Spontaneous and propagated calcium release in isolated cardiac myocytes viewed by confocal microscopy

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Abstract
Laser scanning confocal microscopy of the Ca(2+)-sensitive fluorophore fluo-3 has been used to investigate spontaneous and propagated calcium release at high temporal and spatial resolution in enzymatically dispersed rat cardiomyocytes. Waves of fluorescence which propagated throughout the cytosol were evident in spontaneously contracting cardiac cells containing fluo-3, but not in cells containing Ca(2+)-insensitive fluorophores [2',7'-bis (carboxyethyl)-5,6-carboxyfluorescein, SNARF-1, rhodamine-123, or tetramethylrhodamine-labeled dextran]. These waves represent localized areas of elevated [Ca2+] [975 +/- 13 (SE) nM, range 800-1,500 nM; n = 16 cells]. Ca2+ waves were initiated by the spontaneous release of Ca2+ from the sarcoplasmic reticulum (SR) and propagated through cells at rates of 50-150 microns/s. Ca2+ waves were usually initiated at the cell ends, but multiple and variable initiation foci were observed in some cells. Where waves intersected within a single cell there was extinction of wave propagation, confirming the SR as the direct source of Ca2+ and revealing a refractory period in SR Ca2+ release. In some cells high-frequency Ca2+ waves lead to synchronized elevation of [Ca2+] throughout the entire cytosol and within the time period associated with cell depolarization. These observations support the hypothesis that some cardiac arrhythmias are initiated by spontaneous and propagated Ca2+ release and involve subsequent depolarization, global elevation of intracellular [Ca2+], and cell contraction.