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

Abstract

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.