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A study was made of the acute effects of growth hormone on the rate of leucine incorporation into protein of various tissues of hypophysectomized rats in vivo. The rats were injected with bovine growth hormone intravenously or intraperitoneally for various periods of time, and then the rate of protein synthesis was assessed by pulse-labeling proteins with L-leucine-14C administered intravenously through a tail vein, Growth hormone stimulated leucine incorporation into the total TCA-precipitable protein of thigh muscles 30 min, but not 15 min, after its administration. The stimulatory effect was still evident at 6 and 24 hr after hormone injection. These observations are similar to those previously made with diaphragm muscle. In contrast, no stimulation of leucine incorporation into proteins of the heart was seen 0.25, 0.5, 1, 3, 6, or 24 hr after a single injection of a large dose of growth hormone. In another experiment, the rate of leucine incorporation into proteins of several tissues was assessed by the pulse-labeling technique after 30 min of exposure to growth hormone in vivo. Once again, the hormone increased (by 67%) the rate of leucine incorporation in the protein of both the thigh muscle and the diaphragm. While no significant stimulation of protein labeling was evident in the thymus, kidney or brain at this time, growth hormone enhanced leucine incorporation into hepatic protein by 150%. Neither thymidine factor activity nor the concentration of immunoreactive insulin was increased in the plasma of hypophysectomized rats 30 min after they received an injection of growth hormone. Thus, these results suggest that the acute stimulatory effects of pituitary growth hormone on protein synthesis in skeletal and diaphragmatic muscle and in the liver are not mediated by thymidine factor or insulin.
Kostyo et al. (Mon,) studied this question.