Estrous cycle-related changes of high affinity luteinizing hormone/human chorionic gonadotropin binding sites in the rat uterus

Abstract

LH/human CG (LH/hCG) high affinity binding sites were detected in crude membrane preparations of rat uteri. There was little competition for receptor occupancy between hCG and ovine FSH (oFSH) (0.05%) and no competition between hCG and ovine PRL (less than 0.01%). No similar binding sites were detected in crude membrane preparation of heart, kidney, skeletal muscle, liver, and lung tissues. Concentrations of uterine unoccupied binding sites (RLH) were determined for each stage of the 4-day estrous cycle. The RLH were found in all preparations of metestrus uteri (n = 10) but only in some preparations from the other stages of the estrous cycle (1 of 7 on proestrus, 3 of 4 on estrus, 5 of 7 on diestrus). The concentration of uterine RLH varied throughout the estrous cycle with highest values during the metestrus (1.50 +/- 0.15 fmol/mg protein) and lowest values during the proestrus (less than 0.2 fmol/mg protein). The affinity constant for hCG of uterine RLH remained constant during the estrous cycle (about 0.8 x 10(11) M-1) and was nearly identical to that of rat ovarian receptors. On metestrus, RLH concentration appeared to be approximately 35-fold lower in the uterus than in the ovaries when expressed per mg protein (1.50 +/- 0.15 vs. 52.83 +/- 3.61 fmol/mg protein) but only 20 times lower when expressed per organ (2.2 vs. 48.3 fmol/organ). The estrous cycle-related changes of uterine RLH concentration, together with our data establishing an in vitro influence of hCG on progesterone metabolism in rat uterus, suggest that some uterine functions could be directly regulated either by LH from the pituitary or during early pregnancy by an LH-like substance originating from the embryo.