The reversal potential for an electrogenic sodium pump: a method for determining the free energy of ATP breakdown?

Abstract

Chapman (1973) suggested that the free energy available from ATP breakdown might be estimated in the dialyzed squid axon (Brinley and Mullins, 1967) by measuring isotopically the opposing rates of the reversible Na pump reaction. The free energy dissipated by the reaction was described by a mathematical method, the shortcomings of which are discussed. The author advocates a method making use of the current-carrying property of the electrogenic Na pump. The dialyzed squid axon preparation may offer unique possibilities for demonstrating the principle of the reversal potential. In the event that the normal reversal potential exceeds the limits of feasible hyperpolarization in the squid axon, then the principle of the reversal potential should still be demonstrable by dialyzing the high energy phosphate potential to an appropriately low level. If the predicted effects cannot be demonstrated at all, then a major rethinking of the mechanism and stoichiometry of the Na pump may be required.