Fig. 1. Estimated digestive vacuole pH (pH_DV) of mature trophozoite-stage D10' (CQS, open bars) and 7G8' (CQR, filled bars) parasites as determined using either (A) Fluorescein or (B) Oregon Green 514 calibrated with the nigericin/high-K⁺ method. The pH_DV of 7G8' was significantly higher than the pH_DV of D10' when measured with either Fluorescein (P<0.001) or Oregon Green 514 (P<0.01). However, for both strains, the pH_DV estimated using Fluorescein was significantly higher than that estimated using Oregon Green 514 (P<0.001). The concentration of dextran in each treatment was 28 µM, resulting in a Fluorescein concentration of 40 µM and an Oregon Green 514 concentration of 70 µM as a consequence of the different number of fluorophore molecules per dextran molecule (Haughland, 2002) (see Materials and Methods). The data for each dye represent the means of at least nine (Fluorescein) or at least four (Oregon Green 514) independent experiments, with at least three replicate measurements per experiment. The error bars show s.e.m.