RELATIONSHIPS BETWEEN THE LEVEL OF ATRESIA AND INHIBIN CONTENTS (ALPHA-SUBUNIT AND ALPHA-BETA DIMER) IN MORPHOLOGICALLY DOMINANT FOLLICLES DURING THEIR GROWING AND REGRESSING PHASES OF DEVELOPMENT IN CATTLE

The objectives were to determine the relationships between changes in the levels of histological and biochemical (estradiol [E2]:androstenedione [A], E2:A ratio) atresia and changes in inhibin contents of morphologically dominant follicles collected during the growing or the regressing phase of the first wave of follicular development in cycling crossbred beef heifers. Heifers were slaughtered either when the dominant follicle (greater-than-or-equal-to 9 mm; diameter of the antral cavity as assessed by ultrasonography) of the first wave was still growing (n = 7) or when the first dominant follicle (greater-than-or-equal-to 9 mm; n = 7) was regressing or was at the end of the plateau phase. Following ovary collection, the dominant follicle was dissected and level of histological atresia was determined on sections of follicular walls. Aliquots of follicular fluid from each of the dominant follicles were corrected to measure concentrations of E2, A, and inhibin alpha subunit by RIA and to measure concentrations of dimeric (alpha-beta dimer) inhibin by a two-site immunoradiometric assay. Heifers were slaughtered on Days 5.4 +/- 0.5 (growing phase) and 11.8 +/- 0.5 (regressing phase the estrous cycle, and mean diameter of the dominant follicles was similar in both phases (9.9 +/- 0.4 vs. 10.8 +/- 0.4 mm; p > 0.09). All morphologically dominant follicles collected during the growing phase (7/7) were histologically healthy and estrogen-active (E2:A ratio > 1), white those collected during the regressing phase (7/7) were histologically atretic and estrogen-inactive (E2:A ratio < 1; chi2 = 14.0,p < 0.0001). Concentrations of inhibin alpha subunit in dominant follicles were higher during the growing than the regressing phase ( 1 36.9 +/- 12.9 vs. 60.3 +/- 12.9 mug/ml; p < 0.001) and were positively correlated with E2:A ratios (r = 0.78; p < 0.001). In contrast, dimeric inhibin concentrations were higher in regressing than in growing dominant follicles (1.95 +/- 0.18 vs. 0.43 +/- 0.18 mug/ml; p < 0.001) arid were negatively correlated with E2:A ratios (r = -0.92; p < 0.0001). Follicular fluid levels of dimeric inhibin were substantially lower than inhibin alpha subunit levels, and an inverse relationship (r = -0.69; p < 0.006) was detected between follicular fluid levels of dimeric inhibin and inhibin alpha subunit in both growing and regressing dominant follicles. Morphologically dominant follicles have different levels of atresia during the growing and growing phases of development, which are associated with distinct differences in inhibin alpha subunit and dimeric inhibin levels.