SENSITIVITY OF THE RAT DIAPHRAGM TO GROWTH HORMONE

Abstract

ABSTRACT In vitro administration of bovine growth hormone (GH) to intact hemidiaphragms from hypophysectomized rats was found to produce both an early stimulatory and a late inhibitory effect on the rate of intracellular accumulation of α-aminoisobutyric acid (AIB) and D-xylose. These effects were obtained by using an in vitro system, in which the diaphragms were incubated for periods up to 5 hours, a procedure found suitable for studying early and late effects of GH in vitro. GH (25 μg/ml) was added at the start of incubation and after various lengths of exposure to the hormone the diaphragms were transferred to other flasks and were incubated with AIB-14C (for 60 min) or D-xylose-14C (for 30 min). The stimulatory effect produced by GH on the uptake of the labelled substances in the diaphragm was seen during 2—3 hours after the initial addition of the hormone. The duration of the corresponding stimulatory effect of insulin was at least 4½ hours. Exposure of the diaphragms in vitro to GH in a concentration of 5 μg/ml for 3 hours made the diaphragms almost completely insensitive to further addition of GH (25 μg/ml) later on, when the ability of the muscles to accumulate AIB-14C and D-xylose-14C was studied. Thus, a late inhibitory effect of GH on the membrane transport of AIB and D-xylose in the diaphragm was obtained after in vitro administration of the hormone. The biphasic action of GH in vitro on the accumulation of AIB and D-xylose does not seem to be secondary to GH-produced changes of the glucose utilization in the diaphragm. This phenomenon cannot be explained by different dose levels of GH either, since qualitatively the same effects were obtained by GH at different concentrations from 0.1 to 100 μg/ml medium. The results from the present study are discussed in relation to available observations of the dual nature of GH exerted both in vivo and in vitro.