CBF response was unrelated to the venous-arterial [HCO₃⁻] difference (g); as such, there were no changes in trans-cerebral [HCO₃⁻] exchange during exercise when indexed against arterial pH (h). There are no relationships between the average venous-arterial in vivo buffering capacity (Δ [HCO₃⁻] × Δ pH⁻¹ or −Δ [La] × Δ pH⁻¹) during incremental cycling exercise to exhaustion (i). These data are reflective of the linear relationship between pH, reductions in [HCO₃⁻], and increases in [La] with exercise-induced acidosis and indicate an unaltered in vivo buffering capacity at maximal cycling exercise. Data are individual values during supine incremental cycling exercise to exhaustion for n = 12 (a, b, c, g, h, i) and n=24 (d, e, f). Exercise stages included various relative (0, 20, 40, 60, 80, 100% maximal workload; n=12 males) and fixed exercise intensities (0, 50, 75, 100 watts; n = 5 females and 0, 75, 100, 125 watts; n=7 males). The in vivo buffering capacity was calculated at 60, 80, 100% maximal workload (I).

In vivo buffering capacity during acute respiratory acidosis and exercise

Linear mixed-model analysis revealed no significant relationship between in vivo buffering capacity (−Δ [HCO₃⁻] × Δ pH⁻¹) and the average venous-arterial pH during stepwise respiratory acidosis (P = 0.078; Figure 1(i)). Additionally, there were no relationships between in vivo buffering capacity (Δ [HCO₃⁻] × Δ pH⁻¹ or -Δ [La] × Δ pH⁻¹) and the average venous-arterial pH during incremental cycling exercise to exhaustion (P = 0.187 and P = 0.392, respectively; Figure 2(i)).

Discussion

The results of this study indicate that trans-cerebral [HCO₃⁻] and PCO₂ exchange are differentially regulated in response to acute respiratory and exercise-induced acidosis. This finding is supported by: 1) acute hypercapnic acidosis elevated arterial [HCO₃⁻]; however, trans-cerebral [HCO₃⁻] exchange was reduced (i.e., indicating a shift from net release toward net uptake of [HCO₃⁻]) and this was reflective in narrower venous-arterial [HCO₃⁻] and PCO₂ differences with higher CBF (Figure 1); and 2) arterial [HCO₃⁻] was progressively reduced with maximal exercise-induced acidosis and this was unrelated to venous-arterial [HCO₃⁻] and PCO₂ exchange (Figure 2). Additionally, these results show there are no appreciable changes in the in vivo buffering capacity across a wide range of respiratory acidosis (up to +20 mmHg PaCO₂) or incremental cycling exercise to exhaustion in humans (Figures 1(i) and 2(i)).

Respiratory versus metabolic acidosis – influence of [HCO₃⁻] exchange

The two experimental interventions of acute respiratory acidosis and exercise-induced metabolic acidosis each provoke equivalent reductions in pH with a markedly disparate arterial [HCO₃⁻] response. For example, the reductions in arterial [HCO₃⁻] during exercise are reflective of a 3-fold larger rate of change in [HCO₃⁻] versus the small increases in [HCO₃⁻] observed with respiratory acidosis (e.g., approx. -0.48 vs. +0.15 mmol ⋅ l⁻¹ [HCO₃⁻] per nmol ⋅ l⁻¹ increase in [H⁺]). Additionally, it is noteworthy to consider how alterations in CMRO₂ with exercise and respiratory acidosis will affect $\dot{V}$ CO₂ and, therefore, PvCO₂.^{25,35,64–66}

Increases in PaCO₂ facilitate rapid passive diffusion of CO₂ across the BBB via reduced CSF driving pressure leading to increases in intra- and extracellular [H⁺], thus provoking acidosis through acid-base re-equilibration^67,68 in accordance with Le Chatelier’s Principle (rightward shift in equation (11)).⁶⁹ Overall, CO₂ transport is the sum of the diffusion of 1) dissolved CO₂; and 2) CO₂ bound as HCO₃⁻ (i.e., “facilitated CO₂ diffusion”) – this process involves a flux of H⁺ equivalent to that of HCO₃⁻ as per equation (11).^70–72 ${CO}_{2} + H_{2} O CA H_{2} {CO}_{3} \leftrightarrow {HCO}_{3}^{-} + H^{+}$ (11)

Acute respiratory acidosis increases arterial [HCO₃⁻] via continuous conversion of CO₂ – rapidly catalyzed by carbonic anhydrase (CA) – shifting the equilibrium relationship toward the [HCO₃⁻] buffer reaction. This is reflective of rapid increases in extracellular [HCO₃⁻] due to intracellular [HCO₃⁻] exchange with Cl^- to buffer the [H⁺] produced via CO₂ hydration.⁷³ Importantly, however, the ratio of arterial [HCO₃⁻] to PaCO₂ is lower such that pH is reduced (as per equation (2)).

