TESTICULAR INHIBIN IN THE STALLION - CELLULAR SOURCE AND SEASONAL-CHANGES IN ITS SECRETION

The cellular localization of inhibin alpha, beta(A), and beta(B) subun its, 3 beta-hydroxysteroid dehydrogenase (SP-HSD), and cytochrome P450 aromatase (aromatase) in stallion testes was investigated. In additio n, detailed seasonal changes in circulating immunoreactive (ir)-inhibi n were investigated in correlation with testosterone, estradiol, LH, a nd FSH. Inhibin or subunit-positive staining was observed in Sertoli c ells, and more clearly positive staining was noted in Leydig cells. In hibin beta(A) and beta(B) subunits were also stained in both types of cells. Immunoreactivity of 3 beta-HSD and aromatase was confined to th e Leydig cells. There was no seasonal effect on the percentage of the areas within seminiferous tubules and interstitial tissues that staine d positive for the inhibin or subunit. The highest plasma concentratio ns of ir-inhibin were observed in the breeding season, and the lowest levels were noted during the nonbreeding season. The circulating conce ntrations of ir-inhibin, steroid hormones, and gonadotropins were posi tively correlated with each other throughout the 2 years studied. The presence of the inhibin alpha and beta subunits in Leydig cells and Se rtoli cells in the equine testis suggests that these cells may secrete dimetric (bioactive) inhibin in circulation of stallions, and that th e circulating ir-inhibin may be a useful indicator of the testicular f unction of stallions.