Intracellular calcium transients and arrhythmia in isolated heart cells.

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.