INTRACELLULAR LEVELS OF CYCLIC-AMP AND CYCLIC GMP DIFFERENTIALLY MODIFY PLATELET AGGREGATE SIZE IN HUMAN PLATELETS ACTIVATED WITH EPINEPHRINE OR ADP

We investigated the effects on human platelet aggregation of several a gents that increase either intracellular cyclic AMP or cyclic GMP, usi ng a platelet aggregometer that allows quantification of the size and number of platelet aggregates. During the initial phase of aggregation induced by epinephrine and ADP, small aggregates consisting of <100 c ells predominated; large aggregates formed later. Prostaglandin I-2 (P GI(2)) which increases intracellular cyclic AMP, suppressed the format ion of small as well as large aggregates induced by epinephrine, with ID50 values of 10.7 +/- 2.8 and 3.8 +/- 0.5 nM, respectively. ADP-indu ced formation of small and large aggregates was also inhibited by PGI( 2), with similar ID50 values. Dibutyryl cyclic AMP (db cyclic AMP), a cell-permeant form of cyclic AMP, also inhibited small and large aggre gate formation induced by epinephrine or ADP, with ID50 values of 420- 560 mu M for Small aggregates and 139-166 mu M for large aggregates, r espectively. On the other hand, nitroprusside, which increases intrace llular cyclic GMP, inhibited only the formation of large aggregates, w ith an ID50 value of 454 +/- 191 nM for epinephrine-induced activation and of 2.1 +/- 0.6 mu M for ADP-induced activation. Nitroprusside at 1 mM did not affect the formation of small aggregates induced by epine phrine, whereas that of large aggregates was completely blocked at 10 mu M 8-Bromo cyclic GMP (8-br cyclic GMP) also inhibited only the form ation of large aggregates, with ID50 values of 140-170 mu M, but not t hat of small aggregates induced by epinephrine and ADP. Milrinone, whi ch increases the intracellular level of both cyclic AMP and cyclic GMP , suppressed the formation of small and large aggregates induced by ep inephrine and ADP. These findings suggest that cyclic AMP and cyclic G MP differentially modify the size of aggregates formed during epinephr ine or ADP activation.