THYROID-HORMONES AND THE ENERGETICS OF ACTIVE SODIUM-POTASSIUM TRANSPORT IN MAMMALIAN SKELETAL-MUSCLES

1. The steady‐state heat production rate (E) of soleus muscles obtained from adult mice in various thyroid states was measured in a perfused microcalorimeter. The ouabain‐suppressible fractions of E and 42K influx were compared and the energetic efficiency of active Na‐K transport assessed. 2. Hypothyroidism with plasma thyroxine concentrations below 1 microgram/100 ml. was induced by pretreatment with 131I or perchlorate. In soleus muscles isolated from treated animals, mean E values were 25.1 +/‐ 0.7 and 24.2 +/‐ 0.5 mcal.g wet wt.‐1.min‐1 for the 131I and the perchlorate series respectively, i.e. about 30% lower than the control level (36.3 +/‐ 1.5 mcal.g wet wt.‐1.min‐1). Following triiodothyronine treatment, E was increased by about 45%. 3. In muscles from hypothyroid (131I and perchlorate series), euthyroid and hyperthyroid mice ouabain (10(‐3) M) induced a rapid decrease in E of 1.6 +/‐ 0.1 and 1.4 +/‐ 0.1, 2.5 +/‐ 0.2, and 4.3 +/‐ 0.6 mcal.g wet wt.‐1.min‐1 respectively, i.e. between 6 and 8% of E. 4. 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 micromole. 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. 5. Notwithstanding the parallelism between the changes in basal E and ouabain‐sensitive components of E and K influx with the thyroid status, it is concluded that active NA‐K transport 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 over‐all cellular energy dissipation. 6. The minimum over‐all energetic efficiency of the transport process in the intact muscles (30‐‐35%) was not dependent on the thyroid status. © 1979 The Physiological Society