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.