Fluid shear stress activates Ca(2+) influx into human endothelial cells via P2X4 purinoceptors.

Ca(2+) signaling plays an important role in endothelial cell (EC) responses to shear stress generated by blood flow. Our previous studies demonstrated that bovine fetal aortic ECs showed a shear stress-dependent Ca(2+) influx when exposed to flow in the presence of extracellular ATP. However, the molecular mechanisms of this process, including the ion channels responsible for the Ca(2+) response, have not been clarified. Here, we demonstrate that P2X4 purinoceptors, a subtype of ATP-operated cation channels, are involved in the shear stress-mediated Ca(2+) influx. Human umbilical vein ECs loaded with the Ca(2+) indicator Indo-1/AM were exposed to laminar flow of Hanks' balanced salt solution at various concentrations of ATP, and changes in [Ca(2+)](i) were monitored with confocal laser scanning microscopy. A stepwise increase in shear stress elicited a corresponding stepwise increase in [Ca(2+)](i) at 250 nmol/L ATP. The shear stress-dependent increase in [Ca(2+)](i) was not affected by phospholipase C inhibitor (U-73122) but disappeared after the chelation of extracellular Ca(2+) with EGTA, indicating that the Ca(2+) increase was due to Ca(2+) influx. Antisense oligonucleotides designed to knockout P2X4 expression abolished the shear stress-dependent Ca(2+) influx seen at 250 nmol/L ATP in human umbilical vein ECs. Human embryonic kidney 293 cells showed no Ca(2+) response to flow at 2 micromol/L ATP, but when transfected with P2X4 cDNA, they began to express P2X4 purinoceptors and to show shear stress-dependent Ca(2+) influx. P2X4 purinoceptors may have a "shear-transducer" property through which shear stress is perceived directly or indirectly and transmitted into the cell interior via Ca(2+) signaling.