Demonstration of a (Ca2+ + Mg2+)-ATPase activity probably related to Ca2+ transport in the microsomal fraction of porcine coronary artery smooth muscle.

A (Ca2+ + Mg2+)-ATPase activity is demonstrated in the microsomal fraction of porcine coronary artery. The characteristics of the ATPase activity are compared with those of the Ca2+ transport, both measured in similar solutions. It is concluded that the (Ca2+ + Mg2+)-ATPase is related to the Ca2+ transport because: 1. Both transport and ATPase have similar low Km values for Ca2+ as well as comparable Hill coefficients. The Km values are respectively 0.34 +/- 0.03 microM [4] and 1.17 +/- 0.15 microM [6]. The Hill coefficients are n = 1.69 +/- 0.09 [4] and n = 1.23 +/- 0.17 [6]. 2. Ionophores A23187 and X537A stimulate (Ca2+ + Mg2+)-ATPase activity while they inhibit net Ca2+ accumulation by increasing the Ca2+ permeability of the membranes. 3. The V values for Ca2+ accumulation and for (Ca2+ + Mg2+)-ATPase are comparable.