Developmental Changes in Steroidogenesis by Equine Preovulatory Follicles: Effects of Equine LH, FSH, and CG*

Abstract

Ovulation in mares is preceded by a long and variable estrous period. The differentiation of equine preovulatory follicles with respect to steroidogenic capacity and responsiveness to equine gonadotropins was studied by culturing pieces of follicle wall (FW = theca + attached granulosa cells) from preovulatory follicles isolated during late diestrus (day 14 of cycle, n = 5 mares), early estrus (1st-2nd day of estrus, n = 6) or late estrus (4th or 5th day of estrus, n = 6). FW was cultured with or without equine LH, FSH, LH + FSH, or CG (10 or 100 ng/ml) and medium was collected and replaced at 3, 6, 12, 24, 48, and 72 h of culture. Follicular fluid from presumptive ovulatory follicles and medium from FW cultures were assayed for progesterone, androstenedione, and estradiol-17 beta. The cumulative secretion of all three steroids after 72 h of culture was significantly lower in FW isolated during late diestrus (P less than 0.05) as compared with early or late estrus. Maximal progesterone secretion was observed with FW from late estrus whereas maximal androstenedione and estradiol secretion in vitro occurred with FW from early estrus. In contrast to results obtained in vitro, concentrations of progesterone in follicular fluid were not different among stages of follicular development, and concentrations of androstenedione and estradiol in follicular fluid were maximal in late estrous follicles. Equine gonadotropins had their greatest stimulatory effect on steroidogenesis with FW obtained during late diestrus. As compared with controls, the addition of LH, FSH, or LH + FSH (100 ng/ml) increased progesterone secretion by FW from late diestrus (48x, 64x, and 58x, respectively, P less than 0.01), early estrus (24x, 32x, and 36x, P less than 0.01) and late estrus (9x, 9x, and 9x, P less than 0.01). Equine LH and FSH also increased androstenedione secretion by follicles obtained during diestrus and estrus. In contrast, estradiol secretion showed a more rapid loss in responsiveness to gonadotropin stimulation, with both LH and FSH stimulating estradiol secretion by FW from late diestrous follicles (P less than 0.01), but neither stimulating FW from early or late estrous follicles. Overall, eCG was a less potent stimulator of steroidogenesis in vitro than LH and FSH.(ABSTRACT TRUNCATED AT 400 WORDS)