Testicular secretion of inhibin in stallions.

In this study, clear seasonal changes in the circulating concentrations of immunoreactive inhibin were observed in stallions throughout the year. The highest plasma concentrations of immunoreactive inhibin were observed during the breeding season and the lowest concentrations were observed during the nonbreeding season. There was positive correlation between the circulating concentrations of immunoreactive inhibin, steroid hormones and gonadotrophins. Inhibin alpha subunit positive staining was observed in Sertoli cells and more clearly in Leydig cells. Both cell types also stained positively for inhibin betaA and betaB subunits. Immunoreactivity to 3beta-hydroxysteroid dehydrogenase and aromatase was observed in the Leydig cells only. The testes contained large amounts of inhibin pro-alphaC and small amounts of inhibins A and B, and the inhibin B content was higher than the inhibin A content. These results indicate that inhibin B is the major bioactive form of inhibin in equine testes. However, only inhibin pro-alphaC was detectable in the circulation and the inhibin pro-alphaC concentrations changed with the concentrations of immunoreactive inhibin. In conclusion, both Leydig and Sertoli cells secrete testicular inhibin in stallions. The positive correlation observed between plasma immunoreactive inhibin and inhibin pro-alphaC concentrations and testicular activity during the breeding and nonbreeding seasons indicates that the plasma concentrations of these hormones are potentially useful indicators of testicular activity in stallions.