Serotonin secretion from human platelets may be modified by Ca2+-activated, phospholipid-dependent myosin phosphorylation.

1-(5-Isoquinolinesulfonyl)-2-methylpiperazine (H-7), which has been identified as a potent inhibitor of protein kinase C in vitro (Hidaka, H., Inagaki, M., Kawamoto, S., and Sasaki, Y. (1984) Biochemistry, in press), enhanced serotonin release from human platelets that was induced by the 12-O-tetradecanoyl phorbol 13-acetate and correspondingly decreased incorporation of radioactive phosphate into a 20,000-dalton protein. H-7 did not affect the protein phosphorylation or the serotonin secretion in unstimulated platelets. A phosphopeptide with a molecular weight of 20,000 has previously been identified as a light chain (LC20) of platelet myosin and both protein kinase C and Ca2+-calmodulin-dependent myosin light-chain kinase have been shown to be involved in its phosphorylation. Two-dimensional peptide mapping following tryptic hydrolysis revealed that H-7 selectively inhibited the protein kinase C-catalyzed phosphorylation of myosin light chain. This pharmacological evidence suggests that Ca2+-activated, phospholipid-dependent myosin light-chain phosphorylation may play an inhibitory role in the release reaction.