Pig coronary artery smooth muscle: substrate and pH dependence of the two calcium pumps

The properties of the pig coronary artery smooth muscle Ca transport mechanisms and the effects of pH were examined. The Ca pump in the plasma membrane requires less than 50 microM Mg-ATP2- for half-maximal activity, but the pump in the endoplasmic reticulum requires greater than 700 microM Mg-ATP2-. The Ca uptake by the various subcellular organelles occurs with the following affinity characteristics towards Ca2+: plasma membrane, Km = 0.91 +/- 0.06 microM, and Hill coefficient = 1.31 +/- 0.09; endoplasmic reticulum, Km = 0.58 +/- 0.05 microM, and Hill coefficient = 2.48 +/- 0.34; and mitochondria, Km = 7.1 +/- 1.16, and Hill coefficient = 0.93 +/- 0.10. The active Ca transport systems showed similar pH dependence in that the Ca uptake at pH 6.8 was greater than at 7.6. However, the pH effects were much larger on the Ca uptake by the endoplasmic reticulum and mitochondrial membranes than by the plasma membrane. The plasma membrane also showed a small amount of pH-dependent, high-affinity Ca binding. Ca efflux from passively loaded plasma membrane vesicles was similar at pH 6.4 and 7.4. From these data, it is concluded that the endoplasmic reticulum Ca pump may play a larger role in the vasodilation of large coronary artery, but the role of the plasma membrane may be more in maintenance of relaxation mechanism.