Ionic permeabilities of the cell membranes of sheep tracheal epithelium

Regulation of intracellular pH in Calu-3 human airway cells

The Calu-3 human cell line exhibits features of submucosal gland serous cells and secretes HCO(3)(-). The aim of this study was to identify the HCO(3)(-) transporters present in these cells by studying their role in the regulation of intracellular pH (pH(i)). Calu-3 cells were grown on coverslips, loaded with the pH-sensitive fluorescent dye BCECF, and their fluorescence intensity monitored as an indication of pH(i). Cells were acidified with NH(4)Cl (25 mM, 1 min) and pH(i) recovery recorded. In the absence of HCO(3)(-), initial recovery was 0.208 +/- 0.016 pH units min(-1) (n = 37). This was almost abolished by removal of extracellular Na(+) and by amiloride (1 mM), consistent with the activity of a Na(+)-H(+) exchanger (NHE). In the presence of HCO(3)(-) and CO(2), recovery (0.156 +/- 0.018 pH units min(-1)) was abolished (reduced by 91.8 +/- 6.7 %, n = 7) by removal of Na(+) but only attenuated (by 63.3 +/- 5.8 %, n = 9) by amiloride. 4,4-Dinitrostilbene-2,2-disulfonic acid (DNDS) inhibited recovery by 45.8 +/- 5.0 % (n = 7). The amiloride-insensitive recovery was insensitive to changes in membrane potential, as confirmed by direct microelectrode measurements, brought about by changing extracellular [K(+)] in the presence of either valinomycin or the K(+) channel opener 1-EBIO. In addition, forskolin (10 microM), which activates the cystic fibrosis transmembrane conductance regulator Cl(-) conductance in these cells and depolarises the cell membrane, had no effect on recovery. Removal of extracellular Cl(-) trebled pH(i) recovery rates, suggesting that an electroneutral, DNDS-sensitive, Cl(-)-HCO(3)(-) exchanger together with a NHE may be involved in pH(i) regulation and HCO(3)(-) secretion in these cells. RT-PCR detected the expression of the electrogenic Na(+)-HCO(3)(-) cotransporter NBC1 and the Cl(-)-HCO(3)(-) exchanger (AE2) but not the electroneutral Na(+)-HCO(3)(-) cotransporter NBCn1.

Ion transport by sheep distal airways in a miniature chamber

Ion transport and the electric profile of distal airways of sheep lungs were studied in a miniature polypropylene chamber with a 1-mm aperture. Small airways with an inner diameter < 1 mm were isolated, opened longitudinally, and then mounted as a flat sheet onto the 1-mm aperture where it was glued and secured with an O-ring. Both sides of the tissue were bathed with identical physiological solutions at 37 degrees C and oxygenated. Pooled data from 27 distal airways showed an inner airway diameter of 854 +/- 22 (SE) microm and a transepithelial potential difference (PD) of 1.86 +/- 0.29 mV, lumen negative. Short-circuit current (I(sc)) was 25 +/- 3.5 microA/cm(2), tissue resistance was 96 +/- 14 Omega, and conductance was 15.2 +/- 1.7 mS/cm(2). At baseline, amiloride-sensitive Na transport accounted for 51% of I(sc) (change in I(sc) = 9.7 +/- 2.6 microA/cm(2); n = 8 airways), corresponding to 0.36 microeq. cm(-2). h(-1). Treatment with 0.1 mM bumetanide did not reduce the I(sc) (n = 5 airways). Exposure to 1 microM Ca ionophore A-23187 raised the I(sc) by 9 microA/cm(2) (47%; P < 0.03; n = 6 airways). The latter effect was blunted by bumetanide. Carbachol at 1 microM provoked a biphasic response, an initial rapid rise in I(sc) followed by a decline (n = 3 airways). There was no significant increase in PD or I(sc) in response to isoproterenol or dibutyryl cAMP. The data suggest that Na absorption constitutes at least 50% of baseline transport activity. Cl or other anion secretion such as HCO(3) appears to be present and could be stimulated by raising intracellular Ca.

