Combined alpha-adrenergic stimulation results in biphasic response of growth hormone release in man.

Oral clonidine administration or insulin-induced hypoglycemia may enhance GH secretion through alpha 2-adrenergic stimulation in man. To further characterize the role of adrenergic transmission in the regulation of GH release, the effects of combined administration of clonidine and insulin were investigated in 16 normal men randomly assigned to 2 groups. The first group received 0.1 U/kg regular insulin, iv, followed by placebo or clonidine (0.150 mg), orally, on 2 different days, 2 weeks apart. The second group received oral clonidine (0.150 mg), followed by iv placebo or insulin (0.1 U/kg) on 2 different days, 2 weeks apart. Insulin induced a clear-cut increase in plasma GH (F = 79.88; P less than 0.001) that was not affected by placebo, whereas it was significantly inhibited by oral clonidine (P less than 0.001 at 60, 90, and 120 min). Similarly, oral clonidine administration resulted in a clear-cut rise in plasma GH levels (F = 16.44; P less than 0.001) that was significantly reduced by insulin (P less than 0.001, P less than 0.02, and P less than 0.05 at 60, 90, and 120 min, respectively). These results suggest that while moderate activation of the alpha 2-adrenergic system stimulates GH release, further stimulation of the same system may result in inhibition, rather than further activation, of GH secretion.