Effects of season and phase of the estrous cycle on steroidogenesis and LH-FSH sensitivity of large ovine follicles perfused in vitro

The aims of this study were to compare steroidogenesis (progesterone, andro stenedione and estradiol production) and response to LH and FSH challenge b y whole perifused follicles 4 to 5.5 mm in diameter, obtained at different periods of the breeding season (onset, middle, end), during anestrus and th e luteal phase. We have observed that all follicles do not have the same st eroidogenetic potential and do not respond with the same intensity to LH an d FSH. At the middle of the breeding season, LH and FSH supplementation was ineffective in increasing progesterone secretion by follicles (0.19 +/- 0. 05 vs 0.20 +/- 0.03 ng/mL). In contrast, gonadotrophin challenge elicited s ignificant (P < 0.05) increases in androstenedione (0.94 +/- 0.34 vs 0.35 /- 0.09 ng/mL) and estradiol (120 +/- 11 vs 49 +/- 10 pg/mL) production imm ediately after its administration. At the onset of the breeding season, ste roidogenesis was identical under both basal and gonadotrophin-stimulated co nditions unlike that in middle of the breeding season. However follicles we re more sensitive to the gonadotrophin challenge in terms of estradiol prod uction than those collected at the middle of the breeding season (220 +/- 4 5 vs 120 +/- 11 pg/mL). Follicles obtained at the end of the breeding seaso n featured higher progesterone (2.61 +/- 0.81 vs 0.19 +/- 0.05 ng/mL; P < 0 .05) and lower estradiol production (10 +/- 3 vs 49 +/- 10 pg/mL; P < 0.05) that was not influenced by LH and FSH. Basal androstenedione secretion was comparable to that observed at the middle of the breeding season (0.42 +/- 0.10 vs 0.35 rt 0.09 ng/mL), but the response to stimulation was significa ntly higher (1.82 +/- 0.61 vs 0.94 +/- 0.34 ng/mL; P < 0.05). In anoestrus and the luteal phase, follicles presented higher progesterone and androsten edione and lower estradiol concentrations (P < 0.05) compared with those ob tained during the follicular phase at the middle of the breeding season. In the luteal phase, follicles remained capable of responding to LH-FSH chall enge by increasing estradiol secretion (9 +/- 1 before and 21 +/- 6 pg/mL a fter LH-FSH; P < 0.05). In contrast, in the luteal phase, estradiol product ion was not increased by LH-FSH challenge (7 +/- 2 vs 12 +/- 4 pg/mL). (C) 1999 by Elsevier Science Inc.