Summary
The effects of administration of triiodothyronine (T3) to euthyroid and thyroidectomized rats on hepatic adenine nucleotides were measured using a rapid-freeze technique in situ. The respiratory effects (QO2) of T3 were measured in parallel groups of rats. Total adenine nucleotides (ATP + ADP + AMP) was unchanged after injections of T3 in thyroidectomized or euthyroid rats. Administration of T3 to thyroidectomized rats produced a 119% increase in QO2 but had no statistically significant effect on ATP or ADP content, or on the ATP/ADP ratio—although the mean value of this ratio was slightly lower after T3. In euthyroid rats, T3 stimulated QO2 by 69% and significantly lowered ATP and raised ADP content; resulting in a 38% decrease in the ATP/ADP ratio. Previous studies implicated an increase in energy utilization by the Na+ pump in thyroid thermogenesis. It appears unlikely that a primary action of the hormone on mitochondrial metabolism could account for the increase in energy expenditure for Na+ transport. Combined effects of thyroid hormone via parallel pathways on both mitochondrial oxidative phosphorylation and on the Na+ pump, however, remains a possibility.
The authors are grateful for the able technical assistance provided by Miss Allison Meader.
