Secretory pattern of inhibin during estrous cycle and pregnancy in African (Loxodonta africana) and Asian (Elephas maximus) elephants.

The ovary of female elephants has multiple corpora lutea (CL) during the estrous cycle and gestation. The previous reports clearly demonstrated that inhibin was secreted from lutein cells as well as granulosa cells of antral follicles in cyclic Asian elephants. The aim of this study is to investigate the inhibin secretion during the pregnancy in African and Asian elephants. Two African elephants and two Asian elephants were subjected to this study. Circulating levels of immunoreactive (ir-) inhibin and progesterone were measured by radioimmunoassay. Four pregnant periods of an African elephant and three pregnant periods of an Asian elephant were analyzed in this study. Circulating levels of ir-inhibin started to increase at 1 or 2 week before the ovulation and reached the peak level 3 or 4 weeks earlier than progesterone during the estrous cycle in both African and Asian elephants. After last luteal phase, the serum levels of ir-inhibin remained low throughout pregnancy in both an African and an Asian elephant. The mean levels of ir-inhibin during the pregnancy were lower than the luteal phase in the estrous cycle despite high progesterone levels were maintained throughout the pregnancy. These results strongly suggest that CL secrete a large amount of progesterone but not inhibin during the pregnancy in elephants.