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 mumol/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.