Anti-thrombotic effect of milrinone is caused by inhibition of calcium release from the dense tubular system in human platelets.

Abstract

Milrinone, a phosphodiesterase III inhibitor, exerts positive inotropic effects which induce an increase in the intracellular calcium concentration by raising the cyclic adenosine monophosphate level in cardiac muscle. Milrinone was also reported to inhibit platelet aggregation, however, its mechanism remains unknown. Therefore, we investigated the effects of milrinone on intracellular calcium mobilization when platelets were activated. Washed platelets, obtained from six healthy volunteers, were preincubated with milrinone (0.9 micro M) for 1 min and then exposed to 0.015 i micro ml-1 thrombin for 5 min. The effect of milrinone on changes in the intracellular calcium level using a fluorescent dye, fura-2, was also observed. Calcium mobilizations via plasma membrane calcium channels and the dense tubular system were assessed differentially. Milrinone (0.9 micro M) significantly suppressed the aggregation ratios at 5 min compared with those in controls (86+/-5%) to 75+/-8%. The increase in the intracellular calcium concentration was also significantly suppressed (controls, 915+/-293 nM vs. 405+/-240 nM) when stimulated by thrombin. Milrinone also significantly inhibited the release of calcium from the dense tubular system (controls, 284+/-111 nM vs. 158+/-51 nM). Calcium influx through the plasma membrane was suppressed by milrinone 2.4 micro M. Milrinone (0.9 micro M) inhibited thrombin-induced platelet aggregation. This inhibitory effect was mainly mediated by suppressing calcium release from the dense tubular system.