Shear stress induces a time- and position-dependent increase in endothelial cell membrane fluidity

Blood flow-associated shear stress may modulate cellular processes through its action on the plasma membrane. We quantified the spatial and temporal aspects of the effects of shear stress (τ) on the lipid fluidity of 1,1′-dihexadecyl-3,3,3′-tetramethylindocarbocyanme perchlorate [DiIC₁₆(13)]-stained plasma membranes of bovine aortic endothelial cells in a flow chamber. A confocal microscope was used to determine the DiI diffusion coefficient (D) by fluorescence recovery after photobleaching on cells under static conditions, after a step-τ of 10 or 20 dyn/cm², and after the cessation of τ. The method allowed the measurements of D on the upstream and downstream sides of the cell taken midway between the respective cell borders and the nucleus. In <10 s after a step-τ of 10 dyn/cm², D showed an upstream increase and a downstream decrease, and both changes disappeared rapidly. There was a secondary, larger increase in upstream D, which reached a peak at 7 min and decreased thereafter, despite the maintenance of τ. D returned to near control values within 5 s after cessation of τ. Downstream D showed little secondary changes throughout the 10-min shearing, as well as after its cessation. Further investigations into the early phase, with simultaneous measurements of upstream and downstream D, confirmed that a step-τ of 10 dyn/cm² elicited a rapid (5-s) but transient increase in upstream D and a concurrent decrease in downstream D, yielding a significant difference between the two sites. A step-τ of 20 dyn/cm² caused D to increase at both sites at 5 s, but by 30 s and 1 min the upstream D became significantly higher than the downstream D. These results demonstrate shear-induced changes in membrane fluidity that are time dependent and spatially heterogeneous. These changes in membrane fluidity may have important implications in shear-induced membrane protein modulation.