A specific FSH receptor in rat granulosa cells: properties of binding in vitro.

The specific binding of [125I]iodoFSH to granulosa cells collected from immature, hypophysectomized, DES-treated rats, was studied in vitro. Specific binding occurred after 5 min and reached maximum after 3 h of incubation at 37 C. Non specific binding was very low (less than 10% of the total binding). The [25I]iodoFSH remained tightly associated with the receptor at pH 7.5, but was rapidly dissociated at pH 5. Unlabeled hFSH competitively inhibited [125I]iodoFSH binding. Kinetic analyses of equilibrium binding experiments gave an apparent association constant (Ka) of 1.34 (+/- 0.31) x 10(10)M-1 [mean (+/- SE)] and a number of binding sites per cell of (NB) 1, 130 +/- 70 (mean +/- SE). Rat prolactin, wheat germ agglutinin, and concanavalin-A did not compete with [125I]iodoFSH, but hLH, hCG, and rTSH competed at doses 300- to 900-fold higher than those of hFSH. Granulosa cells isolated from adult DES-treated rats, as well as cells collected from medium and preovulatory follicles of proestrous rats, gave Ka and NB values similar to those described above. A comparative study of rabbit granulosa cells indicated a much lower binding affinity compared with those from the rat.