Energy production of rat soleus muscle.

GIBBS, C. L., AND W. R. GIBSON. Energy production of t-at soleus muscle. Am. J. Physiol. 223(4): 864-871. 1972.-A thermopile was used to record the heat production of rat soleus muscles, weighing between 16 and 96 mg, at 27 C. The muscles were subjected to isotonic and isometric tetani, 2 set in duration, and the stimulus frequency was 30 Hz. As well as recording initial energy production, recovery heat evolution was measured and was usually complete 2 min after a 2-set tetanus. The initial isometric heat rate was 5.1 meal/g muscle per second; this heat rate varied with muscle length falling when either shortening or stretching of the muscles took place. Most soleus muscles could be stretched so that their tension development fell below 0.1 PO and the tensionindependent initial heat rate at these lengths was 2.3 meal/g muscle per second. The activation or tension-independent heat (initial and recovery) in response to a single stimulus was estimated to have a mean value of 0.28 mcaI/g (recovery heat included), between $< and >{o of the value found in frog sartorius. In isotonic experiments, minimum energy expenditure (heat + work) occurred with loads equal to 0.1 PO and maximum expenditure with loads between 0.4 and 0.8 PO. The mechanical efficiency of these muscles (over the complete cycle of contraction, relaxation, and recovery) approached values obtained in amphibian twitch muscles. It was concluded that this mammalian slow muscle has evolved to maintain tension at a low cost and yet to work at a high mechanical efficiency.