Acute hyperglycemia and activation of the β-adrenergic system exhibit synergistic inhibitory actions on growth hormone (GH) releasing hormone-induced GH release

Objective: Acute hyperglycemia stimulates somatostatin (SRIH) release by the hypothalamus which, in turn, suppresses growth hormone (GH) secretion from the anterior pituitary gland. Although it has been suggested that the cholinergic pathway mediates glucose-induced SRIH release, other regulatory systems have not been examined. Therefore, we investigated whether blocking or activating the β-adrenergic pathway alters glucose-mediated inhibition of GH release. Design and methods: One set of experiments was performed with a β-adrenergic antagonist, propranolol, and the other set with a β-adrenergic agonist, isoproterenol. Each set of experiments was performed in ten healthy subjects and consisted of four tests. Test 1, a 100 μg GHRH bolus i.v. at 0 min; test 2, 100 g glucose orally at - 30 min, followed by a 100 μg GHRH bolus at 0 min; test 3, after a 100 μg GHRH bolus i.v. at 0 min, a continuous infusion of propranolol (0.2 mg/kg) or isoproterenol (0.012 μg/kg) was administered between 0 and 120 min; test 4, after a 100 g glucose oral load at - 30 min, and a 100 μg GHRH bolus i.v. at 0 min, a continuous infusion of propranolol (0.2 mg/kg) or isoproterenol (0.012 μg/kg) was administered between 0 and 120 min. Blood was drawn every 10 min from - 30 min to 120 min to measure GH and glucose concentrations. Results: Pretreatment with glucose significantly suppressed GHRH-induced GH secretion. Propranolol infusion significantly increased the GHRH-induced GH secretion, but it did not block glucose-induced suppression of GH secretion. Isoproterenol infusion alone significantly suppressed GHRH-induced GH secretion and augmented the inhibitory action of glucose on GH release. Conclusion: This study demonstrates that glucose-induced suppression of GHRH-stimulated GH release is independent of β-adrenergic tone. Since previous data supports a role for SRIH in both glucose and β-adrenergic suppression of GH release, the current results suggest that subsets of SRIH neurons are differentially responsive to these external cues. Therefore, a combined glucose and isoproterenol test may provide a useful assessment of hypothalamic somatostatinergic activity.