Mechanosensitive Ca²⁺ transients in endothelial cells from human umbilical vein

We have investigated the changes in intracellular calcium concentration ([Ca²⁺](i)) in human endothelial cells induced by mechanical stretch due to osmotic cell swelling. Hypotonic solutions also activate a Cl^- conductance that has been described elsewhere and mainly serves to clamp the membrane potential at negative values to provide a driving force for Ca²⁺ influx. The increase in [Ca²⁺](i) caused by hypotonic solutions is due to release from inositol-1,4,5-trisphosphate-sensitive Ca²⁺ pools and a subsequent Ca²⁺ influx, apparently activated by store depletion. These changes in [Ca²⁺](i) are completely abolished if the phospholipase A₂ (PLA₂) activity is inhibited by either 4-bromophenacyl bromide or cyclosporin A. Arachidonic acid, applied either extracellularly or intracellularly via the patch pipette, mimics the mechanosensitive response even in cells with blocked PLA₂. Metabolites of the lipo- and cyclooxygenase pathways can be excluded. Phospholipase C activation and the protein kinase A pathway are not involved in this mechanical response. Although no specific pharmacological tools for probing the role of PLA₂ are available, our evidence suggests that mechanosensitivity in endothelial cells may be modulated by arachidonic acid.