BACKGROUND CONDUCTANCE ATTRIBUTABLE TO SPONTANEOUS OPENING OF MUSCARINIC K+ CHANNELS IN RABBIT SINOATRIAL NODE CELLS

1. Single myocytes were dissociated from the rabbit sino-atrial node, and the membrane background conductance produced by spontaneous opening of the muscarinic K+ channels was investigated by recording whole-cell and single channel currents in both normal K+ (5.4 mM) and high-K+ (145 mM) external solutions. 2. Increasing external K+ concentration ([K+](o)) from 5.4 to 145 mM induced a large inward shift of the whole-cell current accompanied by considerable current fluctuations at -50 mV. The high-K+-induced current was both K+ selective and voltage dependent, which was examined by varying [K+](o). This current was almost completely suppressed by 1-5 mM Ba2+ or 2-10 mM Cs+ and it was partly blocked by 10 mu M atropine. 3. In high-K+ (l45 mM) solution, 20 nM acetylcholine (ACh) further increased the K+ conductance as well as the current noise. The power density spectrum of the noise was fitted with a sum of two Lorentzian functions. The corner frequencies of both the slow (similar to 5 Hz) and fast (similar to 120 Hz) components were comparable between the noise before and during the ACh application. 4. Internal dialysis with a non-hydrolysable derivative of ATP, 5'-adenylylimidodiphosphate (AMP-PNP) or Mg2+-free solution markedly decreased both the amplitude and fluctuations of the high-K+-induced current. The relation between the variance of the current fluctuations and the mean current amplitude was linear in every experiment using dialysis of AMP-PNP or Mg2+-free internal solution, or using superfusion of ACh. The slopes of these relations gave comparable single channel current amplitudes of -0.7 pA at -50 mV. These results indicate that the spontaneous opening of the muscarinic K+ channels is largely responsible for the high-K+-induced current. 5. In the high-K+ solution, the variance-mean relation at -50 mV showed that the muscarinic K+ channel provides an inward current of 3.12 +/- 2.13 pA pF(-1) (n = 23), which was about 60 % of the total inward background current. 6. In the normal K+ solution, the variance-mean relation at -50 mV indicated that an outward current of 6.0 +/- 2.0 pA (0.33 +/- 0.28 pA pF(-1), n = 8) was provided by the K+ channel. The single channel current amplitude was estimated to be 0.06 +/- 0.02 pA (n = 9). 7. Cell-attached recordings in the absence of ACh demonstrated sporadic and brief openings of channels identical to the ACh-induced channels. The power density spectra of the single channel currents exhibited kinetic properties comparable with those of the whole-cell currents. No obvious channel activities were attributable to K+ channels other than the muscarinic K+ channel in 105 experiments. 8. These results suggest that the K+-selective component of the background current in sinoatrial node cells is entirely due to the spontaneous opening of the muscarinic K+ channels and may play an important role in determining the heart rate under physiological conditions.