The role of endogenous GHRH in arginine-, insulin-, clonidine- and l-dopa-induced GH release in normal subjects.

OBJECTIVE The role of endogenous GHRH in arginine-, insulin-, clonidine- and l-dopa-induced GH secretion was studied in man using a GHRH antagonist (GHRH-Ant). DESIGN Ten healthy adult males were studied for serum GH responses to arginine or insulin singly, or sequentially 120 min after GHRH injection with or without combined administration of GHRH-Ant. Further, GHRH, clonidine or l-dopa were sequentially administered to these subjects 120 min after the GHRH injection. RESULTS The combined administration of GHRH-Ant distinctly inhibited the arginine- and insulin-induced GH release. When these four agents were sequentially administered 120 min after GHRH injection, the GH responses to clonidine and l-dopa disappeared completely while clear responses were observed to arginine and insulin administration. These responses to arginine and insulin were also completely inhibited by the combined administration of GHRH-Ant. CONCLUSIONS These results indicate that clonidine and l-dopa stimulate GH secretion mainly through the release of hypothalamic GHRH, and that arginine- and insulin-induced hypoglycaemia stimulate GH secretion mainly through the inhibition of hypothalamic somatostatin release. However, the presence of endogenous hypothalamic GHRH seems to be essential for the maximal stimulation of GH release induced by arginine and insulin.