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

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Chapter | 10 Hemostasis
TABLE 10-2 Major Platelet Ligands, Integrin/Receptors, and Their Physiological Effects
Integrin/Receptor Major Ligand Effect
α 2 β 1 Collagen Platelet adhesion to ECM
↑ effectiveness GPVI receptor
GPVI Collagen Platelet adhesion to ECM
Activation of PLC r 2
GP1b-IX-V vWF Up-regulation of α 2 β 1 receptor
↑ intracellular calcium
Up-regulation of GPIIb-IIIa receptors
GPIIb-IIIa Fibrinogen Calcium mobilization
Protein kinase activation
Cytoskeleton rearrangement
Platelet aggregation
PAR-1 Thrombin Activation of G q receptor → PLC β 2 activation
PAR-4
Activation of G i receptor → ↓ adenylyl cyclase activity
P2Y 1 ADP Activation of G q receptor → PLC β 2 activation
Activation of MLCK → cytoskeleton rearrangement
P2Y 12 ADP Activation of G i α receptor → inhibition of adenylyl cyclase
P2X 1 ADP ↑ intracellular calcium
PAFR PAF Activation of PLA 2
↓ adenylyl cyclase activity
Abbreviations: ADP, adenosine diphosphate; ECM, extracellular matrix; MLCK, myosin light chain kinase; PAF, platelet activating factor; PAR, protease activated
receptor; PLC, phospholipase C; vWF, von Willebrand factor.
mitochondrial changes consistent with apoptosis ( Pereira
et al ., 2002 ).
2. Platelet Structure
Mammalian platelets circulate as discoid, anucleate subcellular
particles that contain several types of organelles
including α -granules, dense granules, mitochondria, and
lysosomes ( Triplett, 2000 ). The majority of mammalian
platelets exhibit an extensive canalicular system that is
continuous with the plasma membrane, also known as the
open-canalicular system (OCS). The OCS not only plays
an essential role in platelet adhesion and aggregation but
it also facilitates the two-way exchange of biological compounds
between the platelet interior and the surrounding
plasma ( Escolar and White, 1991 ). In platelets of domestic
cattle, horses, and Asian elephants, there is no evidence
for the presence of an OCS. Despite this major ultrastructural
difference, the biochemical mechanisms involved in
platelet aggregation in these species are generally similar
to those of other mammalian species ( Bondy and Gentry,
1988 ; Cheryk et al ., 1997 ; Gentry et al ., 1989 ; Segura
et al ., 2006 ). The biochemical mechanisms required for
normal platelet function are tightly regulated. Multiple
positive feedback reactions facilitate rapid adhesion and
aggregation at sites of vascular injury, whereas negative
feedback responses modulate the continuation of platelet
activation and thus regulate the extent of thrombus formation
once wound healing has been initiated.
Several types of membrane proteins and phospholipids
are essential for platelet adhesion and aggregation ( Tables
10-2 and 10-3 ). Integrins are heterodimeric ( α β ) type 1
transmembrane receptors in which each subunit typically
contains a large extracellular domain and a short cytoplasmic
tail composed of 20 to 60 amino acids. Integrins, like
the membrane G-protein coupled receptors (GPCR), can
be viewed as “ two-faced ” receptors. In the case of integrins,
the face oriented to the platelet exterior interacts
with ligands on the ECM, and the face internally oriented
interacts with platelet cytoplasmic proteins. Ligand binding
to either face can trigger information transfer across
the plasma membrane to initiate biochemical responses
at the other face. This type of cross-membrane signaling
is referred to as “ outside-in ” and “ inside-out ” signaling
depending on the direction of information transfer
( Barkalow et al ., 2003 ; Shattil and Newman, 2004 ).
Current knowledge of integrin biochemistry has come primarily
from the investigation of human and murine platelets
and the impaired aggregation responses in “ knockout ”
mice. The major integrins involved in platelet adhesion and
the stabilization of the primary hemostatic plug are the glycoproteins
GPIIb-IIIa (also termed α IIb β 3 ), α 2 β 1 , the glycoprotein
complex GP1b-V-IX, and GPVI, which is a member
of the Ig receptor superfamily ( Boudreaux, 1996 ; Gibbins,