CA2-HEART CELLS( SPARKS TRIGGERED BY PATCH DEPOLARIZATION RAT)

The goal of this study was to examine the relationship between Ca2+ en try through L-type Ca2+ channels and local [Ca2+](i) transients (Ca2sparks) in single rat cardiac ventricular cells. L-type Ca2+ channels were activated by depolarization of cell-attached membrane patches, an d [Ca2+](i) was measured simultaneously as fluo 3 fluorescence using l aser scanning confocal microscopy. Patch depolarization with Ca2+ as t he charge carrier (10 or 110 mmol.L-1) significantly-increased the pro bability of the occurrence of Ca2+ sparks (Ca2+ spark rate) only in th e volume of cytoplasm located immediately beneath the membrane patch ( basal Ca2+ spark rate, 119 Ca2+ sparks.cell(-1).s(-1); patch depolariz ation Ca2+ spark rate, 610 Ca2+ sparks.cells(-1).s(-1); P<.005). With Ba2+ in the pipette solution (10 mmol.L-1), patch depolarization was n ot associated with an increased Ca2+ spark rate at the position of the pipette or at any other sites distant from the pipette. Therefore, Ca 2+ entry and not voltage per se was a necessary event for the occurren ce of Ca2+ sparks. Under identical experimental conditions, patch depo larization experiments opened single L-type Ca2+ channels with a singl e-channel conductance of 19 pS with Ba2+ as the charge carrier. Althou gh single-channel openings could not be resolved when Ca2+ was the cha rge 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 oil type Ca2+ channels triggers the release of Ca2 only from the sarcoplasmic reticulum in direct proximity to those L-t ype Ca2+ channels. The relatively low probability of triggering Ca2+ s parks may have resulted from some alteration of excitation-contraction coupling associated with the technique of the cell-attached patch cla mp.