Fig. 3. Effects of intravesicular pH (pH 7.5–5.0), extravesicular ATP (1 mmol l^–1), and induced membrane potential on the influx (10 s uptake) of 25 µmol l^{–1 65}Zn²⁺. A transmembrane electrical potential was induced by equilibrating the vesicles with 50 µmol l^–1 valinomycin, varying the extravesicular K⁺ concentration [0 mmol l^–1 (K_i>K_o), 100 mmol l^–1 (K_i=K_o) or 200 mmol l^–1 (K_i<K_o)], and maintaining a constant intravesicular K⁺ concentration (100 mmol l^–1). Vesicles were loaded with 100 mmol l^–1 K₂SO₄, either 20 mmol l^–1 Hepes/Tris (pH 6.5–7.5) or Mes/Tris (pH 5.0–6.0), 50 µmol l^–1 valinomycin, and appropriate mannitol to maintain osmolarity. Loaded vesicles were then incubated in media containing 25 µmol l^{–1 65}Zinc-sulfate, 0, 100 or 200 mmol l^–1 K₂SO₄ 0.2 or 1.0 mmol l^–1 ATP (+ATP), 0.5 mmol l^–1 NTA, 20 mmol l^–1 Hepes/Tris, pH 7.0, and mannitol to maintain osmolarity. The experiment was conducted in triplicate; values are means ± 1 s.e.m. at each time point.