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

Pulmonary epithelia of airbreathing 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.