Human umbilical vein endothelial cells (HUVECs) display hydraulic conductivity (L(P)) responses to shear stress that differ markedly from the responses of bovine aortic endothelial cells (BAECs). In HUVECs, 5, 10, and 20 dyn cm(-2) steady shear stress transiently increased L(P) with a return to preshear baseline after a 2-h exposure to shear stress. Pure oscillatory shear stress of 0 +/- 20 dyn cm(-2) (mean+/-amplitude) had no effect on L(P), whereas superposition of oscillatory shear stress on steady shear stress suppressed the effect induced by steady shear stress alone. Shear reversal (amplitude greater than mean) was not necessary for the inhibitory influence of oscillatory shear stress. The transient increase of L(P) by steady shear stress was not affected by incubation with BAPTA-AM (10 microM), suggesting calcium independence of the shear response. Decreasing nitric oxide (NO) concentration with L-NMMA (100 microM), a nitric oxide synthase (NOS) inhibitor, did not inhibit the HUVEC L(P) response to shear stress. At the protein level, 10 dyn cm(-2) shear stress did not affect the total content of occludin, but it did elevate the phosphorylation level transiently. The positive correlation between occludin phosphorylation and hydraulic conductivity parallels observations in BAECs and suggests that occludin phosphorylation may be a general mediator of shear-L(P) responses in diverse endothelial cell types.