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Different calcium pools in human platelets and their role in thromboxane A2 formation
The Journal of Biological Chemistry
|October 15, 1991
Summary
Human platelets store calcium in two distinct pools. Thapsigargin and other Ca(2+)-ATPase inhibitors reveal these pools, crucial for platelet activation and thromboxane A2 formation.
Area of Science:
- Biochemistry
- Cell Biology
- Hematology
Background:
- Platelet activation involves intracellular calcium increase, a critical signaling event.
- Calcium release from inositol 1,4,5-trisphosphate-sensitive stores and thromboxane A2 formation are key to platelet activation.
- Inhibitors of intracellular Ca(2+)-ATPases, like thapsigargin, can deplete these calcium stores.
Purpose of the Study:
- To investigate the distinct calcium pools within human platelets.
- To elucidate the role of Ca(2+)-ATPase inhibitors in calcium mobilization and platelet activation.
- To understand the interplay between calcium pools, thromboxane formation, and platelet signaling.
Main Methods:
- Utilized Ca(2+)-ATPase inhibitors (thapsigargin, 2,5-di-(tert-butyl)-1,4-benzohydroquinone) to study calcium transients in human platelets.
- Employed EGTA to chelate extracellular calcium and fura-2 for calcium measurements.
- Investigated the effects of thrombin and thromboxane receptor blockade on calcium release.
Main Results:
- Thapsigargin and benzohydroquinone induced different calcium transients, with benzohydroquinone showing inherent cyclooxygenase inhibition.
- Thapsigargin mobilized calcium from a thrombin-responsive store, dependent on an intact platelet self-amplification system.
- Blocking the thromboxane receptor reduced thapsigargin-induced calcium release, while thrombin could still release additional calcium.
- Thrombin pre-treatment did not prevent subsequent thapsigargin-induced calcium release, suggesting distinct pool dynamics.
Conclusions:
- Human platelets possess at least two distinct calcium pools: a rapidly releasable inositol-sensitive pool and a Ca(2+)-ATPase-sensitive pool.
- The inositol-sensitive pool is involved in initial receptor-mediated calcium release and is sensitive to thrombin.
- The Ca(2+)-ATPase-sensitive pool can be refilled and its calcium released independently of thromboxane receptor activation, but is necessary for maximal thromboxane B2 formation.
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