Sodium-pump potentials and currents in guinea-pig ventricular muscles and myocytes

When guinea-pig papillary muscles were depolarized to ca. -30 mV by superfusion with K⁺-free Tyrode's solution supplemented with Ba²⁺, Ni²⁺, and D600, addition of Cs⁺ transiently hyperpolarized the membrane in a reproducible manner. The size of the hyperpolarization (pump potential) depended on the duration of the preceding K⁺-free exposure; peak amplitudes (E(p)max) elicited by 10 mM Cs⁺ after 5-, 10-, and 15-min K⁺-free exposures were 12.9, 17.7, and 23.2 mV, respectively. Pump potentials were unaffected by external Cl^- but suppressed by cardiac glycosides, hyperosmotic conditions, and low-Na⁺ solution. Using E(p)max as an indicator of Na⁺ pump activation, the half-maximal concentration for activation by Cs⁺ was 12-16.3 mM. At 6 mM, Cs⁺ was three times less potent than Rb⁺ or K⁺ and five times more potent than Li⁺. From these findings, and correlative voltage-clamp data from myocytes, we calculate that (i) a pump current of 7.8 nA/cm² generates an E(p)max of 1 mV and (ii) resting pump current in normally polarized muscle (~0.16 μA/cm²) is five times smaller than previously estimated.