Interactions of follicular factors and season in the regulation of circulating concentrations of gonadotrophins in mares

Follicle growth and associated changes in circulating hormone concentrations were evaluated after follicle ablation in mares (n = 13) during four 13 day periods beginning at means of -98.5, -61.4, -26.0 and 10 days from the first ovulation and corresponding to mid-anovulatory, early-transitional, late-transitional and ovulatory periods, respectively. During each period, all follicles > 5 mm in diameter were ablated (day 0) followed by no subsequent ablations (all-follicle group) or the ablation of all follicles > 5 mm when a new follicle reached > 10 mm (0-follicle group). A follicular wave emerged at means of days 1.5-2.5 in all mares of the all-follicle group during each period. Follicle activity increased between the mid-anovulatory and the transitional periods, as indicated by increases in the diameter of the largest follicle and the number of follicles in the all-follicle groups; nonetheless, there were no differences in the FSH concentrations of the wave-stimulating surge throughout the study. Circulating total inhibin increased between day 3 and day 6 in the all-follicle groups but not in the 0-follicle groups, and was higher during the late-transitional and ovulatory periods than during earlier periods in the all-follicle groups. In the all-follicle groups, circulating FSH decreased between day 3 and day 6. In the 0-follicle groups, FSH did not decrease and was higher during the late-transitional and ovulatory periods than during the mid-anovulatory period. Circulating LH was higher during the late-transitional and ovulatory periods than during earlier periods and was not different between the all-follicle and 0-follicle groups. On the basis of temporal relationships, it was concluded that the follicles of a wave secreted inhibin during all periods and the follicular inhibin suppressed circulating FSH. An increase in circulating inhibin induced by the growth of follicles > 21 mm in diameter during the late-transitional and ovulatory periods counteracted a stimulatory effect on FSH mediated by season.