BACKGROUND CURRENT IN SINOATRIAL 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 tetraethyl-ammonium (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+ > K+ > Cs+ > Na+ > Li+, which suggests a poor cation selectivity of this current system. 8. The current-voltage relationship of the Na+-dependent background current was similar to that expressed by the Goldman-Hodgkin-Katz equation. The current amplitude was proportional to the extracellular concentration of Na+ over the range of 0-200 mM. 9. Under physiological conditions, the reversal potential of the background current was around -21 mV at physiological concentrations of intracellular K+ and extracellular Na+. Thus the background current may provide an inward current in the pacemaker potential range.