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

Spontaneous, localized intracellular Ca(2+) concentration ([Ca(2+)](i)) 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+)](i) 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+)](i) 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+)](i) that occasionally triggered a propagating [Ca(2+)](i) 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+)](i) oscillations that always originated from foci with a high spark incidence. The [Ca(2+)](i) oscillations disappeared with the removal of ACh, and Ca(2+) sparks reappeared. We conclude that agonist-induced [Ca(2+)](i) oscillations represent a spatial and temporal integration of local Ca(2+)-release events through RyR channels in TSM cells.