Profile of L-Type Ca2+ Current and Na+/Ca2+ Exchange Current during Cardiac Action Potential in Ventricular Myocytes

Objective: The L-type Ca2+ current (ICa,L) and the Na+/Ca2+ exchange current (INCX) are major inward currents that shape the cardiac action potential (AP). Previously the profiles of these currents during AP were determined from mathematical models based on data from voltage-clamp experiments that used Ca2+ buffer. In this study we aimed to obtain direct experimental measurement of these currents during cardiac AP with Ca2+ cycling. Method: A newly developed AP-clamp sequential dissection method was used to record ionic currents in guinea pig ventricular myocytes under a triad of conditions: (1) using the cell's own AP as the voltage command, (2) using internal and external solutions that mimic the cell's ionic milieu and, importantly, (3) no exogenous Ca2+ buffer was used. Results: The nifedipine-sensitive current (INIFE), which is composed of ICa,L and INCX, revealed hitherto unreported features during AP with Ca2+ cycling in the cell. We identified two peaks in the current profile followed by a long residual current extending beyond the AP, coinciding with a residual depolarization. The second peak and the residual current become apparent only when Ca2+ is not buffered. Pharmacological dissection of INIFE using SEA0400 shows that ICa,L is dominant during AP phase-1&2 whereas INCX contributes significantly to the inward current at phase-3&4. Conclusion: These data provide the first direct experimental visualization of ICa,L and INCX during cardiac AP and Ca2+ cycle. The residual current reported here can serve as a potential substrate for afterdepolarizations when increased under pathological conditions.