Na⁺-Ca²⁺ exchange current and submembrane [Ca²⁺] during the cardiac action potential

Na⁺-Ca²⁺ exchange (NCX) is crucial in the regulation of [Ca²⁺]_i and cardiac contractility, but key details of its dynamic function during the heartbeat are not known. In the present study, we assess how NCX current (I_NCX) varies during a rabbit ventricular action potential (AP). First, we measured the steady-state voltage and [Ca²⁺]_i dependence of I_NCX under conditions when [Ca²⁺]_i was heavily buffered. We then used this relationship to infer the submembrane [Ca²⁺]_i ([Ca²⁺]_sm) sensed by NCX during a normal AP and [Ca²⁺]_i transient (when the AP was interrupted to produce an I_NCX tail current). The [Ca²⁺]_i dependence of I_NCX at -90 mV allowed us to convert the peak inward I_NCX tail currents to [Ca²⁺]_sm. Peak [Ca²⁺]_sm measured via this technique was >3.2 μmol/L within <32 ms of the AP upstroke (versus peak [Ca²⁺]_i of 1.1 μmol/L at 81 ms measured with the global Ca²⁺ indicator indo-1). The voltage and [Ca²⁺]_sm dependence of I_NCX allowed us to infer I_NCX during the normal AP and Ca²⁺ transient. The early rise in [Ca²⁺]_sm causes I_NCX to be inward for the majority of the AP. Thus, little Ca²⁺ influx via NCX is expected under physiological conditions, but this can differ among species and in pathophysiological conditions.