Effect of the scramblase inhibitor R5421 on ionomycin-stimulated microvesicle release. Panel A: Phosphatidylserine migration to the outer leaflet of the cell membrane was assessed by fluorescence spectroscopy as described in Materials and Methods. Cells were treated with ionomycin in the presence or absence of 50 μM R5421 (using DMSO as the control solvent for R5421). Panel B: The 37°C experiment of Figure 1A was repeated with erythrocyte samples incubated for 10 min without or with R5421 prior to addition of ionomycin or DMSO (as control solvent for ionomycin, labeled "Control"). In each case, the data represent the mean ± SEM (n = 13–19). The change in light scatter intensity from the initial minimum to that observed 240 s later (M) was calculated as in Figure 2.