INSULIN-RECEPTORS IN RAT INSULIN-SECRETING BETA-CELLS

The content and functional characteristics of insulin receptors were studied in cultured rat insulinoma RINm5F cells. Insulin receptors content, as measured by a specific radioimmunoassay, was 9.4+/-2.1 ng/10(6) cells (mean+/-SE). The alpha-subunit insulin receptor function was investigated by I-125-insulin binding studies. The calculated binding affinity was lower in intact cells (IC50=0.7 nM) than in a cell-free systems such as purified plasma membranes (IC50=0.25 nM) or wheat germ agglutinin-agarose (WGA) purified receptors (IC50=0.10 nM). Similarly to typical insulin target cells, after binding to cell surface receptors, insulin is internalized into the intracellular compartment. The proportion of bound I-125-insulin that is internalized is temperature-dependent being higher at 37 degrees C than at 4 degrees C (60+/-5.7% and 4+/-0.8%, respectively). Moreover, insulin was able to down regulate cell surface insulin receptors with a half maximal effect at 1 nM. The beta-subunit insulin receptor function was investigated by studying insulin stimulated receptor autophosphorylation and its tyrosine kinase activity toward the exogenous substrate Poly-Glu: Tyr 4:1. The effect of insulin was dose-dependent for both these activities with a half maximal effect at 1 nM insulin. Rat insulinoma RINm5F cells, therefore, have a significant number of functioning insulin receptors fully capable to mediate insulin biological activity. Since in preliminary studies we observed that also freshly isolated rat beta-cells have a similar insulin receptor content (11.7+/-3.1 ng/10(6) cells) than RINm5F cells and that these insulin receptors have an insulin stimulable tyrosine kinase activity, these data are consistent with a potential role of the insulin receptor in the biology and function of pancreatic beta-cells.