Differences in the characteristics and distribution of rat luteal receptors for native and deglycosylated human choriogonadotropin

Previous work has suggested that rat luteal cells have two populations of LH/hCG receptors that are located in different parts of the cell membrane. The possibility that these two receptor pools may have functional differences has been investigated through examination of the binding and action of native and deglycosylated hCG to different membrane fractions. Ovaries from eCG/hCG-primed immature female rats were separated into 1,000 x g (heavy) and 20,000 x g (light) particulate fractions. Increasing concentrations of NaCl had a biphasic effect on the binding of native and deglycosylated hCG to both membrane fractions, causing an increase in binding at low concentrations and a decrease in binding at higher concentrations. The binding of deglycosylated hCG to both membrane preparations and the binding of native hCG to light-membrane preparations was maximal at approximately the same NaCl concentration (50-65 mM). This was higher than the concentration of NaCl necessary for maximal binding of native hCG to the heavy-membrane preparation. In addition, maximal native hCG binding to this preparation occurred over a broader NaCl concentration range (15-65 mM). Equilibrium binding experiments showed differences in hCG binding to both fractions. In light membranes there were significantly more receptor sites for deglycosylated hCG (11.2 +/- 4.8 fmol/mg ovary) than for native hCG (4.8 +/- 0.7 fmol/mg ovary), with no significant different in affinity. In contrast, in heavy membranes the affinity for deglycosylated hCG (6.30 +/- 0.19.10(9) M-1), was significantly higher than that for native hCG (2.60 +/- 0.13.10(9) M-1), with no significant differences in receptor number.