Effect of noradrenaline, sodium nitrite and isosorbide dinitrate on albumin transport in the wall of the excised rabbit common carotid artery

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 pulsation frequency and endothelial integrity on enhanced arterial transmural filtration produced by pulsatile pressure

The role of the endothelium in regulating transmural fluid filtration into the artery wall under pulsatile pressure and the effects of changes in pulsatile frequency on filtration have received little attention. Previous experiments (Alberding JP, Baldwin AL, Barton JK, and Wiley E. Am J Physiol Heart Circ Physiol 286: H1827-H1835, 2004) demonstrated significantly increased filtration after initial onset of pulsatile pressure compared with that predicted by using parameters measured under steady pressure. To determine the role of the endothelium in this phenomenon, the following experiments were performed on five New Zealand White rabbits (anesthetized with 30 mg/kg pentobarbital sodium). One of each pair of carotid arteries was deendothelialized, and filtration measurements under steady and pulsatile pressure were compared with those made in intact vessels (Alberding JP, Baldwin AL, Barton JK, and Wiley E. Am J Physiol Heart Circ Physiol 286: H1827-H1835, 2004). To determine the effect of increasing pulsatile frequency on arterial filtration, transmural filtration was measured by using pulsatile pressure frequencies of 1 Hz, followed by 2 Hz, in another set of intact arteries (6 arteries and 3 animals). For deendothelialized vessels, the initial increase in filtration after onset of pulsatility was similar to that observed in intact vessels, but the subsequent reduction in filtration was less abrupt. When pulsatile frequency was increased from 1 to 2 Hz in intact arteries, an initial increase in filtration was observed, similar to that obtained after onset of pulsatile pressure subsequent to a steady pressure. The observed responses of arteries to pulsatile pressure, with and without endothelium, or undergoing a frequency change, suggest a dynamic role for the endothelium in regulating transvascular transport in vivo.

Onset of pulsatile pressure causes transiently increased filtration through artery wall

Convective fluid motion through artery walls aids in the transvascular transport of macromolecules. Although many measurements of convective filtration have been reported, they were all obtained under constant transmural pressure. However, arterial pressure in vivo is pulsatile. Therefore, experiments were designed to compare filtration under steady and pulsatile pressure conditions. Rabbit carotid arteries were cannulated and excised from male New Zealand White rabbits anesthetized with pentobarbitol sodium (30 mg/kg iv administered). Hydraulic conductance was measured in cannulated excised rabbit carotid arteries at steady pressure. Next, pulsatile pressure trains were applied within the same vessels, and, simultaneously, arterial distension was monitored using Optical coherence tomography (OCT). For each pulse train, the volume of fluid lost through filtration was measured (subtracting volume change due to residual distension) and compared with that predicted from steady pressure measurements. At 60- and 80-mmHg baseline pressures, the experimental filtration volumes were significantly increased compared with those predicted for steady pressure ( P < 0.05). OCT demonstrated that the excess fluid volume loss was significantly greater than the volume that would be lost through residual distension ( P < 0.05). After 30 s, the magnitude of the excess of fluid loss was reduced. These results suggest that sudden onset of pulsatile pressure may cause changes in arterial interstitial hydration.

Increase by adrenaline or angiotensin II of the accumulation of low density lipoprotein and fibrinogen by aortic walls in unrestrained conscious rats

