Receptors for native gonadotropins in amphibian liver

We demonstrate that highly purified bullfrog (f) follicle-stimulating hormone (FSH) and luteinizing hormone (LH) bind specifically and significantly to a crude plasma membrane fraction of bullfrog liver. The other extragonadal organs of the bullfrog showed little or no specific binding. Specific bindings of 125I-fFSH and 125I-fLH to plasma membranes are saturable processes, and are time-, pH-, and temperature-dependent. Scatchard plots of fFSH and fLH were linear. The association constant of equilibrium (Ka) of the specific fFSH binding sites was 4.77 +/- 1.24 X 10(9)M-1 (mean +/- SEM) and the number of sites was 0.262 +/- 0.042 fmol/mg protein (mean +/- SEM). The Ka of the specific fLH binding sites was 5.38 +/- 1.27 X 10(9)M-1 (mean +/- SEM) and the number was 0.315 +/- 0.019 fmol/mg protein (mean +/- SEM). Competition experiments revealed that both fFSH and fLH use the same single class of binding sites. Binding of rat, chicken, bullfrog, and salmon gonadotropins to plasma membranes of the testis and liver of various vertebrates was studied. A significant degree of specific binding was detected only in combinations of bullfrog gonadotropins and amphibian livers. The concentration of adenosine 3'-5'-monophosphate (cAMP) in mince or primary culture cells of bullfrog liver was greatly increased by adding fFSH and fLH to the medium. Bullfrog LH was more potent than fFSH in increasing cAMP concentration, although they were not distinguished by specific binding sites. These data suggest that not only the gonads but also the liver is the target of gonadotropins in the bullfrog, although the final hepatic function controlled by gonadotropins remains unknown.