Vascular Endothelium - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 525

9.6.4.5 Extracellular Constituents

Endothelia are covered by the glycocalyx (thickness 20-3,000 nm depending
on method and vessel type), a meshwork of interacting proteins, glycoproteins,
proteoglycans, and glycolipids, at their luminal surface and rest on a basement
membrane (thickness 40-60 nm) at their abluminal edge. Both structures influence
transendothelial transport. In addition, endothelial cells sense and respond to signals
from blood and neighboring cells that regulate transendothelial transport.
Endothelial permeability increases during inflammation, healing, and angio-
genesis. Many agents, such as platelet-activating factor, thrombin, and vascular
endothelial growth factor, raise endothelial permeability by increasing intercellular
cleft size, i.e., damaging intercellular junctions. Histamine and serotonin provoke
gaps (width 1-8 nm) between venular endothelial cells [ 854 ].
Matrix metallopeptidases and plasminogen activators such as urokinase-type
plasminogen activator degrade the extracellular matrix for cell migration and release
of sequestered growth factors during healing and angiogenesis.
Focal adhesion (Sect. 9.1.2.2 ), a specialized array of integrins, anchors the
endothelial cell to the extracellular matrix. Because focal adhesions are connected
to actin filaments, they transmit tension generated by actomyosin filaments to
the extracellular matrix and conversely. Focal adhesions also serve as signaling
platforms, the activity of which is mechanosensitive. Permeability-increasing me-
diators reorganize punctate focal adhesions into thick foci to strengthen cell-matrix
adhesion.

9.6.4.6 Albumin

Plasma albumin maintains the transendothelial oncotic pressure gradient and
regulates the transport of fatty acids, steroids, thyroxine, and amino acids. Albumin
binding to endothelial surfaces initiates its transcytosis via plasmalemmal vesicles.
Plasmalemmal albumin receptors in vascular endothelia include: (1) secreted
protein, acidic, cysteine-rich protein (SPARC), or osteonectin; (2) 60-kDa albumin-
binding protein located in caveolae (linked to calveolin-1) GP60, or albondin;
and (3) smaller albumin-binding scavenger receptors GP30 and GP18 that target
conformationally modified albumins [ 966 ]. 90
Albumin can be transported in caveolae in free form in the fluid phase or mainly
bound to cell-surface albumin-binding protein that colocalizes with caveolin-1 and

90 Albumin receptors GP30 and GP18, unlike GP60, are expressed by cultured rat fibroblasts,
smooth myocytes, and endothelial cells, whatever the type of endothelial lining, in various
organs (heart, lung, brain, kidney, adrenal, adipose tissue, skeletal muscle, diaphragm, duodenum,
pancreas, and liver).