At rest, the relative contributions of dissolved CO₂, chemically bound to hemoglobin and protein carbamate, and HCO₃⁻ to total CO₂ exchange are approximately 5%, 10% and 85%, respectively. During severe exercise, HCO₃⁻ facilitated CO₂ diffusion is reduced to approximately 2/3 of total CO₂ exchange (as the contribution of dissolved CO₂ increases sevenfold with exercise-induced acidosis).^71,74 With maximal exercise-induced acidosis, arterial [HCO₃⁻] is markedly reduced due to compensatory buffering of [H⁺] leading to production of CO₂ and H₂O (leftward shift in equation (11)); the excess CO₂ is then removed via hyperventilation.^21,22

Acute elevations in cerebral blood flow stabilize [HCO₃⁻] and PCO₂ gradients

Acute respiratory acidosis provokes three key regulatory responses: 1) arterial [HCO₃⁻] increases (via inter-conversion of CO₂) in response to elevated PaCO₂ (Figure 1(d)); 2) there is rapid/transient exchange of HCO₃⁻ and Cl⁻ across the BBB and between brain tissue and extracellular fluid;^47,73 and 3) total CBF increases thus reducing the difference between arterial and extravascular PCO₂, thereby limiting the rise in intracellular tissue PCO₂ (Figure 1(b)). The increase in CBF in response to PaCO₂ is restricted by the maximal vasodilatory response to hypercapnia in vivo in humans; e.g., +15–20 mmHg PaCO₂ equates to 150% increase in CBF.⁵⁴ Elevations in arterial [HCO₃⁻] may theoretically contribute to localized changes in CBF with severe respiratory acidosis – via increases in extravascular [HCO₃⁻] – and thus, regulatory changes in [HCO₃⁻] may help explain the maximal cerebrovascular vasodilatory reserve to PaCO₂. The internal jugular venous-arterial [HCO₃⁻] difference is reduced with acute severe hypercapnic acidosis (Figure 1(e)) – a response likely explained by increased extravascular HCO₃⁻ ‘wash-out’ as the reduction in trans-cerebral [HCO₃⁻] difference is related to the hypercapnic CBF response (P < 0.001; Figure 1(g)). Arvidsson and colleagues (1981) showed that intravenous infusion of NaHCO₃ in hypercapnic dogs causes a 50–70% reduction from the PaCO₂-induced higher CBF. Additionally, cerebrospinal fluid [HCO₃⁻] ([HCO₃⁻]_CSF) was appreciably higher following NaHCO₃; these data indicate that acutely elevated PaCO₂ may facilitate the transport of exogenous HCO₃⁻ across the BBB (via higher CBF) resulting in cerebral vasoconstriction due to increased extravascular pH.⁷⁵ The present results support that the CBF response to acute respiratory acidosis contributes to tight regulation of cerebrovascular pH (via narrowing of the trans-cerebral [HCO₃⁻] and PCO₂ differences) in vivo in humans.

Regulation of cerebrovascular acid-base balance during exercise

Trans-cerebral venous-arterial [HCO₃⁻] exchange was unaffected by maximal exercise-induced acidosis (e.g., pH 7.20; Figure 2(e)) and there was a widening of the venous-arterial PCO₂ difference during incremental cycling exercise to exhaustion (Figure 2(f)). Total CBF increases steadily by 10−20% up to intensities of approximately 60–70% of maximal oxygen uptake ( $\dot{V}$ O_2max) to regulate cerebral substrate delivery,^32,76 and is mediated via relative alveolar hypoventilation (i.e., elevations in PaCO₂) and increases in CMRO₂.^25,36,77 With progressive increases in cycling exercise intensity, the relatively linear CMRO₂ response is coupled to cerebral oxygen delivery (CBF × CaO₂) and oxygen extraction (CaO₂ – CvO₂/CaO₂) × 100%)) rather than CBF per se. At maximal exercise, hyperventilatory-induced reductions in CBF – together with marked acidosis – would conceivably adversely affect intracellular/extravascular [HCO₃⁻] and CO₂ ‘wash-out’. Previous work by Bisgard and colleagues (1978) reported unaltered [HCO₃⁻]_CSF and PaCO₂-mediated increases in CSF pH (via hyperventilatory response) with severe near-maximal exercise in ponies⁷⁸ – these data emphasize the importance of the ventilatory response on cerebrovascular PCO₂/[HCO₃⁻] regulation during exercise.⁶ Additionally, albeit during resting conditions, several studies report that sustained changes in CSF/extracellular [HCO₃⁻] respond slowly (e.g., several hours) and only partially to systemic changes in arterial [HCO₃⁻],^50,79–84 thus explaining the steady [HCO₃⁻]_CSF during acute exercise (<10 minutes). Taken together with the remarkably consistent trans-cerebral [HCO₃⁻] exchange (Figure 2(h)) and in vivo [HCO₃⁻] and [La] buffering capacity throughout exercise (Figure 2(i)), these data indicate that the CBF response to exercise per se likely plays a lesser role in cerebrovascular acid-base regulation (versus respiratory acidosis, for example).

