Fluid shear stress modulates cytosolic free calcium in vascular endothelial cells

Cytosolic free Ca2+ concentration (Ca2+i) was monitored in single and groups of fura-2-loaded bovine aortic endothelial cells (BAEC) during exposure to laminar fluid shear stress. Application of a step increase in shear stress from 0.08 to 8 dyn/cm2 to confluent BAEC monolayers resulted in a transient increase in Ca2+i, which attained a peak value in 15-40 s, followed by a decline to baseline within 40-80 s. The magnitude of the Ca2+i responses increased with applied shear stress over the range of 0.2-4 dyn/cm2 and reached a maximum at greater than 4 dyn/cm2. Transient oscillations in Ca2+i with gradually diminishing amplitude were observed in individual cells subjected to continuous high shear stress. Elimination of extracellular Ca2+ with ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, blockade of Ca2+ entry with lanthanum, depolarization of the cell membrane with high K+, and preconditioning of BAEC in steady laminar flow had little effect on the Ca2+i response. In the presence of ATP or ADP, application of shear stress caused repetitive oscillations in Ca2+i in single BAEC, whose frequency was dependent on both agonist concentration and the magnitude of applied shear stress. However, apyrase, an ATPase and ADPase, did not inhibit the shear-induced Ca2+i responses in standard medium (no added ATP or ADP), suggesting that the shear-induced Ca2+i response is not due to ATP released by endothelial cells.