Figure 4

Electrophysiological response to luminal fluid exchange. A. Original voltage trace from single cell. Effect of changing luminal [Na⁺] from 102 to 7 mmol/l and luminal [K⁺] from 3 to 20 mmol/l. V_m hyperpolarized upon the decrease in luminal [Na⁺] and depolarized upon an increase in luminal [K⁺]. This indicates that the luminal (apical) cell membrane possesses Na⁺ as well as K⁺ conductances. B. Summary data illustrating the effect on V_m of concentration steps in luminal Na⁺ (n = 6) and K⁺ (n = 5).