Changes in ovarian 5α-steroid reductase and 20α-hydroxysteroid dehydrogenase activity produced by induction of first ovulation with gonadotropin

Abstract

A shift in the pathway of steroid synthesis is known to occur in the rat ovary at the time of the first ovulation. In the immature animal progesterone is metabolised mainly through 5α -reduced pregnanes; following the first ovulation, however, 20α-hydroxypregn-4-en-3-one is the main product. In order to determine the mechanism of this shift, the activities of ovarian 5α-steroid reductase and of 20α-hydroxysteroid dehydrogenase were determined during the induction of ovulation by a gonadotropin. A considerable decline in 5α-reductase activity was found following administration of pregnant mare serum gonadotropin: one day after the stimulation, a 40% decrease in activity was measured. On the 3rd day, one tenth of the original activity was detected and by the 7th day only residual activity. No 20α-hydroxysteroid dehydrogenase activity was detectable in the ovary before pregnant mare serum gonadotropin-administration. One day following pregnant mare serum gonadotropin low activity was already present. This activity increased rapidly and was still rising on the 3rd day following the gonadotropin administration. The collective data on the activities of both enzymes throughout the induction of ovulation can explain the shift in steroidogenesis taking place in the ovary. The importance of these findings to the mechanism of the onset of puberty is discussed.