Gonadotrope responsiveness in orchidectomized sheep. II. Effect of gonadotropin-releasing hormone amplitude shift during continuous infusion of estradiol.

Gonadotropin secretion during continuous infusion of estradiol (E2; 5 micrograms/h, i.v.) and pulsatile delivery of GnRH was assessed in orchidectomized sheep (wethers). Wethers received 200 ng GnRH per hourly pulse during the first 12 h of E2 infusion. During the final 36 h of infusion, animals (n = 6/group) received 0, 200, 400, 800, or 1600 ng GnRH/h pulse. Amplitude shift to 1600 ng GnRH/h pulse induced preovulatory surge-like secretion of LH in animals receiving concurrent E2 infusion. In the absence of E2, the same change in GnRH amplitude did not significantly affect serum concentrations of LH. Full expression of the GnRH-induced surge of LH in wethers required prior exposure to E2, but was not dependent on pretreatment with low-amplitude GnRH stimulus. In a companion experiment, anterior pituitary tissue was collected from E2-treated wethers 0, 3, 6, 12, or 24 h after initiation of episodic GnRH delivery (1600 ng/h pulse). Treatment with E2 induced a 5-fold increase in GnRH receptor concentration. The tissue concentration of GnRH receptor was depressed (p < 0.05) within 6 h of initiation of high-amplitude GnRH stimulation and remained depressed for the duration of GnRH delivery. Tissue stores of LH were increased (P < 0.05) by E2 and reduced (p < 0.05) after 3 h of high-amplitude GnRH stimulation. In contrast, tissue stores of FSH were reduced (p < 0.05) by both E2 alone and E2 with concurrent high-amplitude GnRH stimulation. The concentration of mRNA for the FSH beta subunit, but not for the alpha or LH beta subunits, was reduced (p < 0.05) by E2.(ABSTRACT TRUNCATED AT 250 WORDS)