Growth hormone (GH) secretion in the conscious rat: negative feedback of GH on its own release

The negative-feedback effects of GH on its own secretion were studied in conscious male and female rats bearing indwelling double-bore venous cannulae. Intravenous infusions of human GH (hGH; 20-60 micrograms/h) or somatostatin (SS; 10 micrograms/h) were given while frequent serial microsamples of blood were withdrawn using an automatic blood-sampling system. In both sexes, i.v. infusions of hGH for 6 h inhibited endogenous GH secretory pulses, with a slow onset of the inhibition. There was no rebound GH secretion immediately following the removal of the hGH infusion, but spontaneous GH secretion gradually returned to normal. Infusions of hGH did not inhibit the pituitary GH response to repeated GH-releasing factor (GRF) injections (1 microgram) given i.v. every 40 min to female rats. By contrast, infusions of SS, which also blocked spontaneous GH release, dramatically reduced the GH responses to serial GRF injections. When SS Infusions were stopped, the subsequent GRF-induced GH secretory responses were enhanced. These results show that GH can inhibit its own release when given by i.v. infusion to conscious male and female rats. Since GH responses to GRF are maintained during a GH infusion, the feedback effect of GH is unlikely to be exerted directly on the pituitary or by increasing SS release. Our results are consistent with the idea that GH feedback in the conscious rat involves an inhibition of GRF release.