Circadian Rhythms of Glycogen, Free Fatty Acids, and Triglycerides in Rat Heart and Diaphragm

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Abstract
Circadian rhythms entrainable to a 12:12 light-dark cycle were found in rat heart and diaphragm glycogen, free fatty acids and triglycerides. While the diaphragm rhythms of glycogen and lipid exhibit nearly simultaneous peaks and nadirs, the heart substrate rhythms are 180.degree. out of phase. The 2 continuously active muscle types apparently have different control mechanisms for the metabolism and storage of endogenous substrates. These rhythms may reflect different metabolic functions in the 2 muscles. Circadian studies of heart and diaphragm metabolism provide a useful model for in vivo substrate utilization in the normal, unstressed animal.