Human Growth Hormone and Somatomedin C Suppress the Spontaneous Release of Growth Hormone in Unanesthetized Rats*

Abstract

To investigate the feedback control of GH [growth hormone] secretion, the effects of human GH (hGH) and somatomedin C (SmC) on spontaneous GH secretory surges were examined in unanesthetized, freely moving rats. Under pentobarbital anesthesia a right atrial catheter and an intracerebroventricular cannula were placed 7-10 days before the experimentation. For i.v. studies, hGH (0.3 U/ml .cntdot. h) was infused for 6 h after an i.v. loading dose (0.3 U) at the beginning of the experiments. For intraventricular injections, hGH (0.1 U/10 .mu.l) of SmC (500 ng/10 .mu.l) were injected into the lateral ventricle 2 h before the experiments. The equivalent dose of crystalline BSA [bovine serum albumin] diluted in the same vehicle solutions was administered to the same rat as a control on a separate day. Venous blood samples were collected every 20 min for 6 h. I.v. and intraventricular administration of crystalline BSA did not affect the typical rat GH (rGH) surges which appeared about every 3 h and reached peak values of > 300 ng/ml. The i.v. infusion of hGH significantly inhibited the amplitude of rGH surges compared to controls (planimetric areas under the secretory profile 752 .+-. 172 vs. 1921 .+-. 183, P < 0.01, 6). rGH secretion was similarly inhibited by intraventricular hGH (701 .+-. 127 vs. 2208 + 225, P < 0.01, 6) and by intraventricular SmC (537 .+-. 70 vs. 1503 .+-. 114, P < 0.01, 6). Both GH and SmC are active in the feedback regulation of rGH secretion.