The maintenance of endothelial barrier function is important in the regulation of fluid and solute balance between the vascular space and the surrounding tissue. Since fluid flow across endothelial cells stimulates a wide variety of endothelial responses, the effect of shear stress on barrier function was investigated. Bovine pulmonary artery endothelial cells were cultured on permeable microcarrier beads, placed in a chromatography column, and perfused. Indicator-dilution techniques were used to estimate the permeability of the cell-covered beads to low molecular weight tracers (sodium fluorescein-NaFlsc; cyanocobalamin-B12) as a function of flow rate through the column. Permeability values for both tracers were significantly increased (9.3 +/- 0.6 to 19.3 +/- 1.7 for NaFlsc; 8.2 +/- 0.5 to 20.4 +/- 3.1 for B12; mean+/-SEM, x 10(-5) cm/s, P < .05) when the flow rate was increased from 0.9 ml/min to 3.2 ml/min (corresponding to average shear stresses of 4.7 and 16.8 dynes/cm2). The permeability increase occurred within minutes of the flow increase, and was reversed by decreasing the flow rate to 0.9 ml/min. In the presence of cytochalasin D, the flow-induced permeability increase was not reversible. Neither inhibition of nitric oxide synthase (with NG-monomethyl-L-arginine) nor inhibition of cyclooxygenase (with indomethacin) was capable of blocking the flow-induced permeability increase. These results indicate that the rapid modulation of endothelial barrier by flow in vitro is probably not due to prostacyclin or nitric oxide.