Direct Evidence for the Inhibition of Platelet Aggregation and Release by Intracellular Cyclic AMP Produced with a New Photoactivatable Derivative

 

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Summary

An increase in platelet cyclic AMP (cAMP) via stimulation of adenylate cyclase is thought to be the underlying mechanism by which potent prostaglandins i.e. PGD₂ PGI₂ inhibit platelet functions. We report here new and direct evidence for the inhibitory effects of cAMP on platelet aggregation and serotonin release. Washed platelets from rat were incubated with a new photoactivatable cAMP analogue (4,5-dimethoxy-2-nitrobenzyl ester); this compound is almost physiologically inert before irradiation and liberates free cAMP (“cAMP jumps”) following light flashes. A single flash, delivered after 2 min incubation in 100-200 μM of the analogue, dramatically inhibited thrombin-induced aggregation, as compared with controls. Endogenous serotonin release, measured in the same samples by means of an electrochemically treated carbon electrode was undetectable after the cAMP jump. Pre-irradiated solutions added to platelets had no effect. The kinetics of the flash-induced effects were also studied. From these results we can conclude that: i) the photoactivatable cAMP derivative has to permeate through the platelet membrane; ii) the analogue remains photolabile; and, iii) intracellular cAMP, resulting from photolysis dramatically inhibits platelet aggregation and serotonin release. It is possible that cAMP exerts its effects by regulating cytoplasmic free calcium concentration and/or other actions affecting platelet activation.

• References

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