PMA–concentration-dependent responses.

Platelets were labeled with either [³²P]P_i or [³H]5-HT, permeabilized with digitonin, and stimulated with increasing concentrations (1-500 nM) of PMA. [³H]5-HT release and protein phosphorylation were measured. (A) Autoradiography of SDS-PAGE gels of heat-stable platelet extracts (MARCKS) and whole platelet extracts (pleckstrin) is shown. The concentrations of PMA used are indicated at the bottom of the figure. Arrowheads indicate the smallest concentrations of PMA producing a significant increase in the phosphorylation of each protein. (B) PMA–concentration-dependent curves for [³H]5-HT release and pleckstrin and MARCKS phosphorylation are shown. Values represent the mean ± SEM of 6 different platelet preparations.