Somatostatin-28 and somatostatin-14 suppression of arginine-, insulin-, and TRH-stimulated GH and PRL secretion in man.

Abstract

GH and PRL responses to arginine infusion and to the combined pituitary stimulation test (0.1 u/kg of insulin, 200 micrograms of TRH, 100 micrograms of LHRH) were compared in five normal men during infusions of somatostatin-14 (SS-14) and somatostatin-28 (SS-28), and during control infusions of vehicle alone. SS-14 and SS-28 were infused at a rate of 1.8 nmol/kg over 210 min. Arginine (0.5 g/kg) was infused from 30 to 60 min and the combined pituitary stimulation test commenced at 120 min. Arginine (0.5 g/kg) infused from 30 to 60 min induced an increase in GH secretion in all subjects and this increase was completely abolished in these same subjects when infused with SS-14 and SS-28. Arginine-induced hyperglycaemia was significantly greater during infusion of SS-14 and further enhanced by infusion of SS-28. A small increase in PRL secretion occurred after arginine infusion and this was not inhibited by SS-14 or SS-28. Insulin and TRH administration induced marked increases in both GH and PRL secretion. The mean GH increase was significantly inhibited by SS-14 and SS-28 up to 165 min but not thereafter. The PRL increase was significantly inhibited by SS-28 but not by SS-14 and this greater efficacy was also indicated by administering different doses of SS-28 to one subject. Taken together with the demonstration that SS-28 is released from the median eminence, these findings indicate that SS-28 has a hormonal role in the regulation of GH and PRL secretion.