Effect of diverse mammalian and nonmammalian gonadotropins on isolated rat Leydig cells

The isolated rat Leydig cell assay was employed to study species specificity in biological action. Eight mammalian luteinizing hormones (LHs), human chorionic gonadotropin, and pregnant mare serum gonadotropin stimulated testosterone production in this assay with a 250-fold potency range. A high potency correlated with a high carbohydrate content. Comparison of the potencies of these hormones in the Leydig assay, the ovarian ascorbic acid depletion assay, and the Xenopus ovulation assay showed the results of the two rat assays to be the most similar, demonstrating that the receptor as well as the hormone play a role in determining species specificity. The LH specificity of the assay was reexamined and follicle-stimulating hormone (FSH) was found to have a small intrinsic activity. Nonmammalian LHs, which generally possess little or no activity in in vivo mammalian assays, were able to stimulate rat Leydig cells, at 1000x mammalian LH doses. LH specificity was retained with nonmammalian LHs but the potency differences between nonmammalian LHs and FSHs were much less than mammalian LHs and FSHs.