Apical Cl- channels in A6 cells.

Short-circuit current (Isc), transepithelial conductance (Gt), electrical capacitance (CT) and the fluctuation in Isc 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 Gt, CT, Isc and generated a Lorentzian Isc-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 Gt with only moderate peaking of Isc. Phosphodiesterase inhibitors also stimulated Cl- secretion with peaking responses in Gt and Isc. 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 CT, a measure of apical membrane area, to increase in a manner roughly synchronous with Gt. 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.