Background and purpose: Dipyridamole and in particular dipyridamole in combination with low-dose aspirin are very effective in preventing recurrent stroke. However, the mechanism(s) underlying this dipyridamole effect have not been elucidated. Since dipyridamole inhibits the cGMP-specific phosphodiesterase type V in vitro, we hypothesized and tested whether therapeutically relevant dipyridamole concentrations enhance NO/cGMP-mediated effects in intact human platelets studied ex vivo.
Methods: Phosphorylation of vasodilator-stimulated phosphoprotein (VASP), an established marker of NO/cGMP effects in human platelets, was quantified by phosphorylation-specific antibodies and Western blots. Serotonin secretion and thromboxane synthase activity were determined by fluorometric quantification of derivatized serotonin and synthase products, respectively.
Results: Endothelium-derived factors such as NO and prostaglandin I2 are known to elevate both cGMP and cAMP levels with concomitant platelet inhibition and VASP phosphorylation. In our in vitro experiments, therapeutically relevant concentrations (3.5 micromol/L) of dipyridamole amplified only cGMP-mediated VASP phosphorylation due to the NO donor sodium nitroprusside, but not cAMP-mediated effects. Furthermore, thromboxane synthase activity and serotonin secretion, events important for initial platelet activation, were inhibited by sodium nitroprusside, an effect also enhanced by dipyridamole, demonstrating the functional relevance of these observations. Finally, the ex vivo enhancement of NO/cGMP effects was also observed with platelets obtained from healthy volunteers treated with extended-release dipyridamole.
Conclusions: Under therapeutically relevant conditions, dipyridamole enhances platelet inhibition by amplifying the signaling of the NO donor sodium nitroprusside. These data support the concept that enhancement of endothelium-dependent NO/cGMP-mediated signaling may be an important in vivo component of dipyridamole action.