Fig. 5. Effect of luminal Ba²⁺. (A) Effect on V_bl of luminal [K⁺] step (3-20 mmol l^-1) and of 1 mmol l^-1 BaCl₂ in luminal control solution and in high [K⁺] solution. In this cell, Ba²⁺ in the control solution had no effect on V_bl. Depolarization by the high [K⁺] solution was abolished by Ba²⁺. (B) Effect of luminal perfusion with Ba²⁺ on V_bl in the same cell as in A in an experiment where the cell was depolarized by high bath [K⁺]. (C) Summary data showing the effect of a luminal [K⁺] step (3-20 mmol l^-1) and of 1 mmol l^-1 BaCl₂ in luminal control solution and in high [K⁺] solution on V_bl. The asterisk marks the cell shown in A and B. (D) Summary of experiments showing the effect on V_bl of luminal perfusion with Ba²⁺ in the control solution, while depolarizing the cell with a high basal [K⁺] solution. The asterisk marks the cell shown in A and B. Ba_b, Ba²⁺ in bath; Ba_l, Ba²⁺ in lumen; C, control solution; K_20,b, high [K⁺] in bath; K_20,l, high [K⁺] in lumen.