Experiments were designed to evaluate the concept that the activity of the electrogenic Na+-pump is dependent on the transmembrane potential. Cardiac muscle preparations were used because the electrogenic Nay -pump current can be recorded at different potentials with the voltage clamp method in this preparation. Electrogenic Nat-pump current was identified as the membrane current which was abolished by ouabain (5 µM) and induced by the addition of K+ or an alkali metal cation, such as Rb+, Cs,

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... r Lit, to the extracellular K+-free solution. Alkali metal cations other than K+ were used to eliminate the possibility that a passive membrane K+ current might be altered by changes in the K+ concentration in the vicinity of the membrane due to activation of the Na+-pump. It was concluded that the activity of the electrogenic Natpump current is dependent on the membrane potential.