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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^*

OLLE ISAKSSON, DAVID F. NUTTING, JACK L. KOSTYO and CHARLES R. REAGAN

Department of Physiology, Emory University School of Medicine Atlanta, Georgia 30322
Department of Physiology and Biophysics, University of Tennessee Center for the Health Sciences Memphis, Tennessee 38163

Requests for reprints should be addressed to: Dr. Jack L. Kostyo, Department of Physiology, Emory University School of Medicine, Atlanta, Georgia 30322.

Abstract

Groups of young (30-to 32-day-old), freely behaving female rats were decapitated at hourly intervals over a 24-h period, and the concentrations of GH and insulin in the plasma were measured by RIA. 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 µ/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 (based on a rat GH reference standard having a potency in the immunoassay of 2.7 times NIAMDD-RP-1). 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 three 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 1h 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. (Endocrinology 102: 1420, 1978)

Footnotes

^* This work was supported by grants from the NIH (AM12782, AM18995, and HD06580), the Kroc Foundation, and the Southern Regional Education Board.

NIH International Research Fellow (TW02140). Present address: Department of Physiology, University of Goteborg, Goteborg, Sweden.

Received November 14, 1977.

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