title: Mepacrine-induced inhibition of human platelet cyclic-GMP phosphodiesterase.
abstract: The effect of mepacrine (DL-quinacrine-HCI), a specific inhibitor of phospholipase C, on cyclic-GMP levels in human platelets was investigated. The concentrations of mepacrine producing 50% inhibition of human platelet aggregation induced by 5 microM ADP and 3 micrograms/ml of collagen were 50 +/- 8 and 70 +/- 15 microM, respectively. Addition of mepacrine to human platelet suspension resulted in increases in cyclic GMP. In contrast to cyclic-GMP levels, cyclic-AMP content was not affected by mepacrine. Mepacrine did not stimulate guanylate cyclase, but did specifically inhibit human platelet cyclic-GMP phosphodiesterase, separated from cyclic-AMP phosphodiesterase or other forms of phosphodiesterase on DEAE-cellulose columns. Stimulation by cyclic GMP of human platelet cyclic-GMP-stimulated cyclic-AMP phosphodiesterase activity was not inhibited by mepacrine. The IC50 value of the drug for cyclic-GMP phosphodiesterase was 40 microM, and IC50 for cyclic-AMP phosphodiesterase was 1.2 mM. Mepacrine was 30-times more potent as an inhibitor of human platelet cyclic GMP than of cyclic-AMP phosphodiesterase. Mepacrine blocks arachidonate release from human platelets by inhibiting phosphatidylinositol-specific phospholipase C. The increase in cyclic-GMP levels produced by addition of mepacrine will explain part of the pharmacological action of this drug.
mesh_terms: D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000037:antagonists & inhibitors / Q000097:blood*; D000244:Adenosine Diphosphate / Q000494:pharmacology; D000262:Adenylyl Cyclases / Q000097:blood; D001792:Blood Platelets / Q000201:enzymology*; D006162:Guanylate Cyclase / Q000097:blood; D006801:Humans; D007700:Kinetics; D010974:Platelet Aggregation; D011796:Quinacrine / Q000494:pharmacology*
chemical_list: D000244:Adenosine Diphosphate; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D000262:Adenylyl Cyclases; D006162:Guanylate Cyclase; D011796:Quinacrine