Inhibin activity in the mare and stallion

Abstract An overnight double antibody RIA, employing a rabbit antiserum raised to bovine 31 kDa inhibin (rAs-#1989, NICHD) and purified bovine 31 kDa inhibin (bINH-I-90/1, NICHD) as trace and standard, was validated to measure immunoreactive inhibin (iINH) concentrations in equine peripheral plasma, follicular fluid (FF), ovarian vein (OV) plasma, testicular tissue extracts (TTE) and testicular vein (TV) plasma. The dynamic relationship of iINH and follicle stimulating hormone (FSH) was investigated during the estrous cycle of the mare and the annual reproductive cycle of the stallion. In the RIA, parallel dose-response curves were observed between the bovine inhibin standard and serial dilutions of equine FF, OV, TTE, TV and plasma. The average recovery of a known amount of purified bovine inhibin added to gelding plasma was approximately 100%. In the inhibin bioassay, serial dilution of equine FF and TTE were observed to be parallel to the bovine inhibin standard. A five-fold difference (p Concentrations of FSH, estradiol and iINH changed significantly in the mare during the estrous cycle (p In conclusion, a specific and sensitive RIA for iINH has been validated for plasma and biological fluids in the horse. Furthermore, the gonads appear to be the source of bioactive and immunoreactive inhibin as observed in other species. The dynamic relationship between iINH and FSH that is present in both the mare and stallion suggests that iINH may be a useful marker of gonadal activity in this species.