Endocytosis of the lutropin receptor is mediated by a low affinity binding site

Porcine Leydig cells in primary culture were incubated to equilibrium with increasing doses of either porcine luteinizing hormone (pLH) or human chorionic gonadotropin (hCG). Then, free and total high affinity gonadotropin receptors on the cell surface were dosed with [125I]hCG as tracer. In isotonic high salt medium, the pLH concentration for half receptor occupancy (Kocc) and for half receptor endocytosis (Kendo) were nearly indistinguishable (1-3 x 10(-7) M). But, when the medium was changed to an isotonic low salt buffer, Kocc shifted to 1.2 x 10(-9) M and Kendo to 1.5 x 10(-8) M. However, with hCG, both values were largely independent of the ionic strength (Kocc = 10(-10) M and Kendo = 10(-8) M). The fact that Kendo is higher than Kocc suggests that the endocytosis of the high affinity gonadotropin receptor is controlled by the hormone binding to another lower affinity site.