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The secretory pathway of bovine platelets
JG White
Human platelets contain tortuous channels in their cytoplasm, the
surface-connected or open canalicular system (OCS), that communicate
directly with the surrounding medium through openings on the surface
membrane. Some workers have suggested that the OCS serves as the egress
route for products secreted during the release reaction. Others have
proposed alternate secretory pathways. Since bovine platelets lack the OCS
found in human cells, the present study has examined the secretory
mechanism of these cells to see whether it can shed light on the mystery of
human platelet secretion. Bovine platelet granules, in contrast to human
granules, are located more peripherally in resting cells (often in contact
with the plasma membrane), most do not move centrally following thrombin
stimulation as do human platelet granules, and many fuse directly with the
external plasma membrane without any intermediate channel. The lack of
peripheral location of human granules, their central rather than peripheral
movement during secretion, and the presence of extensive channels are all
consistent with the larger importance of the secretory channel to human
platelets. Thus, though studies of bovine secretion do show that platelets
can secrete their granules by direct fusion of granule and surface
membranes, other differences from human platelets emphasize that this
pathway, although important to bovine platelet secretion, is less important
in human platelets. Studies of bovine platelets also show that the OCS is
more dynamic than might have been considered from human studies and can
form rapidly in response to stimulation. Such newly formed channels are
used as a conduit for secretion of granule contents. The finding emphasizes
the importance of channels for granule secretion in platelets generally and
puts a new perspective on the ability of these cells to form channels
rapidly in response to stimulation.
Volume 69,
Issue 3,
pp. 878-885,
03/01/1987
Copyright © 1987 by The American Society of Hematology
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