GONADOTROPIN-SECRETION IN OVARIECTOMIZED CHINESE MEISHAN AND HYBRID LARGE WHITE GILTS - RESPONSES TO CHALLENGES WITH ESTRADIOL BENZOATE, GONADOTROPIN-RELEASING-HORMONE, OR PORCINE FOLLICULAR-FLUID

Castrated Meishan (MS) and hybrid Large White (hLW) gilts were used in a series of challenge experiments to establish differences in the con trol of gonadotropin secretion. Acutely ovariectomized gilts (72 h) we re injected i.m. with 10 mu g/kg estradiol benzoate (EB) and serially bled for evaluation of estradiol clearance from the blood and patterns of LH and FSH release. Estradiol was more rapidly metabolized from th e circulatory system of the acutely ovariectomized MS gilts than from the hLW gilts; as a consequence, hLW gilts had higher concentrations o f estradiol than MS gilts at 24-32 h (p < 0.05) and 48-60 h (p < 0.01) post-EB treatment. The hLW but not the MS gilts exhibited a classic n egative/positive LH response to EB; MS gilts responded with a random r elease of large episodes of LH. Plasma FSH concentrations were consist ently greater (p < 0.05) in acutely ovariectomized MS gilts than in hL W gilts. Both breeds demonstrated a slight negative FSH response to EB and then exhibited increased variability in plasma FSH with time afte r treatment. Mean plasma LH was immediately elevated in chronically ov ariectomized (> 14 days) hLW and MS gilts given 1 mu g/kg of GnRH i.m. , but the magnitude of the LH response was greater (p < 0.01) in the h LW gilts. Comparable differences (p < 0.01) in FSH secretion were also observed, indicating the presence of greater pools of releasable FSH and LH in the hLW gilts, or a greater sensitivity to GnRH. Infusion of 10 ml of porcine follicular fluid (pff) resulted in a greater (p < 0. 001) decline in FSH concentrations in the hLW than in the MS gilts. Pl asma LH concentrations were unaffected, indicating that steroids had b een effectively removed from the pff. Plasma FSH was consistently high er in chronically ovariectomized hLW than in MS gilts, in contrast to the situation in acutely ovariectomized females. However, in both bree ds, plasma LH was similar after acute and chronic ovariectomy. We sugg est, therefore, that although hLW females may have a greater potential for gonadotropin secretion, differences in sensitivity to other facto rs (e.g., inhibin) may allow for an increased FSH:LH ratio at critical times in the estrous cycle of the MS and may be associated with incre ased ovulation rates in older MS females. Differences in the response to estrogen challenges may be related to variation in metabolic cleara nce of estrogen between MS and hLW gilts.