IGF-I, IGF-II and insulin promote differentiation of spermatogonia to primary spermatocytes in organ culture of newt testes

Recombinant human insulin-like growth factors (rhIGF-I and rhIGF-II) and hu man insulin promoted the differentiation of spermatogonia into primary sper matocytes in newt testes fragments cultured in a chemically defined medium. The biological potency for promoting differentiation was dose-dependent fo r all the ligands with the highest potency displayed by IGF-I, followed by IGF-II, and the least by insulin. The difference in potency was larger betw een IGF-II and insulin than that between IGF-I and IGF-II. This order of bi ological potency was in good accordance with the order of affinity in bindi ng specificity of [I-125]IGF-I to the testicular membrane fractions: IGF-II and insulin competed the binding of [I-125]IGF-I only at concentrations 20 -fold and 100-fold higher, respectively, than IGF-I. Specific binding was o bserved in both somatic cells (mostly Sertoli cells) and germ cells (sperma togonia and primary spermatocytes), though the binding to somatic cells was about 2.7 times higher than that to germ cells. These results indicate tha t (1) specific binding sites for IGF-I are present in the newt testes, (2) IGF-II and insulin also bind to these receptors but to a lesser degree, and (3) IGF-II and insulin as well as IGF-I promote spermatogonial differentia tion into primary spermatocytes by binding to the IGF-I receptor.