Spatial and temporal aspects of calcium sparks in porcine tracheal smooth muscle cells.

Spontaneous, localized intracellular Ca(2+) concentration (Ca(2+)) transients (Ca(2+) sparks) in skeletal, cardiac, and smooth muscle cells are thought to represent Ca(2+) release through ryanodine-receptor (RyR) channels. In porcine tracheal smooth muscle (TSM) cells, ACh induces propagating Ca(2+) oscillations that also represent Ca(2+) release through RyR channels. We used real-time confocal imaging to examine the spatial and temporal relationships of Ca(2+) sparks to propagating Ca(2+) oscillations in TSM cells. Ca(2+) sparks within an intracellular region displayed different spatial Ca(2+) distributions with every occurrence. The amplitudes of Ca(2+) sparks within a region were approximately integer multiples of the smallest response. However, across different regions, the attributes of Ca(2+) sparks varied considerably. Individual sparks were often grouped together and coupled across adjacent regions. Fusion of individual sparks produced large local elevations in Ca(2+) that occasionally triggered a propagating Ca(2+) wave. The incidence of sparks was increased by ryanodine and caffeine but was unaffected by removal of extracellular Ca(2+). Exposure to ACh triggered repetitive, propagating Ca(2+) oscillations that always originated from foci with a high spark incidence. The Ca(2+) oscillations disappeared with the removal of ACh, and Ca(2+) sparks reappeared. We conclude that agonist-induced Ca(2+) oscillations represent a spatial and temporal integration of local Ca(2+)-release events through RyR channels in TSM cells.