Abstract
Summary
Circadian rhythms entrainable to a 12:12 light-dark cycle have been found in rat heart and diaphragm glycogen, FFA and TG. While the diaphragm rhythms of glycogen and lipid exhibit nearly simultaneous peaks and nadirs, the heart substrate rhythms are 180° out of phase. The two continuously active muscle types apparently have different control mechanisms for the metabolism and storage of endogenous substrates. It is possible that these rhythms reflect different metabolic functions in the two muscles. Circadian studies of heart and diaphragm metabolism provide a useful model for in vivo substrate utilization in the normal, unstressed animal.
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