RESISTANCE OF GROWTH-HORMONE SECRETION TO SOMATOSTATIN IN MEN WITH TYPE-I DIABETES-MELLITUS

The effect of continuous infusions of somatostatin (SRIF) on growth hormone (GH) secretion induced by GH-releasing hormone (GHRH) bolus was compared in a dose-response manner between diabetic subjects in poor glycemic control and nondiabetic subjects to address the hypothesis that altered pituitary responsiveness to SRIF contributes to the hypersecretion of GH in diabetes mellitus in humans. Studies were conducted with a modification of the euglycemic clamp technique to minimize fluctuations in glucose and insulin. Suppression of GHRH-induced GH secretion was demonstrable in diabetic subjects only at a SRIF dose 15-fold higher than that at which suppression could be detected in nondiabetic subjects. The calculated 50% inhibitory dose (ID50) in diabetic subjects was 4-fold higher than that in nondiabetic subjects (P = 0.03). In diabetic subjects after 2 wk of intensive insulin management, the change in the dose-response curve persisted despite significant decrements in glycosylated hemoglobin and increments in plasma insulinlike growth factor I. The increment in plasma SRIF predicted at the SRIF ID50 from concentrations measured during the SRIF infusions in nondiabetic subjects would result in hypophyseal portal SRIF concentrations in the physiological range reported in recent animal studies, whereas those for the ID50 in diabetic subjects would be approximately 1.5-2.5 times the upper limit of that range. These studies indicate that pituitary resistance to the action of SRIF occurs in men with insulin-dependent (type I) diabetes at physiological concentrations of the hormone and is therefore highly likely to contribute to the hypersecretion of GH in diabetes.