Profile of L-type Ca2+ current and Na+/Ca2+ exchange current during cardiac action potential in ventricular myocytes

ObjectiveThe 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 profile of these currents during the AP was determined 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.MethodA newly developed AP-clamp sequential dissection method was used to record ionic currents in guinea pig ventricular myocytes under a triad of conditions: using the cell's own AP as the voltage command, using internal and external solutions that mimic the cell's ionic composition, and, importantly, not using any exogenous Ca2+ buffer.ResultsThe nifedipine-sensitive current (INIFE), which is composed of ICa,L and INCX, revealed hitherto unreported features during the AP with Ca2+ cycling in the cell. We identified 2 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 by using SEA0400 shows that ICa,L is dominant during phases 1 and 2 whereas INCX contributes significantly to the inward current during phases 3 and 4 of the AP.ConclusionThese data provide the first direct experimental visualization of ICa,L and INCX during cardiac the AP and Ca2+ cycle. The residual current reported here can serve as a potential substrate for afterdepolarizations when increased under pathologic conditions.