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

Although intracellular calcium ([Ca²⁺]_i) 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 [Ca²⁺]_i transients in isolated guinea pig cardiac cells. Single guinea pig cells were enzymatically isolated, loaded with the [Ca²⁺]_i 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 [Ca²⁺]_i transients that had an earlier onset at the anodal-facing end, suggesting that [Ca²⁺]_i gradients (∇[Ca²⁺]_i) develop during the rising phase of the [Ca²⁺]_i transients. With the assumption that the peak change in [Ca²⁺]_i is 980 nM, ∇[Ca²⁺]_i was estimated to be ∼3.4 nM/μm 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 [Ca²⁺]_i transients and produced [Ca²⁺]_i gradients directed from the center to either end of the cell. Mean ∇[Ca²⁺]_i in the anodal half of the cell (∼4.2 nM/μm) was found to be statistically higher than in the cathodal half (∼2.8 nM/μm).