1. Drugs that cause vasodilatation tend to inhibit platelet behaviour whereas compounds that cause vasoconstriction stimulate platelet behaviour. Thus there appears to be a similarity between platelet behaviour and vascular tone. To investigate this relationship we have compared the maximum vasoconstrictor response in the forearm induced by cold stimulation with some aspects of platelet behaviour.
2. In 26 normal volunteers there was a close correlation between the maximum vasoconstrictor response to cold and the threshold concentration of sodium arachidonate that was needed to induce platelets from each individual to aggregate and to undergo a release reaction (r = 0.774, P<0.001).
3. Infusion of epoprostenol (prostacyclin) in five volunteers altered both the maximum vasoconstrictor response to cold and the threshold concentration of sodium arachidonate needed to induce platelet aggregation in a manner parallel to this relationship.
