Excitation of murine cardiac myocytes by nanosecond pulsed electric field

Introduction Opening of voltage-gated sodium channels takes tens to hundreds of microseconds, and mechanisms of their opening by nanosecond pulsed electric field (nsPEF) stimuli remain elusive. This study was aimed at uncovering the mechanisms of how nsPEF elicits action potentials (APs) in cardiomyocytes. Methods and Results Fluorescent imaging of optical APs (FluoVolt) and Ca2+-transients (Fluo-4) was performed in enzymatically isolated murine ventricular cardiomyocytes stimulated by 200-nanosecond trapezoidal pulses. nsPEF stimulation evoked tetrodotoxin-sensitive APs accompanied or preceded by slow sustained depolarization (SSD) and, in most cells, by transient afterdepolarization waves. SSD threshold was lower than the AP threshold (1.26 +/- 0.03 vs 1.34 +/- 0.03 kV/cm, respectively, P < 0.001). Inhibition of l-type calcium and sodium-calcium exchanger currents reduced the SSD amplitude and increased the AP threshold (P < 0.05). The threshold for Ca2+-transients (1.40 +/- 0.04 kV/cm) was not significantly affected by a tetrodotoxin-verapamil cocktail, suggesting the activation of a Ca2+ entry pathway independent from the opening of Na+ or Ca2+ voltage-gated channels. Removal of external Ca2+ decreased the SSD amplitude (P = 0.004) and blocked Ca2+-transients but not APs. The incidence of transient afterdepolarization waves was decreased by verapamil and by removal of external Ca2+ (P = 0.002). Conclusions The study established that nsPEF stimulation caused calcium entry into cardiac myocytes (including routes other than voltage-gated calcium channels) and SSD. Tetrodotoxin-sensitive APs were mediated by SSD, whose amplitude depended on the calcium entry. Plasma membrane electroporation was the most likely primary mechanism of SSD with additional contribution from l-type calcium and sodium-calcium exchanger currents.