SELECTIVE-INHIBITION OF COLLAGEN-INDUCED ARACHIDONIC-ACID LIBERATION BY NAPHTHALENE-1-SULFONYL)-1H-HEXAHYDRO-1,4-DIAZEPINE HYDROCHLORIDE (ML-7), A MYOSIN LIGHT-CHAIN KINASE INHIBITOR, IN WASHED RABBIT PLATELETS

Effects of myosin light chain (MLC) kinase inhibitor ML-7 aphthalene-1 -sulphonyl)-1H-hexahydro-1,4-diazepine hydrochloride] and protein kina se C inhibitor H-7 [1-(5-isoquinolinesulphonyl)2-methylpiperazine dihy dro-chloride] on collagen-induced platelet activation were investigate d in washed rabbit platelets. Upon stimulation with collagen (1 mu g/m L), H-7 decreased protein kinase C-mediated pleckstrin phosphorylation , but had no inhibitory effect on thromboxane (TX) A(2) formation or p latelet aggregation. In contrast, ML-7 produced a concentration-depend ent inhibition of the collagen-induced platelet aggregation and TXA(2) formation by preventing arachidonic acid (AA) liberation from membran e phospholipids. However, ML-7 had little effect on AA liberation indu ced by thrombin, Ca2+ ionophore A-23187 or melittin, suggesting that M L-7 may affect the signal transduction pathway specific for collagen-i nduced AA liberation, without direct inhibition of phospholipase A, ac tivity. In indomethacin-treated platelets, collagen caused MLC phospho rylation and AA liberation in the absence of a significant increase in intracellular Ca2+ concentration ([Ca2+](i)) or protein tyrosine phos phorylation. ML-7 inhibited both MLC phosphorylation and AA liberation induced by collagen in indomethacin-treated platelets. These results demonstrate that MLC phosphorylation and AA liberation are early event s detectable in collagen-stimulated platelets, and suggest that ML-1 i nhibits these early steps of collagen-induced signal transduction path way in rabbit platelets. (C) 1997 Elsevier Science Inc.