Polymyxin B (PMB), a cyclic polycationic peptide antibiotic, inhibited dose-dependently human platelet aggregation induced by a high level (45μg/ml) of arachidonate (AA) or eicosadienoic acid (C20: 2), whereas this drug had no effect on a low level (0.45μg/ml) of AA-induced platelet responses. Colistin (polymyxin E), a derivative of PMB, had no inhibitory effect on platelet aggregation induced by a high level of AA or C20: 2. When human platelets were stimulated with AA, phosphorylation of 40K and 20K proteins preceded the aggregation response and platelet response by AA correlated closely with the phosphorylation of these proteins. PMB (100μM) inhibited the phosphorylation of 40K and 20K proteins induced by 45μg/ml AA, but not the phosphorylation event induced by 0.45μg/ml AA. PMB also inhibited dose-dependently [Ca2+]i elevation in response to a high level (20μg/ml) of AA, whereas PMB had no effect on a transient increase in [Ca2+]i induced by a low level (0.45μg/ml) of AA. The present results indicate that PMB has no effect on protein phosphorylation systems in human platelets although the drug has been shown to inhibit protein kinase C in vitro, and suggest that PMB selectively inhibit platelet aggregation induced by a high level of AA, possibly by interacting the lipid matrix of platelet membranes.
