Rapid effects of phorbol ester on platelet shape change, cytoskeleton and calcium transient

Platelets are rapidly activated by several agonists. When phorbol 12-myristate 13-acetate (PMA) was added to washed platelet suspensions 10 s prior to either thrombin or ADP, it caused a dose-dependent inhibition of shape change correlated with decreased myosin association with the cytoskeleton and with inhibition of the calcium transient measured in fura-2-loaded platelets. PMA added 5-10 s after agonists did not reverse shape change or the association of myosin with the cytoskeleton, but markedly increased the rate at which the calcium signal returned to the baseline. The analogue, 4 alpha-phorbol didecanoate did not cause these effects. Our results suggest that one effect of C-kinase activation is to provide negative feedback in sequential responses.