Abstract 16206: Sinoatrial Node Pacemaker Activity in Murine Myocardium Requires the Sodium-Calcium Exchanger
Sodium-calcium exchange (NCX1) has been identified as a possible requirement for sinoatrial node (SAN) pacemaker activity in the heart via a “Calcium Clock” mechanism. To test this hypothesis, we used cre/loxP technology to generate mice with an atrial-specific knockout (KO) of NCX1 using a sarcolipin promoter. At 8 weeks, the mice appear healthy, but exhibit cardiac enlargement. The ventricular weight to body weight ratio is increased in KO (5.68 ± 0.16 (n = 21)) compared to WT (4.04 ± 0.20 (n = 18); p < 0.001). The atrial weight to body weight ratio is almost doubled in the KO (0.41 ± 0.03 (n = 21) in KO versus 0.22 ± 0.02 (n = 18) in WT; p < 0.001) and the atrial walls are thinned in the KO as well. By immunostaining and immunoblot, the atrial tissue, including the SAN, shows no detectable NCX1. Echocardiography reveals left ventricular hypertrophy and dilation in KO with a trend towards impaired systolic function. Surface ECG shows a markedly lower heart rate in KO (231 ± 16 bpm, n = 4) compared to WT (422 ± 63 bpm, n = 2; p = 0.01), with no P waves and a junctional escape rhythm in KO. We confirmed the absence of spontaneous atrial electrical activity in the KO by recording cardiac electrograms in Langendorff-perfused hearts. Unlike WT atria, which display rhythmic Ca transients when loaded with Fluo3-AM, KO atria have no spontaneous activity. However, Ca2+ transients and contraction in KO can be elicited by external pacing, indicating intact electrical coupling in KO atrial tissue. In patch clamped SAN cells isolated from KO mice, we confirmed that there is no NCX activity in response to caffeine-induced SR Ca2+ release. L-type calcium current is decreased in KO by ~50% (similar to the ventricular NCX1 KO mouse) but there is no significant difference in funny current (If) between WT and KO. Spontaneous action potentials, which are routinely observed in enzymatically isolated patch clamped SAN cells from WT (n=5), are rare or absent in KO (n=4). We conclude that NCX1 is required for normal pacemaker activity in the murine SAN.
- © 2011 by American Heart Association, Inc.