A fast passive Ca2+ efflux mediated by the (Ca2+ + Mg2+)-ATPase in reconstituted vesicles (original) (raw)

The (Ca 2+ + Mg2+)-ATPase from skeletal muscle sarcoplasmic reticulum was reconstituted into phospholipid bilayers. The permeability of lipid bilayers to Co z+ and glucose was increased slightly by incorporation of the ATPase, and the permeability of mixed bilayers of phosphatidylethanolamine and phosphatidylcholine increased with increasing content of phosphatidylethanolamine both in the presence and absence of the ATPase. The presence of the ATPase, however, resulted in a marked increase in permeability to Ca 2+, the permeability decreasing with increasing phosphatidylethanolamine content. Permeability to Ca 2 + was found to be dependent on pH and the external concentrations of Mg 2+ and Ca 2+, was stimulated by adenine nucleotides but was unaffected by inositol trisphosphate. A kinetic model is presented for Ca 2+ eftlux mediated by the ATPase. It is shown that the kinetic parameters that describe Ca 2 + efflux from vesicles of sarcoplasmic reticulum also describe efflux from the vesicles reconstituted from the purified ATPase and phosphatidylcholine. It is shown that the effects of phosphatidylethanolamine on efflux can be simulated in terms of changes in the rates of the transitions linking conformations of the ATPase with inward-and outward-facing Ca2+-binding sites, and that effects of phosphatidylethanolamine on the ATPase activity of the ATPase can also be simulated in terms of effects on the corresponding conformational transitions. We conclude that the ATPase can act as a specific pathway for Ca z+ efflux from sarcoplasmic reticulum.