Reoxygenation-induced arrhythmogenic transient inward currents in isolated cells of the guinea-pig heart

Abstract

Transient inward currents (I _ti), activated by a rise in intracellular Ca concentration, are believed to trigger cardiac arrhythmias in reperfused hearts. In this report, I _ti in isolated cardiocytes from the guinea-pig were evoked by reoxygenation following a period of anoxia of between 4 min and 35 min. Reoxygenation was performed 1 min after the full development of an anoxia-induced time-independent K current. This current disappeared within 2–6 s and in the following 10 s I _ti developed to maximum amplitude. I _ti were evoked using a constant pulse pattern (holding potential V _h=−45 mV; test potential V _t=+10 mV; pulse duration 350 ms; frequency 1 Hz). In more than 95% of the cells, I _ti at the holding potential I _ti (−45 mV) declined with a time constant of τ=670±240 ms (mean±SD, n=17). In two cells, undamped oscillatory currents were observed. The amplitude of I _ti (-45 mV) was proportional to the amplitude and duration of the preceding depolarizing test pulse. Test pulses of long duration (500 ms and 1000 ms, mean ± SD) to potentials positive to +10 mV produced slowly decaying tail currents (τ=391±51 ms, mean ± SD), which superimposed with I _ti (−45 mV). The current/voltage relationship of I _ti peaked between −30 mV and −10 mV and approximated zero at the most positive potentials, i.e. no reversal of I _ti was found up to +80 mV. Using double-pulse protocols (prepulse potential +40 mV), I _ti were enhanced at potentials negative to −30 mV and were also present in the range of the normal resting potential of ventricular heart cells. The instantaneous current-voltage relationship was monotone between −50 mV and +40 mV. Because of the dependence of I _ti on the preceding depolarization, the instantaneous current-voltage relationship provides more reliable information on the voltage dependence of I _ti. The interval between two subsequent I _ti (−45 mV) values was 237±35 ms (mean ± SD, n=27) and depended on the amplitude of I _ti (−45 mV) to increase by 5.2±0.5% (mean ± SD) per 100 pA decrease in I _ti (−45 mV). A simple noise analysis showed that if one assumes that ionic channels are responsible for the generation of I _ti (−45 mV), their unitary conductance cannot exceed 0.36 pS. We conclude that reoxygenation-induced I _ti are triggered by a cyclic release of Ca from the sarcoplasmic reticulum and provide evidence that they are mediated by the electrogenic Na/Ca exchanger. The arrhythmogenic potency of reoxygenation-induced I _ti is demonstrated under current-clamp conditions.