To investigate the feedback control of GH secretion, we examined the effects of human GH (hGH) and somatomedin C (SmC) on spontaneous GH secretory surges 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 iv studies, hGH (0.3 U/ml-h) was infused for 6 h after an iv loading dose (0.3 U) at the beginning of the experiments. For intraventricular injections, hGH (0.1 U/ 10 μl) or SmC (500 ng/10 μl) were injected into the lateral ventricle 2 h before the experiments. The equivalent dose of crystalline BSA 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. Intravenous 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 more than 300 ng/ml. The iv 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, n = 6). rGH secretion was similarly inhibited by intraventricular hGH (701 ± 127 vs. 2208 ± 225, P < 0.01, n = 6) and by intraventricular SmC (537 ± 70 vs. 1503 ± 114, P < 0.01, n = 6). These findings suggest that both GH and SmC are active in the feedback regulation of rGH secretion.
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