Effect of acetyl choline on ion transport in sheep tracheal epithelium

The action of acetylcholine (ACh) on sheep tracheal epithelium has been investigated. ACh increases transiently the short-circuit current (ISC). The same response is obtained in tissues in which the apical membrane has been permeabilized with amphotericin B in the presence of a potassium gradient. Microelectrode studies show that the majority of tracheal epithelial cells depolarize as the apical fractional resistance decreases on application of ACh. These results, together with the finding that bumetanide decreases the initial ACh-induced ISC increase, are consistent with an initial activation by ACh of apical Cl- channels and basolateral K+ channels. Following the initial increase, ISC declines to values lower than in control conditions both in untreated and in amphotericin-permeabilized tissues, suggesting that the basolateral K+ conductance falls during this phase. The late decrease in ISC induced by ACh is significantly reduced in tissues pretreated with amiloride, suggesting that the apical Na+ channels are also involved in this response. ACh abolishes the net Na+ absorption by decreasing the mucosal to serosal Na+ flux. This effect is possibly a result of a down-regulation of apical Na+ channels and basolateral K+ channels.

Effects of purified Pseudomonas rhamnolipids on bioelectric properties of sheep tracheal epithelium

The effects of purified rhamnolipids from Pseudomonas aeruginosa on short circuit current in respiratory epithelium have been studied in sheep tracheal epithelium mounted in Ussing chambers under short circuit conditions. In low concentrations (100 microM) mucosal addition of rhamnolipids produces a decrease in short circuit current of 22% and in conductance of 4%. At higher concentrations (> 100 microM), large increases in tissue conductance accompany a greater reduction in short circuit current, suggesting disruption of the intercellular junctions. Serosal addition of rhamnolipids has no effect on ion transport. Pretreatment of the tissues with the sodium channel blocker amiloride (100 microM) or bathing the mucosal surface with sodium-free solution significantly decreased the rhamnolipid-induced fall in short circuit current but did not prevent it completely. Inhibition of chloride transport, sodium-glucose cotransport, and bicarbonate secretion with bumetanide, phloridzin, and acetazolamide, respectively, did not significantly alter the rhamnolipid effect. This suggests that the effect of rhamnolipids on short circuit current is mediated predominantly but not exclusively by an effect on sodium transport. The effects of rhamnolipids on ion transport occur at concentrations within the range occurring in the sputum of patients with cystic fibrosis. The changes in ion transport may explain some of the known effects of rhamnolipid on mucociliary clearance.

Second-messenger regulation of sodium transport in mammalian airway epithelia

1. Sodium absorption is the dominant ion transport process in conducting airways and is a major factor regulating the composition of airway surface liquid. However, little is known about the control of airway sodium transport by intracellular regulatory pathways. 2. In sheep tracheae and human bronchi mounted in Ussing chambers under short circuit conditions, the sodium current can be isolated by pretreating tissues with acetazolamide (100 microM) to inhibit bicarbonate secretion, bumetanide (100 microM) to inhibit chloride secretion and phloridzin (200 microM) to inhibit sodium-glucose cotransport. This sodium current consists of amiloride-sensitive (57%) and amiloride-insensitive (43%) components. 3. The regulation of the isolated sodium current by three second messenger pathways was studied using the calcium ionophore A23187 to elevate intracellular calcium, a combination of forskolin and the phosphodiesterase inhibitor zardaverine to elevate intracellular cyclic AMP, and the phorbol ester 12,13-phorbol dibutyrate (PDB) to stimulate protein kinase C. 4. In sheep trachea, A23187 produces a dose-related inhibition of the sodium current with maximal effect (38% of ISC) at 10 microM and IC50 1 microM. This response affects both the amiloride-sensitive and insensitive components of the sodium current and is not altered by prior stimulation of protein kinase C or elevation of intracellular cyclic AMP. In human bronchi, A23187 (10 microM) produced a significantly greater inhibition of ISC (68%), a response which was unaffected by prior treatment with PDB or forskolin-zardaverine. 5. In sheep trachea, stimulation of protein kinase C with PDB produced a dose-related inhibition of ISC maximal (56% of ISC) at 50 nM (IC50 7 nM). This response was abolished by amiloride (100 microM) pretreatment suggesting a selective effect on the amiloride-sensitive component of the sodium current. The response was not altered by prior elevation of intracellular calcium or cyclic AMP. PDB (10 nM) caused a similar inhibition of ISC in human bronchi (43%). The effect of PKC stimulation following pretreatment with A23187 was diminished in human bronchi. Elevating intracellular cyclic AMP did not alter this response. 6. Addition of forskolin (1 microM) together with the phosphodiesterase inhibitor zardaverine (100 microM) produced a mean 35-fold increase in intracellular cyclic AMP in sheep trachea. This was associated with a small, but significant, 6% transient increase in ISC followed by a significant 4% fall. Neither effect could be abolished by amiloride pretreatment. In human bronchi, a small decrease in ISC which could not be distinguished from that occurring in controls was observed.(ABSTRACT TRUNCATED AT 400 WORDS)