1. Earlier experiments of ours with anaesthetized rabbits showed that infusions of catecholamines into the carotoid blood stream significantly increased the uptake of radioiodinated low density lipoprotein (LDL) by the artery wall after as little as 2 h. This observation has now been extended to much longer time periods, i.e., 6 days, and another species, viz. conscious and unrestrained rats; also to another pressor agent, angiotensin II, as well as to another plasma protein, fibrinogen. 2. Groups of rats were infused from subcutaneously implanted osmotic minipumps for 6 days. The infusions were either into a carotid artery or into the surrounding tissues, with essentially the same effects. Control animals were infused with saline, and test animals with either adrenaline or angiotensin II. The minipump concentration of adrenaline of 4-5 microM, which gave blood concentrations of 25-41 nM, increased the blood pressure significantly after 3 days. The minipump concentration of angiotensin II of 9.9 mg ml-1 was chosen to produce similar increases in blood pressure. 3. Five days after starting the infusion, rats were injected i.v. with either homologous or human LDL labelled with [125I]-tyramine cellobiose (TC), or with [131I]-TC labelled human fibrinogen. Twenty-four hours later, the animals were killed and the radioactivities determined in the whole aorta. The labelled TC radioactivities represent primarily metabolised protein (because TC is trapped intracellularly), but also include the fraction of intact, i.e., non-metabolized protein in transit through the vessel wall. To determine the contribution of the latter, in some experiments we injected double-labelled [131I]-[125TC]-LDL only. These experiments showed that the [131I]-LDL counts representing protein in transit accounted for approx. 20% of the total 125TC counts, and that this percentage was not significantly affected by adrenaline or angiotensin II. Therefore, the bulk of the experiments was carried out with single labelled proteins, using 125I to label TC-LDL and 131I to label TC-fibrinogen. In these experiments, the radioactivity of the arterial wall thus provides a cumulative measure of the uptake and degradation of proteins. 4. Aortic wall radioactivities from rat and human LDL and from human fibrinogen were significantly increased by both agents. Adrenaline at 25-41 nM increased the radioactivities by 52 and 47% for rat and human LDL respectively, and by 31% for human fibrinogen; these differences were highly significant (P < 0.01). Angiotensin II at ca. 10 nM also increased the radioactivities significantly, by 21% for human LDL and by 109% for human fibrinogen (P < 0.05). 5. The results suggest that the accumulation of LDL and of fibrinogen by rat aorta is increased by adrenaline or by angiotensin II at concentrations which raise the blood pressure progressively and significantly after 3 or 5 days respectively.

Arterial mechanics in spontaneously hypertensive rats. Mechanical properties, hydraulic conductivity, and two-phase (solid/fluid) finite element models

To characterize the interaction between mechanical and fluid transport properties in hypertension, we measured in vivo elastic material constants and hydraulic conductivity in intact segments of carotid arteries in normal and spontaneously hypertensive rats (SHR). With the use of a finite element model, the arterial wall was modeled as a large-deformation, two-phase (solid/fluid) medium, which accounts for the existence and motion of the tissue fluid. Measurements of internal diameter and transmural pressures were obtained during continuous increases in pressure from 0 to 200 mm Hg. Strain and stress components were calculated based on a pseudostrain exponential energy density function. To measure the hydraulic conductivity, segments of the carotid artery were isolated, filled with a 4% oxygenated albumin-Tyrode's solution, and connected to a capillary tube. The movement of the meniscus of the capillary tube represented the fluid filtration across the artery. To study the influence of transmural pressure on hydraulic conductivity, measurement of fluid filtration across the arterial wall was obtained at transmural pressures of 50 and 100 mm Hg. The material constants in the SHR (n = 9) were higher (p less than 0.05 for all variables) than in normal rats (n = 10): c = 1,343 +/- 96 versus 1,158 +/- 65 mm Hg, b1 = 1.84 +/- 0.24 versus 1.22 +/- 0.22, b2 = 0.769 +/- 0.114 versus 0.616 +/- 0.11, b3 = 0.017 +/- 0.005 versus 0.0065 +/- 0.002, b4 = 0.206 +/- 0.04 versus 0.083 +/- 0.03, b5 = 0.0594 +/- 0.007 versus 0.0217 +/- 0.006, and b6 = 0.22 +/- 0.09 versus 0.123 +/- 0.02, respectively. The hydraulic conductivity of the total wall, calculated from the filtration data, was lower (p less than 0.05) at both 50 and 100 mm Hg in the SHR (n = 6) compared with normal rats (n = 7): 1.12 +/- 0.31 x 10(-8) and 0.72 +/- 0.23 x 10(-8) versus 1.95 +/- 0.53 x 10(-8) and 1.35 +/- 0.47 x 10(-8) cm/(sec.mm Hg), respectively. The intergroup comparisons between 50 and 100 mm Hg in both SHR and normal rats were also different (p less than 0.05). The finite element model was used to predict tissue fluid pressure distribution, tissue fluid velocity distribution, and total Cauchy stress gradients developed in the arterial wall during fluid pressurization in both species.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of pressure on aortic hydraulic conductance