Experimental considerations

A key strength of this study was the invasive direct Kety-Schmidt technique to quantify trans-cerebral venous-arterial exchange of [HCO₃⁻], PCO₂, and [H⁺] in vivo in healthy humans; we recognize that this relies on the assumption that these sampling sites are reflective of trans-cerebral exchange due to practical and ethical experimental constraints.⁸⁵ Additionally, the Duplex ultrasound derived indexes of extra-cranial blood flow utilized in the current experiment are assumed to indicate cerebral tissue nutritive flow. Although we verified PaCO₂ values with blood gas sampling, without temperature correcting to adjust for higher blood temperature during exercise, we may have underestimated PaCO₂ by 1–2 mmHg (due to reduced CO₂ solubility at higher temperatures).^86,87 Additionally, the influences of exercise^88–90 and temperature^91–95 may conceivably have a small effect on PCO₂, H⁺, and HCO₃⁻ via alterations in carbonic anhydrase activity in the exercise trial^96,97 – however, this is unlikely to differentially affect the trans-cerebral venous-arterial exchange values within participants during this trial.

A subset of female participants completed the +8 mmHg PaCO₂ respiratory acidosis protocol as well as the fixed intensity submaximal exercise protocol (n = 5 females and n = 7 males). Previous literature is equivocal, with studies reporting that cerebrovascular CO₂ reactivity is higher in females,^98–100 higher in males,^101–103 or not different between sexes.^104,105 Whether sex-related differences in cerebrovascular CO₂ reactivity exist between females and males in response to a fixed targeted elevation in PaCO₂, this is unlikely to affect our results as we saw no change in the in vivo buffering capacity (Δ [HCO₃⁻] × Δ pH⁻¹) when indexed against the average venous-arterial pH during respiratory acidosis. These results indicate that any within-subject change in the sensitivity of CBF to PaCO₂ would be reflected in an equivalent pH response (with which we indexed against our dependent variables). Additionally, recent studies have shown no difference in the CBF response to moderate-intensity exercise between females and males^106,107 with sex-related differences in CBF regulation only apparent at severe intensity exercise (80–100% maximal workload).¹⁰⁸ As such, we do not expect that the inclusion of 5 female participants in the fixed intensity submaximal exercise protocol (0, 50, 75, 100 watts) would affect the variability of our results. Future investigations are merited to address any sex-related differences in cerebrovascular acid-base regulation in response to respiratory and exercise-induced metabolic acidosis.

Conclusion

Taken together, these results indicate that increases and decreases in systemic arterial [HCO₃⁻] – during acute respiratory/exercise-induced metabolic acidosis, respectively – differentially contribute to cerebrovascular acid-base balance. The previously recognized tight regulation of cerebral interstitial pH during acute hypercapnic acidosis^19,20 is likely attributable to: 1) narrower internal jugular venous-arterial [HCO₃⁻] and PCO₂ differences; 2) higher CBF; and 3) reductions in trans-cerebral [HCO₃⁻] exchange indicating less venous [HCO₃⁻] efflux/release. These results are supportive of unaltered trans-cerebral [HCO₃⁻] exchange with exercise-induced acidosis – consistent with the unchanged in vivo buffering capacity – which was unrelated to the CBF response throughout progressive cycling exercise to exhaustion.

Footnotes

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Funding

The author(s) declared the following potential conflicts of interest with respect to the research,authorship,and/or publication of this article: Philip N. Ainslie was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and a Canada Research Chair. Hannah G. Caldwell was funded by a NSERC PGS-Doctoral Scholarship. Patrice Brassard was funded by the Foundation of the Institut universitaire de cardiologie et de pneumologie de Québec and a NSERC Discovery Grant. Damian M. Bailey was supported by a Royal Society Wolfson Research Fellowship (#WM170007) and Higher Education Funding Council for Wales (Postdoctoral Fellowship for Benjamin S. Stacey).

