Platelets, endothelium and blood vessel wall.

Aggregating platelets cause contraction of vascular smooth muscle, because they release serotonin and thromboxane A2. If the platelets aggregate in a blood vessel with intact intima, the platelet-products cause endothelium-dependent relaxation of the underlying smooth muscle. Hence, the presence of an intact intima considerably reduces the vasospastic response to platelet-aggregation. The major platelet products which trigger endothelium-dependent relaxations are the adenine nucleotides and serotonin. The ability of the endothelium to prevent platelet-induced vasospasm is augmented after chronic intake of cod liver oil, but is reduced after previous intimal injury.