Epinephrine potentiates calcium mobilization and activation of protein kinases in platelets stimulated by ADP through a mechanism unrelated to phospholipase C

ADP, added to suspensions of aspirinized 32P-labelled washed platelets, induced reversible platelet aggregation, the rapid elevation of cytosolic Ca 2+ (maximum at 2 s), 20 kDA myosin light chain phosphorylation (maximum faster than 3 s), 40 kDa protein phosphorylation (maximum at 3–10 s) and phosphatidic acid formation (maximum at 30 s). Prior addition of epinephrine potentiated platelet aggregation, cytosolic Ca 2+-elevation, 20 and 40 kDa protein phosphorylation evoked by ADP, but it did not enhance phosphatidic acid formation induced by ADP. The potentiating effect of epinephrine on aggregation, cytosolic Ca 2+-increase and 20 and 40 kDa protein phosphorylation induced by ADP was also observed in the presence of EGTA. Ethylisopropylamiloride, an inhibitor of Na +/H +-exchange, did not affect the potentiation of ADP-induced platelet aggregation by epinephrine. We conclude that epinephrine primes platelets to increase Ca 2+-influx and Ca 2+-immobilization in response to ADP. The potentiation of cytosolic Ca 2+-elevation by epinephrine leads to further stimulation of myosin light chain phosphorylation and protein kinase C activation and ultimately to enhanced platelet aggregation. These effects of epinephrine do not seem to take place at the level of phospholipase C.