Binding of prostaglandin F2 alpha and 20 alpha-hydroxysteroid-dehydrogenase activity of immature rat ovaries throughout pseudopregnancy.

PGF2 alpha is involved in the initiation of progesterone decline in the late luteal phase in the pseudopregnant rat. 20 alpha-Hydroxysteroid- dehydrogenase (20 alpha-OH-SDH), an important enzyme in progesterone metabolism, may participate in luteal regression. It was therefore of interest to investigate whether prostaglandin F2 alpha (PGF2 alpha) receptor binding in membrane particles and 20 alpha-OH-SDH activity in a cytosolic fraction in superovulated ovaries of immature rats are related. Scatchard analysis of the radioligand binding data revealed two classes of PGF2 alpha receptors (KD 10(-10) mol/l and 10(-8) mol/l). The number of binding sites varied from day 1 through day 21 of pseudopregnancy. Maximal binding/mg protein was obtained on day 7 with a gradual decrease until day 21 after HCG administration. 20 alpha-OH-SDH activity was low in 1 to 9 days old corpora lutea and increased markedly during days 11 to 13. Maximal enzyme activity was monitored on days 15 to 21 after administration of HCG. The time dependent increase in 20 alpha-OH-SDH activity was partially suppressed by the treatment of the rats with indomethacin, a potent inhibitor of prostaglandin biosynthesis. Since PGF2 alpha-receptor content does not coincide with the increase in enzyme activity (maximal receptor content preceded maximal enzyme activity by about 6 days), PGF2 alpha may not be the only direct factor responsible for the induction of ovarian 20 alpha-OH-SDH, to terminate luteal function.