Effects of Atipamezole, an .ALPHA.2-Adrenergic Antagonist, and Somatostatin on Xylazine-lnduced Growth Hormone Release in Calves.

In order to clarify the mechanism of xylazine-induced GH release, we investigated the effects of atipamezole, a selective alpha 2-adrenergic antagonist, and somatostatin (SRIF) on xylazine-stimulated GH release in calves. Xylazine injection (0.30 mg/kg BW, iv) induced a rapid increase in the GH concentration. When atipamezole was used in combination with xylazine, it blunted the increase in the plasma GH concentration induced by the xylazine injection. The GH levels at 15-50 min after the simultaneous injection of xylazine and atipamezole were significantly (P < 0.05) lower than the corresponding values in the animals given xylazine alone. The area under the GH response curve for 120 min after the simultaneous injection of xylazine and atipamezole was significantly (P < 0.05) smaller than that for the xylazine alone. A series of five intravenous injections of 1 mg of SRIF at 10-min intervals also blunted xylazine-stimulated GH release. Atipamezole partially suppressed xylazine-induced hyperglycemia, but SRIF completely suppressed the hyperglycemia for the first 60 min after the xylazine injection and the suppression by SRIF was stronger than that by atipamezole. On the other hand, both atipamezole and SRIF failed to blunt xylazine-induced hypoinsulinemia. The present results suggest that xylazine stimulates GH release via the alpha 2-adrenergic pathway in cattle, but the mechanism of xylazine-induced hyperglycemia remains to be determined.