Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC159
Conduction velocity changes contribute to arrhythmogenicity in a murine model of catecholaminergic polymorphic ventricular tachycardia, RyR2-P2328S
Y. Zhang1, J. Wu4, K. Jeevaratnam1, J. H. King1, L. Guzadhur1, Z. Ren4, M. Lei2, A. A. Grace3, C. L. Huang1, J. A. Fraser1
1. Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge, United Kingdom. 2. School of Clinical and Laboratory Sciences, University of Manchester, Manchester, United Kingdom. 3. Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom. 4. Department of Paediatrics, Xi’an Jiaotong University, Xi’an, China.
Fig. 1. Mean relative latency of AP firing over an 8x8 electrode array in an RyR+/S heart under control conditions showing relatively uniform epicardial conduction approximating a regression plane with multiple-R of 0.92.
Figure 2. Conduction velocities of WT, RyR+/S and RyRS/S hearts under control conditions and during exposure to iso or iso + caffeine. *: difference from control conditions; #: difference from control conditions and WT/RyR+/S under similar conditions, all P<0.01.
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a potentially lethal familial disease characterized by bidirectional ventricular tachycardia (BVT), polymorphic VT (PVT), and ventricular fibrillation. Cardiac ryanodine receptor (RyR2) mutations associated with autosomal dominant forms of CPVT (Priori et al., 2001) result in increased diastolic sarcoplasmic reticular (SR) Ca2+ release (Jiang et al., 2002; George et al., 2003; Lehnart et al., 2004), but the mechanisms by which this might trigger and sustain fatal arrhythmias are incompletely understood. ECGs were recorded from mice under terminal anaesthesia (ketamine 3.6 mg/10g + xylazine 0.16 mg/10g I.P.) Isoprenaline (iso, 2 mg/kg) and caffeine (120 mg/kg) challenge revealed a significant arrhythmic phenotype for homozygous (s/s) RyR2-P2328S mice (Goddard et al., 2008), with 21.2±9.7 (n=6) episodes of bigeminy, BVT or PVT per 10 min, compared to ≤2 for WT or +/s mice (P<0.05), and 2.5±1.1 per 10 min (n=6) for s/s mice exposed to iso alone. No arrhythmic episodes were recorded under control conditions (n=12 in each case). Similarly, PR intervals, QRS durations, QT intervals and heart rate (HR) were not significantly influenced by genotype under control conditions. Iso challenge increased HR by ~20% in each case (P<0.01), with no other significant electrocardiographic changes. However, iso and caffeine together markedly reduced HR and prolonged QT and corrected QT interval specifically in the s/s mice relative to control conditions and to WT (each P<0.01), suggesting a possible conduction velocity (CV) abnormality that was investigated further. As shown in Fig. 1, CV was measured by fitting a regression plane to mean relative ventricular activation times recorded using a 64-electrode array in intrinsically beating Langendorff-perfused hearts, (n≥6 in all cases). Multiple-r values of such planes ranged from 0.85 to 0.98, consistent with an acceptable planar approximation. Fig. 2 shows that CVs were indistinguishable in WT, +/s and s/s ventricles before pharmacological manipulation and were increased by iso in WT and s/s. However, the iso + caffeine combination markedly reduced CV in the s/s relative to control conditions, iso exposure and WT. These findings were corroborated by conventional 20 MΩ intracellular glass microelectrode AP recordings. Maximum AP upstroke rate was reduced specifically in s/s hearts following application of iso + caffeine. We thus correlate the increased arrhythmic tendency of RyR2S/S hearts following adrenergic and caffeine challenge with significantly decreased epicardial CV and AP upstroke velocity.
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