Evidence for basolateral Cl−channels as modulators of apical Cl−secretion in pulmonary epithelia of Xenopus laevis

Abstract

Pulmonary epithelia of air-breathing vertebrates are covered by a thin, fluid layer that is essential for immune defense and gas diffusion. The composition of this layer is maintained by ion transport mechanisms, including Cl−transport. The present study focuses on the function of basolateral Cl−channels in Xenopus pulmonary epithelia, since knowledge concerning this issue is limited. Therefore, Ussing chamber measurements were performed, and transepithelial short-circuit currents ( ISC) were monitored. Basolateral application of the Cl−channel inhibitor N-phenylanthranilic acid (DPC) resulted in an increase of the ISC, indicating a DPC-sensitive Cl−conductance. This observation was confirmed in experiments using an apical-to-basolateral Cl−gradient, with and without nystatin (apical side) to permeabilize the epithelia as well as by establishing an iodide gradient. The DPC-sensitive Cl−conductance was influenced by procedures interfering with apical Cl−secretion. For example, the effect of forskolin was increased when basolateral Cl−channels were blocked by the simultaneous application of DPC. Activation of apical Cl−secretion by forskolin/IBMX and subsequent DPC application resulted in a significantly reduced DPC effect. Accordingly, DPC led to an increased apical Cl−secretion estimated by an increased 5-nitro-2-(3-phenylpropylamino)benzoic acid-sensitive ISC. Furthermore, inhibition of basolateral anion exchangers responsible for Cl−uptake resulted in a decreased DPC-sensitive current. Taken together, we have evidence concerning the function of basolateral Cl−channels in Xenopus pulmonary epithelium and that these channels play a significant role in mediating apical Cl−secretion involving a novel Cl−recycling mechanism across the basolateral membrane.