The platelet dense tubular system: its relationship to prostaglandin synthesis and calcium flux

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.