The atrioventricular node (AVN) plays a critical role in normal cardiac conduction and can also take over pacemaking of the ventricles should the sinoatrial node fail. The cellular electrophysiological basis of AVN pacemaking is incompletely understood, although it is likely to involve multiple ionic conductances (Inada et al., 2009). The potential importance of a background inward conductance in sinoatrial node cells has long been recognised (Hagiwara et al., 1992), and we recently presented preliminary evidence for a sodium-dependent background current (IB,Na) in rabbit AVN cells (Cheng et al., 2012). The present study was designed to investigate further the characteristics of IB,Na from rabbit AVN cells. Adult male New Zealand White rabbits were killed in accordance with UK Home Office legislation and AVN cells were isolated as described previously (Cheng et al., 2009). Whole-cell voltage-clamp using voltage-ramp protocols was used to record the background current, after the Ca2+, K+, Na+-K+ pump and Na+-Ca2+ exchange (NCX) currents were inhibited by appropriate blockers (Hagiwara et al., 1992). Values are means±SEM, compared by ANOVA or t-test. Since IB,Na is Na+-dependent, in order to preclude the involvement of NCX current (INCX), Ni2+ was applied at a concentration (10 mM) which produces profound block of INCX (Hinde et al., 1999). 10 mM Ni2+ caused only a small decrease in the amplitude of IB,Na at -100 mV, from -1.27±0.11 pA/pF (in 2 mM Ni2+, which was present in all solutions and substantially inhibits the NCX; Hagiwara et al, 1992; Hinde et al, 1999) to -0.98±0.06 pA/pF (P<0.05, n=6 and7). IB,Na also persisted following application of a second NCX inhibitor (KB-R7943; 5 μM). These data demonstrate that IB,Na in rabbit AVN cells cannot be attributed to INCX. Amiloride (1 mM), which was reported to produce partial inhibition of IB,Na in sinoatrial node cells (Hagiwara et al., 1992), produced a modest reduction in IB,Na (-0.91±0.11 vs. -1.52±0.13 pA/pF in control at -100 mV, P<0.01, n=8). In contrast, changing extracellular solution pH from 7.4 to 6.3 markedly decreased IB,Na (to -0.43±0.08 pA/pF from -1.11±0.12 pA/pF at -100 mV, P<0.01, n=12). The sensitivity of IB,Na to extracellular acidosis is consistent with the notion that modulation of IB,Na is likely to contribute to previously reported effects of pH on AVN cell electrophysiology (Cheng et al., 2009).