Acknowledgements

We would like to extend our thanks to the volunteers who participated in these studies. Additionally,we would like to thank past and present members of the Centre for Heart,Lung and Vascular Health at UBCO for their contribution to this series of studies by our group.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Authors’ contributions

Study design: RLH,KJS,PB,ARB,DMB,PNA. Data collection: HGC,RLH,KJS,PB,ARB,MMT,CAH,JMJRC,BSS,DMB,AD,MSS,DBM,PNA. Data analysis: HGC,RLH,KJS,PB,ARB. Data interpretation: HGC,RLH,JMJRC,DMB,PNA. Drafted manuscript: HGC. Critically reviewed manuscript: HGC,RLH,KJS,PB,ARB,MMT,CAH,JMJRC,BSS,DMB,AD,MSS,DBM,PNA. Approved final version: HGC,RLH,KJS,PB,ARB,MMT,CAH,JMJRC,BSS,DMB,AD,MSS,DBM,PNA.

ORCID iD

Hannah G Caldwell

References

Lennox

Gibbs

EL.

The blood flow in the brain and the leg of man, and the changes induced by alteration of blood gases. J Clin Invest 1932; 11: 1155–1177.

Kety

Schmidt

CF.

The effects of active and passive hyperventilation on cerebral blood flow, cerebral oxygen consumption, cardiac output, and blood pressure of normal young men. J Clin Invest 1946; 25: 107–119.

Kety

Schmidt

CF.

The effects of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men. J Clin Invest 1948; 27: 484–492.

Hoiland

Fisher

Ainslie

PN.

Regulation of the cerebral circulation by arterial carbon dioxide. Compr Physiol 2019; 9: 1101–1154.

Fencl

Vale

Broch

JA.

Respiration and cerebral blood flow in metabolic acidosis and alkalosis in humans. J Appl Physiol 1969; 27: 67–76.

Carr

JMJR

Caldwell

Ainslie

PN.

Cerebral blood flow, cerebrovascular reactivity and their influence on ventilatory sensitivity. Exp Physiol 2021; 106: 1425–1448.

Shapiro

Wasserman

Patterson

JL.

Mechanism and pattern of human cerebrovascular regulation after rapid changes in blood CO2 tension. J Clin Invest 1966; 45: 913–922.

Severinghaus

Lassen

Step hypocapnia to separate arterial from tissue PCO2 in the regulation of cerebral blood flow. Circ Res 1967; 20: 272–278.

Poulin

Liang

Robbins

PA.

Dynamics of the cerebral blood flow response to step changes in end-tidal PCO2 and PO2 in humans. J Appl Physiol (1985) 1996; 81: 1084–1095.

10.

Hoiland

Smith

Carter

, et al. Shear-mediated dilation of the internal carotid artery occurs independent of hypercapnia. Am J Physiol Heart Circ Physiol 2017; 313: H24–H31.

11.

Willie

Macleod

Smith

, et al. The contribution of arterial blood gases in cerebral blood flow regulation and fuel utilization in man at high altitude. J Cereb Blood Flow Metab 2015; 35: 873–881.

12.

Posner

Swanson

Plum

Acid-base balance in cerebrospinal fluid. Arch Neurol 1965; 12: 479–496.

13.

Siesjö

BK.

Symposium on acid-base homeostasis. The regulation of cerebrospinal fluid pH. Kidney Int 1972; 1: 360–374.

14.

Lambertsen

CJ.

Carbon dioxide and respiration in acid-base homeostasis. Anesthesiology 1960; 21: 642–651.

15.

Merwarth

Sieker

Manfredi

Acid-base relations between blood and cerebrospinal fluid in normal subjects and patients with respiratory insufficiency. N Engl J Med 1961; 265: 310–313.

16.

Manfredi

Acid-base relations between serum and cerebrospinal fluid in man under normal and abnormal conditions. J Lab Clin Med 1962; 59: 128–136.

17.

Pontén

Siesjö

BK.

Gradients of CO2 tension in the brain. Acta Physiol Scand 1966; 67: 129–140.

18.

Messeter

Siesjö

BK.

Regulation of the CSF pH in acute and sustained respiratory acidosis. Acta Physiol Scand 1971; 83: 21–30.

19.

Fencl

Miller

Pappenheimer

JR.

Studies on the respiratory response to disturbances of acid-base balance, with deductions concerning the ionic composition of cerebral interstitial fluid. Am J Physiol 1966; 210: 459–472.

