Hourly Variations in Plasma Concentrations of Growth Hormone and Insulin and in Amino Acid Uptake and Incorporation into Protein in Diaphragm Muscle of the Rat*

Abstract

Groups of young (30-32 day old), freely behaving female rats were decapitated at hourly intervals over a 24 h period, and the concentrations of GH [growth hormone] and insulin in the plasma were measured by RIA [radioimmunoassay]. Paired hemidiaphragms from these rats were incubated for 40 min in buffer containing 2-[14C]aminoisobutyric acid (AIB) to measure amino acid transport and [3H]-leucine to assess protein synthesis. One member of each pair of hemidiaphragms was also exposed to rat GH (5 .mu.g/ml). Several episodes of GH secretion were evident, as suggested by accumulations of rats having high or low levels of plasma GH at certain times of the day. Out of 192 rats studied, 46 had values above 50 ng/ml. Peaks of GH concentration were observed at 0300 h, 0600-0700 h and 2200 h. No clear peaks were seen at other times, perhaps due to a loss of synchrony in GH secretion among rats during the middle of the day. Mean plasma insulin levels did not vary appreciably over the 24 h period. Amino acid transport and leucine incorporation into protein in hemidiaphragms not exposed to exogenous rat GH varied markedly from hour to hour. During the time when the 3 peaks of plasma GH were observed, there was a significant correlation between the mean plasma GH concentration and the rate of protein synthesis in diaphragms taken from rats killed 1 h later. Adding rat GH to diaphragms in vitro stimulated both AIB transport and leucine incorporation into protein several times during the day, but effects were most evident during the dark period. There was a significant correlation between the effect of exogenous GH on protein synthesis and its effect on AIB uptake. These observations suggest that in prepubertal female rats the plasma concentration of GH reaches levels of 50-150 ng/ml several times during the day and that these episodic peaks in circulating GH may have transient effects on muscle metabolism.