A diurnal rhythm of incorporation of L-[3H] leucine in myocardium of the rat.

The influence of the time of day on myocardial protein synthesis and hypertrophy was examined in rats entrained to a 12-12 hr light-dark cycle. Analysis of ventricular weight to body weight ratio indicates that isoproterenol was a more effective stimulus of hypertrophy if given during the dark period, when the rat is most active, than when administered during the light hours. The difference was not attributed to alterations in percent water of the myocardium or nutritional status of the animal. Differential incorporation of labeled leucine into protein throughout the day indicated that myocardial protein may be synthesized at the greatest rate late in the light period with least synthesis occurring during the late hours of the dark period. The implications of these results are discussed.