Myofilament Ca sensitization increases cytosolic Ca binding affinity, alters intracellular Ca homeostasis, and causes pause-dependent Ca-triggered arrhythmia.
BACKGROUND
Ca binding to the troponin complex represents a major portion of cytosolic Ca buffering. Troponin mutations that increase myofilament Ca sensitivity are associated with familial hypertrophic cardiomyopathy and confer a high risk for sudden death. In mice, Ca sensitization causes ventricular arrhythmias, but the underlying mechanisms remain unclear.


OBJECTIVE
To test the hypothesis that myofilament Ca sensitization increases cytosolic Ca buffering and to determine the resulting arrhythmogenic changes in Ca homeostasis in the intact mouse heart.


RESULTS
Using cardiomyocytes isolated from mice expressing troponin T (TnT) mutants (TnT-I79N, TnT-F110I, TnT-R278C), we found that increasing myofilament Ca sensitivity produced a proportional increase in cytosolic Ca binding. The underlying cause was an increase in the cytosolic Ca binding affinity, whereas maximal Ca binding capacity was unchanged. The effect was sufficiently large to alter Ca handling in intact mouse hearts at physiological heart rates, resulting in increased end-diastolic [Ca] at fast pacing rates, and enhanced sarcoplasmic reticulum Ca content and release after pauses. Accordingly, action potential (AP) regulation was altered, with postpause action potential prolongation, afterdepolarizations, and triggered activity. Acute Ca sensitization with EMD 57033 mimicked the effects of Ca-sensitizing TnT mutants and produced pause-dependent ventricular ectopy and sustained ventricular tachycardia after acute myocardial infarction.


CONCLUSIONS
Myofilament Ca sensitization increases cytosolic Ca binding affinity. A major proarrhythmic consequence is a pause-dependent potentiation of Ca release, action potential prolongation, and triggered activity. Increased cytosolic Ca binding represents a novel mechanism of pause-dependent arrhythmia that may be relevant for inherited and acquired cardiomyopathies.
Division of Clinical Pharmacology, Department of Medicine, Vanderbilt University Medical School, Nashville, TN, USA.
Tilmann
T
Schober
Sabine
S
Huke
Raghav
R
Venkataraman
Oleksiy
O
Gryshchenko
Dmytro
D
Kryshtal
Hyun Seok
HS
Hwang
Franz J
FJ
Baudenbacher
Björn C
BC
Knollmann
D000200:Action Potentials / Q000235:genetics; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000235:genetics / Q000378:metabolism* / Q000503:physiopathology; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D020013:Calcium Signaling / Q000235:genetics; D003600:Cytosol / Q000378:metabolism* / Q000473:pathology; D006706:Homeostasis / Q000235:genetics*; D006801:Humans; D007424:Intracellular Fluid / Q000378:metabolism*; D051379:Mice; D008822:Mice, Transgenic; D009154:Mutation; D009210:Myofibrils / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding / Q000235:genetics; D015854:Up-Regulation / Q000235:genetics
D002118:Calcium