The synergistic effect of serotonin and epinephrine on the human platelet at the level of signal transduction.

Simultaneous addition to platelets of submaximal amounts of excitatory agonists acts synergistically in provoking secretory and aggregatory responses. By measuring changes in intracellular free Ca2+ concentration, inositol phospholipid metabolism and protein phosphorylation, we verified whether synergism could be evidenced at the level of signal transduction. Challenging platelets with epinephrine only induced minor changes on the measured parameters. However, when added together with serotonin, epinephrine amplified mobilisation of intracellular Ca2+, PA formation, PIP formation, protein kinase C and myosin light chain kinase activity as compared to the alterations induced by serotonin alone. It is concluded that synergistic effects on simultaneous addition of serotonin and epinephrine might originate at the level of signal transduction.