Differences in the properties of FSH and LH binding sites in the avian gonad revealed by homologous radioligands

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the turkey, Meleagris gallopavo, were used as radioligands to study the characteristics of gonadotropin binding sites in avian and reptilian gonads. Scatchard analysis indicated the presence of multiple binding sites for each radioligand in the turkey and chicken gonad; the association constants ( K a ) for the high affinity, low capacity sites (10 9 to 10 10 M −1) were similar to those observed in gonads of other species. Phylogenetic differences were evident in the ability of gonads to bind turkey and mammalian radioligands; e.g., radioiodinated turkey hormones showed little binding to the pig ovary and radioiodinated human FSH showed little binding to the turkey gonad. Radioreceptor assays (RRAs) revealed the presence of separate binding sites with differential specificity for the two types of gonadotropins in the avian gonad. In particular, when turkey hormones were studied in the homologous RRA with 125I-labeled turkey LH as radioligand, gonadotropin binding sites appeared to be highly specific for LH. With 125I-labeled turkey FSH as radioligand, binding sites showed a relatively high affinity for FSH; but gonadotropin specificity of these sites was incomplete since turkey LH continued to show considerable activity in this FSH-RRA. Comparative studies with heterologous hormones (from chicken, sea turtle, and sheep) and heterologous receptor preparations (from chicken, sea turtle, lizard, and pig) gave variable results, depending on the source of both the hormone and receptor. For example, the specificity of FSH binding sites in the chicken testis was less pronounced than in the homologous turkey system; in fact, turkey LH was more potent than FSH in the chicken FSH-RRA. Binding in the turtle ovary did not show a clear specificity for either gonadotropin with either radioligand. The implications of these results for physiological actions of gonadotropins and evolution in gonadotropin structure are discussed.