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Abstract
It has been previously proposed that platelet stimulation may involve two platelet-thrombin
complexes: (1) an initial platelet-thrombin complex (P-T) which is converted to (2)
an activated plateletthrombin complex (P∗-T). By using the release of radioactive
serotonin as a measure of thrombin stimulation, we have demonstrated that under appropriate
conditions, a hirudin sensitive and a hirudin insensitive complex can be differentiated.
At short platelet-thrombin preincubation times (0–2 minutes) at 4°C, in a buffer containing
18.7 mM phosphate, added hirudin almost completely inhibited the release of radioactive
serotonin obtained upon subsequent warming to 37°C (only the hirudin sensitive complex
exists). If platelets were ṕreincubated with thrombin for longer periods of time
(30 minutes), hirudin became less effective in inhibiting the release obtained upon
subsequent warming to 37°C. The same results were obtained whether or not the platelets
were washed after incubation at 4°C and before warming to 37°C. We postulate that
this change in hirudin sensitivity may reflect a slow conversion of the first platelet-thrombin
complex (P-T) to an activated platelet-thrombin complex (P∗-T) which can undergo release
upon warming. This transition appears to be much faster in acetatetris buffer since
at short platelet-thrombin incubation times at 4°C, added hirudin had little or no
effect on the release obtained upon warming to 37°C. The difference in the ability
of hirudin to inhibit thrombin-induced release in the two buffers was shown to depend
upon the presence of phosphate and on variations in ionic strength, and not due to
a change in the inhibition constant (Ki) for hirudin.
Keywords
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Article info
Publication history
Accepted:
December 1,
1984
Received:
November 7,
1984
Identification
Copyright
© 1985 Published by Elsevier Inc.