Microvascular permeability to water is independent of shear stress, but dependent on flow direction

Abstract

Endothelial cells in a cultured monolayer change from a "cobblestone" configuration when grown under static conditions to a more elongated shape, aligned with the direction of flow, after exposure to sustained uniform shear stress. Sustained blood flow acts to protect regions of large arteries from injury. We tested the hypothesis that the stable permeability state of individually perfused microvessels is also characteristic of flow conditioning. In individually perfused rat mesenteric venular microvessels, microvascular permeability, measured as hydraulic conductivity (Lp), was stable [mean 1.0 × 10(-7) cm/(s × cmH2O)] and independent of shear stress (3-14 dyn/cm(2)) for up to 3 h. Vessels perfused opposite to the direction of normal blood flow exhibited a delayed Lp increase [ΔLp was 7.6 × 10(-7) cm/(s × cmH2O)], but the increase was independent of wall shear stress. Addition of chondroitin sulfate and hyaluronic acid to perfusates increased the shear stress range, but did not modify the asymmetry in response to flow direction. Increased Lp in reverse-perfused vessels was associated with numerous discontinuities of VE-cadherin and occludin, while both proteins were continuous around the periphery of forward-perfused vessels. The results are not consistent with a general mechanism for graded shear-dependent permeability increase, but they are consistent with the idea that a stable Lp under normal flow contributes to prevention of edema formation and also enables physiological regulation of shear-dependent small solute permeabilities (e.g., glucose). The responses during reverse flow are consistent with reports that disturbed flows result in a less stable endothelial barrier in venular microvessels.

Fig. 1.

Hydraulic conductivity (L_p) plotted over the duration of the experiment for each vessel perfused with BSA/Ringer solution. A: vessels perfused in forward direction have a stable L_p for up to 3 h (n = 12). B: reverse-flow group was more variable; several vessels showed a gradual increase in L_p starting at about 90–120 min of perfusion (n = 11). Individual vessels are identified by a symbol and color.

Fig. 2.

Final ΔL_p relative to baseline L_p plotted as a function of the average wall shear stress, τ. A: vessels perfused with BSA solutions. None of the vessels in the forward-flow (open symbols) group had a ΔL_p higher than 4. The reverse-flow (closed symbols) group was more variable with five vessels having ΔL_p below 4 and six vessels with ΔL_p above 4. There was no clear relationship with wall shear stress. B: vessels perfused with CSHA solutions. None of the vessels in the forward-flow group (open symbols) had ΔL_p higher than 4. The reverse-flow group (closed symbols) was variable with three vessels having ΔL_p below 4 and four vessels with ΔL_p above 4. There was no clear relationship with wall shear stress. CS, chondroitin sulfate; HA, hyaluronic acid. See results for description of CSHA solution.

Fig. 3.

L_p plotted over time for vessels perfused with CSHA solutions. A: vessels perfused in forward direction have a stable L_p for up to 3 h (n = 5). B: reverse-flow group was variable; several vessels showed a rapid increase in L_p starting at about 60–90 min of perfusion (n = 7). Individual vessels are identified by a symbol and color.

Fig. 4.

Representative segments of perfused microvessels labeled for VE-cadherin and occludin. The reverse-flow perfusions (bottom panels) showed numerous small gaps in both the occludin and VE-cadherin (arrowheads). Similar gaps were not seen in vessels perfused in the forward direction (top panels). A segment of tight junction occludin in the overlying mesothelium is visible (arrows) as a nonconnecting green shape. Scale bar, 20 μm for all images.