Intercellular calcium signaling induced by extracellular adenosine 5'-triphosphate and mechanical stimulation in airway epithelial cells

Airway epithelial cells in culture respond to extracellular adenosine 5'-triphosphate (ATP) by increasing their intracellular Ca²⁺ concentration ([Ca²⁺](i)). The effective concentration of ATP that elicited a Ca²⁺ response equal to 50% of the maximal response (EC₅₀) was 0.5 μM. Release of ATP from a pipette to form a local gradient of ATP increased [Ca²⁺](i) of individual cells in a sequential manner. Cells closest to the pipette showed an immediate increase in [Ca²⁺](i) while more distal cells displayed a delayed increase in [Ca²⁺](i). This response to the local release of ATP appeared as a wave of increasing [Ca²⁺](i) that spread to several cells and, in this respect, was similar to the intercellularly communicated Ca²⁺ waves initiated by mechanical stimulation in airway epithelial cells. In the presence of a unidirectional fluid flow, the Ca²⁺ response to a local release of ATP was biased such that virtually all the cells responding with an increase in [Ca²⁺](i) were downstream of the release site. By contrast, an identical fluid flow did not bias the radial propagation of intercellular Ca²⁺ waves induced by mechanical stimulation. Suramin, a P₂-purinergic receptor antagonist, did attenuate the Ca²⁺ response induced by ATP but did not block the propagation of mechanically induced Ca²⁺ waves. Cells from young cultures (3-5 days) or those at the leading edge of an outgrowth elevated their [Ca²⁺](i) in response to ATP. However, these cells do not respond to mechanical stimulation by the propagation of a Ca²⁺ wave. From these results we conclude that the intercellular Ca²⁺ waves elicited by mechanical stimulation are not the result of ATP or another compound released from the stimulated cell, diffusing through the extracellular fluid. This conclusion is consistent with previous experimental evidence suggesting that intercellular Ca²⁺ signaling in epithelial cells is mediated by the movement of inositol trisphosphate through gap junctions.