Early afterdepolarization formation in cardiac myocytes: analysis of phase plane patterns, action potential, and membrane currents.

INTRODUCTION: Early afterdepolarizations (EADs) are among the mechanisms proposed to underlie ventricular arrhythmias. Sea anemone toxin, ATXII, known to delay Na inactivation and to induce plateau level voltage oscillations, was used to study the formation of EADs.

METHODS AND RESULTS: Action potential and membrane currents were studied in rat ventricular myocytes using whole cell current and voltage clamp techniques. Phase plane trajectories were generated by plotting membrane potential (V) versus the first time derivative of membrane potential (dV/dt). Under current clamp conditions, ATXII (40 nM) consistently prolonged the action potential and induced EADs. The EADs developed at a plateau voltage between -10 and -40 mV. Calcium channel blockers, verapamil 10 microM and cobalt 4 mM, and the sarcoplasmic reticulum modulator, ryanodine (1 microM), did not antagonize ATXII effects on the action potential and EADs. However, Na channel blockers, tetrodotoxin 0.3 microM and lidocaine 40 microM, and rapid stimulation consistently shortened the prolonged action potential and suppressed EADs. Under voltage clamp conditions in the presence of ATXII, a slowly decaying inward current followed the fast inward current during depolarizing pulses. Membrane currents flowing at or later than 100 msec after the test pulse were analyzed. The control isochronal current-voltage (I-V) curves showed no late inward currents. In the presence of ATXII, all the isochronal I-V curves showed an inward current that was more prominent between -40 and 0 mV. The ATXII-induced current at the 100-msec isochrone activated at a potential of approximately -60 mV, peaked at about -20 mV, and reversed at +40 mV consistent with the Na current I-V curve. The isochronal I-V curves obtained after lidocaine superfusion resembled those of the control. The phase plane trajectory of the action potential obtained with ATXII showed an oscillatory behavior corresponding to the EAD range of potential; within this voltage range, the isochronal I-V curves were shown to cross the abscissa three times instead of once.

CONCLUSION: These results suggest that, in this experimental model, neither sarcolemmal L-type Ca current nor sarcoplasmic reticulum Ca release plays a significant role in the genesis of ATXII-induced EADs. EADs are generated by a voltage-dependent balance between a markedly prolonged Na inward current and K outward currents within the voltage plateau range of the action potential but not by Ca current reactivation and inactivation.