METHSCOPOLAMINE BROMIDE BLOCKS HYPOTHALAMIC-STIMULATED RELEASE OF GROWTH-HORMONE IN EWES

Twenty-five nonlactating ewes were used to test the hypothesis that me thscopolamine bromide (MB) blocks secretion of growth hormone (GH) by affecting hypothalamic rather than pituitary mechanisms. Ewes were ran domly assigned to receive a s.c. injection of 96 mg of MB or 2 mL of s aline at min = 0. Saline-treated ewes were assigned to receive a subse quent (at +60 min) i.v. injection of 10 mu g of growth hormone-releasi ng hormone (GHRH) to test pituitary responsiveness or .3 mg of clonidi ne to test hypothalamic responsiveness. Methscopolamine bromide-treate d ewes were assigned to receive a subsequent (at +60 min) i.v. injecti on of 10 mu g of GHRH, .3 mg of clonidine, or 2 mL of saline. Jugular blood samples were collected at 10-min intervals from -120 min to +240 min, and serum concentrations of GH were quantified with a RIA. No di fference was detected in serum concentrations of GH with respect to MB vs saline treatment (P = .20). Concentrations of GH increased in sali ne-pretreated ewes following injections of clonidine or GHRH (P < .01) . Treatment of ewes with MB, however, limited the ability of clonidine -induced mechanisms to increase concentrations of GH, but did not affe ct pituitary responsiveness to GHRH (P < .01). These data support the hypothesis that MB inhibits hypothalamic and not pituitary mediated me chanisms that regulate the secretion of GH.