Concentrations Of cAMP Phosphodiesterase Inhibitors That Block The ADP-Induced Shape Change Of Rabbit Platelets Do Not Increase Platelet cAMP

S C T Lam; M A Guccione; M A Packham; J F Mustard

doi:10.1055/s-0038-1652408

Thrombosis and Haemostasis, Table of Contents

Thromb Haemost 1981; 46(01): 149
DOI: 10.1055/s-0038-1652408

Platelets – XIII: Inhibitors, Activators, Chemiluminescence

Schattauer GmbH Stuttgart

Concentrations Of cAMP Phosphodiesterase Inhibitors That Block The ADP-Induced Shape Change Of Rabbit Platelets Do Not Increase Platelet cAMP

S C T Lam

Dept. of Biochem., University of Toronto, Toronto, Canada, and Dept. of Pathol., McMaster University, Hamilton, Canada

,

M A Guccione

Dept. of Biochem., University of Toronto, Toronto, Canada, and Dept. of Pathol., McMaster University, Hamilton, Canada

,

M A Packham

Dept. of Biochem., University of Toronto, Toronto, Canada, and Dept. of Pathol., McMaster University, Hamilton, Canada

,

J F Mustard

Dept. of Biochem., University of Toronto, Toronto, Canada, and Dept. of Pathol., McMaster University, Hamilton, Canada

› Author Affiliations

Abstract

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When the cyclic AMP (cAMP) concentration in platelets is increased, platelet functions are inhibited. It is generally assumed that inhibitors of cAMP phosphodiesterase inhibit platelet reactions by increasing cAMP, but this is not established. We have studied the effects of cAMP phosphodiesterase inhibitors on ADP-induced platelet shape change, nucleoside diphosphokinase (NDK) activity, and cAMP concentrations. Papaverine (0.08 mM), dipyridamole (0.2 mM), caffeine (10 mM), or theophylline (8 mM) prevented the shape change of washed rabbit platelets induced by 1 μM ADP. We showed previously that PGEi, which increases cAMP, inhibits platelet NDK activity. Therefore we investigated the effects of cAMP phosphodiesterase inhibitors on platelet NDK activity and cAMP. At concentrations that prevented ADP-induced shape change, papaverine and dipyridamole had no effect on the formation of ¹⁴C-ATP from ¹⁴C-ADP by washed rabbit platelets. The methylxanthines partially inhibited NDK activity of washed rabbit platelets and of isolated membranes, probably due to the structural similarity between the adenine ring of ADP and these substances. However, it seems unlikely that these substances exert their inhibitory effects through interference with platelet NDK. None of these phosphodiesterase inhibitors increased platelet cAMP above basal levels, measured both by a protein binding assay and by prelabeling the platelet adenine nucleotides by incubation with ¹⁴C-adenine. When adenylate cyclase was stimulated with PGE₁ (1.2 μM), the cAMP concentration was increased from 7.8 to 27.2 pmol/10⁸ platelets, and in the presence of phosphodiesterase inhibitors,the cAMP concentration was greater than 50 pmol/10⁸ platelets at 90 sec. Since the phosphodiesterase inhibitors by themselves had no detectable effect on cAMP at concentrations that inhibit ADP-induced shape change, it seems likely that they act through other mechanisms. Whether or not they all act in the same way remains to be determined.

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