Ca²⁺ sparks triggered by patch depolarization in rat heart cells

The goal of this study was to examine the relationship between Ca²⁺ entry through L-type Ca²⁺ channels and local [Ca²⁺](i) transients (Ca²⁺ sparks) in single rat cardiac ventricular cells. L-type Ca²⁺ channels were activated by depolarization of cell-attached membrane patches, and [Ca²⁺](i) was measured simultaneously as fluo 3 fluorescence using laser scanning confocal microscopy. Patch depolarization with Ca²⁺ as the charge career (10 or 110 mmol · L^-1) significantly increased the probability of the occurrence of Ca¹²⁺ sparks (Ca²⁺ spark rate) only in the volume of cytoplasm located immediately beneath the membrane patch (basal Ca²⁺ spark rate, 119 Ca²⁺ sparks · cell^- · s^-1; patch depolarization Ca²⁺ spark rate, 610 Ca²⁺ sparks · cell^-1 · s^-1; P<.005). With Ba²⁺ in the pipette solution (10 mmol · L^-1), patch depolarization was not associated with an increased Ca²⁺ spark rate at the position of the pipette or at any other sites distant from the pipette. Therefore, Ca²⁺ entry and not voltage per se was a necessary event for the occurrence of Ca²⁺ sparks. Under identical experimental conditions, patch depolarization experiments opened single L-type Ca²⁺ channels with a single-channel conductance of 19 pS with Ba²⁺ as the charge career. Although single-channel openings could not be resolved when Ca²⁺ was the charge carrier, ensemble averages yielded an inward current of up to 0.75 pA. The results suggest that voltage-activated Ca²⁺ entry through one or a small number of L type Ca²⁺ channels triggers the release of Ca²⁺ only from the sarcoplasmic reticulum in direct proximity to those L-type Ca²⁺ channels. The relatively low probability of triggering Ca²⁺ sparks may have resulted from some alteration of excitation-contraction coupling associated with the technique of the cell- attached patch clamp.