Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C38
Right ventricular origin of arrhythmias in Brugada Syndrome is due to lower Na+ channel expression and function
C. A. Martin1, U. Siedlecka2, K. Kemmerich3, J. Cartledge2, J. Lawrence4, L. Guzadhur5, C. Rada3, A. A. Grace5, C. Schwiening1, C. Terracciano2, C. L. Huang1
1. Physiological Laboratory, University of Cambridge, Cambridge, United Kingdom. 2. Laboratory of Cell Electrophysiology, Heart Science Centre, Imperial College London, Harefield, United Kingdom. 3. Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom. 4. Takeda Cambridge Limited, Cambridge, United Kingdom. 5. Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom.
Figure 1. Nav1.5 protein expression in LV and RV of WT and Scn5a+/- hearts. (A) Representative Western blot. (B) Graph comparing mean relative expression levels normalised to myosin expression. Significant differences: * effect of genotype; # effect of ventricle.
Figure 2. INa current measurements. (A) Representative traces in myocytes from LV and RV of WT and Scn5a+/- hearts. (B) I-V relationship. (C) Max INa densities. Significant differences: * effect of genotype; # effect of ventricle.
Brugada syndrome (BrS) is associated with loss of Na+ channel function and ventricular tachycardia originating in the right ventricle (RV). Two major mechanisms have been postulated: conduction delays (1) or increased repolarization heterogeneities (2). We use a heterozygotic Scn5a+/- murine model, which has previously shown both conduction and repolarization abnormalities consistent with BrS (3, 4), to assess the underlying molecular mechanisms. Expression levels of mRNA for Nav1.5 and a range of K+ channels were measured from the LV and RV of homogenised WT and Scn5a+/- hearts using real time RT-PCR and equivalent protein levels measured by Western blotting. Action potential durations (APDs), fast Na+ currents (INa), persistent Na+ currents (IpNa) and transient outward K+ currents (Ito) were measured in isolated myocytes. Nav1.5 mRNA and protein expression were both lower in Scn5a+/- than WT, but particularly so in the RV (protein expression relative to WT LV: 0.91+/-0.05 vs 0.53+/-0.07, p<0.05, n=4). While there were no significant differences in mRNA or protein expression for Kir2.1, Kv1.4 or Kv1.5, there were significantly higher mRNA and protein levels of Kv4.2 in RV than LV in both WT and Scn5a+/-, and higher protein levels also for Kv4.3 and KChIP2 (protein expression in Scn5a+/- relative to WT LV: 1.90+/-0.09 vs 1.04+/-0.08 (Kv4.2), 1.75+/-0.06 vs 1.10+/-0.06 (Kv4.3), 1.20+/-0.08 vs 0.77+/-0.09 (KChIP2), p<0.05, n=4). APDs were smaller in the RV than LV of both WT and Scn5a+/-, but with a larger interventricular gradient in the Scn5a+/-, and upstroke velocity was decreased in the RV of Scn5a+/- isolated hearts. Ito currents were greater in the RV of both WT and Scn5a+/- (WT: 22.5+/-2.7pA/pF vs 32.4+/-0.3pA/pF, p<0.05, n=16), with similar I-V relationships, kinetics and voltage dependence of activation and inactivation. Max INa was similar in LV (-44.0+/-1.6 pA/pF) and RV (-44.7+/-0.7 pA/pF) of WT hearts, but was significantly decreased to -31.8+/-1.3 pA/pF in the LV and to -20.9+/-1.0 pA/pF in the RV of Scn5a+/- (p<0.05, n=12). The I-V relationship and voltage dependence of activation were unchanged, but inactivation was shifted to more positive values (RV: V1/2 of -91.0+/-0.7 mV vs -74.2+/-0.3 mV, p<0.05, n=12). Max IpNa was also decreased in a similar pattern in Scn5a+/- hearts. Our findings suggest that in the LV there is upregulation of the single Scn5a allele, which does not occur in the RV. The reduced expression of Na+ channels in the RV leads to smaller INa currents, which results in slowed conduction, and smaller IpNa currents, which in combination with increased Ito currents, results in shorter APDs and greater heterogeneity of repolarization. These insights could prove useful in finding new drug treatments for a disease where the current mainstay of therapy is cardioverter defibrillator implantation.
Where applicable, experiments conform with Society ethical requirements