Insulin binding to differentiating muscle cells in culture

Saturable binding of 125I-labelled insulin was detected on L6 myoblasts grown in monolayers. Binding was proportional to the number of cells and was pH sensitive, decreasing above pH 7.0. Binding also decreased with increasing temperature in the range 22-37 degrees C. Binding of insulin over a range of hormone concentrations (10(-10)-10(-6) M) was analyzed by Scatchard plots. The data are compatible with the coexistence of two components with Kd of 3 and 190 nM. The number of both types of binding sites increased with cell differentiation (upon alignment and cell fusion), when expressed either per unit protein, DNA, or surface membrane cholesterol. On the other hand, their association constants did not vary. Binding of insulin to a nonfusing mutant of L6 cells resembled binding to the parent myoblasts before alignment.