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⁺](i) oscillations 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⁺](i) oscillations 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⁺](i) that occasionally triggered a propagating [Ca²⁺](i) wave. 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⁺](i) oscillations that always originated from loci with a high spark incidence. The [Ca²⁺](i) oscillations disappeared with the removal of ACh, and Ca²⁺ sparks reappeared. We conclude that agonist-induced [Ca²⁺](i) oscillations represent a spatial and temporal integration of local Ca²⁺-release events through RyR channels in TSM cells.