Positive cooperative action of follicle-stimulating hormone on binding of luteinizing hormone to testicular receptors from the bullfrog (Rana catesbeiana).

We found that the bullfrog testis has specific gonadotropin-binding sites of only a single type which have the same affinity for both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the bullfrog. The affinity (the equilibrium constant of dissociation or Kd) and capacity (the number of binding sites) of the binding sites for FSH were 0.49 nM and 0.27 fmol/mg fresh tissue, respectively, and those for LH were 0.53 nM and 0.23 fmol/mg fresh tissue, respectively, when these parameters were determined independently. Specific binding of labeled FSH was completely displaced by both FSH and LH, and their competition curves were identical. However, when labeled LH was used, the slope of the curve for competition by FSH was much gentler than that of the curve for competition by LH. A large amount of FSH (about 5 micrograms/ml) was required to replace labeled LH and to reduce the binding of LH to the nonspecific binding level. To explain this result, we assumed the model of nonlinear positive cooperativity by heterologous hormones, in which the affinity for LH of the binding sites is increased in proportion to the number of sites occupied by FSH, raised to the cth power, where c is a constant. When the observed equilibrium parameters were employed and appropriate values for c and for Kf (the Kd for completely occupied binding sites) were chosen as 1.45 and 0.00045 nM, respectively, the theoretical curve for competition by FSH against labeled LH coincided almost exactly with the observed curve. The positive cooperative action of FSH on the binding of LH was also shown by association experiments, but not by dissociation experiments. The positive cooperative action of a heterologous hormone may be a device developed in anurans to overcome the problem that one of the two gonadotropins competitively inhibits the action of the other when both gonadotropins are secreted simultaneously.