Mechanism of inhibition of platelet aggregation by HCL-31D.

The antiplatelet effect of the pyridazinone analogue, 4, 5-dihydro-6-[4-[2-hydroxy-3-(3,4 dimethoxybenzylamino)propoxy]naphth-1-yl]-3(2H)-pyridazinone (HCL-31D), was investigated in vitro with rabbit platelets. HCL-31D dose-dependently inhibited the platelet aggregation and ATP release induced by collagen (10 microg/ml), arachidonic acid (100 microM) or thrombin (0.1 U/ml) with an IC(50) of about 0.95-5.41 microM. HCL-31D (0.5-5 microM) increased the platelet cyclic AMP level in a dose-dependent manner. Furthermore, HCL-31D potentiated cyclic AMP formation caused by prostaglandin E(1) but not that caused by 3-isobutyl-1-methylxanthine (IBMX). HCL-31D also attenuated phosphoinositide breakdown and intracellular Ca(2+) elevation induced by collagen, arachidonic acid or thrombin. HCL-31D inhibited the formation of thromboxane B(2) induced by collagen or thrombin but not by arachidonic acid. In addition, HCL-31D did not affect platelet cylooxygenase and thromboxane synthase activity. These data indicate that HCL-31D is an inhibitor of phosphodiesterase and that its antiplatelet effect is mainly mediated by elevation of cyclic AMP levels.