Ca2+ Sparks Triggered by Patch Depolarization in Rat Heart Cells

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The goal of this study was to examine the relationship between Ca (2+) 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 conforcal microscopy. Patch depolarization with Ca2+ as the charge carrier (10 or 110 mmol [center dot] L-1) significantly increased the probability of the occurrence of Ca2+ sparks (Ca2+ spark rate) only in the volume of cytoplasm located immediately beneath the membrane patch (basal Ca2+ spark rate, 119 Ca2+ sparks [center dot] cell-1 [center dot] s-1; patch depolarization Ca2+ spark rate, 610 Ca2+ sparks [center dot] cell-1 [center dot] s-1; P<.005). With Ba2+ in the pipette solution (10 mmol [center dot] 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 carrier. 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. (Circ Res. 1998;82:424-429.)

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