Summary
Dipyridamole possesses antithrombotic properties in the animal and in man but it does
not inhibit platelet aggregation in plasma. We evaluated the effect of dipyridamole
ex vivo and in vitro on platelet aggregation induced by collagen and adenosine- 5’-diphosphate
(ADP) in human whole blood with an impedance aggregometer. Two hundred mg dipyridamole
induced a significant inhibition of both ADP- and collagen-induced aggregation in
human blood samples taken 2 hr after oral drug intake. Administration of the drug
for four days, 400 mg/day, further increased the antiplatelet effect. A significant
negative correlation was found between collagen-induced platelet aggregation in whole
blood and dipyridamole levels in plasma (p <0.001). A statistically significant inhibition
of both collagen (p <0.0025) and ADP-induced (p <0.005) platelet aggregation was also
obtained by incubating whole blood in vitro for 2 min at 37° C with dipyridamole (3.9
μM). No such effects were seen in platelet-rich plasma, even after enrichment with
leukocytes. Low-dose adenosine enhanced in vitro inhibition in whole blood.
Our results demonstrate that dipyridamole impedes platelet aggregation in whole blood
by an interaction with red blood cells, probably involving adenosine.
Keywords
Adenosine - Dipyridamole - Impedance aggregometry - Platelet aggregation - Red blood
cells - Whole blood
