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Intracellular calcium transients and arrhythmia in isolated heart cells
F T Thandroyen1, A C Morris, H K Hagler
1Department of Internal Medicine Cardiology, University of Texas Southwestern Medical Center, Dallas.
Insights
High extracellular calcium triggers cardiac arrhythmias by altering intracellular calcium levels. Reducing intracellular calcium effectively reversed these arrhythmias in a novel myocyte model.
Area of Science:
- Cardiology
- Cell Physiology
- Biophysics
Background:
- Intracellular calcium ([Ca2+]i) elevation is implicated in cardiac arrhythmias.
- Direct measurement of rapid [Ca2+]i changes in vivo is challenging.
- A suitable isolated myocyte model for studying arrhythmias is needed.
Purpose of the Study:
- To measure rapid [Ca2+]i alterations in isolated, spontaneously beating neonatal rat ventricular myocytes.
- To characterize the relationship between [Ca2+]i and arrhythmia development.
- To establish an in vitro model for cardiac arrhythmia research.
Main Methods:
- Utilized isolated neonatal rat ventricular myocytes with cell-to-cell communication.
- Measured rapid [Ca2+]i changes with high temporal resolution.
- Induced arrhythmias using elevated extracellular calcium (10.8 mM) and assessed interventions like BAPTA-AM and ryanodine.
Main Results:
- Elevated extracellular calcium induced premature beats, tachyarrhythmia, and fibrillation.
- Systolic [Ca2+]i increased to 1-3 microM, and diastolic oscillations preceded arrhythmia.
- Tachyarrhythmia led to severe intracellular calcium overload.
- Reducing [Ca2+]i with BAPTA-AM reversed the induced arrhythmias.
Conclusions:
- An isolated multicellular myocyte model mimics in vivo cardiac arrhythmias.
- Diastolic [Ca2+]i oscillations and elevated systolic [Ca2+]i are key triggers.
- Intracellular calcium overload perpetuates arrhythmias.
- Reducing intracellular calcium is a potential therapeutic strategy for calcium-mediated arrhythmias.
Abstract:
Intracellular calcium ([Ca2+]i) elevation may mediate cardiac arrhythmias. However, direct measurement of the rapid alterations of [Ca2+]i on a beat-to-beat basis using fast temporal resolution and without signal averaging in the spontaneously beating in vivo heart is lacking. Furthermore, data from an isolated spontaneously beating myocyte preparation that develops arrhythmia similar to that in the in vivo heart are unavailable. We measured rapid changes of [Ca2+]i with fast temporal resolution in isolated spontaneously beating neonatal rat ventricular myocytes with cell-to-cell communication and characterized the interrelation between [Ca2+]i and arrhythmia. An elevated extracellular calcium ([Ca2+]o) concentration of 10.8 mM induced premature beats, a rapid beating rate (tachyarrhythmia), and chaotic or fibrillatory beating activity in a small group of myocytes. [Ca2+]i levels during systole increased from the nanomolar to micromolar concentration range before arrhythmia development. Spontaneous oscillations of [Ca2+]i during diastole could evoke a spontaneous tachyarrhythmia. In the presence of [Ca2+]i elevation, a spontaneous tachyarrhythmia could induce severe [Ca2+]i overload. Reduction of [Ca2+]i with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid AM (5 microM) in the presence of 10.8 mM [Ca2+]o reversed the arrhythmia. In single ventricular myocytes superfused with 10.8 mM [Ca2+]o, oscillations of membrane potential characteristic of transient inward current occurred that were prevented by ryanodine (0.1 microM), an inhibitor of Ca2+ flux across the sarcoplasmic reticulum. This study characterizes 1) an isolated multicellular myocyte model of arrhythmia similar to that evident in in vivo hearts, 2) elevation of [Ca2+]i with systolic [Ca2+]i levels of 1-3 microM and diastolic [Ca2+]i oscillations before the initiation of arrhythmia, 3) tachyarrhythmia as a cause of severe [Ca2+]i overload, which may be important in the perpetuation and degeneration of arrhythmias, and 4) reversal of arrhythmia with reduction of [Ca2+]i. The results in the isolated myocyte model may have relevance to the generation and perpetuation of certain cardiac arrhythmias associated with calcium overload.