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This chapter discusses the isolation procedure of sarcoplasmic membrane adenosine triphosphatase (ATPases). It also discusses its membrane proteins, membrane lipids composition, and its physical properties. The sarcoplasmic membranes are presumably not the only membranes equipped with a calcium transporting mechanism. Other, less active ATP-dependent calcium transport systems have been found in membranes of erythrocytes and blood platelets. Non muscular cells are calcium sensitive because calcium interferes with oxidative phosphorylation, membrane permeability, or glycolysis at concentrations of 1-10 μM. Thus, membranes of all cells can be expected to be capable of eliminating calcium ions, which have invaded the cells. Direct evidence for an ATP-driven calcium pump in the plasma membrane is difficult to obtain because the fragmented plasma membranes do not form tightly sealed vesicles as do the sarcoplasmic membranes. From experiments with intact cells, however, one can infer the existence of a calcium transport mechanism in the plasma membrane, which is energized by the sodium gradient. The concentration ratio of calcium inside: calcium outside that is maintained by these transport systems has been estimated to be approximately 1,000-10,000, which is in the same order of magnitude as the concentration ratio that can be produced by the sarcoplasmic membranes.
