Mediators of Increased Permeability
Vascular permeability to both water and solutes can be increased by inflammatory mediators released by neurons (e.g., substance P), mast cells (e.g., histamine), and platelets (e.g., ATP). These agonists act through stimulation of seven transmembrane domain G-protein coupled receptors, resulting in phospholipase C-b activation, inositol 1,4,5-trisphosphate production, release of Ca2+ from intracellular stores, and subsequent Ca2+-mediated Ca2+ entry across the cytoplasm. This Ca2+ increase is common to many agonists that increase Ps. It leads to a series of intracellular signaling events, the relative roles of which are still being established. It appears that activation of nitric oxide synthesis, production of NO, and subsequent activation of guanylyl cyclase is critical in many agonist-mediated increases in both Lp and P .
Other factors that increase permeability include growth factors such as VEGF that act through receptor tyrosine kinases and result in activation of phospholipase C-g, and hence diacylglycerol production and Ca2+ entry but in this case, independently of Ca2+ stores.
Ps to small and intermediate-sized solutes, such as K+ and glucose-sized molecules, can be increased by shear stress. Ps to albumin and Lp are not affected. These increases in permeability are NO dependent and can be abolished by increasing cAMP concentrations. Recent evidence has implicated the activation of NO through a VEGF-R2 mediated signaling pathway possibly involving an integrin-mediated activation of Src.
There are a number of ultrastructural alterations in endothelial cells in vessels in which permeability is increased. These include intercellular gaps (between separate endothelial cells), transcellular gaps (through individual endothelial cells), induction of fenestrations, collections of vesicle-like structures known as vesiculo-vacuolar organelles (VVOs), vacuole formation, and reduction in cleft length and tight junction overlap. Each of these ultrastructural events has been linked to increased permeability stimulated by one or more agonists, but the molecular mechanisms regulating these are not yet clear.
The molecules that contribute to the formation of the cleft—the occludins in the tight junction, and the cadherins in the adherens junctions—can be regulated by anchoring proteins on the cytoplasmic side of the membrane. These proteins, such as ZO-1 and the catenins, are phosphorylated in response to many permeability enhancing agonists. Furthermore, prevention of their phosphorylation can prevent permeability increases in some model systems—particularly in vitro. Although there is some evidence that this may also be true in vivo, the significance of phosphorylation of these proteins is still not clear.
Endothelial gaps were for many years thought to be intercellular, indicating a breakdown in the cleft. However, in the vast majority of these studies only single sections have been examined using transmission electron microscopy or scanning electron microscopy. Serial section reconstruction is necessary to prove that a gap is between rather than through the cell. When serial sections have been carried out, both intercellular and transcellular gaps have been shown, and the relative frequency of these appears to alter with the agonist. Some agonists, such as VEGF, result primarily in intra-cellular gaps, whereas others, such as substance P, appear to create intercellular gaps.
Fenestrations, Vacuoles, and Vesiculovacuolar Organelles
The induction of fenestrations by release of paracrine factors has been shown in response to VEGF, but does not appear to be a widespread mechanisms for inducing increased permeability. The induction of clusters of vesicles (or caveolae), or the formation of large vacuoles that span the cell occurs in response to VEGF and in tumor vessels, presumably formed from a Ca2+-mediated fusion of vesicles to form continuous pathways through the cell. Ferritin tracer experiments show that these caveolae are open to perfusing substances. VVOs, vacuoles, fenestrations, and transcellular gaps may all be part of a single cascade, resulting from Ca2+-mediated activation of vesicle fusion.
Hydraulic conductivity (Lp): The convective permeability of the vessel wall to fluid flow. Can be defined as the filtration rate per unit pressure difference per unit area. Dependent on relative viscosity of fluid as it flows through the cleft, the functional pore radius (to the power 4), and the path length.
Oncotic reflection coefficient (a): The ratio of the osmotic pressure exerted by a molecule across the vascular wall to that exerted by the solute across an ideal semipermeable membrane. is a function of the solute area relative to the pore area, or the true solute concentration in the pore relative to the free concentration.
Solute flux (/s): The rate of solute movement across the vessel wall. This is driven by both convection (dependent on filtration rate, reflection coefficient, and plasma solute concentration), and diffusion (dependent on the concentration gradient, the surface area, and the solute permeability).
Solute permeability (P.): The diffusive permeability to a solute. Can be defined as the diffusive solute flux per unit concentration gradient per unit area for small solutes. Depends on the restricted diffusion coefficient, the equilibrium partition coefficient, the mean pore area, and the path length.
Adamson, R. H., and Michel, C. C. (1993). Pathways through the intercellular clefts of frog mesenteric capillaries. J. Physiol. (Lond.) 466, 303-327. The first, and still most clear, description of the tight junctional strands in the endothelial cell cleft.
Bates, D. O., Hillman, N. J., Williams, B., Neal, C. R., and Pocock, T. M. (2002). Regulation of microvascular permeability by vascular endothelial growth factors. J. Anat. 200, 581-597. A comprehensive review of how vascular growth factors increase permeability.
Feng, D., Nagy, J. A., Pyne, K., Hammel, I., Dvorak, H. F., and Dvorak, A.M. (1999). Pathways of macromolecular extravasation across microvascular endothelium in response to VPF/VEGF and other vasoactive mediators. Microcirculation 6, 23-44. A review of pathways through the cleft and the article that crystallizes the unifying hypothesis of transcellular transport during endothelial activation.
Levick, J. R., and Smaje, L. H. (1987). An analysis of the permeability of a fenestra. Microvasc. Res. 33, 233-256.
Michel, C. C., and Curry, F. E. (1999). Microvascular permeability. Physiol. Rev. 79, 703-761. This is the essential description of vascular permeability states. It describes, in close detail and a concise and easy-to read-style, what permeability is and how it is regulated.
Renkin, E. M., and Tucker, V. L. (1998). Measurement of microvascular transport parameters of macromolecules in tissues and organs of intact animals. Microcirculation 5, 139-152. A review of how to correctly measure vascular permeability.
Dr. David Bates is a British Heart Foundation Lecturer, University of Bristol Senior Research Fellow, and Director of the Microvascular Research Laboratories in the Department of Physiology at the University of Bristol, United Kingdom. He received his Ph.D. in Physiology from the university of London and postdoctoral training in the Department of Genetics, University of Glasgow, and Human Physiology, University of California at Davis. He was a lecturer in Medicine at the University of Leicester, before moving to Bristol in 1999. His research focuses on the vascular endothelial growth factors and their effects on permeability, angiogenesis, and lymphangiogenesis.
1. Michel, C. C., and Phillips, M. E. (1987). Steady-state fluid filtration at different capillary pressures in perfused frog mesenteric capillaries. J. Physiol. 388, 421-435.
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