Inhibition of 125I-Human Follicle-Stimulating Hormone Binding to Receptor by a Low Molecular Weight Fraction of Bovine Follicular Fluid: Inhibitor Concentration is Related to Biochemical Parameters of Follicular Development

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Abstract
Pools of follicular fluid (FF) were obtained from large or small follicles of cows which were pregnant or in the luteal phase of the estrous cycle. Cells present in each FF pool were collected by centrifugation and measured for content of FSH and luteinizing hormone (LH) receptors. Steroid levels in FF were quantitated by radioimmunoassay (RIA). Since the quantity of bovine follicular cells (mostly granulosa cells) was limited, FSH binding inhibition was studied utilizing a calf testis receptor system. Low (< 6000) MW fractions prepared by dialysis were shown to account for most (76-94%) of the FSH binding inhibition (FSH-BI) present in unfractionated FF. The concentration of low MW FSH-BI was higher in pools of FF from cows in the luteal phase of the estrous cycle than in pools of FF from pregnant cows. The concentration of low MW FSH-BI was also higher in FF pooled from small follicles than in FF pooled from large follicles of either pregnant or luteal phase cows. Relative concentrations of receptors for gonadotropins (FSH, LH) on granulosa cells were used to rank the pools according to relative degree of follicular maturation. Other parameters of follicular maturation were concentration of estrogens and the ratio of estrogens to androgens in FF. Biochemical parameters for follicular atresia were the concentration of androgens and the ratio of estrogens to androgens in FF. Based on these parameters, concentration of low MW FSH-BI in pools of FF was not significantly correlated with relative degree of follicular maturation, but was positively correlated (r = 0.967, no. = 4, P .simeq. 0.02) with relative degree of follicular atresia. Low MW FSH-BI may play a role in the process of follicular atresia in the bovine ovary.