Summary
The platelet dense tubular system, an internal smooth endoplasmic reticulum membrane
system occupies a pivotal position in the initiation and modulation of platelet activation.
The best available evidence suggests platelet prostaglandin and thromboxane synthesis
and an internal calcium store critical to platelet activation are both found in the
platelet dense tubular system. Studies of the structural, physiologic and chemical
properties of thromboxane A2 support the concept that this product of platelet prostaglandin synthesis acts to
carry the calcium from the dense tubular system into the cytoplasm where the calcium
is released to initiate contraction of the platelet contractile proteins. Evaluation
of platelet phospholipase A2 suggests that calcium may be critical to activation of this enzyme. Two of the steps
occurring during platelet activation which appear to require calcium 1) phospholipase
A2 activation and 2) internal contraction may be linked up in series with the thromboxane
A2 acting as the critical carrier of calcium from one location to the other.
