Specific gonadotrophin binding sites in the bullfrog testis

We demonstrated the presence of specific binding sites for bullfrog luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in a testicular crude plasma membrane fraction of the bullfrog, Rana catesbeiana. The equilibrium analysis of the binding showed that a straight line was fitted to the Scatchard plot of LH and a curvilinear line to that of FSH, suggesting the presence of single-type binding sites for LH and the presence of multiple-type binding sites or negative cooperactivity of the binding for FSH. Competition experiments showed that bullfrog LH could replace the specific binding of bullfrog FSH and vice versa. Binding affinity of bullfrog LH to radioiodinated bullfrog FSH binding sites was as high as that of intact bullfrog FSH to the same sites. In contrast, binding properties of bullfrog FSH to radioiodinated bullfrog LH binding sites were not simple: the competition curve obtained with FSH against the binding of bullfrog LH had an extremely low slope value and was not parallel to that obtained with LH. These results suggest that differentiation of LH and FSH receptors is not complete in this species, although it has been reported that these hormones have separate actions on the testis. Competition experiments further showed that bullfrog testicular LH and FSH receptors possessed higher affinities for gonadotrophins of homologous or closely related animal species compared with phylogenically distant groups. Species specificity of the hormone-receptor interaction may cause species specificity of the gonadotrophin action.