Synergistic responses and receptor occupancy in rabbit blood platelets.

Several observations indicate that simultaneous receptor occupancy is necessary for generation of a synergistic response of rabbit platelets to two excitatory agonists. First, an aggregatory response to adrenaline induced by prior addition of 5HT reverses rapidly unless stabilised by inhibition of 5HT uptake. The extent of response correlates with the extracellular 5HT concentration. Second, addition of an ADPase following adrenaline enhances the rate of disaggregation if the response to this agonist has been induced by prior addition of ADP. Third, disaggregation is induced, or enhanced, if an antagonist selective to the inducing agonist is added following completion of the induced aggregatory response. These data suggest marked lability in the mechanisms responsible for generation of synergistic responses.