Apical Cl- channels in A6 cells

Short-circuit current (I-sc), transepithelial conductance (G(t)), electrica l 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 surfa ce. Basolateral perfusate was NaCl-Ringer. Agents that increase cellular cA MP evoked increases in G(t), C-T, I-sc and generated a Lorentzian I-sc-nois e. The responses could be related to active, electrogenic secretion of Cl-. Arginine-vasotocin and oxytocin caused a typical peak-plateau response pat tern. 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 Lorentzi an noise, originating from the activated apical Cl- channel, with almost id entical corner frequency (40-50 Hz). Repetitive challenge with the hormones led to a refractory behavior of all parameters. Activation of the cAMP rou te 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 f lufenamate depressed Cl- current and conductance and apparently generated b locker-noise. However, blocking kinetics extracted from noise experiments c ould not be reconciled with those obtained from current inhibition, suggest ing the drug does not act as simple open-channel inhibitor.