This study was performed to determine whether the transmural hydraulic conductance (Lp) of the rabbit aortic wall depends on its distension. In 19 rabbits, the aorta was cannulated in situ and perfused at a given pressure with a physiologically buffered solution containing 4% bovine serum albumin. The output cannula was then occluded to limit fluid flow to that traversing the artery wall. External diameter and transmural fluid flow were measured at three pressures (eight rabbits, group 1) or at four pressures (12 rabbits, group 2) in each vessel. Transmural fluid flow was determined by monitoring the velocity of an air bubble within a buffer-filled tube leading to the input cannula. From group 1 measurements, Lp values (mean +/- SD) at 50, 100, and 150 mm Hg were calculated to be 3.8 +/- 2.8, 3.5 +/- 1.3, and 4.1 +/- 1.2 x 10(-8) cm/sec/mm Hg, respectively. Group 2 measurements gave values of 4.2 +/- 1.6, 3.8 +/- 1.1, 3.8 +/- 1.1, and 4.2 +/- 1.1 x 10(-8) cm/sec/mm Hg at 75, 100, 125, and 150 mm Hg, respectively. Paired Student's t tests indicated no significant change in Lp with pressure. However, linear regression analysis demonstrated a weak correlation between Lp values obtained at 50 and 100 mm Hg (r2 = 0.30) and at 75 and 100 mm Hg (r2 = 0.36). Values of Lp at 100 and 150 mm Hg and at 125 and 150 mm Hg were closely correlated in each case. These results suggest that between 50 and 100 mm Hg the structural properties of the aortic wall change so as to alter Lp but not in the same way in each vessel. Lp may increase or decrease depending on which structural change predominates in a particular vessel.

Transport of 125I-albumin across normal and deendothelialized rabbit thoracic aorta in vivo

Transmural concentration profiles of 125I-albumin in vivo were measured across the normal and balloon catheter-deendothelialized rabbit descending thoracic aorta as a function of time following intravenous injection. A tracer was injected 5 or 60 minutes after deendothelialization, and the animals were sacrificed after circulation times of 10, 30 or 60 minutes. The aorta was immediately excised and frozen flat between glass slides. Samples were serially sectioned parallel to the intimal surface in a refrigerated microtome, washed with trichloroacetic acid (TCA), and counted. Relative tissue concentration profiles of TCA-precipitable radioactivity from the media of control animals showed entry from both luminal and adventitial sides, as previously found with conscious normal rabbits, but spatial gradients at both luminal and medial-adventitial borders were less steep. Relative concentration levels in ballooned animals were 10- to 40-fold higher than in controls, and the profiles were flatter. Uptake rates at equivalent circulation times were greater in experiments initiated 60 minutes, as compared with 5 minutes, after deendothelialization, suggesting that progressive medial edema may have occurred following balloon injury. These results show that the intact endothelium is the dominant mass transfer resistance for 125I-albumin transport across the aortic wall. The data also suggest that the incomplete monolayer of platelets adherent to the subendothelium after balloon deendothelialization is not a substantial resistance to transport, as compared to that of the media, and that convection plays a more important role than diffusion for 125I-albumin transport across the deendothelialized aortic wall.