Actions of emigrated neutrophils on Na(+) and K(+) currents in rat ventricular myocytes

Interactions between neutrophils and the ventricular myocardium can contribute to tissue injury, contractile dysfunction and generation of arrhythmias in acute cardiac inflammation. Many of the molecular events responsible for neutrophil adhesion to ventricular myocytes are well defined; in contrast, the resulting electrophysiological effects and changes in excitation-contraction coupling have not been studied in detail. In the present experiments, rat ventricular myocytes were superfused with either circulating or emigrated neutrophils and whole-cell currents and action potential waveforms were recorded using the nystatin-perforated patch method. Almost immediately after adhering to ventricular myocytes, emigrated neutrophils caused a depolarization of the resting membrane potential and a marked prolongation of myocyte action potential. Voltage clamp experiments demonstrated that following neutrophil adhesion, there was (i) a slowing of the inactivation of a TTX-sensitive Na(+) current, and (ii) a decrease in an inwardly rectifying K(+) current. One cytotoxic effect of neutrophils appears to be initiated by enhanced Na(+) entry into the myocytes. Thus, manoeuvres that precluded activation of Na(+) channels, for example holding the membrane potential at -80 mV, significantly increased the time to cell death or prevented contracture entirely. A mathematical model for the action potential of rat ventricular myocytes has been modified and then utilized to integrate these findings. These simulations demonstrate the marked effects of (50-fold) slowing of the inactivation of 2-4% of the available Na(+) channels on action potential duration and the corresponding intracellular Ca(2+) transient. In ongoing studies using this combination of approaches, are providing significant new insights into some of the fundamental processes that modulate myocyte damage in acute inflammation.