Effect of cations on the blood platelet release reaction.

1. Stimulus-secretion phenomena in blood platelets incubated with trypsin or thrombin have been investigated.2. Secretion occurs in at least two stages: (a) induction, obtained when the platelets are incubated with proteolytic enzymes in the presence of the chelating agent EDTA, and (b) release, which occurs with the subsequent addition of Ca(2+) to the incubation medium.3. Ca(2+) was the only essential divalent cation required for release. Ba(2+) and Mg(2+) inhibited release and Sr(2+) could only partially substitute for Ca(2+).4. Blood platelets exhibited recovery from enzymic perturbation by trypsin or thrombin by a mechanism that was temperature dependent, but did not require the synthesis of new protein.5. The results are discussed in relationship to stimulus-secretion in other cells.