Effects of internal and external Na+ ions on inwardly rectifying K+ channels in guinea-pig ventricular cells

1. The effects of internal and external Na+ ions on the inwardly rectifying K+ channel were studied in guinea-pig ventricular cells. 2. Single-channel currents through the inwardly rectifying K+ channel were recorded in the open cell-attached or inside-out configuration at 150 mM internal K+ and either 150 or 25 mM external K+. Internal Na+, at a concentration of 5-40 mM, reduced the unitary amplitude of the outward current. No increase in open-channel current noise was detected with the filter cut-off frequency of 3 kHz. Substate behaviour seen with internal Mg2+ at a micromolar level was not observed. The inward currents were little affected by internal Na+. 3. The unitary current-voltage relation rectified inwardly in the presence of internal Na+ in a concentration-dependent manner. 4. Outward unitary currents were normalized to those measured in the absence of Na+. The normalized current-voltage relation was shifted in the negative direction by 20-25 mV by decreasing external K+ from 150 to 25 mM, indicating that the blocking effect increases with low external K+ when compared at a fixed voltage. 5. The normalized current-Na+ concentration curve was fitted by a one-to-one binding curve at each voltage. In a semi-logarithmic plot of dissociation constant versus membrane potential, data points for 150 and 25 mM external K+ were fitted by straight lines with nearly the same slope. The dissociation constant at 0 mV is 154 mM in 150 mM external K+ and 89 mM in 25 mM external K+. The voltage dependence of dissociation constants gives a value for the effective valency of the Na+ ion of around 0.5. 6. To study effects of external Na+, single-channel currents were recorded with pipette solutions containing 125 mM Na+, 125 mM choline or 125 mM N-methyl-D-glucamine (NMDG) in addition to 25 mM K+. Current amplitude was smaller with choline than with Na+ or NMDG. The reduction in current amplitude with choline was more evident in the inward current, resulting in a stronger outward rectification of the current-voltage relation. This finding and prolonged mean open time (see Summary point 7) was interpreted by assuming that choline is an open-channel blocker. 7. The lifetimes of the openings in the inward currents were distributed according to a single exponential. The mean open time with Na+ was similar to that with NMDG, which decreased with hyperpolarization. The mean open time with choline was much longer and less voltage dependent.(ABSTRACT TRUNCATED AT 400 WORDS)