Mechanism of inhibition of platelet aggregation by HCL-31D

Abstract

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 μg/ml), arachidonic acid (100 μM) or thrombin (0.1 U/ml) with an IC 50 of about 0.95–5.41 μM. HCL-31D (0.5–5 μM) 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.