EFFECTS OF ATIPAMEZOLE, AN ALPHA(2)-ADRENERGIC ANTAGONIST, AND SOMATOSTATIN ON XYLAZINE-INDUCED GROWTH-HORMONE RELEASE IN CALVES

In order to clarify the mechanism of xylazine-induced GH release, we i nvestigated the effects of atipamezole, a selective alpha(2)-adrenergi c antagonist, and somatostatin (SRIF) on xylazine-stimulated GH releas e in calves. Xylazine injection (0.30 mg/kg BW, iv) induced a rapid in crease in the GH concentration. When atipamezole was used in combinati on with xylazine, it blunted the increase in the plasma GH concentrati on induced by the xylazine injection. The GH levels at 15-50 min after the simultaneous injection of xylazine and atipamezole were significa ntly (P<0.05) lower than the corresponding values in the animals given xylazine alone. The area under the GH response curve for 120 min afte r the simultaneous injection of xylazine and atipamezole was significa ntly (P<0.05) smaller than that for the xylazine alone. A series of fi ve intravenous injections of 1 mg of SRIF at 10-min intervals also blu nted xylazine-stimulated GH release. Atipamezole partially suppressed xylazine-induced hyperglycemia, but SRIF completely suppressed the hyp erglycemia for the first 60 min after the xylazine injection and the s uppression by SRIF was stronger than that by atipamezole. On the other hand, both atipamezole and SRIF failed to blunt xylazine-induced hypo insulinemia. The present results suggest that xylazine stimulates GH r elease via the alpha(2)-adrenergic pathway in cattle, but the mechanis m of xylazine-induced hyperglycemia remains to be determined.