CALCIUM SPARKS - ELEMENTARY EVENTS UNDERLYING EXCITATION-CONTRACTION COUPLING IN HEART-MUSCLE

Spontaneous local increases in the concentration of intracellular calc ium, called ''calcium sparks,'' were detected in quiescent rat heart c ells with a laser scanning confocal microscope and the fluorescent cal cium indicator fluo-3. Estimates of calcium flux associated with the s parks suggest that calcium sparks result from spontaneous openings of single sarcoplasmic reticulum (SR) calcium-release channels, a finding supported by ryanodine-dependent changes of spark kinetics. At restin g intracellular calcium concentrations, these SR calcium-release chann els had a low rate of opening (approximately 0.0001 per second). An in crease in the calcium content of the SR, however, was associated with a fourfold increase in opening rate and resulted in some sparks trigge ring propagating waves of increased intracellular calcium concentratio n. The calcium spark is the consequence of elementary events underlyin g excitation-contraction coupling and provides an explanation for both spontaneous and triggered changes in the intracellular calcium concen tration in the mammalian heart.