Genetic Inhibition of Na ⁺ -Ca ²⁺ Exchanger Current Disables Fight or Flight Sinoatrial Node Activity Without Affecting Resting Heart Rate

Abstract

Rationale The sodium-calcium exchanger 1 (NCX1) is predominantly expressed in the heart and is implicated in controlling automaticity in isolated sinoatrial nodal (SAN) pacemaker cells, but the potential role of NCX1 in determining heart rate in vivo is unknown. Objective Determine the role of Ncx1 in heart rate. Methods and Results We employed global myocardial and SAN-targeted conditional Ncx1 knockout (Ncx1−/−) mice to measure the effect of the NCX current (INCX) in pacemaking activity in vivo, ex vivo and in isolated SAN cells. We induced conditional Ncx1−/− using a Cre/loxP system. Unexpectedly, in vivo and ex vivo hearts and isolated SAN cells showed that basal rates in Ncx1−/− (retaining ~20% of control level INCX) and control mice were similar, suggesting that physiological NCX1 expression is not required for determining resting heart rate. However, heart rate and SAN cell automaticity increases in response to isoproterenol or the dihydropyridine Ca2+ channel agonist BayK8644 were significantly blunted or eliminated in Ncx1−/− mice, indicating that NCX1 is important for fight or flight heart rate responses. In contrast the ‘pacemaker’ current (If) and L-type Ca2+ currents were equivalent in control and Ncx1−/− SAN cells under resting and isoproterenol-stimulated conditions. Ivabradine, an If antagonist with clinical efficacy, reduced basal SAN cell automaticity similarly in control and Ncx1−/− mice. However, ivabradine decreased automaticity in SAN cells isolated from Ncx1−/− mice more effectively than in control SAN cells after isoproterenol, suggesting that the importance of INCX in fight or flight rate increases is enhanced after If inhibition. Conclusion Physiological Ncx1 expression is required for increasing sinus rates in vivo, ex vivo and in isolated SAN cells but not for maintaining resting heart rate.

Burst pacemaker activity of the sinoatrial node in sodium–calcium exchanger knockout mice

“…Both of these models exhibit a much less severe phenotype in SAN cell function than our mice (2), which may be due to incomplete ablation of NCX. Nevertheless, there are certain similarities: Gao et al (6) showed an impaired response to ISO and Bay K, as we did; Herrmann et al (7) reported bursts alternating with pauses in ECG traces, comparable with our NCX KO SAN pattern.…”

“…BayK caused an increase in Ca transient frequency in WT SAN, consistent with prior studies (6,19). However, in KO, we obtained the opposite response (Fig.…”

“…In SAN pacemaker cells, NCXmediated extrusion of Ca during late diastole also generates a depolarizing current in response to local Ca release by RyRs, a mechanism we and others have previously shown to be important for normal SAN cell automaticity (2,6,7). Although the contribution of NCX to late SDD has been widely investigated, less attention has been focused on the importance of NCX-mediated Ca efflux during SAN activity.…”

“…Several studies have demonstrated the importance of NCX in the mechanism of spontaneous SAN cell depolarization (2,3,(6)(7)(8). However, NCX is also the dominant Ca efflux mechanism in cardiomyocytes (9), and thus an essential mechanism for maintaining cellular Ca balance.…”

Two‐Pore K + Channel TREK‐1 Regulates Sinoatrial Node Membrane Excitability

“…Moreover, 1 μmol/L glibenclamide and 100 μmol/L CdCl 2 were added to block ATP‐sensitive K + channels and voltage‐gated Ca 2+ channels. Pacemaker current (funny current [ I f ]) was recorded, as described 23. All current recording experiments were conducted at room temperature (21–23°C).…”