Effect of albumin on hydraulic conductivity of pulmonary artery endothelial monolayers

A novel method for measuring hydraulic conductivity at the human blood-nerve barrier in vitro

Microvascular barrier permeability to water is an essential biophysical property required for the homeostatic maintenance of unique tissue microenvironments. This is of particular importance in peripheral nerves where strict control of ionic concentrations is needed for axonal signal transduction. Previous studies have associated inflammation, trauma, toxin exposure and metabolic disease with increases in water influx and hydrostatic pressure in peripheral nerves with resultant endoneurial edema that may impair axonal function. The regulation of water permeability across endoneurial microvessels that form the blood-nerve barrier (BNB) is poorly understood. Variations exist in apparatus and methods used to measure hydraulic conductivity. The objective of the study was to develop a simplified hydraulic conductivity system using commercially available components to evaluate the BNB. We determined the mean hydraulic conductivity of cultured confluent primary and immortalized human endoneurial endothelial cell layers as 2.00×10^-7 and 2.17×10^-7cm/s/cm H₂O respectively, consistent with restrictive microvascular endothelial cells in vitro. We also determined the mean hydraulic conductivity of immortalized human brain microvascular endothelial cell layers, a commonly used blood-brain barrier (BBB) cell line, as 0.20×10^-7cm/s/cm H₂O, implying a mean 10-fold higher resistance to transendothelial water flux in the brain compared to peripheral nerves. To our knowledge, this is the first reported measurement of human BNB and BBB hydraulic conductivities. This model represents an important tool to further characterize the human BNB and deduce the molecular determinants and signaling mechanisms responsible for BNB hydraulic conductivity in normal and disease states in vitro.

Fluorescence correlation spectroscopy can probe albumin dynamics inside lung endothelial glycocalyx

The endothelial glycocalyx is believed to play a major role in capillary permeability by functioning as a macromolecular barrier overlying the intercellular junction. Little is known about the functional attributes of the glycocalyx (i.e., porosity and permeability) or which constituents contribute to its overall structure-function relationship. In this report, we demonstrate the utility of fluorescence correlation spectroscopy (FCS) to measure albumin diffusion rates and concentration profiles above the cell surface and overlying the intercellular junctions of lung capillary endothelial cells. Albumin diffusion rates and concentration profiles were obtained before and after enzymatic digestion of the glycocalyx with pronase, heparanase, or hyaluronidase. The results suggest a structure interacting with albumin located from 1.0 to 2.0 microm above the cell membrane capable of reducing albumin diffusion by 30% while simultaneously increasing albumin concentration fivefold. Digestion of the glycocalyx with pronase or heparanase resulted in only modest changes in albumin diffusion and concentration profiles. Hyaluronidase digestion completely eliminated albumin-glycocalyx interactions. These data also suggest that hyaluronan is a major determinant for albumin interactions with the lung endothelial glycocalyx. Confocal images of heparan sulfate and hyaluronan confirm a cell-surface layer 2-3 mum in thickness, thus supporting FCS measurements. In summary, we report the first use of FCS to probe extracellular structures and further our understanding of the structure-function relationship of the lung microvascular endothelial glycocalyx.

Transport in rat vessel walls. I. Hydraulic conductivities of the aorta, pulmonary artery, and inferior vena cava with intact and denuded endothelia

In this study, filtration flows through the walls of the rat aorta, pulmonary artery (PA), and inferior vena cava (IVC), vessels with very different susceptibilities to atherosclerosis, were measured as a function of transmural pressure (DeltaP), with intact and denuded endothelium on the same vessel. Aortic hydraulic conductivity (L(p)) is high at 60 mmHg, drops approximately 40% by 100 mmHg, and is pressure independent to 140 mmHg. The trends are similar in the PA and IVC, dropping 42% from 10 to 40 mmHg and flat to 100 mmHg (PA) and dropping 33% from 10 to 20 mmHg and essentially flat to 60 mmHg (IVC). Removal of the endothelium renders L(p)(DeltaP) flat: it increases L(p) of the aorta by approximately 75%, doubles L(p) of the PA, and quadruples L(p) of the IVC. Specific resistance (1/L(p)) of the aortic endothelium is approximately 47% of total resistance; i.e., the endothelium accounts for approximately 47% of the DeltaP drop at 100 mmHg. The PA value is 55% at >40 mmHg, and the IVC value is 23% at 10 mmHg. L(p) of the intact aorta, PA, and IVC are order 10(-8), 10(-7), and 5 x 10(-7) cm.s(-1).mmHg(-1), and wall thicknesses are 145.8 microm (SD 9.3), 78.9 microm (SD 3.3), and 66.1 microm (SD 4.1), respectively. These data are consistent with the different wall structures of the three vessels. The rat aortic L(p) data are quantitatively consistent with rabbit L(p)(DeltaP) (Tedgui A and Lever MJ. Am J Physiol Heart Circ Physiol 247: H784-H791, 1984; Baldwin AL and Wilson LM. Am J Physiol Heart Circ Physiol 264: H26-H32, 1993), suggesting that intimal compression under pressure loading may also play a role in L(p)(DeltaP) in these other vessels. Despite very different driving DeltaP, nominal transmural water fluxes of these three vessels are very similar and, therefore, cannot alone account for their differences in disease susceptibility. The different fates of macromolecular tracers convected by these water fluxes into the walls of these vessels may account for this difference.

Effects of albumin supplementation on microvascular permeability in septic patients

Albumin has a stabilizing effect on endothelium and helps maintain capillary permeability to macromolecules. Critically ill patients with sepsis may have profound hypoalbuminemia, but the effect of this hypoalbuminemia on microvascular permeability is unknown. To determine the degree and potential importance of this effect, we measured the transcapillary escape rate (TER) of (125)I-labeled albumin in 12 adult patients fulfilling American College of Chest Physicians/Society of Critical Care Medicine criteria for septic shock. We measured TER over a 90-min baseline period and then repeated these measurements immediately after the rapid infusion of 200 ml of 20% albumin. At baseline, patients had a mean serum albumin concentration of 10.3 +/- 3.8 g/l, which, at 30 min after the albumin infusion, was 18.5 +/- 3.7 g/l. The baseline TER was 6.7 +/- 1.5%/h, with a postinfusion TER of 6.4 +/- 2.1%/h (P = 0.550). Albumin supplementation sufficient to nearly double serum concentrations in profoundly hypoalbuminemic septic patients had no clinically significant effect in reducing microvascular permeability.