Secretion of Inhibin during the Estrous Cycle in the Female Asian Elephant (Elephas maximus)

Abstract

To define the source of circulating inhibin of female Asian elephants, the immunolocalizations of inhibin α, β_A, and β_B subunits, 3β-hydroxysteroid dehydrogenase (3β-HSD), aromatase cytochrome P450 (P450arom) and cytochrome 17α-hydroxylase P450 (P450 c17) were investigated. Concentrations of immunoreactive (ir-) inhibin, progesterone and follicle-stimulating hormone (FSH) during the estrous cycle were measured by radioimmunoassay. Inhibin immunoreactivity in follicular fluid and homogenate of corpora lutea were also measured. Immunolocalization of inhibin subunits, 3β-HSD, P450arom and P450c17 were detected in granulosa cells of antral follicles and luteal cells. The follicular fluid contained high level of ir-inhibin and bioactive inhibin. The homogenate of corpora lutea also contained ir-inhibin. Serum ir-inhibin remained low levels during the early non-luteal phase, began to increase from the late non-luteal phase and continued to increase during the early luteal phase. Serum ir-inhibin showed maximal levels at the middle of luteal phase, and gradually decreased to the baseline three weeks prior to progesterone decline. The serum ir-inhibin levels were positively correlated with progesterone throughout the estrous cycle. On the other hand, ir-inhibin was negatively correlated with FSH during late non-luteal and early luteal phases. These findings strongly suggest that corpus luteum is one of the sources of inhibin as well as granulosa cells in the Asian elephant.