Gonadotropin stimulation of steroid synthesis and metabolism in the Rana pipiens ovarian follicle: sequential changes in endogenous steroids during ovulation, fertilization and cleavage stages.
Steroid synthesis and metabolism have been followed in Rana pipiens ovarian follicles, denuded oocytes and eggs during ovulation, fertilization and cleavage stages (blastula formation). Under physiological conditions, gonadotropin stimulation of the fully grown follicle leads to progesterone synthesis from [(3)H]acetate as well as formation of much smaller amounts of 17alpha-hydroxyprogesterone, androstenedione, pregnanedione and pregnanediol. Progesterone levels increase during completion of the first meiotic division, but by ovulation progesterone disappears from the egg. Plasma membrane-bound progesterone is taken up into the oocyte cortical granules and is largely metabolized to 5alpha-pregnane-3alphaol,20-one and 5beta-pregnane-3alpha,17alpha,20beta-triol coincident with internalization of 60% of the oocyte surface (and >90% of bound progesterone) by the end of the hormone-dependent period. The principal steroid in the ovulated egg is 5beta-pregnane-3alpha,17alpha,20beta-triol. There is a rapid efflux of 5beta-pregnane-3alpha,17alpha,20beta-triol into the medium immediately following fertilization and residual steroid levels remain low in the developing blastula. Dissociated blastulae cells prepared from stage 9 1/2 embryos concentrate both pregnenolone and progesterone from the medium with minimal metabolism. The results indicate that the ovarian follicle has the ability to synthesize and metabolize progesterone but that this ability disappears in the ovulated egg. The progesterone metabolites formed during meiosis are largely released at fertilization.
Morrill
Gene A
GA
Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461, USA. morrill@aecom.yu.edu
Schatz
Frederick
F
Kostellow
Adele
A
Bloch
Eric
E