FLUID-FLOW INCREASES MEMBRANE-PERMEABILITY TO MEROCYANINE-540 IN HUMAN ENDOTHELIAL-CELLS

Fluid shear stress is a ubiquitous stimulus of mammalian cell metaboli sm; however, its signal transduction pathway is unknown. We hypothesiz ed that shear stress may alter some physical properties of the cell me mbrane. Using primary human umbilical vein endothelial cells (HUVECs), we investigated the effects of shear on the cell membrane by monitori ng flow-induced changes in the uptake of the amphipath merocyanine 540 (MC540). Under static conditions, MC540 was rapidly internalized by H UVECs at 37 degrees C, and so was the membrane impermeant dye lucifer yellow, suggesting that the MC540 uptake was partly due to endocytosis . However, exposure to steady flow for 5 min at 37 degrees C induced a n increase in MC540 uptake while that of lucifer yellow was unchanged, suggesting that the flow-induced increase in MC540 uptake was not end ocytosis-related. The increase in MC540 uptake was significant for lev els of steady shear of 6 dyne/cm(2) and above. Pulsatile flow was more stimulatory than steady flow at 2 dyne/cm(2), but no significant diff erence between the two was seen at higher shear stress levels. We conc lude that fluid shear stress enhanced the uptake of MC540 by a mechani sm other than endocytosis, suggesting an increase in plasma membrane p ermeability during exposure of the cells to shear stress.