Characterization of insulin receptors in cardiac sarcolemmal and sarcoplasmic reticular membranes

Specific insulin binding sites in cardiac sarcolemmal (SL) and sarcoplasmic reticular (SR) membranes were examined. Under in vitro conditions, the specific insulin binding to SL was two to three times greater than that to SR isolated from myocardium of different species. Insulin binding to these membranes was rapid, saturable, and temperature dependent. Scatchard plots were nonlinear and revealed high-affinity sites with K1 of 1.2 +/- 0.1 and 2.3 +/- 0.4 nM and low-affinity sites with K2 of 34.0 +/- 11.6 and 87.7 +/- 18.7 nM for SR and SL membranes, respectively. Unlabeled insulin accelerated the dissociation of previously bound labeled insulin, reflecting negative cooperativity of site-to-site interaction in both membranes; however, the dissociation rate in SR was about twofold higher than that in SL. The optimum binding of insulin to SR showed a plateau between pH 6.5 and 7.5 in comparison with a sharp peak at pH 8.0 for SL. Insulin significantly enhanced ATP-dependent Ca2+ binding to SL and SR membranes by approximately 100 and 40%, respectively. The Na+-dependent Ca2+ uptake in SL and the oxalate-supported Ca2+ uptake in SR were also stimulated by insulin; the maximum increase in Ca2+ accumulation in these vesicles was approximately 150 and 25%, respectively. These studies indicate that specific insulin binding sites in SL are characteristically distinct from those in SR and that insulin is capable of stimulating Ca2+ pumps located in these membranes.