Fig. 3. The normalised ratiometric pH-dependent change in fluorescence of either Fluorescein (open symbols) or Oregon Green 514 (closed symbols). In situ measurements (squares) were made using isolated mature trophozoite-stage D10' parasites suspended in high-K⁺ saline and exposed to 50 µM nigericin. The concentration of dextran in the loading solution was 28 µM. Cell-free measurements (circles) were made as described in Materials and Methods. The pH range over which the Fluorescein response is linear was 5.0-7.5, whereas the linear response of Oregon Green 514 was in the pH range 3.5-5.5.