Fig. 6. Coupled and uncoupled proton movement can flow in opposite directions. Oocytes injected with SN1 cRNA were incubated in BCECF-AM and simultanous recordings made of pH_i and currents at pH_o 8. (A) At +30 mV, asparagine (10 mM) induces outward currents and strong alkalinization, presumably due to the H⁺ efflux coupled to amino acid uptake plus the uncoupled H⁺ conductance. (B) At –100 mV, addition of asparagine to the same oocyte shows alkalinization but inward currents. An inwardly directed uncoupled H⁺ conductance presumably contributes to the reduced alkalinization by H⁺ efflux coupled to amino acid uptake. (C) At –120 mV, the oocyte shows even larger inward currents and barely detectable alkalinization after the addition of asparagine. In this case, the magnitude of the uncoupled inward H⁺ conductance presumably approaches that of the coupled outward H⁺ flux. V = voltage output of the p100s photomultiplier at 490 nm excitation, normalized to fluorescence at 440 nm.