The present experiments describe the use of an in vitro perifusion system to observe the time course of the GH response to extracts of porcine stalkmedian eminence (pSME). It was presumed that both releasing and inhibiting activities were present in the pSME and that the characterization of a response profile would provide more information than measurement of the net response alone. Release of 3H-labeled growth hormone ([3H]GH) from perifused rat pituitaries preincubated in [3H]leucine was measured by specific immunoprecipitation. A methanol extract of pSME elicited a complex [3H]GH release response: initial stimulation of release was followed by inhibition and then by a rebound release, the latter occurring after termination of the pSME pulse. When pituitary explants were simultaneously exposed to pulses of pSME during continuous, maximal somatostatin (SRIF) inhibition, a blunted initial stimulatory response was observed and the rebound release phase was abolished. Continuous exposure to pSME produced only a brief response in both the SRIF-inhibited (blunted) and the uninhibited explant. These data suggest that extracts of pSME contain principles which both stimulate and inhibit rGH release. The pSME inhibitory effect is indistinguishable from that of SRIF because 1) continuous exposure to pSME does not further inhibit [3H]GH release from pituitary explants already maximally inhibited by SRIF, and 2) the post pSME rebound does not occur in the presence of maximal inhibitory concentrations of SRIF. Maximal SRIF inhibition does not prevent the early stimulation of rGH release by pSME. The perifusion-immunoprecipitation system is a discriminating technique which can demonstrate the simultaneous presence of two hypothalamic factors with opposing effects upon rGH secretion.
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