Effect of withdrawal of somatostatin plus GH-releasing hormone as a stimulus of GH secretion in obesity.

Abstract

Somatostatin (SS) may not merely be inhibitory to GH secretion but, under appropriate temporal conditions, may act in a paradoxically positive manner to sensitize somatotroph responsiveness to GH-releasing hormone (GHRH). SS infusion withdrawal (SSIW) produces a rebound GH rise in humans and increases GHRH-induced GH release. Theoretically, SSIW leaves the somatotroph cell in a situation of low endogenous SS. In obesity, there is markedly decreased GH secretion. In both children and adults, the greater the body mass index (BMI), the lower the GH response to provocative stimuli. It has been postulated that increased hypothalamic somatostatin secretion is the main mechanism responsible for the blunted GH secretion of obesity. There are no data evaluating GH responsiveness to SSIW plus GHRH in obese adults. The aim of the present study was to evaluate the GH response to SSIW plus GHRH in a group of control subjects and a group of obese patients. Seven obese patients (six female, one male) with a BMI of 36.1 +/- 7.7 kg/m2 were studied. As a control group, seven normal subjects (six female, one male) with a BMI of 20.3 +/- 0.9 kg/m2 were also studied. Two tests were performed. On one day, somatostatin (SS) i.v. infusion (500 microg from 0-90 min) was performed followed by a placebo i.v. bolus 90 min after SS withdrawal (SSIW). On another day, SS i.v. infusion (500 microg from 0-90 min) was performed followed by a GHRH (100 microg) i.v. bolus 90 min after SS withdrawal. A second group of seven obese patients (six female, one male) with a BMI of 32.2 +/- 2.3 kg/m2 were studied. As a second control group, seven normal healthy subjects (six female, one male) with a BMI of 20.1 +/- 0.6 kg/m2 were also studied. On one day, saline infusion was performed followed by a placebo i.v. bolus at 90 min. On another day, saline infusion was performed followed by a GHRH (100 microg) i.v. bolus at 90 min. Blood samples were taken at appropriate intervals for determination of GH. Serum GH was measured by chemiluminescent immunometric assay. Statistical analysis was performed by Wilcoxon and Mann-Whitney tests. GHRH-induced GH secretion in normal subjects showed a mean peak of 15.8 +/- 2.1 microg/l. Normal control subjects had a mean peak of 3.1 +/- 1.5 microg/l after SSIW-induced GH secretion. When GHRH was administered after SSIW there was an increased GH secretion with a mean peak of 23.3 +/- 4.4 microg/l, significantly greater than the response after SSIW alone (P < 0.05) and GHRH alone (P < 0.05). GHRH-induced GH secretion in obese patients was decreased with a mean peak of 3.9 +/- 1.5 microg/l. In obese patients, GH secretion after SSIW was markedly decreased with a mean peak of 1.0 +/- 0.4 microg/l. When GHRH was administered after SSIW, an increase in GH secretion was observed with a mean peak of 4.3 +/- 0.9 microg/l, significantly greater than SSIW alone (P < 0.05) but not GHRH alone (P = NS), and significantly less than in normal subjects (P < 0.05). This study demonstrates a significantly blunted peak GH response to somatostatin infusion withdrawal plus GHRH in obese patients compared to normal subjects. In this theoretical situation of decreased somatostatinergic tone, there is persistence of GH hyposecretion in obesity, suggesting the existence of multiple defects responsible for decreased GH secretion in obesity. We also found that in obese patients, in contrast to normal subjects, SSIW did not increase GHRH-induced GH secretion.