20.

Fencl

Rossing

TH.

Acid-base disorders in critical care medicine. Annu Rev Med 1989; 40: 17–29.

21.

Nielsen

Hein

Svendsen

, et al. Bicarbonate attenuates intracellular acidosis. Acta Anaesthesiol Scand 2002; 46: 579–584.

22.

Robergs

Ghiasvand

Parker

Biochemistry of exercise-induced metabolic acidosis. Rockville: American Physiological Society, 2004.

23.

Scheinberg

Blackburn

Rich

, et al. Effects of vigorous physical exercise on cerebral circulation and metabolism. Am J Med 1954; 16: 549–554.

24.

Smith

Wildfong

Hoiland

, et al. Role of CO2 in the cerebral hyperemic response to incremental normoxic and hyperoxic exercise. J Appl Physiol 2016; 120: 843–854.

25.

Smith

Ainslie

PN.

Regulation of cerebral blood flow and metabolism during exercise. Exp Physiol 2017; 102: 1356–1371.

26.

Ide

Schmalbruch

Quistorff

, et al. Lactate, glucose and O2 uptake in human brain during recovery from maximal exercise. J Physiol 2000; 522: 159–164.

27.

Nybo

Møller

Pedersen

, et al. Association between fatigue and failure to preserve cerebral energy turnover during prolonged exercise. Acta Physiol Scand 2003; 179: 67–74.

28.

Larsen

Rasmussen

Overgaard

, et al. Non‐selective β‐adrenergic blockade prevents reduction of the cerebral metabolic ratio during exhaustive exercise in humans. J Physiol 2000; 586: 2807–2815.

29.

Brassard

Seifert

Wissenberg

, et al. Phenylephrine decreases frontal lobe oxygenation at rest but not during moderately intense exercise. J Appl Physiol 2010; 108: 1472–1478.

30.

Rasmussen

Nielsen

Overgaard

, et al. Reduced muscle activation during exercise related to brain oxygenation and metabolism in humans. J Physiol 2010; 588: 1985–1995.

31.

Rasmussen

Overgaard

Bjerre

, et al. The effects of normoxia, hypoxia, and hyperoxia on cerebral haemoglobin saturation using near infrared spectroscopy during maximal exercise. Int J Ind Ergon 2010; 40: 190–196.

32.

Fisher

Hartwich

Seifert

, et al. Cerebral perfusion, oxygenation and metabolism during exercise in young and elderly individuals. J Physiol 2013; 591: 1859–1870.

33.

Trangmar

Chiesa

Stock

, et al. Dehydration affects cerebral blood flow but not its metabolic rate for oxygen during maximal exercise in trained humans. J Physiol 2014; 592: 3143–3160.

34.

Dalsgaard

Quistorff

Danielsen

, et al. A reduced cerebral metabolic ratio in exercise reflects metabolism and not accumulation of lactate within the human brain. J Physiol 2014; 554: 571–578.

35.

Dalsgaard

MK.

Fuelling cerebral activity in exercising man. J Cereb Blood Flow Metab 2006; 26: 731–750.

36.

Smith

MacLeod

Willie

, et al. Influence of high altitude on cerebral blood flow and fuel utilization during exercise and recovery. J Physiol 2014; 592: 5507–5527.

37.

Oldendorf

Braun

Cornford

pH dependence of blood-brain barrier permeability to lactate and nicotine. Stroke 1979; 10: 577–581.

38.

Knudsen

Paulson

Hertz

MM.

Kinetic analysis of the human blood-brain barrier transport of lactate and its influence by hypercapnia. J Cereb Blood Flow Metab 1991; 11: 581–586.

39.

Hertz

Dienel

GA.

Lactate transport and transporters: general principles and functional roles in brain cells. J Neurosci Res 2005; 79: 11–18.

40.

Volianitis

Fabricius-Bjerre

Overgaard

, et al. The cerebral metabolic ratio is not affected by oxygen availability during maximal exercise in humans. J Physiol 2008; 586: 107–112.

41.

Seifert

Brassard

Jørgensen

, et al. Cerebral non-oxidative carbohydrate consumption in humans driven by adrenaline. J Physiol 2009; 587: 285–293.

42.

van Hall

Strømstad

Rasmussen

, et al. Blood lactate is an important energy source for the human brain. J Cereb Blood Flow Metab 2009; 29: 1121–1129.

43.

Bailey

Evans

McEneny

, et al. Exercise-induced oxidative-nitrosative stress is associated with impaired dynamic cerebral autoregulation and blood-brain barrier leakage. Exp Physiol 2011; 96: 1196–1207.

