Effect of minoxidil on platelet function and the synthesis of prostaglandins in platelets.

At the 12.5 micrograms level, minoxidil prevents the irreversible aggregation of platelets by 2 x 10(-6) mol/L adenosine diphosphate (ADP). Levels of minoxidil greater than 12.5 micrograms cause a reversal of primary aggregation by 2 x 10(-6) mol/L ADP. Aggregation of platelets in response to 125 micrograms of arachidonic acid is measurably reduced by 12.5 micrograms of minoxidil and totally suppressed by 30 micrograms. Concurrent with the inhibition of platelet aggregation, increasing concentrations of minoxidil cause a gradual reduction in the synthesis of prostaglandin E2 (PGE2) and thromboxane B2 (TxB2). In the presence of 100 micrograms of minoxidil, PGE2 is reduced from a control value of 87.7 +/- 2.2 pg/ml to 23.9 +/- 3.2 pg/ml. At this level of minoxidil, TxB2 drops from 105 +/- 3.3 ng/ml to 10.5 +/- 2.6 ng/ml. The effect of minoxidil on platelet aggregation is not associated with increased cyclic adenosine monophosphate synthesis. All data support the conclusion that minoxidil functions (in platelet metabolism) primarily as a cyclooxygenase inhibitor.