Time-Course of the Effects of Thyroid Hormone on Respiration and Na + + K + -ATPase Activity in Rat Liver 1

Abstract

Summary

As a further test of the hypothesis that increased energy expenditure in active Na⁺ transport mediates a major part of thyroid thermogenesis, time-course measurements were made of hepatic oxygen consumption (Qo₂), Na⁺-dependent oxygen consumption [Qo₂(t)] and Na⁺ + K⁺ adenosine-triphosphatase (NaK-ATPase) activity after a single or repeated injections of triiodothyronine (T₃) into thyroidectomized and euthyroid rats. In both groups, after single injections, Qo₂ and Qo₂(t) increased to peak values at 48 hr and declined in parallel to close to the control level by the 6th day. In euthyroid rats, NaK-ATPase activity manifested the same time-course with a peak increase at 48 hr. In both groups, after repeated injections (every other day ×3), there were curvilinear increases in Qo₂, Qo₂(t) and NaK-ATPase activity to steady-state levels in 2-3 days. These findings are in accord with the inference that thyroidal stimulation of the Na⁺ pump mediates the increase in Qo₂(t). In addition, we found that in hypothyroid rats there was an increase in Na⁺-independent respiration and Mg-ATPase activity with a peak response at 48 hr; this effect was transient even with repeated injections of T₃.