Basic and Clinical Studies of Regulatory Mechanism of GH Secretion

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Several provocative tests for GH secretion such as insulin tolerance, 1-dopa, arginine infusion and GRF tests have been used in clinical endocrinology. However, the precise mechanisms of GH secretion are not clear in these tests. The mechanisms were, therefore, investigated in this study. Intravenous (iv) injection of 200ug GRF or 0.1U/kg b.w. regular insulin, oral administration of 500mg 1-dopa or infusion of 30g arginine for 30min was preformed 120min, 120min, 90min, or 105nin, respectively, after iv administration of 200ug GRF. The effect of prior subcutaneous administration of SMS201-995, a long acting somatostatin analog, on GH secretion in each provocative test was examined in the same manner. The secretion of GH in response to 200ug GRF was completely inhibited by the prior administration of 200ug GRF, whereas that induced by insulin, 1-dopa or arginine was not affected by the prior sdministration of GRF. The secretion of GH induced by 1-dopa or arginine was completely inhibited by the prior administration of 50ug SMS201-995, and that induced by GRF was completely inhibited by 100ug SMS201-995. The response of GH secretion to insulin was small but significantly preserved even after administration of 100ug SMS201-995. Taken together with the previous reports that 1-dops elevates plasma GRF levels and arginine has no influence on plasma GRF levels, it is suggested that 1-dops stimulates GH secretion probably by suppression of somatostatin release in addition to stimulation of GRF and that arginine stimulates GH secrtion probably by suppression of somatostatin release. It is also suggested that some factor (s) other than GRF and somatostatin is involved in the mechanism whereby insulin stimulates GH secretion since the effect of insulin still exists in the presence of exogenous GRF or somatostatin.