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

Spontaneous, localized intracellular Ca2+concentration ([Ca2+]i) transients (Ca2+ sparks) in skeletal, cardiac, and smooth muscle cells are thought to represent Ca2+ release through ryanodine-receptor (RyR) channels. In porcine tracheal smooth muscle (TSM) cells, ACh induces propagating [Ca2+]ioscillations that also represent Ca2+ release through RyR channels. We used real-time confocal imaging to examine the spatial and temporal relationships of Ca2+ sparks to propagating [Ca2+]ioscillations in TSM cells. Ca2+sparks within an intracellular region displayed different spatial Ca2+ distributions with every occurrence. The amplitudes of Ca2+sparks within a region were approximately integer multiples of the smallest response. However, across different regions, the attributes of Ca2+ sparks varied considerably. Individual sparks were often grouped together and coupled across adjacent regions. Fusion of individual sparks produced large local elevations in [Ca2+]ithat occasionally triggered a propagating [Ca2+]iwave. The incidence of sparks was increased by ryanodine and caffeine but was unaffected by removal of extracellular Ca2+. Exposure to ACh triggered repetitive, propagating [Ca2+]ioscillations that always originated from foci with a high spark incidence. The [Ca2+]ioscillations disappeared with the removal of ACh, and Ca2+ sparks reappeared. We conclude that agonist-induced [Ca2+]ioscillations represent a spatial and temporal integration of local Ca2+-release events through RyR channels in TSM cells.