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

When guinea-pig papillary muscles were depolarized to ca. -30 mV by superfu sion 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) elici ted 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 C l- but suppressed by cardiac glycosides, hyperosmotic conditions, and low-N a+ 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+ w as three times less potent than Rb+ or K+ and five times more potent than L i+. From these findings, and correlative voltage-clamp data from myocytes, we calculate that (i) a pump current of 7.8 nA/cm(2) generates an E(p)max o f 1 mV and (ii) resting pump current in normally polarized muscle (similar to 0.16 mu A/cm(2)) is five times smaller than previously estimated.