Hormonal Alterations of the Sensitivity of Amino Acid Transport to Growth Hormone in Muscle of Young Rats

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Abstract
The ability of growth hormone [GH] in vitro to stimulate amino acid and sugar uptake by diaphragm muscle from normal rats declines from age 15-30 days. In 15 day old intact rats, as in older, hypophysectomized rats, GH has a biphasic effect on anino acid transport: a transient stimulatory phase, followed by a period of refractoriness to stimulation by additional GH. After age 15 days the refractoriness phase becomes progressively dominant. GH actions are not affected by prolactin or by adrenalectomy. During midlactation, exogenous glucocorticoid may reduce refractoriness, enhancing the response to additional GH.