Stretch increases inositol 1,4,5-trisphosphate concentration in airway epithelial cells

Mechanical stimulation of airway epithelial cells with a microprobe leads to an increase in cytoplasmic [Ca2+] that appears to be due, in part, to release of Ca2+ from inositol 1,4,5-trisphosphate (IP3)-sensitive stores (Boitano et al., Science 258:292 [1992]). To investigate whether intracellular IP3 concentration ([IP3](i)) increases in response to mechanical stimulation, we grew confluent monolayers from rabbit tracheal mucosal explants on flexible substrates and measured [IP3](i) after stretching the substrate. The effect of stretch on [IP3](i) was measured in the presence of Li+, an inhibitor of IP3 degradation. In unstretched cells, IP3 measured approximately 5.1 pmol/10(6) cells, from which we estimated [IP](3i) to be 1.8 mu M. Addition of Li+ had no effect on resting [IP3](i). When the flexible cell support was stretched to increase its surface area by 13%, mean [IP3](i) increased about 3-fold with a half-time of approximately 1 s. The increased [IP3](i) was maintained in a plateau phase for approximately 8 s and then decayed to near the unstretched level over the next 10 s, despite the sustained application of stretch. A transient stretch (0.5 s) induced a similar rate of increase and peak [IP3](i); however, [IP3](i) subsided without a plateau phase. The magnitude of the [IP3](i) increase was proportional to stimulus intensity between 0 and 13% increase in substrate surface area. In addition, dissociated airway epithelial cells were exposed to hypotonic solution to induce cell swelling. [IP3](i) increased about 4-fold above control levels after 10 s of exposure to hypotonic solution. Basal [IP3](i) of dissociated cells in isotonic solution was estimated to be 0.7 mu M. These results are consistent with mechanical stimulation leading to phospholipase C synthesis of IP3, which mediates intracellular and intercellular Ca2+ signaling.