Specific stimulation of heart sarcolemmal Ca2+/Mg2+ ATPase by concanavalin A

The effects of concanavalin A (Con A) on membrane Ca2+/Mg2+ ATPase activities as well as the characteristics of Con A binding were examined by employing rat heart sarcolemmal preparations. Con A stimulated the Ca2+ ATPase and Mg2+ ATPase activities in sarcolemma; maximal stimulation in these parameters was seen at a concentration of 10 micrograms/ml. The observed effects of Con A were blocked by alpha-methylmannoside. Sarcolemmal Na+-K+ ATPase and Ca2+-stimulated ATPase were not affected by Con A. Likewise, Con A did not alter the mitochondrial, sarcoplasmic reticular, and myofibrillar ATPase activities. Con A was found to bind to sarcolemma; alpha-methylmannoside prevented this binding. The Scatchard plot analysis of the data on specific Con A binding showed a straight line with a Kd of about 530 nM and a Bmax of 235 pmol/mg protein, thus indicating that there was only one kind of binding site for Con A in sarcolemma. These results suggest that Con A is a specific activator of the low affinity Ca2+/Mg2+ ATPase system in the heart sarcolemmal membrane.