Gonadotrope responsiveness in orchidectomized sheep: III. Effect of estradiol withdrawal after continuous infusion.

The magnitude of GnRH-induced (1600 ng/hourly pulse for 24 h) preovulatory surge-like secretion of LH was assessed in orchidectomized sheep (wethers) during infusion of estradiol (E2, 5 micrograms/h in 10% ethanol-saline [vehicle]) or at 0, 12, 24, or 48 h after E2 withdrawal (n = 6 wethers/group). In one group, infusion of E2 was continued for 48 h, with concurrent circhoral delivery of GnRH during the final 24 h of the E2 delivery period. In other treatment groups, infusion of E2 or vehicle was continued for 24 h. Circhoral delivery of GnRH was initiated at the conclusion of E2 delivery or 12, 24, or 48 h thereafter, or at 48 h after cessation of vehicle infusion. Total GnRH-induced LH secretion in wethers receiving concurrent E2 was significantly augmented relative to the GnRH-induced secretory response in wethers receiving vehicle alone. The magnitude of GnRH-induced LH release was significantly reduced in animals in which E2 delivery was halted at the beginning of the GnRH challenge period. Further reductions in gonadotrope responsiveness were noted 12, 24, and 48 h after cessation of E2 delivery. Responsiveness 48 h after halting the E2 infusion did not differ (p > 0.05) from the responsiveness of wethers that had not been treated with E2. In a companion study, anterior pituitary tissue was collected at the end of E2 infusion (5 micrograms/h for 24 h) or at 12, 24, or 48 h thereafter (n = 6 wethers/group). Anterior pituitary tissue of control animals (n = 6) was collected after halting the infusion of vehicle alone.(ABSTRACT TRUNCATED AT 250 WORDS)