Time Course of the Effect of Growth Hormonein Vitroon Amino Acid and Monosaccharide Transport and on Protein Synthesis in Diaphragm of Young Normal Rats*

Abstract

The in vitro responsiveness of diaphragms from 18 day old fasted normal rats to bovine growth hormone (bGH) with time was studied by determining the accumulation of .alpha.-aminoisobutyric acid (AIB) and 3-O methylglucose (3-OMG) and by determining the incorporation of phenylalanine into diaphragm proteins. An estimate of the time course of the effect of GH was obtained by adding the different labeled substances to the incubation medium at different time periods after the start of the incubation. bGH (5 .mu.g/ml) stimulated the uptake of AIB and 3-OMG and the incorporation of phenylalanine between 0-60 min after the start of the incubation. This period was then followed by a period of 60-120 min during which no further stimulation occurred in spite of the continuous presence of the hormone, i.e., the diaphragm was refractory to GH. When the incubation period was extended beyond the refractory phase, the uptake of AIB and the incorporation of phenylalanine once again increased in diaphragms exposed to bGH. The present experiments demonstrate that the membrane transport system(s) and protein synthesis in diaphragms of 18 day old fasted rats pass through a sequence of changes in the response to GH. The fact that GH exerts a biphasic action on membrane transport and protein synthesis in diaphragm muscle of normal rats suggests that this responsiveness pattern to GH with time is a generalized cellular phenomenon in response to the hormone.