NEUROENDOCRINE REGULATION OF GROWTH-HORMONE SECRETION IN SHEEP .2. EFFECT OF SOMATOSTATIN ON GROWTH-HORMONE AND GLUCOSE-LEVELS

The effect of intravenous (iv) and intracerebroventricular (icv) administration of somatostatin on the plasma levels of growth hormone (GH) and glucose was studied in sheep. Intravenous somatostatin decreased (P < 0.001) circulating GH when infused at the rate of 5-mu-g/min (150 ng/kg/min) over 1 hr, but when used at 1-mu-g/min there was no effect on plasma GH levels during infusion. At both doses used there was an indication of an increase in GH following the cessation of somatostatin infusion. Somatostatin given at both these doses iv had no effect on plasma glucose levels. When given icv neither 1.8-mu-g, 18-mu-g nor 180-mu-g somatostatin had any significant effect of plasma GH levels, although there was a significant (P < 0.05) elevation in GH levels 75 min after 180-mu-g somatostatin icv. Plasma glucose levels did not increase following injection of somatostatin icv at 1.8 or 18-mu-g, but there was a clear hyperglycaemic episode following 180-mu-g icv. Despite a lack of effect of somatostatin on GH release when given icv, there was a clear elevation (P < 0.05) in plasma GH levels immediately following icv administration of a somatostatin antiserum. These data indicate that iv administration of somatostatin at pharmacological levels can depress unstimulated GH levels in sheep while administration icv does not. Central administration of somatostatin increases plasma glucose levels only at high doses and seems unlikely to be of physiological importance in glucose homeostasis.