Differences between outward currents of human atrial and subepicardial ventricular myocytes.

1. Outward currents were studied in myocytes isolated from human atrial and subepicardial ventricular myocardium using the whole‐cell voltage clamp technique at 22 degrees C. The Na+ current was inactivated with prepulses to ‐40 mV and the Ca2+ current was eliminated by both reducing extracellular [Ca2+] to 0.5 mM and addition of 100 microM CdCl2 to the bath solution. 2. In human myocytes, three different outward currents were observed. A slowly inactivating sustained outward current, I(so), was found in atrial but not ventricular myocytes. A rapidly inactivating outward current, I(to), of similar current density was observed in cells from the two tissues. An additional uncharacterized non‐inactivating background current of similar size was observed in atrial and in ventricular myocytes. 3. I(to) and I(so) could be differentiated in atrial myocytes by their different kinetics and potential dependence of inactivation, and their different sensitivities to block by 4‐amino‐pyridine, suggesting that two individual channel types were involved. 4. In atrial cells, inactivation of I(to) was more rapid and steady‐state inactivation occurred at more negative membrane potentials than in ventricular cells. Furthermore, the recovery of I(to) from inactivation was slower and without overshoot in atrial myocytes. In addition, 4‐aminopyridine‐induced block of I(to) was more efficient in atrial than in ventricular cells. These observations suggest that the channels responsible for atrial and ventricular I(to) were not identical. 5. We conclude that the differences in outward currents substantially contribute to the particular shapes of human atrial and ventricular action potentials. The existence of I(so) in atrial cells only provides a clinically interesting target for anti‐arrhythmic drug action, since blockers of I(so) would selectively prolong the atrial refractory period, leaving ventricular refractoriness unaltered.