Intracellular K+ activity and its relation to basolateral membrane ion transport in Necturus gallbladder epithelium.
Conventional and Cl-selective liquid ion-exchanger intracellular microelectrodes were employed to study the effects of extracellular ionic substitutions on intracellular Cl activity (aCl i ) inNecturus gallbladder epithelium. As shown previously (Reuss, L., Weinman, S.A., 1979;J. Membrane Biol. 49:345), when the tissue was exposed to NaCl-Ringer on both sidesaCl i was about 30mm, i.e., much higher than the activity predicted from equilibrium distribution (aCleq) across either membrane (5–9mm). Removal of Cl from the apical side caused a reversible decrease ofaCl i towards the equilibrium value across the basolateral membrane. A new steady-stateaCl i was reached in about 10 min. Removal of Na from the mucosal medium or from both media also caused reversible decreases ofaCl i when Li, choline, tetramethylammonium or N-methyl-d-glucamine (NMDG) were employed to replace Na. During bilateral Na substitutions with choline the cells depolarized significantly. However, no change of cell potential was observed when NMDG was employed as Na substitute. Na replacements with choline or NMDG on the serosal side only did not changeaCl i . When K substituted for mucosal Na, the cells depolarized andaCl i rose significantly. Combinations of K for Na and Cl for SO4 substitutions showed that net Cl entry during cell depolarization can take place across either membrane. The increase ofaCl i in depolarized cells exposed to K2SO4-Ringer on the mucosal side indicates that the basolateral membrane Cl permeability, (P Cl) increased. These results support the hypothesis that NaCl entry at the apical membrane occurs by an electroneutral mechanism, driven by the Na electrochemical gradient. In addition, we suggest that Cl entry during cell depolarization is downhill and involves an increase of basolateral membraneP Cl.