Platelets are able to secrete SP from internal stores following activation. Levels of SP-like immunoreactivity released from platelets were determined following stimulation with the GPVI collagen receptor agonist convulxin (Cvx), and thrombin using an SP immunoassay. Values shown are mean percent of total SP released (7.08 ± 1.65 ng/1010 platelets) ± SEM (n = 3).