Stimulation of somatostatin release in vitro by synthetic human growth hormone-releasing factor by a nondopaminergic mechanism.

The effect of synthetic human GH-releasing factor (hGRF-40) on somatostatin (SRIF) release from the median eminence of the hypothalamus was evaluated in rats with use of an in vitro incubation system. hGRF-40 stimulated SRIF release in a dose-related manner. This effect was significant at concentrations varying from 10(-11)-10(-7) M, with a minimal effective dose of 10(-11) M. Maximal stimulation was observed at 10(-10) M. Pimozide was added in vitro at a concentration of 10(-6) M to block dopamine (DA) receptors, since DA is a known stimulator of SRIF release. Pimozide was without effect on SRIF release and did not alter the stimulatory effect of hGRF-40. To evaluate the possibility that DA and GRF may share a common pathway to stimulate SRIF release, median eminence fragments were simultaneously exposed to submaximal concentrations of both DA (6 X 10(-7) M) and hGRF-40 (10(-12) M). By themselves, each of these doses had little effect on SRIF release. When they were added together, a marked stimulation was noted, which was not, however, significantly greater than the sum of the responses to each agent alone. These results suggest that DA and GRF act by separate mechanisms to stimulate SRIF release. GRF may be physiologically involved in the regulation of SRIF release. Stimulation of SRIF release may be a mechanism by which GRF exerts a negative ultrashort-loop feedback to inhibit GH release.