Profile of L-type Ca ²⁺ current and Na ⁺/Ca ²⁺ exchange current during cardiac action potential in ventricular myocytes

The L-type Ca ²⁺ current (I _Ca,L) and the Na ⁺/Ca ²⁺ exchange current (I _NCX) 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 Ca ²⁺ buffer. In this study, we aimed to obtain direct experimental measurement of these currents during cardiac AP with Ca ²⁺ cycling. A 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 Ca ²⁺ buffer. The nifedipine-sensitive current (I _NIFE), which is composed of I _Ca,L and I _NCX, revealed hitherto unreported features during the AP with Ca ²⁺ 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 Ca ²⁺ is not buffered. Pharmacological dissection of I _NIFE by using SEA0400 shows that I _Ca,L is dominant during phases 1 and 2 whereas I _NCX contributes significantly to the inward current during phases 3 and 4 of the AP. These data provide the first direct experimental visualization of I _Ca,L and I _NCX during cardiac the AP and Ca ²⁺ cycle. The residual current reported here can serve as a potential substrate for afterdepolarizations when increased under pathologic conditions.