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Inhibin Secretion in the Mare: Localization of Inhibin , ß_A, and ß_B Subunits in the Ovary¹

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

To determine the source of circulating inhibin and estradiol-17ß during the estrous cycle in mares, the cellular localization of the inhibin , ß_A, and ß_B subunits and aromatase in the ovary was determined by immunohistochemistry. Concentrations of immunoreactive (ir-) inhibin, estradiol-17ß, progesterone, LH, and FSH in peripheral blood were also measured during the estrous cycle in mares.

Immunohistochemically, inhibin subunits were localized in the granulosa cells of small and large follicles and in the theca interna cells of large follicles, whereas inhibin ß_A and ß_B subunits were localized in the granulosa cells and in the theca interna cells of large follicles. On the other hand, aromatase was restricted to only the granulosa cells of large follicles. Plasma ir-inhibin concentrations began to increase 9 days before ovulation; they remained high until 2 days before ovulation, after which they decreased when the LH surge was initiated. Thereafter, a further sharp rise in circulating ir-inhibin concentrations occurred during the process of ovulation, followed by a second abrupt decline. After the decline, plasma concentrations of ir-inhibin remained low during the luteal phase. Plasma estradiol-17ß concentrations followed a profile similar to that of ir-inhibin, except during ovulation, and these two hormones were positively correlated throughout the estrous cycle. Plasma FSH concentrations were inversely related to ir-inhibin and estradiol-17ß. These findings suggest that the dimeric inhibin is mainly secreted by the granulosa cells and the theca cells of large follicles; granulosa cells of small follicles may secrete inhibin subunit, and estradiol-17ß is secreted by the granulosa cells of only large follicles in mares.

¹ The work was supported in part by a grant-in-aid from the Equine Research Institute of the Japan Racing Association and by a Co-operative Research Grant from the Ministry of Education of Japan (08306015).

² Correspondence: Kazuyoshi Taya, Laboratory of Veterinary Physiology, Tokyo University of Agriculture and Technology, 3–5-8 Saiwai-cho, Fuchu, Tokyo 183–8509, Japan. FAX: 81 423 67 5767; taya{at}cc.tuat.ac.jp

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