Myofilament Ca2+ sensitization causes susceptibility to cardiac arrhythmia in mice
Franz Baudenbacher, Tilmann Schober, Jose Renato Pinto, Veniamin Y. Sidorov, Fredrick Hilliard, R. John Solaro, James D. Potter, Björn C. Knollmann
Franz Baudenbacher, Tilmann Schober, Jose Renato Pinto, Veniamin Y. Sidorov, Fredrick Hilliard, R. John Solaro, James D. Potter, Björn C. Knollmann
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Research Article

Myofilament Ca2+ sensitization causes susceptibility to cardiac arrhythmia in mice

Abstract

In human cardiomyopathy, anatomical abnormalities such as hypertrophy and fibrosis contribute to the risk of ventricular arrhythmias and sudden death. Here we have shown that increased myofilament Ca2+ sensitivity, also a common feature in both inherited and acquired human cardiomyopathies, created arrhythmia susceptibility in mice, even in the absence of anatomical abnormalities. In mice expressing troponin T mutants that cause hypertrophic cardiomyopathy in humans, the risk of developing ventricular tachycardia was directly proportional to the degree of Ca2+ sensitization caused by the troponin T mutation. Arrhythmia susceptibility was reproduced with the Ca2+-sensitizing agent EMD 57033 and prevented by myofilament Ca2+ desensitization with blebbistatin. Ca2+ sensitization markedly changed the shape of ventricular action potentials, resulting in shorter effective refractory periods, greater beat-to-beat variability of action potential durations, and increased dispersion of ventricular conduction velocities at fast heart rates. Together these effects created an arrhythmogenic substrate. Thus, myofilament Ca2+ sensitization represents a heretofore unrecognized arrhythmia mechanism. The protective effect of blebbistatin provides what we believe to be the first direct evidence that reduction of Ca2+ sensitivity in myofilaments is antiarrhythmic and might be beneficial to individuals with hypertrophic cardiomyopathy.

Authors

Franz Baudenbacher, Tilmann Schober, Jose Renato Pinto, Veniamin Y. Sidorov, Fredrick Hilliard, R. John Solaro, James D. Potter, Björn C. Knollmann

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Figure 7

Blebbistatin prevents AP triangulation, normalizes CV dispersion, and prevents VT induction in Ca2+-sensitized mouse hearts.

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Blebbistatin prevents AP triangulation, normalizes CV dispersion, and pr...
(A) Optical AP records from an NTg heart perfused with solutions containing 3 μM blebbistatin before and after 3 μM EMD. Note that blebbistatin prevents the EMD-induced AP changes shown in Figure 5. (B) Average effect of EMD on murine ventricular APD in the presence of blebbistatin. n = 4 hearts; PCL, 150 ms. (C and D) Isochronal activation maps obtained at a PCL of 60 ms from an NTg heart perfused with solutions containing 3 μM blebbistatin before (C) and after (D) addition of 3 μM EMD to the perfusate. Isolated mouse hearts were stained with the voltage-sensitive fluorescent indicator di-4-ANEPPS and paced at stepwise-shorter PCLs as described in Figure 2. Note that in the presence of blebbistatin, EMD no longer altered ventricular isochronal activation maps. (E) Maximal CV dispersion before and after treatment with blebbistatin. For control conditions, the TnT mutations and the group size follow: NTg, n = 7; WT, n = 6; R278C, n = 6; F110I, n = 8; I79N, n = 8; EMD, n = 6. The group sizes of TnT mutations in the presence of 3 μM blebbistatin follow: NTg, n = 3; WT, n = 7; R278C, n = 5; F110I, n = 7; I79N, n = 7; EMD, n = 3; *P < 0.05, **P < 0.01 by Mann-Whitney U test. (F) Average incidence of sustained VT (>30 s duration) in the different transgenic mice under control conditions and in the presence of 3 μM blebbistatin. For control conditions, the TnT mutations and the group size follow: NTg, n = 8; WT, n = 10; R278C, n = 6; F110I, n = 11; I79N, n = 13; EMD, n = 7. The group sizes of TnT mutations in the presence of blebbistatin follow: NTg, n = 4; WT, n = 8; R278C, n = 5; F110I, n = 7; I79N, n = 8; EMD, n = 4; **P < 0.01, by Fisher’s exact test.