Secretion of HCO₃^– by airway submucosal glands is essential for normal liquid and mucus secretion (Inglis et al. 1998). Since the liquid bathing the airway surface is fairly acidic, we have proposed that the surface epithelium may acidify HCO₃^–-rich glandular fluid. We previously showed that isolated distal bronchi, containing both glandular and surface epithelium, can both acidifiy and alkalinise their lumen (Inglis et al. 2003). The aim of the current study was to investigate whether the acidification arises from ion transport in the surface or glandular epithelium.

Porcine distal bronchi were isolated from pigs humanely killed with an overdose of sodium pentobarbital, cannulated in a bath containing HCO₃^–-buffered solution and perfused (3ml min^-1) with similar solution, in which NaCl replaced NaHCO₃^–. This solution was lightly buffered (buffering capacity 0.6 mM pH unit^-1) with KH₂PO₄ and NaOH to pH ~7, gassed with 100 % O₂ to eliminate dissolved CO₂ and stirred vigorously. The pH of this circulating luminal solution (10 ml) was monitored continuously. Transepithelial PD was measured using a luminal microelectrode referenced to the bathing solution, connected to an electrometer.

As previously shown (Inglis et al. 2003), upon perfusion through the airway lumen pH initially fell by 0.057 ± 0.009 pH units ([H⁺] increase 2.54 ± 0.45 µmol h^-1 (means ± S.E.M., n = 7)) before stabilising. Since removing the surface epithelium removes the primary resistive barrier to passive HCO₃^– transport into the lumen, subsequent experiments were carried out in the absence of HCO₃^–. HCO₃^– removal significantly reduced the acidification (pH fell by 0.033 ± 0.007 pH units; [H⁺] increase 0.782 ± 0.1 µmol h^-1 (means ± S.E.M., n = 7, P < 0.05, Student’s paired t test used throughout). The acidification was further inhibited (65.0 ± 3.8 %, n = 5, P < 0.05) by luminal bafilomycin A₁ (100 nM), a vH⁺-ATPase blocker. Previous results showed that bafilomycin A₁ had similar effects in the presence of HCO₃^– (Inglis et al. 2003). To remove surface epithelium, a nylon brush was pushed through the airway lumen. Histological analyses revealed this successfully removed surface epithelium and left underlying glandular epithelium relatively intact (see Fig. 1). Such treatment abolished transepithelial PD (control bronchi -6.3 ± 1.7 mV; epithelium-stripped bronchi 0.1 ± 0.4 mV, n = 5, P < 0.05) and significantly reduced the acidification (control bronchi 0.022 ± 0.003 pH units; epithelium-stripped bronchi 0.009 ± 0.006 pH units, n = 5, P < 0.05).

Figure 1. The effect of removing surface epithelium on acid secretion in isolated bronchi. HIstological sections show that the surface epithelium was successfully removed by brushing. Transepithelial potential difference was abolished and the fall in luminal pH was greatly reduced. Data are means ± S.E.M., n = 5. Asterisks indicate significant difference from control bronchi.

These data suggest that the surface epithelium has the major role in luminal acidification in intact distal bronchi, that the acidification occurs mainly through activity of vH⁺-ATPase, and that the protons excreted may be generated within the epithelial cells through formation of carbonic acid from HCO₃^– and water.

This work was supported by the Wellcome and Tenovus Trusts.