Testicular Inhibin in the Stallion: Cellular Source and Seasonal Changes in Its Secretion¹

^a Laboratory of Racing Chemistry, Tokyo 158-0098, Japan ^b Shadai Corporation, Hokkaido 059-1432, Japan ^c Laboratory of Veterinary Physiology, Tokyo University of Agriculture and Technology, Tokyo 183-8509, Japan ^d Rakuno Gakuen University, Hokkaido 069-0836, Japan ^e Japan Racing Association, Equine Research Institute, Tochigo 320-0856, Japan

The cellular localization of inhibin , ß_A, and ß_B subunits, 3ß-hydroxysteroid dehydrogenase (3ß-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 subunit-positive staining was observed in Sertoli cells, and more clearly positive staining was noted in Leydig cells. Inhibin ß_A and ß_B subunits were also stained in both types of cells. Immunoreactivity of 3ß-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 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 and ß 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.

¹ This work was supported in part by a grant-in-aid for Co-operative Research from the Ministry of Education of Japan (08306015).

² Correspondence: Shun-ichi Nagata, Laboratory of Racing Chemistry, 3-47-6 Kamiyoga Setagaya-ku, Tokyo 158-0098, Japan. FAX: 81-3-3429-4402; BXD01742{at}niftyserve.or.jp

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