CALCIUM-BINDING AND ATPASE ACTIVITIES OF HEART SARCOLEMMA

Rat heart sarcolemma prepared by the hypotonic shock-LiBr treatment method bound Ca2+ by a concentration-dependent and saturable process. The Ca2+ binding values at 50 .mu.M and 1.25 mM Ca2+ concentrations were about 30 and 250 nmol/mg protein, respectively. Both Mg2+ and ATP inhibited Ca2+ binding and no evidence for energy-linked Ca2+ binding with sarcolemma was found. Maximal ATP hydrolysis by heart sarcolemma was seen at 4 mM Mg2+ or Ca2+. The Ca2+-ATPase [EC 3.6.1.3] activity was depressed by the presence of Mg2+ or excess ATP. Low concentrations (10-100 .mu.M) of Ca2+ failed to stimulate ATP hydrolysis in the presence of various concentrations of Mg-ATP. These results indicate the absence of a Ca2+ pump mechanism in the heart sarcolemmal membrane preparation.