INHIBIN SECRETION IN THE STALLION

To examine the physiological role of inhibin in the stallion, a hetero logous radioimmunoassay (RIA) based on a bovine RTA was validated and used to measure immunoreactive (ir)inhibin concentrations in plasma an d testicular homogenates, The bioactivity of equine testicular inhibin was also examined using an assay for suppression of FSH secretion fro m rat anterior pituitary cells. In addition, to identify the cell resp onsible for secreting testicular inhibin, the localisation of inhibin in the testis was investigated by an immunohistochemical method using a polyclonal antibody against (Tyr30)-porcine inhibin alpha(1-30) NH2. In the RIA, parallel dose response curves were obtained for the bovin e inhibin standard and serial dilutions of stallion plasma and equine testicular homogenates. Parallel FSH inhibition curves were also obser ved for the bovine inhibin standard and serial dilutions of equine tes ticular homogenates in the bioassay, The inhibition of FSH secretion f rom rat pituitary cells by equine testicular homogenates was neutralis ed by an antiserum against bovine inhibin in vitro. Plasma concentrati ons of ir-inhibin, testosterone and oestradiol-17 beta in stallions de creased abruptly after bilateral gonadectomy and FSH and LH concentrat ions in the plasma subsequently increased, Therefore, circulating inhi bin in the stallion appeared to be largely of testicular origin. The h istochemical results showed for the first time that strong immunoposit ive staining for inhibin occurred in the Leydig cells of the testes. S ertoli cells were also stained by the inhibin antibody but the reactio n was weaker than that in Leydig cells. These results indicate clearly that both Leydig and Sertoli cells are potential sources of testicula r inhibin in the stallion. A clear increase in plasma ir-inhibin conce ntrations was observed during the natural breeding season. Similar sea sonal changes in the plasma concentrations of testicular steroid hormo nes and pituitary gonadotrophins occurred throughout the year. In conc lusion, the testes appear to be the main source of inhibin, and testic ular inhibin is secreted by Leydig and Sertoli cells in stallions. The positive correlations between plasma ir-inhibin and testicular activi ty during both the breeding and nonbreeding seasons indicate that plas ma ir-inhibin is a useful indicator of reproductive activity in the st allion.