Fig. 1. Examples of recordings of basolateral membrane potential (V_bl) and intracellular activity of (A) Na⁺, (B) K⁺ and (C) Cl^- in serotonin-stimulated tubules. V_bl and ion activity were measured simultaneously using double-barrelled ion-selective microelectrodes. In this and subsequent figures, impalement is indicated by the downward-pointing arrows and the removal of the electrode from the cell is indicated by upward-pointing arrows.