Characterization of insulin and insulin-like growth factor-I ovarian receptors during the reproductive cycle of carp (Cyprinus carpio)

Abstract

Insulin and insulin-like growth factor-I (IGF-I) receptors were characterized in glycoprotein fractions prepared by wheat germ agglutinin- agarose affinity chromatography from the ovaries of carp. Insulin-specific overall binding in carp ovaries was 6- to 11-fold lower than IGF-I binding (2.7 ± 0.48% vs. 22.8 ± 3.6% per 20 μg glycoprotein). Cold IGF-I displaced radiolabeled IGF-I binding in doses 1000- to 3000-fold lower than cold insulin. On the other hand, cold insulin displaced radiolabeled insulin binding at concentrations 5- to 30-fold lower than cold IGF-I. The α- subunit molecular masses of carp insulin and IGF-I receptors were smaller than the α-subunit molecular mass of rat insulin receptor (125 and 120 vs. 135 kDa, respectively). Autophosphorylation of carp β-subunit insulin and IGF-I receptors showed similar molecular masses that did not differ from the molecular mass of rat insulin β subunit. Receptor tyrosine kinase activity was stimulated in a dose-dependent manner by insulin and IGF-I. Insulin and IGF-I stimulated tyrosine kinase activity and reached a maximum, respectively, of 224 ± 14% and 279 ± 7% of basal phosphorylation. Insulin and IGF-I binding characteristics were measured through different stages of follicular development. High specific binding of both peptides in primary oocyte growth (5.6 ± 0.8% and 50 ± 10% per 20 μg glycoprotein for insulin and IGF-I, respectively) decreased to a minimum at the end of vitellogenesis, followed by a slight increase later, in the preovulatory stage. The presence of insulin and IGF-I receptors in carp ovaries and the changes in percentage of binding throughout the reproductive cycle suggest that, in carp, the roles of insulin and IGF-I depend on the ovarian maturation stage.