Complex and rate-dependent beat-to-beat variations in Ca²⁺ transients of canine Purkinje cells

Purkinje fibers play an essential role in transmitting electrical impulses through the heart, but they may also serve as triggers for arrhythmias linked to defective intracellular calcium (Ca²⁺) regulation. Although prior studies have extensively characterized spontaneous Ca²⁺ release in nondriven Purkinje cells, little attention has been paid to rate-dependent changes in Ca²⁺ transients. Therefore we explored the behaviors of Ca²⁺ transients at pacing rates ranging from 0.125 to 3Hz in single canine Purkinje cells loaded with fluo3 and imaged with a confocal microscope. The experiments uncovered the following novel aspects of Ca²⁺ regulation in Purkinje cells: 1) the cells exhibit a negative Ca²⁺-frequency relationship (at 2.5Hz, Ca²⁺ transient amplitude was 66±6% smaller than that at 0.125Hz); 2) sarcoplasmic reticulum (SR) Ca²⁺ release occurs as a propagating wave at very low rates but is localized near the cell membrane at higher rates; 3) SR Ca²⁺ load declines modestly (10±5%) with an increase in pacing rate from 0.125Hz to 2.5Hz; 4) Ca²⁺ transients show considerable beat-to-beat variability, with greater variability occurring at higher pacing rates. Analysis of beat-to-beat variability suggests that it can be accounted for by stochastic triggering of local Ca²⁺ release events. Consistent with this hypothesis, an increase in triggering probability caused a decrease in the relative variability. These results offer new insight into how Ca²⁺ release is normally regulated in Purkinje cells and provide clues regarding how disruptions in this regulation may lead to deleterious consequences such as arrhythmias.