Fig. 6. pH_DV of mature trophozoite-stage D10' (circles) and 7G8' (squares) parasites as determined using either Fluorescein (open symbols) or Oregon Green 514 (closed symbols) over a range of concentrations and calibrated with the nigericin/high-K⁺ method. For both D10' and 7G8' parasites, the pH_DV reported by Fluorescein was significantly lower (P<0.01) at the lowest concentration tested than at the highest (10 µM and 156 µM, respectively). However, there was no significant change in reported pH_DV for either strain over the range of concentrations of Oregon Green 514 tested (4-70 µM). At Fluorescein loading concentrations of 40 µM and above, the reported pH_DV of 7G8' parasites was significantly higher than that of D10' parasites (P<0.01). Similarly, at Oregon Green 514 loading concentrations of 35 µM and above, the reported pH_DV of 7G8' parasites was significantly higher than that of D10' parasites (P<0.05). The data represent the means (± s.e.m.; where not shown, error bars fall within the symbol) of at least three independent experiments, each derived from at least three replicate measurements per experiment. Curves (rectangular hyperbolic and simple linear) were fitted by least-squares regression with SigmaPlot 2001 (SPSS).