Properties of follicle-stimulating hormone-receptor interactions. Specific binding of human follicle-stimulating hormone to rat testes.

Some properties of the interaction between radioiodinated human follicle stimulating hormone (lz51-hFSH) and its specific receptors in preparations of rat seminiferous tubule homogenate are described. Specific binding of lz51-hFSH by seminiferous tubule homogenate was appreciably greater than to Leydig cell homogenate of rat testes. The uptake of rz51-hFSH was hormone specific and uptake inhibition seen with purified human luteinizing hormone (hLH) could be attributed to residual FSH activity in the latter. While biologically active FSH inhibited the specific uptake of Y-hFSH by seminiferous tubule homogenate, biologically inactive FSH, denatured by heat, did not. The system could also be used to follow formation of intact FSH from its (Y and /3 subunits, thus further validating the relationship between specific binding and bioiogic activity. The uptake of 12SI-hFSH was tissue specific, since negligible specific binding was seen when a variety of rat tissues other than seminiferous tubule homogenate were examined. The uptake of 1251-hFSH was 1.6fold greater in seminiferous tubule homogenate derived from testes of mature rats compared to immature (16-day-old) rats. Specific 12SI-hFSH uptake was affected by variations in ionic strength, with a maximum uptake, 2.17 X lo-l6 moles per mg of homogenate, being noted in 1 rnrd phosphate buffer, pH 7.5, and no specific uptake in 0.5 M phosphate buffer. Specific uptake was not affected by substitution of Mg2+ for CaZf at the molar concentrations of phosphate described above. Specific binding is temperature-dependent and maximum specific uptake of lz51-hFSH was seen at 37”. Heating of seminiferous tubule homogenate to 60” and above prior to addition of lz51-hFSH abolished specific uptake, whereas nonspecific uptake increased as the temperature was elevated. Specific binding was pH-dependent, with an optimum at pH 7.5. Extremes of pH caused irreversible damage to the receptors and this was a time-dependent process. In our system, specific uptake of 1251-hFSH was a saturable process with respect to hormone concentration, and significant inhibition of uptake by unlabeled hFSH could be detected at 1.06 x lo-l6 moles per mg of homogenate using 7.5 x lo-l4 moles of 1251-hFSH. Scatchard analysis