Delayed rectifier potassium current (I_K) in latent atrial pacemaker cells isolated from cat right atrium

Whole-cell recording techniques were used to study the delayed rectifier K⁺ current (I_K) in latent pacemaker cells isolated from cat right atrium, From a holding potential of -40 mV, depolarizing clamp steps elicited L-type Ca²⁺ current followed by an increasing outward current (I_K). The time course of tail current amplitudes parallelled that of the time-dependent activation of outward current. Activation of I_K exhibited a sigmoidal time course that was best fit by a power function where the activation variable was raised to the second power. The voltage-dependence of I_K activation exhibited a sigmoidal relationship between -40 and +30 mV. The half-maximal activation voltage and slope factor were -21.9±1.3 and 13.8±0.9 mV respectively (n=6). The fully activated I/V relationship of I_K was linear between -100 and -30 mV and inwardly rectified at more positive voltages. Following I_K activation, hyperpolarizations more negative than about -50 mV elicited tail currents that consisted of both I_K deactivation and I_f activation. A subtraction protocol was used to isolate I_K tail currents. In 5.4 mM extracellular [K⁺], I_K tail currents exhibited a reversal potential of -78.2±0.3 mV (n=6). The reversal potential of I_K was linearly related to log extracellular [K] and the slope was 51.5 mV per ten-fold change in extracellular [K]. At -70 mV, I_K tail currents decayed as a single exponential function with a time constant of 159±16 ms (n=6). These results indicate that latent atrial pacemakers exhibit I_K activated by depolarization. I_K appears to consist of one current component. At voltages compátible with the maximum diastolic potential (-70 mV) I_K tail currents are relatively small and I_K decays much more rapidly than I_f activation. We conclude that in right atrial latent pacemakers I_K may contribute to repolarization of the action potential and the time course of the initial phase of the pacemaker potential.