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.