Inhibition of the Na+/Ca2+ exchange current abbreviated the action potential duration by 50%, suggesting it is a major physiological determinant of APD and can account for DADs and EADs.
Does Na+/Ca2+ exchange activation by SR Ca2+ release modulate action potential duration and afterdepolarizations in guinea pig ventricular myocytes?
Na+/Ca2+ exchange is a major physiological determinant of action potential duration, and its activation by spontaneous SR Ca2+ release can account for both delayed and early afterdepolarizations during SR Ca2+ overload.
In cardiac cells, evoked Ca2+ releases or spontaneous Ca2+ waves activate the inward Na+/Ca2+ exchange current (INaCa), which may modulate membrane excitability and arrhythmogenesis. In this study, we examined changes in membrane potential due to INaCa elicited by sarcoplasmic reticulum (SR) Ca2+ release in guinea pig ventricular myocytes using whole cell current clamp, fluorescence, and confocal microscopy. Inhibition of INaCa by Na+-free, Li+-containing Tyrode solution reversibly abbreviated the action potential duration at 90% repolarization (APD90) by 50% and caused SR Ca2+ overload. APD90 was similarly abbreviated in myocytes exposed to the Na+/Ca2+ exchange inhibitor KB-R7943 (5 microM) or after inhibition of SR Ca2+ release with ryanodine (20 microM). In the absence of extracellular Na+, spontaneous SR Ca2+ releases caused minimal changes in resting membrane potential. After the myocytes were returned to Na+-containing solution, the potentiated intracellular Ca2+ concentration (Ca2+i) transients dramatically prolonged APD90 and Ca2+i oscillations caused delayed and early afterdepolarizations (DADs and EADs). Laser-flash photolysis of caged Ca2+ mimicked the effects of spontaneous Ca2+i oscillations, confirming that APD prolongation, DADs, and EADs could be ascribed to intracellular Ca2+ release. These results suggest that Na+/Ca2+ exchange is a major physiological determinant of APD and that INaCa activation by spontaneous SR Ca2+ release/oscillations, depending on the timing, can account for both DADs and EADs during SR Ca2+ overload.
Spencer et al. (Mon,) reported a other. Inhibition of INaCa or SR Ca2+ release vs. Baseline / Na+-containing solution was evaluated on Action potential duration at 90% repolarization (APD90) and membrane potential changes. Inhibition of the Na+/Ca2+ exchange current abbreviated the action potential duration by 50%, suggesting it is a major physiological determinant of APD and can account for DADs and EADs.