44.

van Hall

Lactate kinetics in human tissues at rest and during exercise. Acta Physiol (Oxf) 2010; 199: 499–508.

45.

Overgaard

Rasmussen

Bohm

, et al. Hypoxia and exercise provoke both lactate release and lactate oxidation by the human brain. Faseb J 2012; 26: 3012–3020.

46.

Hoiland

Howe

Coombs

, et al. Ventilatory and cerebrovascular regulation and integration at high-altitude. Clin Auton Res 2018; 28: 423–435.

47.

Ahmad

Loeschcke

HH.

Fast bicarbonate-chloride exchange between plasma and brain extracellular fluid at maintained PCO2. Pflugers Arch 1982; 395: 300–305.

48.

Friis

Paulson

Hertz

MM.

Carbon dioxide permeability of the blood-brain barrier in man. The effect of acetazolamide. Microvasc Res 1980; 20: 71–80.

49.

Johnson

Hoop

Kazemi

Movement of CO2 and HCO-3 from blood to brain in dogs. J Appl Physiol Respir Environ Exerc Physiol 1983; 54: 989–996.

50.

Bradley

Semple

SJ.

A comparison of certain acid-base characteristics of arterial blood, jugular venous blood and cerebrospinal fluid in man, and the effect on them of some acute and chronic acid-base disturbances. J Physiol 1962; 160: 381–391.

51.

van Heijst

Maas

Visser

BF.

Comparison of the acid-base balance in cisternal and lumbar cerebrospinal fluid. Pflugers Arch Gesamte Physiol Menschen Tiere 1966; 287: 242–246.

52.

Bain AR, Hoiland RL, Donnelly J, et al. Cerebral metabolism, oxidation, and inflammation in severe passive hyperthermia with and without respiratory alkalosis. J Physiol 598: 943–954.

53.

Hoiland

Caldwell

Howe

, et al. Nitric oxide is fundamental to neurovascular coupling in humans. J Physiol 2020; 598: 4927–4939.

54.

Willie

Macleod

Shaw

, et al. Regional brain blood flow in man during acute changes in arterial blood gases. J Physiol 2012; 590: 3261–3275.

55.

Lumb

AB.

Nunn's applied respiratory physiology eBook. 8 ed. 2016.

56.

West

Luks

AM.

West's respiratory physiology. 11 ed. 2020.

57.

Cullen

Keeler

Robinson

HW.

The pK′ of the Henderson-Hasselbalch equation for hydrogen concentration of serum. J Biol Chem 1925; 66: 301–322.

58.

Böning

Maassen

Thomas

, et al. Extracellular pH defense against lactic acid in normoxia and hypoxia before and after a Himalayan expedition. Eur J Appl Physiol 2001; 84: 78–86.

59.

Nordsborg

Siebenmann

Jacobs

, et al. Four weeks of normobaric ‘live high-train low’ do not alter muscular or systemic capacity for maintaining pH and K⁺ homeostasis during intense exercise. J Appl Physiol (1985) 2012; 112: 2027–2036.

60.

Tymko

Ainslie

Macleod

, et al. End tidal-to-arterial CO2 and O2 gas gradients at low- and high-altitude during dynamic end-tidal forcing. Am J Physiol Regul Integr Comp Physiol 2015; 308: R895–R906.

61.

Tymko

Hoiland

Kuca

, et al. Measuring the human ventilatory and cerebral blood flow response to CO2: a technical consideration for the end-tidal-to-arterial gas gradient. J Appl Physiol (1985) 2016; 120: 282–296.

62.

Howe

Caldwell

Carr

, et al. Cerebrovascular reactivity to carbon dioxide is not influenced by variability in the ventilatory sensitivity to carbon dioxide. Exp Physiol 2020; 105: 904–915.

63.

Groeneveld

AB.

Interpreting the venous-arterial PCO2 difference. Crit Care Med 1998; 26: 979–980.

64.

Siesjö

BK.

Cerebral metabolic rate in hypercarbia – a controversy. Anesthesiology 1980; 52: 461–465.

65.

Dalsgaard

Ogoh

Dawson

, et al. Cerebral carbohydrate cost of physical exertion in humans. Am J Physiol Regul Integr Comp Physiol 2004; 287: R534–R540.

66.

Yablonskiy

DA.

Cerebral metabolic rate in hypercapnia: controversy continues. J Cereb Blood Flow Metab 2011; 31: 1502–1503.

67.

