Effects of adenosine analogs and adenine nucleotides on adenosine 5′-diphosphate-induced rat platelet aggregation

Abstract

Various adenosine analogs and adenine nucleotides have been tested as inhibitors of ADP-induced aggregation of rat platelets. The potent inhibitors of human platelet aggregation, adenosine, 2-fluoroadenosine, 2-chloroadenosine, carbocyclic adenosine and N 6-phenyl adenosine, had little effect on rat platelet aggregation (0–30 per cent inhibition). The effects of adenosine or its analogs on ADP-induced aggregation of cross-species platelet-rich plasmas (PRPs) (human platelets suspended in rat plasma or rat platelets in human plasma) were similar to those with the native PRPs, indicating that these species differences were due to intrinsic factors in the platelets and not in the plasma. When these analogs were tested in the presence of the cyclic AMP phosphodiesterase inhibitor papaverine, strong inhibiton of rat platelet ADP-induced aggregation was seen. 2′-Deoxyadenosine and 3′-deoxyadenosine were not inhibitory to ADP-induced aggregation of rat PRP even in the presence of papaverine. Adenosine 5′-tetraphosphate strongly inhibited both human and rat platelet aggregation. AMP, like adenosine, did not inhibit rat platelet aggregation but became strongly inhibitory in the presence of papaverine. This inhibitory effect was abolished by preincubating rat PRP with an adenylate cyclase inhibitor, 2′, 5′-dideoxyadenosine or adenosine deaminase. In the later case, however, if the adenosine deaminase inhibitor 2′-deoxycoformycin was included in the incubation mixture, the inhibition by AMP plus papaverine was similar to adenosine plus papaverine. About 50 per cent of [ 14C]AMP was converted to [ 14 C]adenosine in rat platelet-free plasma or PRP after a 10-min incubation. α,β-Methylene-ADP and β,γ-methylene-ATP (200 μM) inhibited rat platelet aggregation by 50 and 64 per cent, respectively. Cyclic AMP phosphodiesterase of rat and human platelets gave comparable K m , and V max values ( K m 0.53 and 0.21μM and V max 6.0 and 6.7 pmoles/min/10 7 platelets, respectively).