Platelet kinetics in the pulmonary microcirculation in vivo assessed by intravital microscopy

Growing evidence supports the substantial pathophysiological impact
of platelets on the development of acute lung injury. Methods for
studying these cellular mechanisms in vivo are not present yet. The
aim of this study was to develop a model enabling the quantitative
analysis of platelet kinetics and platelet-endothelium interaction
within consecutive segments of the pulmonary microcirculation in
vivo. New Zealand White rabbits were anesthetized and ventilated.
Autologous platelets were separated from blood and labeled ex vivo
with rhodamine 6G. After implantation of a thoracic window,
microhemodynamics and kinetics of platelets were investigated by
intravital microscopy. Velocities of red blood cells (RBCs) and
platelets were measured in arterioles, capillaries and venules, and
the number of platelets adhering to the microvascular endothelium
was counted. Kinetics of unstimulated platelets was compared with
kinetics of thrombin-activated platelets. Velocity of unstimulated
platelets was comparable to RBC velocity in all vessel segments.
Unstimulated platelets passed the pulmonary microcirculation
without substantial platelet-endothelial interaction. In contrast,
velocity of activated platelets was decreased in all vascular
segments indicating platelet margination and temporal
platelet-endothelium interaction. Thrombin-activated platelets
adhered to arteriolar endothelium; in capillaries and venules
adherence of platelets was increased 8-fold and 13-fold,
respectively. In conclusion, using intravital microscopy platelet
kinetics were directly analyzed in the pulmonary microcirculation
in vivo for the first time. In contrast to leukocytes, no
substantial platelet-endothelium interaction occurs in the
pulmonary microcirculation without any further stimulus. In
response to platelet activation, molecular mechanisms enable
adhesion of platelets in arterioles and venules as well as
retention of platelets within capillaries. Copyright (C) 2002 S.
Karger AG, Basel.