Equilibrium state of ATP-driven ion pumps in relation to physiological ion concentration gradients

Dear Sir: The purpose of this communication is to show that the Ca ion concentration gradient across the sarcoplasmic reticulum membrane of a resting skeletal muscle cell is close to what would be predicted for the equilibrium state of the ATP-driven Ca pump. The Na and K gradients across the plasma membranes of erythrocytes or of a resting squid axon, however, fall far short of the predicted equilibrium of the ATP-driven Na,K pump. The basis for the apparent inefficiency of the Na,K pump is not obvious. It may be physiological, arising from competing ion fluxes produced by other ion-translocating systems in the membrane, or it may be biochemical, arising from a factor such as imperfect discrimination by the pump protein between Na and K ions. The calculated equilibria are based on the assumption that ATP hydrolysis and ion transport are tightly coupled by the pump proteins, but no assumption about the detailed mechanism is required. With the generally accepted stoichiometries (Glynn and Karlish, 1975; Hasselbach, 1979), the coupled overall reactions are