Effect of pH and ionophores on the calcium-pumping ATPase of endoplasmic reticulum microsomes from guinea pig pancreas.

The effects of the proton-ionophore FCCP, the electroneutral calcium-proton ionophore A23187, pH and oxalate on calcium uptake and (Ca-Mg)ATPase activity were assayed in microsomal vesicles prepared from guinea pig pancreas. The results can be summarized as follows: 1) The optimum pH for ATP-dependent calcium uptake was 7, whereas the (Ca-Mg)ATPase activity was maximal at pH 7.5-8; 2) after a time lag of oxalate-insensitivity, oxalate stimulated strongly the ATP-dependent calcium uptake; 3) FCCP uncoupled the process of calcium uptake and ATP hydrolysis; 4) the effects of FCCP on calcium transport and ATPase activity were quite similar to those induced by the calcium ionophore A23187. These results can be explained by the formation of a proton gradient through membrane vesicles during the calcium pumping process.