(587d) “Aquaporin-1: Its Presence, Distribution and Quantification In Arterial Endothelium and How These Change with Chronic Hypertension – with Relevance to Early Atherogenesis”

Atherosclerosis is responsible for over 50% of all deaths in the US and in all Western countries. The earliest events leading to atherosclerosis occur when low-density lipoproteins (LDLs) are transported, mainly by pressure driven convection, from the blood and deposited into the inner layers of high-pressure, large and intermediate sized arteries. Subsequent events narrow and harden these vessels' walls, leading possibly to strokes and heart attacks.

Permeability studies have shown that, at least over short time scales, macromolecular tracers appear to cross the endothelium of these atherosclerosis-prone vessels non-uniformly via rare focal leaks that are associated with transient leaky junctions of endothelial cells, some of which are either dying or dividing.

Pressure-driven transmural water transport advects these macromolecules across the endothelium into the subendothelial intimal space and initially spreads them in a direction parallel to the endothelium away from the leakage sites. The overall trans-wall water flux, not just the part through the few widened junctions, can dilute the local subendothelial macromolecular concentration, thereby causing the LDL to spread away from the focal leaky regions, eventually through the wall. Since this water flux affects the local LDL concentration in the intima, it very likely affects the kinetics of LDL attachment to the intima's extracellular matrix, which is believed to be one of the earliest and most critical events leading to lesion formations. The nature of this convective water flux is therefore a central factor in the prelesion events of atherosclerosis.

It has been accepted that water crosses the endothelium paracellularly, that is through tight and leaky junctions, the former, due to their numbers, being far more important than the latter to the overall trans-wall water flow. Since 1990, however, a family of ubiquitous transmembrane proteins called aquaporins (AQP) has been identified. AQPs are very specific and facilitate very efficient transcellular water transport. It is therefore natural to ask whether AQPs are present in arterial endothelial cells and, if so, if their presence implies a transcellular contribution in transmural water transport. Particularly, if water transport is not simply a passive process, that goes up and down with a changing transmural pressure, but is rather a protein-gated process, then a vessel might be able to actively regulate this transport by differential protein expression. Such transcellular transport, if it indeed is present, would play an important role in LDL transport once it has entered the subendothelial region through the leaky junctions and in its binding kinetics in the subendothelial portion of the vessel wall.

In our previous study, we used immunohistochemistry to identify and show the existence of Aquaporin-1 (AQP1) on both the luminal and abluminal membranes of rat aortic endothelial cells.

A known risk factor for atherosclerosis is hypertension and we postulated that one external condition that may influence a vessel's AQP expression is its transmural pressure. We previously confirmed this hypothesis in both genetically modified normotensive Wistar Kyoto rats and their hypertensive cousins Spontaneously Hypertensive rats (SHR) showing that endothelial cells from the chronically hypertensive rats appear to express far more AQP than their normotensive analogues.

In this study, we repeat our study in non- genetically modified normotensive (Sprague-Dawley rats without any operation, Sprague-Dawleys that have undergone a sham operation) and chronically hypertensive (Sprague-Dawleys that have undergone the Goldblatts procedure to induce hypertension by partially crimping one renal artery). We again show similar qualitative results to our previous genetically modified models study.

We also show evidence that blocking aquaporins, either chemically or by a knockdown experiment, reduces endothelial hydraulic conductivity, both in cultured monolayers and in whole vessels ex vivo.

This evidence begins to support the hypothesis that aortic endothelial cells may be able to actively regulate their aquaporin expression in response to chronic hypertensive conditions so as to control their transmural transport processes. Improved understanding of these processes may lead to novel therapies, not just in the case of atherosclerosis, but also for a wide variety of human diseases.