Effects of uniform electric fields on intracellular calcium transients in single cardiac cells.

Although intracellular calcium (Ca(2+)) transients in cardiac cells have been well studied in the uniformly polarized cell membrane, how these transients are modified during field stimulation when the cell membrane is nonuniformly polarized has not been investigated. In this study we characterized the effects of uniform field stimuli on Ca(2+) transients in isolated guinea pig cardiac cells. Single guinea pig cells were enzymatically isolated, loaded with the Ca(2+) fluorescent indicator fluo-3, and stimulated along their longitudinal axes with S1 or S1-S2 (S1-S2 = 50 ms) pulses. The fluorescence signals were recorded simultaneously from up to 12 sites along the cell length using a multisite mapping system. S1 pulse, applied during the resting phase of the action potential, induced Ca(2+) transients that had an earlier onset at the anodal-facing end, suggesting that Ca(2+) gradients (nablaCa(2+)) develop during the rising phase of the Ca(2+) transients. With the assumption that the peak change in Ca(2+) is 980 nM, nablaCa(2+) was estimated to be approximately 3.4 nM/microm in the anodal half of the cell for a nominal 10 V/cm field and negligible in the cathodal half. The S2 pulse that was applied during the plateau of the action potential also perturbed the Ca(2+) transients and produced Ca(2+) gradients directed from the center to either end of the cell. Mean nablaCa(2+) in the anodal half of the cell (approximately 4.2 nM/microm) was found to be statistically higher than in the cathodal half (approximately 2.8 nM/microm).