Na+–Ca2+ exchange current from rabbit isolated atrioventricular nodal and ventricular myocytes compared using action potential and ramp waveforms

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Abstract
We measured and compared Na–Ca exchanger current (INa–Ca) from rabbit isolated ventricular and atrioventricular (AV) nodal myocytes, using action potential (AP) and ramp voltage commands. Whole cell patch‐clamp recordings were made at 35–37 °C; INa–Ca was measured as 5 m M nickel (Ni)‐ sensitive current with major interfering voltage and calcium‐activated currents blocked. In ventricular cells a 2‐s descending ramp elicited INa–Ca showing outward rectification and a reversal potential (Erev) of –13.1 ± 1.2 mV (n = 12; mean ± SEM). With a ventricular AP as the voltage command, the profile of INa–Ca followed the applied waveform closely. The current–voltage relation during AP repolarization was almost linear and showed an Erev of –38.3 ± 5.3 mV (n = 6). As INa–Ca depended on the applied voltage waveform, comparisons between the two cell types utilized the same command waveform (a series of AV nodal APs). In ventricular myocytes this elicited INa–Ca that reversed near –38 mV and was inwardly directed during the pacemaker potential. This command was also applied to AV node cells; mean INa–Ca density at all voltages encompassed by the AP (–70 to +30 mV) did not differ significantly from that in ventricular myocytes (P > 0.05, ANOVA). This finding was confirmed using brief (250 ms) voltage ramp protocols (P > 0.1 ANOVA). These data represent the first direct measurements of AV nodal INa–Ca and suggest that the exchanger may be functionally expressed to similar levels in the two cell types. They may also suggest a possible role for INa–Ca during the pacemaker potential in AV node as inward INa–Ca was observed over the pacemaker potential range even with bulk internal Ca buffered to a low level.

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