Apical Cl− Channels in A6 Cells

Short-circuit current (I sc ), transepithelial conductance (G t ), electrical capacitance (C T ) and the fluctuation in I sc were analyzed in polarized epithelial cells from the distal nephron of Xenopus laevis (A6 cell line). Tissues were incubated with Na+- and Cl−-free solutions on the apical surface. Basolateral perfusate was NaCl-Ringer. Agents that increase cellular cAMP evoked increases in G t , C T , I sc and generated a Lorentzian I sc -noise. The responses could be related to active, electrogenic secretion of Cl−. Arginine-vasotocin and oxytocin caused a typical peak-plateau response pattern. Stimulation with a membrane-permeant nonhydrolyzable cAMP analogue or forskolin showed stable increases in G t with only moderate peaking of I sc . Phosphodiesterase inhibitors also stimulated Cl− secretion with peaking responses in G t and I sc . All stimulants elicited a spontaneous Lorentzian noise, originating from the activated apical Cl− channel, with almost identical corner frequency (40–50 Hz). Repetitive challenge with the hormones led to a refractory behavior of all parameters. Activation of the cAMP route could overcome this refractoriness. All agents caused C T , a measure of apical membrane area, to increase in a manner roughly synchronous with G t . These results suggest that activation of the cAMP-messenger route may, at least partly, involve exocytosis of a vesicular Cl− channel pool. Apical flufenamate depressed Cl− current and conductance and apparently generated blocker-noise. However, blocking kinetics extracted from noise experiments could not be reconciled with those obtained from current inhibition, suggesting the drug does not act as simple open-channel inhibitor.