Studies on the regulatory mechanism of ovarian hCG receptors and placental luteotropin in rats (author's transl)

Abstract

The influences of exogenous hCG on rat (pseudopregnant and pregnant) ovarian hCG receptors and the presence of hCG-like material in the rat placenta were investigated. Ovarian hCG receptors in immature rats induced by PMS-hCG priming maintained a binding activity for about 20 days after hCG injection. The administration of hCG to pseudopregnant rats caused time and dose related changes in ovarian 125I-hCG binding percentage and the concentration of serum progesterone. The injection of hCG produced a slight decrease in the binding percentage and was followed by a marked decrease. The measurement of ovarian tissue hCG concentration by radioimmunoassay suggested that the initial loss of binding percentage was associated with the occupancy of receptors by the exogenous hCG, but the major loss of binding percentage might not be attributed to the receptor occupancy. The active process might be involved in the mechanism of receptor loss. The progesterone concentration in sera had a tendency to increase after the injection of 25IU hCG, while it decreased after the injection of 125IU hCG. The 125I-hCG binding percentage to ovaries and the serum concentration of progesterone reached their maximal values from mid to late pregnancy and declined as parturition approached. After the injection of hCG into pregnant rats, the binding percentage rapidly decreased and the serum concentration of progesterone increased. Thereafter the binding percentage recovered on the 4th day after the injection of hCG. On that day, though the concentration of hCG in ovarian tissue was barely detectable, the binding percentage was significantly low compared to the control level. The rat placenta (8th to 21st day of pregnancy) contained hCG-like activities, but they could not be detected in peripheral blood. In the treatment of hCG-antibody in pregnant rats, the binding percentage did not change but the serum concentration of progesterone showed a tendency to decrease to the control level. These studies show that hCG receptors exist as free sites in pregnant and pseudopregnant rat ovaries and may be regulated by down regulation after the treatment of large amounts of exogenous hCG. We found that hCG-like material in rat placenta determined by radioimmunoassay might serve as a luteotropin during pregnancy.