Mechanism of current-induced early afterdepolarizations in guinea pig ventricular myocytes

We investigated possible ionic mechanisms that cause early afterdepolarizations (EADs) following the injection of constant inward current in guinea pig ventricular myocytes by several interventions that affect failure of action potential repolarization. The amount of constant current was adjusted to measure the threshold potential (V(th)) associated with the minimum inward current required for inducing EADs [threshold current (I(th))] and also the magnitude of EADs at V(th) and following adjustment of current to generate takeoff potentials of -30 and -20 mV. Interventions associated with either inhibition of Ca²⁺ release from the sarcoplasmic reticulum (ryanodine 5 x 10^-6 M) or L-type membrane Ca²⁺ channel current (verapamil 1.1 x 10^-5 M and nisoldipine 5 x 10^-7 M) reduced or abolished EADs arising from -30 or -20 mV. Cells that generated delayed afterdepolarizations (DADs) in the absence of depolarizing current after 20 stimulations at 5 Hz either in control solution or following interventions associated with Ca²⁺ loading (reduced extracellular [K⁺] or increased extracellular [Ca²⁺]) also developed a marked shift in V(th) of current- induced EADs at 1-Hz stimulation to more negative potentials [-60.3 ± 10.7 mV (mean ± SD, n = 17) vs. -41.7 ± 6.4 mV in cells without DADs in control solution (n = 25), P < 0.001]. Ca²⁺ loading also increased the magnitude of EADs arising from V(th) and -20 mV. Exposure to quinidine (1.23 x 10^-5 M), which blocks both Na⁺ and delayed rectifier K⁺ channels, significantly reduced I(th) but had only minimal effect on the magnitude of EADs. Our results suggest that L-type Ca²⁺ channel current and [Ca²⁺]-sensitive inward current associated with release of Ca²⁺ from the sarcoplasmic reticulum are the major currents that cause this form of EADs, and that Ca²⁺ loading promotes the development of large EADs likely to propagate to normal tissue.