Gonadotropic hormone (GtH) receptors in the testis of the troutSalmo gairdneri: in vitro studies

A particulate fraction obtained from trout testis at the time of spermiation shows saturable binding sites for¹²⁵I-labeled salmon gonadotropin (¹²⁵I-GtH). Non-gonadal tissues (liver, muscle and spleen) did not demonstrate specific¹²⁵I-GtH binding. The tracer's specific activity was determined by the self-displacement method (18 to 30 Ci/g). Maximal specific binding ability of¹²⁵I-GtH varied from 20 to 30% of the labelled ligand added, depending on the hormone preparation. Specific binding of¹²⁵I-GtH to 20 mg of the testis membrane varied from 40 to 85% of the total binding depending on the method of membrane prepratation, and was competitively inhibited by concentrations of unlabelled GtH ranging from ca 1 to 1000 ng/ml of incubate. Gonadotropin of mammalian origin, ovine TSH or salmon prolactin competed only weakly, or not at all, for testicular gonadotropin binding sites (relative potencies s-GtH>>FSH=hCG>s-PRL>bTSH). Scatchard analysis of equilibrium binding studies shows that saturable gonadotropin binding was due to a class of high affinity binding sites (sites I Ka3×10¹⁰ M^–1) and possibly to a second class of lower affinity binding sites (sites II Ka=5 to 14×10⁸ M^–1). The binding capacity of sites I, as measured in enriched membrane preparations, was 45±18 fmoles/g of testis during the period of spermiation. The concentration of GtH required to obtain half maximal displacement of¹²⁵I-GtH in the binding studies was of the same order of magnitude as the apparent ED50 for GtH stimulation of 11-Cetotestosterone (11KT) secretion by trout testesin vitro. Mammalian LH and FSH were 100 to 1000 folds less potent than salmor GtH to increase 11 KT secretion.