Inhibin binding sites in bovine pituitary membranes

Membranes derived from bovine pituitary glands free of the neural lobe were used to investigate the presence of binding sites for inhibin, a glycoprotein produced by the ovarian granulosa cells capable of selectively suppressing FSH secretion from the pituitary gland. Optimal concentration of membranes (400 micrograms prot) and 125I-bovine inhibin (2 nM) were incubated in a medium containing 50 mM Tris-HCl pH 7.4, 0.01 M MgCl2 and BSA 0.01% in a final assay volume of 200 microliters at 37 degrees C for different time intervals. Non-specific binding was estimated using unlabelled inhibin in excess. The time course of specific 125I-bovine inhibin (2 nM) binding to bovine pituitary membranes is slow with 50% binding at approximately 20 min of incubation and reaching equilibrium at 90 min of incubation. The kinetic analysis shows an apparent pseudo first order association rate constant (Kob) equivalent to 4 x 10(-2) min-1. Following equilibrium with the tracer, a large excess of unlabelled inhibin (1.27 microM) was able to displace 84% of the specific binding within 120 min of incubation and 50% of the binding at approximately 40 min. The analysis under displacing conditions showed an apparent dissociation rate constant (K2) equals to 1.5 x 10(-2) min-1 and an apparent association rate constant (K1) equals to 1.3 x 10(9) M min-1. Thus, the estimation of the apparent kinetic equilibrium dissociation constant (Kd = K2/K2) of the binding of inhibin to bovine pituitary membranes was 1.2 nM. These results show for the first time the existence of bovine inhibin specific binding sites in bovine pituitary, and also that such a binding can take place in the absence of either gonadal and/or hypothalamic influences. They also contribute to the better understanding of the role of non-steroidal hormones such as inhibin, in the regulation of gonadotrophin secretion.