Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 10 Hemostasis
TABLE 10-1 Antithrombotic and Prothrombotic
Properties of Endothelium
Antithrombotic
Properties
Prostacyclin (PGI 2 )
Nitric oxide
Thrombomodulin
Tissue factor pathway
inhibitor (TFPI)
Heparan sulfate
Tissue plasminogen activator
Prothrombotic
Properties
Tissue factor release/expression
von Willebrand factor
Plasminogen activator
inhibitor-1 (PAI-1)
Factor V
Platelet activating factor
P-selectin expression
During homeostasis, endothelial cells promote an anticoagulant
and anti-inflammatory state. This is facilitated
by several factors including the production of prostacyclin
(PGI 2 ), adenosine, and nitric oxide, which act together to
inhibit the association of platelets with the endothelium
and with other platelets ( Becker et al ., 2000 ). The expression
of thrombomodulin on the lumenal surface binds any
thrombin that may be formed and activates protein C to
down-regulate the effects of any factor V (FV) and factor
VIII (FVIII) that may be activated (see Section II.C.5.c) .
Tissue factor pathway inhibitor (TFPI) is synthesized by
endothelial cells and is important for preventing co-localization
of tissue factor with activated FVII (FVIIa), which
would stimulate downstream production of thrombin and
subsequently fibrin (see Section II.C.5.a). Tissue plasminogen
activator (tPA) from endothelial cells activates
plasmin, which results in lysis of any fibrin that is formed
(see Section II.D.3). Proteoglycans such as heparin, heparan
sulfate, and dermatan sulfate inhibit clotting factors
and platelet aggregation. Finally, there is a relative lack
of expression of adhesion molecules (e.g., P selectin) that
would facilitate tethering of platelets to the endothelial surface
( Becker et al ., 2000 ).
The initial reaction to vessel injury is vasoconstriction.
This is a transient effect that minimizes blood flow
to the affected area and is mediated by an autonomic neurogenic
reflex and vasoactive mediators including endothelin.
Within minutes of vascular injury or activation, the
anticoagulant effect can change to a procoagulant milieu
with resultant clot formation. Several factors contribute
to this alteration. The expression of TF on the endothelial
cell luminal surface is usually limited, but if expression is
enhanced, activation of the clotting mechanism can occur.
At the same time, expression of thrombomodulin and heparan
sulfate can be lost, removing an important inhibitor to
fibrin clot formation. Release of von Willebrand factor
(vWF) from endothelial Wiebel-Palade bodies facilitates
the binding of platelets to subendothelial collagen. This
contributes to platelet activation and release of granule contents,
facilitating platelet aggregation and plug formation
(see Section II.B.3.a). Plasminogen activator inhibitor-1
is released, negating the activation of plasmin and thereby
minimizing fibrinolysis (see Section II.D.4.a). Thromboxane
A2 and platelet activating factor (PAF) are released, which
encourage further platelet aggregation and activation.
Endothelial cells also contain FV, which, when available,
greatly amplifies thrombin formation. Additionally, there is
locally enhanced expression of adhesion molecules such as
P-selectin, which promote tethering of platelets to the endothelial
surface. If the endothelial cell becomes apoptotic,
the exposure of phosphatidylserine (PS) on the outer cell
membrane leaflet can support the direct formation of thrombin
because it can act as the phospholipid source for the
prothrombinase complex ( Becker et al ., 2000 ).
Besides traumatic injury, endothelial dysfunction can
occur in various disease states, which may have clinical
consequences on a proper functioning hemostatic system.
Examples include systemic inflammatory mediators such
as tumor necrosis factor (TNF) and interleukin-1 (IL-1),
various systemic viral infections, Gram-negative bacteria,
rickettsial agents, and in people, cholesterol and oxidative
lipoproteins as observed in atherogenesis (see Cullen et al .,
2005 for a thorough review on atherosclerosis). The outcome
of endothelial dysfunction observed in these disease
states includes local or disseminated formation of thrombi,
vascular permeability causing accumulation of extravascular
fluid, and petechiae or hemorrhage.
B. Platelets
Platelet plug formation involves a complex, interrelated
sequence of events that overcome local resistance to platelet
activation long enough to permit the cessation of bleeding. It
is not possible to describe the biochemical events involved
in platelet function, activation, and aggregation in a strictly
temporal sequence because of the numerous positive feedback
reactions, multiple agonists, and complementary intracellular
pathways that function in a cooperative, coordinated
fashion ( Fig. 10-1 ). For simplicity, the process that results
from activation, and leads to platelet plug formation, can be
divided into three major events. The initiating response is
the tethering of platelets to ligands exposed in the perturbed
extracellular matrix (ECM) and subsequent formation of a
platelet monolayer around the site of damage. The platelet
plug grows when additional activated platelets accumulate
on top of this monolayer in a complex series of reactions
involving “outside-in ” and “inside out ” signaling (see
Section II.B.2). These events involve platelet shape change,
the release of granule contents, and the formation of plateletplatelet
aggregates that initiate contact-dependent signaling