Thyroid hormones and the energetics of active sodium‐potassium transport in mammalian skeletal muscles.

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Abstract
The steady-state heat production rate (.ovrhdot.E) of soleus muscles from adult mice in various thyroid states was measured in a perfused microcalorimeter. The ouabain-suppressible fractions of .ovrhdot.E and 42K influx were compared and the energetic efficiency of active Na-K transport assessed. Hypothyroidism with plasma thyroxine concentrations below 1 .mu.g/100 ml was induced by pretreatment with 131I or perchlorate. In soleus muscles isolated from treated animals, mean .ovrhdot.E values were 25.1 .+-. 0.7 and 24.2 .+-. 0.5 mcal .cntdot. g wet wt-1 min-1 for the 131I and the perchlorate series, respectively, .apprx. 30% lower than the control level (36.3 .+-. 1.5 mcal .cntdot. g wt wt-1 min-1). Following triiodothyronine treatment, .ovrhdot.E was increased .apprx. 45%. In muscles from hypothyroid (131I and perchlorate series), euthyroid and hyperthyroid mice, ouabain (10-3 M) induced a rapid decrease in .ovrhdot.E of 1.6 .+-. 0.1 and 1.4 .+-. 0.1, 2.5 .+-. 0.2 and 4.3 .+-. 0.6 mcal .cntdot. g wet wt-1 min-1, respectively, between 6 and 8% of .ovrhdot.E. In muscles obtained from hypothyroid, euthyroid and hyperthyroid mice, the ouabain-suppressible component of 42K influx was 0.17 .+-. 0.04, 0.31 .+-. 0.02 and 0.45 .+-. 0.02 .mu.mol .cntdot. g wet wt-1 min-1, respectively. Whereas the total number of ouabain binding sites varied appreciably with the thyroid status, the Na-K contents of soleus or diaphragm muscles showed no significant changes. Despite the parallelism between the changes in basal .ovrhdot.E and ouabain-sensitive components of .ovrhdot.E and K influx with the thyroid status, active Na-K transport apparently cannot be considered a primary effector of thyroid thermogenesis in intact mammalian skeletal muscle. The direct contribution of active Na-K transport to this thermogenesis was indeed small compared with the overall cellular energy dissipation. The minimum overall energetic efficiency of the transport process in the intact muscle (30-35%) was not dependent on the thyroid status.

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