Characteristics of insulin receptors in the heart muscle: binding of insulin to isolated muscle cells from adult rat heart

Abstract

Adult rat heart muscle cells obtained by perfusion of the heart with collagenase have been used to characterize the insulin receptors by equilibrium binding and kinetic measurements. Binding of 125I-labelled insulin to heart cells exhibited a high degree of specificity; it was dependent on pH and temperature, binding at steady state increased with decreasing temperatures. Above 70% of the radioactivity bound at equilibrium at 25 degrees C could be dissociated by addition of an excess of unlabelled insulin. 54 and 40% of 125I-labelled insulin was degraded by isolated heart cells after 2 h at 37 degrees C and 4 h at 25 degrees C, respectively. This degrading activity was effectively inhibited by high concentrations of albumin. Equilibrium binding studies were conducted at 25 degrees C using insulin concentrations ranging from 2.5 x 10(-11) mol/l to 10(-6) mol/l. Scatchard analysis of the binding data resulted in a curvilinear plot (concave upward), which was further analyzed using the average affinity profile. The empty site affinity constant was calculated to be 9.5 x 10(7) l/mol with a total receptor concentration of 3.4 x 10(6) sites per cell. The presence of site-site interactions of the negative cooperative types among the insulin receptors has been confirmed by kinetic experiments. The rate of dilution induced dissociation was enhanced in the presence of native insulin (5 x 10(-9) mol/l), both, under conditions of low and high fractional saturation of receptors. These studies demonstrate the presence of specific insulin receptors in isolated muscle cells from adult rat heart and provide a useful model for the study of insulin action on the heart.