FIGURE 2.

ADP and P_i activate an inward current (vacuole to cytoplasm) through the short-circuited vacuolar membrane (V_m = 0), when the vacuole is sufficiently acidic. (A) Experiment similar to the left-hand third of Fig. 1 A, except that vacuolar pH was 3.1 compared with cytosolic pH of 7.5. Bar below the trace (AAP) indicates perfusion of the chamber with 5 mM ATP + 5 mM ADP + 10 mM P_i. Again, the arrows a and b indicate times when the 5 s voltage ramps were applied to the tonoplast. The downward current elicited by AAP indicates that the pump runs backward under these experimental conditions. (B) I-V analysis of A, similar to Fig. 1 C, but describing the ATPase reaction to the very acid vacuole (pH_V = 3.1). The difference curve (b–a) is constant over the voltage range −80 mV to +40 mV. Under these conditions, the membrane becomes rather unstable, evidenced by the large current fluctuations at extreme voltages. Under these conditions the reversal voltage for the pump could not be determined.