Ca2+/Mg2+ ATPase Activities of Heart Sarcolemma, Microsomes, and Mitochondria12

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Abstract
Several divalent cations such as Ca2+ Mg2+ Mn2+ Co2+ Ba2+ and Sr2+ were able to stimulate ATP hydrolysis by rat heart sarcolemmal, mitochondrial, and microsomal fractions; however, the order of their potency was different for each fraction. Maximal activities in the presence of Ca2+ and Mg2+ were obtained at 4–8 mM concentrations; however, the mitochondrial ATPase activity was higher than that of sarcolemma but lower than that of microsomes. The pH optima for mitochondrial and microsomal ATPases varied between 8.0–8.5 whereas those for sarcolemma were observed at 7.5–8.0. Unlike sarcolemma and microsomes, mitochondrial Ca2+ or Mg2+ ATPases were markedly stimulated by DNP and inhibited by sodium azide; sarcolemmal Mg2+ ATPase was slightly inhibited by sodium azide. Although NaF, iodoacetate and ruthenium red inhibited sarcolemmal, mitochondrial, and microsomal ATPases, some differences in sensitivities to these agents were apparent. Lanthanum in low concentrations significantly inhibited sarcolemmal Ca2+ or Mg2+ ATPase only. Ca2+ ATPase and Mg2+ ATPase activities of sarcolemma, microsomes, and mitochondria were decreased by PCMB except that this agent had a biphasic effect on sarcolemmal Mg2+ ATPase and decreased the microsomal enzyme activities to a greater extent than those of the mitochondria. lodoacetamide stimulated mitochondrial Ca2+ ATPase and sarcolemmal Mg2+ ATPase but decreased microsomal Mg2+ ATPase activity. Carbodiimide inhibited microsomal Ca2+ or Mg2+ ATPase activity. On the other hand, sarcolemmal Ca2+ ATPase and mitochondrial Mg2+ ATPase activities were depressed and stimulated by maleic anhydride, respectively. These results indicate some similarities and differences among ATPase systems of cardiac sarcolemma, mitochondria and sarcoplasmic reticulum.