Cardiac myocyte excitation by ultrashort high-field pulses

In unexcitable, noncardiac cells, ultrashort (nanosecond) high-voltage (megavolt-per-meter) pulsed electrical fields (nsPEF) can mobilize intracellular Ca²⁺ and create transient nanopores in the plasmalemma. We studied Ca²⁺ responses to nsPEF in cardiac cells. Fluorescent Ca²⁺ or voltage signals were recorded from isolated adult rat ventricular myocytes deposited in an electrode microchamber and stimulated with conventional pulses (CPs; 0.5-2.4 kV/cm, 1 ms) or nsPEF (10-80 kV/cm, 4 ns). nsPEF induced Ca ²⁺ transients in 68/104 cells. Repeating nsPEF increased the likelihood of Ca²⁺ transient induction (61.8% for <10 nsPEF vs. 80.6% for ≥10 nsPEF). Repetitive Ca²⁺ waves arising at the anodal side and Ca²⁺ destabilization occurred after repeated nsPEF (12/29) or during steady-state single nsPEF delivery at 2 Hz. Removing extracellular Ca²⁺ abolished responses to nsPEF. Verapamil did not affect nsPEF-induced Ca²⁺ transients, but decreased responses to CP. Tetrodotoxin and KB-R7943 increased the repetition threshold in response to nsPEF: 1-20 nsPEF caused local anodal Ca²⁺ waves without Ca ²⁺ transients, and ≥20 nsPEF caused normal transients. Ryanodine-thapsigargin and caffeine protected against nsPEF-induced Ca ²⁺ waves and showed less recovery of diastolic Ca²⁺ levels than CP. Voltage recordings demonstrated action potentials triggered by nsPEF, even in the presence of tetrodotoxin. nsPEF can mobilize intracellular Ca ²⁺ in cardiac myocytes by inducing action potentials. Anodal Ca ²⁺ waves and resistance to Na²⁺ and Ca²⁺ channel blockade suggest nonselective ion channel transport via sarcolemmal nanopores as a triggering mechanism.