Effects of peptides MPSD and Ala-MPSD on PKC-induced [³H]5-HT release from permeabilized platelets.

[³H]5-HT–labeled platelets were permeabilized for 5 minutes with 15 μM of digitonin in K⁺-glutamate buffer. Permeabilization was done in the absence or presence of 10 μM of either peptide MPSD or peptide Ala-MPSD. MPSD has a 25–amino acid sequence corresponding to the phosphorylation site domain of MARCKS (A). This is the site for the binding of F-actin and calmodulin (A). In Ala-MPSD, the 4 serine residues of MPSD have been substituted by alanines (A). When indicated, PMA (100 nM) was present in the permeabilization medium during the last 3 minutes. Platelets were recovered by centrifugation and resuspended in the same medium for 45 seconds. At the end of this stimulation period, [³H]5-HT content was measured in the medium and in platelets. [³H]5-HT outputs were expressed as a percentage of total content after subtraction of spontaneous release (B). Each bar represents the mean ± SEM of results obtained from 3 different experiments (bottom). A minimum of 24 samples per condition were measured.