The macromolecular transport of lipoproteins into the artery wall and their spread in the subendothelial intima are believed to be critical prelesion events in atherogenesis. Large molecules (e.g., lipoprotein cholesterol) cross the arterial endothelium via very rare (~1 in 2000-6000) endothelial cells, many associated with widened junctions around endothelial cells that are either dividing or dying^2,5,6,11. Our group has established that^2,3 this focal transport is due to the advection of macromolecules by transmural pressure-driven water transport through these leaky junctions. In contrast, water is known to cross the normal endothelium uniformly e.g., through normal tight junctions. Thus water transport, together with the diffusion, can further transport lipoproteins that have already entered the vessel's intima. Hence, the total transmural water transport, and not just the portion through leaky junctions, appears to play a central role in delivering low-density lipoprotein (LDL) cholesterol to the subendothelial space. The basic aim behind this work is to develop a theory, that explains the transcellular pathway for water transport, alongside the generally accepted paracellular route, and to quantify its percentage contribution to the vessel endothelium's hydraulic conductivity Lp, the ratio of the tranmural water flux to the driving pressure difference, as a function of transmural pressure. First some background.

Experimental results of Tedgui & Lever¹⁰ and Baldwin & Wilson¹ show a higher value of Lp at low pressures (60-70 mmHg) which drops by ~40% at higher pressures and remains pressure-insensitive beyond, up to ~180mmHg. Endothelial removal doubles Lp and renders it pressure-insensitive. Huang et al ⁴ explained this behavior by postulating that intimal compression under transmural pressure loading causes the endothelium to partially block internal elastic lamina fenestrae, thereby causing the observed Lp lowering with ΔP. Endothelial removal eliminates both a layer of resistance and thus fenestral blocking potential, thereby increasing Lp and explaining the denuded vessel Lp's pressure insensitivity. Jimmy Toussaint of our group has established that rat aortic endothelial cells avidly express Aquaporin-1 (AQP), a membrane protein that acts as a specific water channel. These highly selective water channels allow high throughputs of water (~3x10⁹ molecules/sec) in response to, e.g., osmotic gradients, at little or no cost in ATP⁷. Tieuvi Nguyen of our group has shown experimentally⁹, by measuring the hydraulic conductivity of an excised vessel ex vivo as a function of pressure, first with functioning and then with chemically blocked AQPs (using HgCl₂ as a blocker ⁸ ), on the same vessel, that these AQPs play a significant functional role in transmural water transport in rat aortas.

New theory to determine AQP contribution to endothelial Lp vs ΔP: Nguyen's experiments showed significant decreases of ~36%, 13% & 8% in intact vessel's total hydraulic conductivity, Lp_t (1/ Lp_t = 1/Lp_e+i + 1/ Lp_m+I, where Lp_e+i is the hydraulic conductivity of endothelium plus subendothelial intima and Lp_m+I is the hydraulic conductivity of the denuded vessel, representing media and internal elastic lamina contributions) at 60,100,140 mmHg respectively, when HgCl₂ is used to chemically block AQP. The variation in these numbers clearly shows that more is going on than a simple blocking of AQP. Recall Huang et al's ⁴ intimal compression theory (without blocker) argued that above 80 mmHg the intima remains fully compressed and therefore Lp is ΔP-independent. Note that in case of blocked AQPs, Nguyen showed nearly ΔP-independent values of Lp that extend to her lowest value of 60 mmHg, where the unblocked intima is not compressed. This suggests that AQP-blocking may lead to intima compaction at a lower overall ΔP. We propose a theory to explain this Hg++ dependence of Lp vs ΔP. We shall also extract the percentage of intrinsic Lp due to AQP, by extending the filtration model given by Huang et al ⁴ . The idea behind the new model is that blocking the endothelial AQPs decreases the intima's intrinsic Lp, which decreases intimal pressure (P_i) at a fixed ΔP. Thus there is a larger endothelial force per unit area (P_L-P_i),where P_L is lumen pressure,which can compress the intima at lower total ΔP. Specifically, we extend the transport model of Huang et al ⁴ by including transcellular flow characterized by Lp_EC (hydraulic conductivity of endothelial cell), which is the fraction of Lp_e(hydraulic conductivity of endothelium) due to the AQPs. From this model, we calculate the experimentally measured Lp_e+i, which depends on the thickness of intima, and compare with Nguyen's corresponding experimental values.

References:

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(2) Chuang P.T, “Macromolecular transport across arterial and venous endothelium in rats: Studies with Evans blue-albumen and horseradish peroxidase”, Arteriosclerosis, 10, 188-197,1990.

(3) Huang Y., Rumschitzki D.,Chein S.,Weinbaum S., “A fiber matrix model for the growth of macromolecular leakage spots in the arterial intima”, ASME Journal of Biomechanical Engineering,116,430-445,1995.

(4) Huang Y., Rumschitzki D.,Chein S.,Weinbaum S., “A fiber matrix model for the filtration through fenestral pores in a compressible arterial intima”, American Journal of Physiology,272,H2023-2039,1997.

(5) Lin S.L., Jan K.M., Schuessler G.B., Weinbaum S., Chein S., “Transensendothelial transport of LDL in association with cell mitosis in rat aorta”, Arteriosclerosis, 9,230-236,1989.

(6) Lin S L., Jan K M., Schuessler G B., Weinbaum S., Chein S., “ Enhanced macromolecular permeability of aortic endothelial cells in association with mitosis”, Atherosclerosis, 73,223-232,1988.

(7) Murata K., “ Structural determinants of water permeation through aquaporin-1”, Nature, 407,599-605,2000.

(8) Preston G. M., Jung J. S., Guggino W. B., Agre P., “ The mercury sensitive residue at cystine 189 in the CHIP28 water channel”, Journal of Biological Chemistry, 268,17-20,1993.

(9) Shou Y., Jan K.M., Rumschitzki D., “Transport in rat vessel wallsI: The hydraulic conductivities of aorta, pulmonary artery and inferior vena cava with intact and denude endothelia”, American Journal of Physiology, 291, H2758-2771, 2006.

(10) Tedgui A., Lever M J., “Filtration through damaged and undamaged rabbit thoracic aorta”, American Journal of Physiology, 247, H784-791,1984.

(11) Weinbaum S., Tzeghai G, Ganatos P, Pfeffer R,Chein S, “Effect of cell turnover and leaky junctions on arterial macromolecular transport”, American Journal of Physiology, 248, H945-960,1985.