TEMPORAL RELATIONSHIP OF THE GROWTH HORMONE EFFECTS ON AMINO ACID TRANSPORT AND PROTEIN SYNTHESIS IN ISOLATED RAT DIAPHRAGM
ABSTRACT Experiments were performed to study whether the influence of bovine growth hormone (GH) on the mebrane transport of labelled leucine and glycine in the isolated rat diaphragm was similar to that previously found for α-aminoisobutyric acid (Hjalmarson & Abrén 1967a, b). The relationship between the effects of GH on amino acid transport and protein synthesis was also studied. Addition of GH in vitro (25 μg/ml) to intact hemidiaphragms from hypophysectomized rats increased the accumulation of glycine in the intracellular water after 2 hours of incubation, while that of leucine was reduced. GH in vitro increased the incorporation rate into muscle protein of both glycine and leucine. An Intravenous (i. v.) injection of GH (10 μg) to hypophysectomized rats 60 min. before incubation increased the distribution ratio of leucine, while no significant effect was found on the incorporation into protein of this amino acid. On the other hand, an injection of GH (10 μg) 180 min. before incubation increased the in vitro incorporation of both leucine and glycine. This injection did not change the distribution ratio of glycine and that of leucine was significantly decreased. Repeated injections of GH (50 μg × 4 days) increased the incorporation of both glycine and leucine. This treatment also increased the accumulation of glycine after 2 hours of incubation, while no such effect was seen on the accumulation of leucine. In vitro addition of GH (25 μg/ml) did not significantly change the distribution ratio of glycine and leucine in diaphragms from hypophysectomized rats previously treated with GH. However, addition of GH in vitro to the diaphragms from these rats further increased the incorporation of glycine into protein. In addition, GH in vitro increased the accumulation of glycine also when the incorporation of this amino acid into protein was completely blocked by puromycin (500 μg/ml). The present results show that GH, at least in certain doses, may have a biphasic action on the membrane transport of normal amino acids. The results also indicate that GH may have separate effects on the membrane transport and the incorporation into protein of amino acids.