Testicular Inhibin in the Stallion: Cellular Source and Seasonal Changes in Its Secretion

The cellular localization of inhibin (alpha), (beta)A, and (beta)B subunits, 3(beta)-hydroxysteroid dehydrogenase (3(beta)-HSD), and cytochrome P450 aromatase (aromatase) in stallion testes was investigated. In addition, detailed seasonal changes in circulating immunoreactive (ir)-inhibin were investigated in correlation with testosterone, estradiol, LH, and FSH. Inhibin (alpha)subunit-positive staining was observed in Sertoli cells, and more clearly positive staining was noted in Leydig cells. Inhibin (beta)A and (beta)B subunits were also stained in both types of cells. Immunoreactivity of 3(beta)-HSD and aromatase was confined to the Leydig cells. There was no seasonal effect on the percentage of the areas within seminiferous tubules and interstitial tissues that stained positive for the inhibin (alpha)subunit. The highest plasma concentrations of ir-inhibin were observed in the breeding season, and the lowest levels were noted during the nonbreeding season. The circulating concentrations of ir-inhibin, steroid hormones, and gonadotropins were positively correlated with each other throughout the 2 years studied. The presence of the inhibin (alpha)and (beta) subunits in Leydig cells and Sertoli cells in the equine testis suggests that these cells may secrete dimetric (bioactive) inhibin in circulation of stallions, and that the circulating ir-inhibin may be a useful indicator of the testicular function of stallions.