Characterization of insulin receptors and hormone induced augmentation of Ca2+ pools in cardiac sarcolemma and sarcoplasmic reticulum

Although insulin is known to bind with heart cells, no information on specific binding sites for insulin in cardiac sarcolemma (SL) or sarcoplasmic-reticulum (SR) is available in the literature. Subcellular distribution of 125I-insulin in myocardium after an intravenous injection of the hormone indicated highest specific activity of the radio-ligand in the 40,000 g fraction. However, insulin binding to SL was 2 to 3 folds more than that to SR membranes. Scatchard plots were nonlinear in both cases but the apparent affinity constants and receptor population in SR were substantially different than that in SL. Insulin binding to both the membranes obeyed the phenomenon of negative cooperativity; however, dissociation rate of the high affinity component of binding was 2 fold faster in SR than SL. The optimum pH for insulin binding to SR was between 6.5 to 7.5 in comparison to a sharp peak at pH 8.0 in SL. In physiological concentrations insulin enhanced ATP dependent Ca2+ binding and oxalate supported Ca2+ uptake of SL and SR, respectively. These studies indicate that specific insulin binding sites on SR are characteristically distinct from those in SL and insulin is capable of stimulating Ca2+ pumps located in both membranes