Fibroblastic cultures from the diabetic db/db mouse. Demonstration of decreased insulin receptors and impaired responses to insulin.

Binding of 125I-insulin to cells cultured from the skin of nondiabetic and diabetic (db/db) mice was 80 to 90% specific, time- and temperature-dependent, and maximal at pH 8.0. Porcine insulin and desalanine insulin competed equally for 125I-insulin binding, while proinsulin and desoctapeptide insulin were 35% and 20% as potent, respectively. 125I-Insulin dissociated from both types of fibroblasts with a T 1/2 of 7.5 min. Analysis of the dissociation data resolved two rate constants of 3.0 and 1.0 X 10(-4) s-1 for nondiabetic, and 2.0 and 0.8 X 10(-4) s-1 for diabetic fibroblasts. Binding of 125I-insulin to diabetic fibroblasts was 35 to 50% of that to nondiabetic fibroblasts during at least 17 passages. Scatchard analysis of the binding data resolved high (K1 = 2 X 10(10( M-1) and low affinity K2 = 2 X 10(9) M-1) sites. Nondiabetic fibroblasts possessed 7.7 X 10(4) sites/cell, while diabetic fibroblasts possessed 2.9 X 10(4)/cell. Incubation of nondiabetic fibroblasts with insulin resulted in a time- and concentration-dependent decrease in the binding of 125I-insulin. Binding activity returned to normal when insulin was removed and it was prevented by cycloheximide. In contrast, diabetic fibroblasts did not exhibit down-regulation of receptors. A half-maximal and maximum (85%) stimulation of 2 deoxy-D-glucose uptake was observed with 0.75 nM and 16.7 nM insulin in nondiabetic fibroblasts. In contrast, diabetic cultures required 3.5 nM insulin for half-maximal stimulation of 2 deoxy-D-glucose uptake, and maximum stimulation was 32% with 16.7 nM insulin. Similarly, diabetic fibroblasts required higher concentrations of insulin (20 nM) to stimulate ornithine decarboxylase activity to 42% of nondiabetic cells. These results indicate that in comparison with fibroblastic cultures from nondiabetic animals, those from diabetic animals expressed differences in insulin receptor numbers which are maintained in culture over many generations and are accompanied by diminished insulin responses.