Evidence that Calu-3 human airway cells secrete bicarbonate.

The Calu-3 cell line is being investigated as a model for human submucosal gland serous cells. In a previous investigation of basal short-circuit current (Isc) in Calu-3 cells, high levels of bumetanide-insensitive basal Isc (approximately 60 microA/cm2) were measured in cells grown at an air interface. Basal Isc was reduced only 7% by bumetanide, and the largest component of basal Isc required both Cl- and HCO3- in the bathing solutions. Because Isc could be partially inhibited by basolateral 4,4'-dinitrostilbene-2,2'-disulfonic acid and because the only known apical exit pathway for anions is the cystic fibrosis transmembrane conductance regulator, which has a relatively poor conductance for HCO3-, it was concluded that most basal Isc is HCO3(-)-dependent Cl- secretion [M. Singh, M. Krouse, S. Moon, and J. J. Wine. Am. J. Physiol. 272 (Lung Cell. Mol. Physiol. 16): L690-L698, 1997]. We have now measured isotopic fluxes of 36Cl- and 22Na+ across short-circuited Calu-3 cells and found that virtually none of the basal Isc is Cl- secretion or Na+ absorption. Thus, in contrast to the earlier report, we conclude that the major component of basal Isc is HCO3- secretion. Stimulation recruits primarily Cl- secretion, as previously proposed.

Differential effects of polycationic protein on Cl- secretory and Na+ absorptive airways.

Effects of cationic proteins on electrolyte transport depend on the specific channels and electrochemical driving forces expressed by epithelia. The bioelectric responses of canine tracheal and bronchial epithelia (CTE and CBE, respectively) to a polycationic protein, protamine, were therefore compared. CTE exhibited a brief transient inhibition of shortcircuit current (I(SC)) followed by a prolonged increase of 18 microA/cm2. The apical membrane transiently hyperpolarized and then depolarized by 11 mV. The increase in I(SC) was inhibited by bumetanide. Adenosine 3',5'-cyclic monophosphate, ionomycin, and thapsigargin attenuated the response whereas indomethacin or hypotonic solution had no effect, indicating that latent cystic fibrosis transmembrane regulator Cl- channels were activated. CBE preparations exhibited a 4-microA/cm2 decrease in I(SC), 2 mV hyperpolarization of the apical membrane, and an increase in fractional resistance of the apical membrane on exposure to protamine. These results were consistent with inhibition of the Na+ conductance in the apical membrane of CBE and confirmed that polycationic proteins exert differential effects on Cl- secretory and Na+ absorptive airways.

CFTR-like chloride channels in non-ciliated bronchiolar epithelial (Clara) cells.

Distal airways are believed to be a major site of disease in cystic fibrosis. The product of the CF gene, CFTR, is expressed in non-ciliated bronchiolar epithelial (Clara) cells. Clara cells in primary culture are capable of Cl- secretion which can be stimulated by cAMP and extracellular nucleotides. We used the patch clamp technique to look for Cl- channels in the apical membrane of rabbit Clara cells. In cell-attached patches, we recorded Cl- channels with a conductance for outward currents of 7.5 +/- 0.4 pS (n = 10) and for inward currents of 3.2 +/- 0.5 pS (n = 10); these channels typically exhibited slow kinetics and were not inhibited by the Cl- channel blockers NPPB and DIDS. Channel activity was not noticeably dependent on pipette potential. Addition of chlorophenylthio-cyclic AMP (cpt-cAMP) to the bath increased the percentage of cell-attached patches with active channels (33.8% vs 56.7%; p< 0.05) and the channel open probability (0.49 +/- 0.03 vs 0.84 +/- 0.02; p< 0.05). Extracellular ATP increased the percentage of cell-attached patches with active channels (28.7% to 50.0%; p< 0.05) but had no significant effect on the channel open probability (0.62 +/- 0.07 vs 0.60 +/- 0.06). In conclusion, rabbit non-ciliated bronchiolar epithelial cells express low-conductance Cl- channels that share many similarities with the CFTR-related Cl- channel and are regulated by cAMP and extracellular ATP.

Beta-adrenergic regulation of Cl- and HCO3- secretion by Clara cells.

Previous studies on beta-adrenergic agonist regulation of ion transport in distal airways yielded discordant results. The present study was performed to further investigate this process in isolated bronchiolar epithelial cells and resolve the discrepancies. Epithelial enriched in rabbit nonciliated bronchiolar epithelial (Clara) cells responded to isoproterenol with a biphasic increase in transepithelial short circuit current (Isc) and decrease in transepithelial resistance (Rt). The first phase of the Isc response consisted of a transient, 11 microA/cm2 increase in current that was inhibited by HCO3(-)-free bathing solutions, but was not inhibited by amiloride, bumetanide, or Cl(-)-free bathing solutions. The ED50 for isoproterenol stimulation of the initial peak was 81 pM. The second phase was a prolonged, 27 microA/cm2 elevation in Isc. Amiloride in the apical bath inhibited basal Isc and the prolonged change in Isc induced by isoproterenol. Bumetanide in the basolateral bath and bilateral Cl(-)-free bathing solutions likewise inhibited the plateau phase of the isoproterenol response, and the inhibition was accentuated in the presence of amiloride. HCO3(-)-free bathing solutions did not inhibit the plateau phase. The ED50 for isoproterenol stimulation of the plateau phase was 6.3 nM. The bioelectric response to isoproterenol was mimicked by isobutylmethylxanthine (IBMX) and, to a lesser degree, by dibutyryl-cAMP. Culturing the cells in medium containing cholera toxin completely inhibited the bioelectric response, yet the preparations continued to respond to isoproterenol with an increase in cAMP production. These results indicated that beta-adrenergic stimulation of Clara cells induced electrogenic transepithelial secretion of Cl- and HCO3- and resolved discrepancies between previous studies.