Muscle physiology and metabolism vary between day and night, but whether the diurnal variation is dependent on the circadian clock or due to differences in nutrition or physical activity remains unclear.

To examine circadian regulation of muscle growth independent of exercise and feeding, the authors measured muscle growth over 12-hour periods in live pre-feeding larval zebrafish.

The authors found that larval zebrafish muscle grows more during the day than night and that the differences are independent of muscle activity. Muscle anabolism was greater in daytime, promoted by physical activity, and correlated with TORC1 activity, whereas catabolism was greater at night and correlated with the accumulation of Muscle RING Finger (murf) mRNAs, which are markers of protein degradation.

Rapamycin, a TORC1 inhibitor, reduced muscle growth during the day, whereas proteasome inhibitors enhanced muscle growth at night. Inhibiting molecular clock function abolished circadian differences and reduced muscle growth. After entrainment of larvae, diurnal variations in muscle growth and protein turnover persisted under constant conditions.

According to the authors, muscles in zebrafish larvae show circadian differences in growth and are regulated by the circadian clock independent of physical activity and food intake.

https://www.pnas.org/content/early/2020/11/20/2012450117

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