Calcium-tolerant myocytes from the adult rat heart were used to study the effects of EDTA on insulin binding and insulin action. Treatment of cardiocytes with EDTA resulted in a 60% inhibition of insulin binding. This effect was partially reversible by subsequent addition of calcium or magnesium. Scatchard analysis of equilibrium binding data in the presence of calcium and magnesium showed a curvilinear plot with a high-affinity segment having a Kd of 5.7 × 10−10 mol/L. In the presence of EDTA a linear Scatchard plot was observed with a Kd of 5.6 × 10 −9 mol/L. The total number of insulin receptors remained unaltered under these conditions. In contrast to insulin binding, insulin internalizaron was not affected by EDTA treatment.
Insulin action was studied by measuring the effect of the hormone on the transport of 3-O-methylglucose. Half-maximal action occurred at an insulin concentration of 3 × 10−10 mol/L and 10−8 mol/L in control and EDTA-treated cells, respectively. Maximal transport stimulation, however, was not significantly different in both groups (130% and 106%, respectively).
In conclusion, low-affinity insulin receptors in cardiac myocytes mediate a biologic response comparable to that of high-affinity sites; moreover, they may be involved in the process of internalization in this tissue. The data suggest a functional role of low-affinity insulin receptors in cardiac muscle.