Comparison between the biological effects of LH-RH and buserelin on the induction of LH release from hemi-pituitary glands of female rats in vitro

The biological effects of LH-RH and the agonist D-Ser(But)6-des Gly10]-LH-RH(1-9)-ethylamide (buserelin) were compared during 8 h of incubation with female rat hemi-pituitary glands. Similar dose-response relationships were found for LH-RH and buserelin as concerns the release of luteinizing hormone (LH) by pituitary glands from intact and ovariectomized rats. Also the LH secretion patterns from glands of intact rats were similar: an initial low response was followed by a fast increase (priming effect) after which the response declined again (desensitization). In a subsequent experiment pituitary glands from ovariectomized rats were first exposed to LH-RH or buserelin for 4 h and then further incubated in medium only. After discontinuation of the stimuli the rate of LH release decreased in all cases, but this decrease was significantly greater when the glands had been exposed to LH-RH. Short-term (1/2, 1 or 2 h) exposures to LH-RH or buserelin followed by an intervening period (1 1/2, 1 or 0 h, respectively) of incubation in medium only resulted in an almost similar, significant increase in the subsequent protein synthesis-independent LH response to LH-RH (priming effect). Only preincubation with LH-RH for 2 h was significantly more effective. The results demonstrate equal intrinsic activities for LH-RH and buserelin. Differences in the biopotencies for LH-RH and buserelin in vivo and in vitro may occur only after discontinuation of the external stimuli.