Lung epithelia of air breathing vertebrates are covered by a thin liquid layer (airway surface layer, ASL), being essential for immune defence and gas exchange in the respiratory system. The height and the viscosity of this layer are strictly regulated by active ion transport processes of the underlying epithelia – water following passively. Disorders in the involved transport mechanisms are associated with severe diseases of the lung like pulmonary edema or cystic fibrosis. In addition to transepithelial Na⁺ absorption, the transepithelial transport of Cl^- plays a crucial role in the regulation of the ASL. Until now investigations mainly focused on apical Cl^- transport mechanisms. Comparatively, less is known about the function and the regulation of Cl^- transporting channels at the basolateral side of pulmonary epithelial cells. Recently, roles in the regulation of transepithelial Cl^- secretion as well as in transepithelial Cl^- reabsorption are discussed for basolateral Cl^- channels in pulmonary epithelia. In former studies we were able to demonstrate a Cl^- secreting as well as a Cl^- reabsorptive component in native lung preparations of Xenopus laevis. Here we focus on the role of basolateral Cl^- channels in this amphibian lung.

Electrophysiological investigations via the Ussing chamber revealed a basolateral N-phenylanthranilic acid (DPC) sensitive Cl^- conductance. Experiments, using basolaterally directed Cl^- gradients, indicated that this conductance is involved in transepithelial Cl^- reabsorption. Furthermore, its inhibition under basal conditions led to an increase in forskolin induced chloride secretion and therefore implicates a role in the modulation of Cl^- secretion by recycling the anion at the basolateral membrane. Additionally, we detected a 5-nitro-2-(3-phenylpropyl-amino)benzoic acid (NPPB) and niflumic acid (NFA) sensitive basolateral Cl^- conductance which is activated by cellular depolarisation via apical membrane permeabilisation. This might indicate its putative role in cellular repolarisation and Cl^- delivery under anion secretion conditions. Taken together these data demonstrate that, similar to the situation in mammals, Xenopus lung epithelia possess basolateral Cl^- conductances and that these conductances are involved in the modulation of transepithelial Cl^- secretion as well as in Cl^- reabsorption. These results confirm our former notion that the lung of Xenopus laevis might serve as an additional alternative model system for the investigation of pulmonary ion transport processes.