SENSITIVITY OF THE RAT DIAPHRAGM TO GROWTH HORMONE

Abstract

ABSTRACT The in vivo and in vitro effects of bovine growth hormone (GH) on the rate of intracellular accumulation of D-xylose and α-aminoisobutyric acid (AIB) were studied in the intact rat hemidiaphragm preparation. GH was added in vitro or injected intravenously (i. v.) to hypophysectomized rats 45 min before incubation. It markedly increased the accumulation of both D-xylose-14C and AIB-14C by the isolated diaphragm. This stimulatory effect of GH was almost completely abolished by pretreatment of the rats with GH, either intramuscularly for 4 days or i. v. 16 hours before the in vitro experiment. An i. v. injection of GH (10 μg) to hypophysectomized rats 45 or 90 min before incubation gave a two-fold increase in the rate of AIB uptake by the isolated diaphragm while a similar injection 3½, 24 or 38 hours before incubation did not produce such stimulation. These injections, however, significantly decreased the sensitivity of the diaphragm to GH, added later in vitro. This type of inhibitory effect was seen after a single i. v. injection of as little as 1.0 μg of GH. The results indicate that acute administration of GH to hypophysectomized rats may first produce a stimulatory effect for 2–3 hours, which is then followed by an inhibitory phase lasting for as long as 35 hours. The nature of these two effects of GH are discussed in connection with other experimental and clinical observations on the diversity of the GH action. It is concluded that the contrasting effects of GH found in the present study on the isolated diaphragm may be due, at least in part, to changes in the transport mechanisms of the cell membrane to amino acids and sugars.