Recovery heat production of mammalian fast- and slow-twitch muscles.

Relationships between initial heat and recovery heat in fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles of the rat have been investigated by estimating the ratio of total heat (initial + recovery) to estimated initial heat. Results obtained from SOL agreed well with earlier data from amphibian skeletal muscle and indicated that in tetanic contractions of SOL the ratio,total heat/initial heat, was slightly greater than 2. Experiments on EDL revealed complexities not previously reported for amphibian muscle. Anaerobic total heat production by EDL was as much as 75% of aerobic heat production. When the initial heat was estimated under anaerobic conditions and with iodacetate present, the apparent ratio of total heat/initial heat in tetanic contractions of EDL approached a value of 2. The results are discussed in relation to the enzymatic characteristics of these muscles, and it is suggested that the recovery heat of SOL arises mainly from mitochondrial oxidative metabolism while that of EDL arises, to a large extent, from glycolytic activity.