FOLLICLE-STIMULATING-HORMONE STIMULATES RAT SERTOLI CELLS VIA RELATIVELY LOW-AFFINITY BINDING-SITES

FSH receptor binding and hormone-induced activation of the FSH receptor have been studied in the absence and presence of NaCl. Physiological concentrations of NaCl inhibited FSH receptor binding to either intact rat Sertoli cells or Sertoli cell membrane preparations. Analysis of FSH receptor binding, after a 3 h incubation period at 37 degrees C to both FSH receptor sources revealed a single class of high affinity binding sites (K-d approximate to 30 pM) in the absence of NaCl. A gradual increase in NaCl concentration resulted in a progressive decrease in the ligand binding affinity of the FSH receptor and a concomitant three to fourfold reduction in the actual number of FSH binding sites. Estimation of the specific FSH receptor binding became less accurate at physiological NaCl concentrations, since total binding declined while non-specific binding was not affected. Therefore, accurate estimations of the ligand binding affinity and the number of FSH binding sites were not possible at physiological NaCl concentrations. NaCl also affected functional properties of the FSH receptor. The half-maximal FSH concentration for hormone-induced receptor endocytosis was shifted from 31 +/- 4 pM, when tested in the absence of NaCl, up to 660 +/- 27 pM at physiological NaCl concentrations. Similarly, the half-maximal concentration for FSH-induced stimulation of cAMP production was increased from 14 +/- 2 pM up to 259 +/- 21 pM when NaCl was added. Forskolin-stimulated cAMP production was independent of the NaCl concentration. It thus appears that the functional FSH receptor is of relatively low affinity, as illustrated by the 15-20-fold higher ED(50) values for hormone-induced stimulation of adenylyl cyclase and receptor endocytosis in the presence of NaCl. Moreover, in the presence of physiological concentrations of NaCl only approximately 30% of the FSH binding sites that are detectable under low ionic strength conditions are bound by the ligand. The interaction between FSH and its receptor, as occurring under low and physiological ionic strength conditions are so different that it is not warranted to apply the high ligand binding properties of the receptor at low ionic strength conditions to evaluate physiological receptor function.