Jensen

Thomsen

Henriksen

In vivo measurement of intracellular pH in human brain during different tensions of carbon dioxide in arterial blood. A 31P-NMR study. Acta Physiol Scand 1988; 134: 295–298.

68.

Vorstrup

Jensen

Thomsen

, et al. Neuronal pH regulation: constant normal intracellular pH is maintained in brain during low extracellular pH induced by acetazolamide–31P NMR study. J Cereb Blood Flow Metab 1989; 9: 417–421.

69.

L, Chatelier

HL.

Comptes rendus. France: French Academy of Sciences, 1884, p.99.

70.

Longmuir

Forster

Woo

C-Y.

Diffusion of carbon dioxide through thin layers of solution. Nature 1966; 209: 393–394.

71.

Geers

Gros

Carbon dioxide transport and carbonic anhydrase in blood and muscle. Physiol Rev 2000; 80: 681–715.

72.

Swenson

ER.

Does aerobic respiration produce carbon dioxide or hydrogen ion and bicarbonate?

Anesthesiology 2018; 128: 873–879.

73.

Ahmad

Loeschcke

HH.

Fast bicarbonate-chloride exchange between brain cells and brain extracellular fluid in respiratory acidosis. Pflugers Arch 1982; 395: 293–299.

74.

Bangsbo

Johansen

Graham

, et al. Lactate and H+ effluxes from human skeletal muscles during intense, dynamic exercise. J Physiol 1993; 462: 115–133.

75.

Arvidsson

Häggendal

Winsö

Influence on cerebral blood flow of infusion of sodium bicarbonate during respiratory acidosis and alkalosis in the dog. Acta Anaesthesiol Scand 1981; 25: 146–152.

76.

Ide

Secher

NH.

Cerebral blood flow and metabolism during exercise. Prog Neurobiol 2000; 61: 397–414.

77.

Nybo

Møller

Volianitis

, et al. Effects of hyperthermia on cerebral blood flow and metabolism during prolonged exercise in humans. J Appl Physiol 2002; 93: 58–64.

78.

Bisgard

Forster

Byrnes

, et al. Cerebrospinal fluid acid-base balance during muscular exercise. J Appl Physiol Respir Environ Exerc Physiol 1978; 45: 94–101.

79.

Robin

Whaley

Crump

, et al. Acid-base relations between spinal fluid and arterial blood with special reference to control of ventilation. J Appl Physiol 1958; 13: 385–392.

80.

Bradley

Semple

Spencer

GT.

Rate of change of carbon dioxide tension in arterial blood, jugular venous blood and cisternal cerebrospinal fluid on carbon dioxide administration. J Physiol 1965; 179: 442–455.

81.

Hornbein

Pavlin

EG.

Distribution of H+ and HCO3 minus between CSF and blood during respiratory alkalosis in dogs. Am J Physiol 1975; 228: 1149–1154.

82.

Pavlin

Hornbein

TF.

Distribution of H+ and HCO3 minus between CSF and blood during metabolic alkalosis in dogs. Am J Physiol-Legacy Content 228: 1141–1144.

83.

Nattie

Romer

CSF HCO3- regulation in isosmotic conditions: the role of brain PCO2 and plasma HCO3. Respir Physiol 1978; 33: 177–198.

84.

Abeysekara

Zello

Lohmann

, et al. Infusion of sodium bicarbonate in experimentally induced metabolic acidosis does not provoke cerebrospinal fluid (CSF) acidosis in calves. Can J Vet Res 76: 16–22.

85.

Kety

Schmidt

CF.

The nitrous oxide method for the quantitative determination of cerebral blood flow in man: theory, procedure and normal values. J Clin Invest 1948; 27: 476–483.

86.

Bacher

Effects of body temperature on blood gases. Intens Care Med 2005; 31: 24–27.

87.

Losa-Reyna

Torres-Peralta

Henriquez

JJG

, et al. Arterial to end‐tidal PCO2 difference during exercise in normoxia and severe acute hypoxia: importance of blood temperature correction. Physiol Rep 2015; 3: e12512.

88.

Swenson

Maren

TH.

A quantitative analysis of CO2 transport at rest and during maximal exercise. Respir Physiol 1978; 35: 129–159.

89.

Wistrand

PJ.

The importance of carbonic anhydrase B and C for the unloading of CO2 by the human erythrocyte. Acta Physiol Scand 1981; 113: 417–426.

90.

Swenson

ER.

Respiratory and renal roles of carbonic anhydrase in gas exchange and acid-base regulation. EXS 2000; 90; 281–341.

91.

Meldrum

Roughton

FJ.

Carbonic anhydrase. Its preparation and properties. J Physiol 1933; 80: 113–142.

