FIG. 3. P-selectin surface expression is calpain-dependent.

A, calpastat, but not calpastat-Ala nor the calpastatin 24-mer, inhibits P-selectin surface expression on gel-filtered platelets. The extent of P-selectin surface expression from SFLLR-treated platelets was determined from the geometric mean of P-selectin fluorescence, measured by flow cytometry (see “Experimental Procedures”). The SFLLR concentration utilized, 50 µm, results in maximal P-selectin expression. B, the dose response curve for inhibition of platelet P-selectin surface expression by calpastat. The IC₅₀ for calpastat inhibition of SFLLR-induced P-selectin presentation on gel-filtered platelets is 20 µm. Measurement of P-selectin surface expression was performed as in A.