Contribution of skeletal muscle to the regulatory non-shivering thermogenesis in small mammals

Energy dissipation (E) was measured by direct microcalorimetry in perifused resting soleus muscles from cold adapted, euthyroid, hypothyroid and hyperthyroid mice, before and during exposure to noradrenaline (NA), lipid substrates or ouabain. The thermogenic effect of NA on the muscle was transitory and it did not exceed 5% of basal E, in all groups of preparations. The substrate effects were larger than that of NA and were sustained. They were the largest in hypothyroid animals and were not potentiated by NA. Basal E and the thermogenic effects of NA and the lipid substrates were identical in preparations from mice adapted to 23 degrees C and to 8 degrees C. The inhibitory effect of ouabain in resting muscles was very small, but it was increased by adaptation to the lower temperature. Experiments performed on rat muscles perfused in situ showed much larger thermogenic effects of NA than that observed in perifused mouse muscles. It is suggested that the NA thermogenic effect in resting muscles from small mammals is essentially mediated by hemodynamic changes which tend to suppress a hypoxic and acidotic restriction of the metabolic rate, rather than by any direct effect of NA on skeletal muscle cells.