A role for phospholipase C activity but not ryanodine receptors in the initiation and propagation of intercellular calcium waves

Mechanical stimulation of a single cell in an airway epithelial culture initiates an increase in intracellular Ca2+ concentration ([Ca2+]i) that propagates from cell to cell as an intercellular Ca2+ wave. These Ca2+ waves appear to require an increase in intracellular inositol 1,4,5-trisphosphate (IP3) concentration ([IP3]i) in the stimulated cell and are propagated between cells by the diffusion of IP3 through gap junctions. To test the hypothesis that the activation of phospholipase C (PLC) contributes to the elevation of [IP3]i and initiation of an intercellular Ca2+ wave, changes in [Ca2+]i induced by mechanical stimulation were measured by digital fluorescence microscopy in the presence of the PLC inhibitor, aminosteroid U73122. Following exposure to U73122 mechanical stimulation elevated [Ca2+]i of the stimulated cell, but did not initiate the propagation of an intercellular Ca2+ wave. By contrast, in the presence of U73343, a similar aminosteroid that does not inactivate PLC, mechanical stimulation increased the [Ca2+]i of the stimulated cell and initiated an intercellular Ca2+ wave. U73122 also blocked the elevation of [Ca2+]i of airway epithelial cells in response to ATP, a P2-receptor agonist that activates PLC to elevate [IP3]i and [Ca2+]i. In addition, the propagation of intercellular Ca2+ waves was not affected by the ryanodine-receptor agonists, caffeine or ryanodine. The hypotheses that: (1) an elevation of [IP3]i is required to initiate intercellular Ca2+ waves; (2) mechanical stimulation activates PLC; and (3) Ca2+ wave propagation in airway epithelial cells involves Ca2+ release from intracellular stores primarily via IP3 receptors are supported by these results.