(202g) Aquaporin-1 and Transendothelial Water Transport: An Oncotic Paradox

The transport of macromolecules like low-density lipoproteins (LDL) across the blood vessel wall, leading to atherosclerotic lesions, has been the focus of intense theoretical and experimental studies in the past. This macromolecular transport process is known to occur due to advection by transmural pressure (pressure difference between inside and outside the vessel) driven water transport, characterized by hydraulic conductivity Lp, the ratio of transmural water flux to the driving pressure difference. It is very crucial to understand the details of transmural water transport because, on one hand, it advects LDL into the arterial subendothelial intima (SI) and spreads it there, thereby giving it the chance to bind to extra cellular matrix and possibly trigger the start of lesion formation. On the other hand, it dilutes LDL's local concentration, thereby likely slowing binding kinetics, and washes not-yet-bound lipid further into the wall. Water, being very small, can easily cross the endothelium via inter-endothelial cell (EC) junctions and enter the vessel wall (paracellular route), while macromolecules can only enter through ?leaky' junctions, associated with ECs that are either dying or dividing. Besides this, the presence of highly specific water channel proteins like aquaporin-1 (AQP) in verity of epithelial and endothelial cells, suggests a new possibility of water transport via AQPs (transcellular route). Preliminary data from our group show that bovine aortic ECs express AQP in cultured monolayers and that rat aortic ECs express AQP in excised whole vessels. Chemically blocking AQPs using HgCl₂ or knocking down AQP expression showed a significant reduction in monolayer Lp. Moreover, blocking EC AQPs reduced the Lp of excised vessel by ~32%, 11% and 5% at 60, 100 and 140 mmHg, respectively, suggesting significant AQP-mediated transcellular flow. In addition, the Lp was found to be pressure independent starting at as low as 60 mmHg. We have developed a model for filtration through fenestral pores and considered, for the first time, the role played by AQPs in modulating the total hydraulic conductivity of an intact arterial wall with changes in transmural pressure. Our hypothesis suggests that blocking the AQPs will decrease the number of available pathways for water transport, thereby decreasing the intimal pressure (P_i) at fixed ΔP. Thus, there is larger force per unit area [P_L (lumen pressure) ? <P_i>] acting on the endothelium that can compress the intima and lead to IEL fenestral blockage at lower overall ΔP. We have successfully shown that AQPs indeed play a significant role in overall transport across the arterial wall and they contribute about 25-30% to the intrinsic hydraulic conductivity of the endothelium. We have also found that the force acting on the endothelium at 60 mmHg with functioning AQPs is same as that at 44 mmHg for blocked AQPs. In other words, AQP blocking, when done at pressures where the SI would otherwise be uncompressed, can cause the SI compression and initiate fenestral blocking.

The transmural pressure driven trans-AQP mediated water flow presents an oncotic paradox. The idea is that normal EC junctions transport isotonic fluid, i.e., water and small solutes (e.g. albumin) responsible for osmotic pressure. In contrast, AQPs specifically transport pure (hypotonic) water, excluding nearly everything else. This pure water inflow should both raise albumin concentration on the lumen side and lower it on the SI side of the EC, thereby setting up an osmotic pressure gradient against the ΔP-driven flow through the cell and prevent the intima from getting more hypotonic. So the question comes, how can the vessel maintain trans-AQP water flow given its effect on intima tonicity. To understand the effect of such osmotic gradients on overall transport and resolve the paradox, we develop a model for water and small solute transport across the glycocalyx layer (which sits on top of the EC) and endothelial interface and through the entire wall. Here, we combine our filtration theory (discussed above) with the mass transfer problem and solve the two problems simultaneously using finite difference method. The results from this study will show us if/how osmotic gradients across an EC are reduced to a level consistent with observed transendothelial flows. Given AQP's presence in high-pressure cardiac endocardium and in heart, lung and renal epithelium, resolving such flow paradoxes may have broader impact.