Relaxing Factor in Extracts of Blood Platelets and its Function in the Cells

THE known release of 5-hydroxytryptamine from blood platelets exposed to thrombin1 has led to further investigations in this laboratory concerning the mechanism of action of the enzyme on the cells. Recently published results of these investigations2 show that a number of intracellular constituents, including 5-hydroxytryptamine, are simultaneously released from the cells through the operation of a sequential reaction system for which the term ‘release reaction of the platelets’ was suggested. In this process a proteolytic reaction catalysed by thrombin represents the first step. It is followed by a reaction which requires calcium ions. Observations indicate that the final step consists in active contraction of the platelet, by which water and solutes are extruded into the surrounding medium.