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Background current in sino‐atrial node cells of the rabbit heart.

1. The Ca2+ current, K+ current, hyperpolarization‐activated current, Na(+)‐K+ pump current and the Na(+)‐Ca2+ exchange current were all blocked by appropriate blockers and the remaining time‐independent currents were investigated in single pacemaker cells of the rabbit sino‐atrial node using the whole‐cell patch clamp technique. 2. Exchanging the bathing solution from Tris‐hydroxymethyl‐aminomethane hydrochloride (Tris) Na+ free to 150 mM‐Na+ induced an inward current and the slope conductance of the current‐voltage relationship increased from 0.45 +/‐ 0.18 to 0.87 +/‐ 0.33 nS (n = 71) at ‐50 mV. The remaining conductance in Tris Na(+)‐free solution was essentially the same when Tris was substituted with tetraethylammonium (TEA) or N‐methyl‐D‐glucamine (NMG). The current density of the Na(+)‐dependent inward current obtained by subtracting the current in Tris Na(+)‐free from that in 150 mM‐Na+ solution was 0.73 +/‐ 0.21 pA/pF (n = 71) at ‐50 mV. We called this current the Na(+)‐dependent background current. 3. The membrane conductance was reduced by lowering the temperature of the external solution from 36 to 23 degrees C. In Tris Na(+)‐free solution, the temperature‐sensitive component was outward at all potentials, whereas it showed a reversal potential at around ‐20 mV in 150 mM‐Na+ solution. This reversal potential was interpreted as a sum of the Cs+ efflux and Na+ influx, by comparing the Na(+)‐dependent inward currents obtained at 36 degrees C and those at 23 degrees C. 4. Divalent cations (2 mM‐Ni2+, 1 mM‐Ba2+ or 2 mM‐Ca2+) reduced only the outward current in the Tris Na(+)‐free solution, while in the 150 mM‐Na+ solution, they reduced both the inward and outward components of the current which had a reversal potential of around ‐10 mV. 5. Amiloride depressed the membrane conductance in 150 mM‐Na+, Cs+ or Rb+ external solution, though only at negative membrane potentials, which suggests amiloride has a voltage‐dependent effect on the background current. 6. Removal of Cl‐ from the external solution or the addition of a Cl‐ channel blocker (4,4'‐dinitrostilbene‐2,2'‐disulphonic acid disodium salt, DNDS) failed to affect the membrane conductance. 7. When the monovalent cation‐dependent inward current was measured by subtracting the current in the Tris solution from those recorded in the various monovalent cation solutions, the current amplitude decreased in the order: Rb+ greater than K+ greater than Cs+ greater than Na+ greater than Li+, which suggests a poor cation selectivity of this current system.(ABSTRACT TRUNCATED AT 400 WORDS)