Abstract 12747: A Null Mutation of the Neuronal Sodium Channel Isoform Nav1.6 Disrupts Action Potential Propagation and EC Coupling in the Mouse Heart
Several lines of evidence support the expression of the brain-type sodium channels NaV1.1, NaV1.2, NaV1.3 and NaV1.6 in the heart. Their functional role, however, remains controversial. We studied cardiac function in the ataxia3 null mouse with a missense mutation that causes a defect in protein trafficking and death by 21 days of age (Sharkey et al, J. Neurosci. 2009). We hypothesized that expression of NaV1.6 in ventricular myocytes contributes to EC coupling and maintenance of propagation in the depolarized myocardium. Using splice variant specific primers and quantitative RT-PCR, we confirmed the expression of full length NaV1.6 TRANSCRIPT in ventricular tissue. Immunohistochemistry showed a t-tubular pattern of fluorescence in the myocytes. Hearts from Nav1.6 null mice were smaller than wildtype (WT) littermates. In addition, myocytes from null hearts had significantly reduced length and width. Acetylcholinesterase staining suggested that the innervation of the ventricles is unaltered in null hearts, at the macroscopic level. ECG recording in conscious mice, demonstrated prolongation of the PR interval (42.3±6.7 ms in null vs. 30±3.7 ms in WT, p<0.01) (mean ± SD) and the QRS interval (20.1±1.2 ms vs. 14.7±3.8 ms, p<0.01, n=8). Ca2+ transients in single cells paced at 1 Hz were prolonged in null vs. WT (456±84 ms vs. 357±108 ms, p<0.01, n=28 cells). In patch clamping, at holding potential (HP)= -120mV, peak inward INa was similar in the two phenotypes. However, at HP= -70mV, which inactivates most NaV1.5 channels, the peak inward INa was smaller in null compared to WT cells (3±0.3 pA/pF, n=4 vs. 5.5±0.28 pA/pF, n=5, p<0.05). Finally, optical mapping of isolated perfused hearts revealed that with 4mM K+ Tyrode's solution, ventricular conduction velocity (CV) was slightly (7%) but significantly slower in 17 null compared to 16 WT mice (p<0.05). At 12mM K+, CV was 25% slower in null compared to WT. These results highlight the importance of neuronal sodium channels in the heart. NaV1.6 and other neuronal channels appear to participate in EC-coupling, and represent an intrinsic depolarizing reserve that contributes to the maintenance of excitation under conditions that compromise the resting membrane potential, such as hyperkalemia and ischemia.
- © 2010 by American Heart Association, Inc.