Separation of the Effects of Growth Hormone on Muscle Amino Acid Transport and Protein Synthesis

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Abstract
The in vitro accumulation of glycine-14C and AIB-14C by intact diaphragms of hypophysectomized rats was inhibited when all of the sodium in the Krebs bicarbonate incubation medium was replaced by choline. Only moderate shifts in tissue fluid distribution occurred in diaphragms incubated in sodium-free buffer, but cellular sodium and potassium were gradually lost. Glycine-14C incorporation into diaphragm protein was also reduced in muscles incubated in the absence of extracellular sodium, but it could be reduced further by DNP and puromycin, and stimulated by insulin. Bovine growth hormone added in vitro stimulated the accumulation of glycine and AIB by diaphragms incubated in Krebs bicarbonate medium but had no effect in the absence of sodium. However, growth hormone stimulated glycine incorporation into diaphragm protein in the presence or absence of extracellular sodium. Thus, growth hormone can stimulate amino acid incorporation into muscle protein when its effect on amino acid transport is blocked, suggesting that the action of growth hormone on muscle protein biosynthesis is not mediated solely through its action on membrane amino acid transport. (Endocrinology75: 113, 1964)