Na+-activated K+ channels in small dorsal root ganglion neurones of rat

1. Whole-cell Na+-activated K+ (K-Na) channel currents and single K-Na channels were studied with the patch-clamp method in small (20-25 mu m) dorsal root ganglion (DRG) neurones in slices of rat dorsal root ganglia. 2. The whole-cell K-Na channel current was identified as an additional K+-selective leakage current which appeared after cell perfusion with internal solutions containing different [Na+]. The concentration for half-maximal activation of K-Na channel current was 39 mM and the Hill coefficient was 3.5. At [Na+](i) above 12 mM, K-Na channel current dominated the unspecific leakage current. The ratio of maximum Ii,, channel current to unspecific leakage current was 45. 3. K-Na channel current was not activated by internal Li+. It was suppressed by external 20 mM Cs+ but not by 10 mM tetraethylammonium. 4. Single K-Na channels with a conductance of 142 pS in 155 mM external K+ (K-o(+))-85 mM internal K+ (K-i(+)) solutions were observed at a high density of about 2 channels mu m(-2) 5. In two-electrode experiments, a direct correlation was seen between development of whole-cell K-Na channel current and activation of single K-Na channels during perfusion of the neurone with Na+-containing internal solution. 6. Under current-clamp conditions, K-Na channels did not contribute to the action potential. However, internal perfusion of the neurone with Na+ shifted the resting potential towards the equilibrium potential for K+ (E-K). Varying external [K+] indicated that in neurones perfused with Na+-containing internal solution the resting potential followed the E-K values predicted by the Nernst equation over a broader voltage range than in neurones perfused with Na+-free solution. 7. It is concluded that the function of K-Na channels has no links to firing behaviour but that the channels could be involved in setting or stabilizing the resting potential in small DRG neurones.