PLATELET-AGGREGATION AND FIBRINOGEN BINDING IN HUMAN, RHESUS-MONKEY, GUINEA-PIG, HAMSTER AND RAT-BLOOD - ACTIVATION BY ADP AND A THROMBIN RECEPTOR PEPTIDE AND INHIBITION BY GLYCOPROTEIN-IIB IIIA ANTAGONISTS/

Platelet aggregation and fibrinogen binding in whole blood, induced ei ther by ADP or by a 14 amino acid peptide mimicking an N-terminal regi on of the platelet thrombin receptor (TRP, thrombin receptor activatin g peptide), have been studied with blood from different species. Aggre gation was assessed by counting the number of single platelets in bloo d before und after addition of the agonist with an automated cell coun ter. Both ADP (0.02-0.5 muM) and TRP (1-10 muM) were found to be poten t agonists of platelet aggregation in human, rhesus monkey and guinea- pig blood, causing a near-maximal aggregatory response within 2 min of agonist addition. In contrast, hamster and rat platelets were much le ss responsive to both ADP and TRP in terms of the whole blood aggregat ion. Echistatin, RGDW and a synthetic glycoprotein (GP) IIb/IIIa antag onist, Ro 43-8857, inhibited fibrinogen binding to purified immobilize d human GP-II/IIIa with IC50's of 1.6, 88 and 11.4 nM, respectively. I n whole human blood, the respective IC50's (as determined by flow cyto metric analysis of platelet fibrinogen binding) were 4.4, 1700 and 29. 5 nM. The affinities of these three compounds for inhibiting fibrinoge n binding in whole blood from rhesus monkeys and guinea-pigs were simi lar to the affinities for human platelets, but with rat blood echistat in, RGDW and Ro 43-8857 were all around 100-fold less potent. Ro 43-88 57 was a potent inhibitor of ADP- or TRP-induced platelet aggregation in human, rhesus monkey and guinea-pig whole blood (IC50 of 69-320 nM) but was much less active in hamster blood. These results highlight im portant species differences in the response of platelets to activation by two different agonists and also in their inhibition by GP-IIb/IIIa antagonists. In particular, platelets from the rat and hamster were i nsensitive to agonists and antagonists, whereas guinea-pig and rhesus monkey platelets responded with an affinity similar to human platelets . Since these studies were performed in whole blood, the results shoul d be representative of those expected in animal experiments. These rec ently developed methods for studying platelet responses in small aliqu ots of whole blood are simple to perform and provide important informa tion concerning the optimal choice of species for subsequent in vivo s tudies with these compounds.