92.

Roughton

FJ.

Proceedings of the physiological society. J Physiol 107: 12–13.

93.

Maren

TH.

Carbonic anhydrase kinetics and inhibition at 37 degrees: an approach to reaction rates in vivo. J Pharmacol Exp Ther 1963; 139: 129–139.

94.

Kernohan

JC.

The effect of temperature on the catalytic activity of bovine carbonic anhydrase. Adv Exp Med Biol 1972; 28: 189–199.

95.

Sanyal

Maren

TH.

Thermodynamics of carbonic anhydrase catalysis. A comparison between human isoenzymes B and C. J Biol Chem 1981; 256: 608–612.

96.

Roughton

Booth

VH.

The effect of substrate concentration, pH and other factors upon the activity of carbonic anhydrase. Biochem J 1946; 40: 319–330.

97.

Krebs

HA.

Carbonic anhydrase as a tool in studying the mechanism of enzymic reactions involving H2CO3, CO2 or HCO3. Biochem J 1948; 42: lxi.

98.

Kastrup

Thomas

Hartmann

, et al. Sex dependency of cerebrovascular CO2 reactivity in normal subjects. Stroke 1997; 28: 2353–2356.

99.

Kastrup

Happe

Hartmann

, et al. Gender-related effects of indomethacin on cerebrovascular CO2 reactivity. J Neurol Sci 1999; 162: 127–132.

100.

Minhas

Panerai

Robinson

TG.

Sex differences in cerebral haemodynamics across the physiological range of PaCO2. Physiol Meas 2018; 39: 105009.

101.

Kassner

Winter

Poublanc

, et al. Blood-oxygen level dependent MRI measures of cerebrovascular reactivity using a controlled respiratory challenge: reproducibility and gender differences. J Magn Reson Imaging 2010; 31: 298–304.

102.

Barnes

JN.

Sex-specific factors regulating pressure and flow. Exp Physiol 2017; 102: 1385–1392.

103.

Miller

Howery

Rivera-Rivera

, et al. Age-Related reductions in cerebrovascular reactivity using 4D flow MRI. Front Aging Neurosci 11: 281.

104.

Madureira

Castro

Azevedo

Demographic and systemic hemodynamic influences in mechanisms of cerebrovascular regulation in healthy adults. J Stroke Cerebrovasc Dis 2017; 26: 500–508.

105.

Barnes

Charkoudian

Integrative cardiovascular control in women: Regulation of blood pressure, body temperature, and cerebrovascular responsiveness. Faseb J 2021; 35: e21143.

106.

Murrell

Cotter

Thomas

, et al. Cerebral blood flow and cerebrovascular reactivity at rest and during sub-maximal exercise: effect of age and 12-week exercise training. Age (Dordr) 2013; 35: 905–920.

107.

Ward

Craig

Liu

, et al. Effect of healthy aging and sex on Middle cerebral artery blood velocity dynamics during moderate-intensity exercise. Am J Physiol Heart Circ Physiol 2018; 315: H492–H501.

108.

Ashley

Shelley

Sun

, et al. Cerebrovascular responses to graded exercise in young healthy males and females. Physiol Rep 2020; 8: e14622.

109.

Carr JMJR, Caldwell HG, Carter H, et al. The stability of cerebrovascular CO₂ reactivity following attainment of physiological steady-state. Exp Physiol 106: 2542–2555.

Trans-cerebral HCO 3 − and PCO 2 exchange during acute respiratory acidosis and exercise-induced metabolic acidosis in humans

Abstract

Keywords

Introduction

Methods

Ethical approval

Participants

Experimental overview

Study 1: Respiratory acidosis protocols

Study 2: Exercise protocols

Blood sampling

Acid-base balance data analysis

Cardiorespiratory

Cerebrovascular

Statistical analyses

Results

Study 1: Respiratory acidosis

Study 2: Exercise-induced metabolic acidosis

In vivo buffering capacity during acute respiratory acidosis and exercise

Discussion

Respiratory versus metabolic acidosis – influence of [HCO3−] exchange

Acute elevations in cerebral blood flow stabilize [HCO3−] and PCO2 gradients

Regulation of cerebrovascular acid-base balance during exercise

Experimental considerations

Conclusion

Footnotes

Data availability statement

Funding

Acknowledgements

Declaration of conflicting interests

Authors’ contributions

ORCID iD

References

Respiratory versus metabolic acidosis – influence of [HCO₃⁻] exchange

Acute elevations in cerebral blood flow stabilize [HCO₃⁻] and PCO₂ gradients