Aldosterone regulation of basolateral potassium channels in alveolar epithelium

A phospholipid-apolipoprotein A-I nanoparticle containing amphotericin B as a drug delivery platform with cell membrane protective properties

Amphotericin B (AMB), a potent antifungal agent, has been employed as an inhalable therapy for pulmonary fungal infections. We recently described a novel nano-sized delivery vehicle composed of phospholipid (PL) and apolipoprotein A-I, NanoDisk (ND), to which we added AMB as a payload (ND-AMB). The goal of the present study was to evaluate whether ND-AMB, compared to other formulations, preserves lung cell integrity in vitro, as AMB can be toxic to mammalian cells and reduce lung function when inhaled. Epithelial integrity was assessed by measuring K(+) ion flux across a model airway epithelium, Calu-3 cells. In this assay ND-AMB was at least 8-fold less disruptive than AMB/deoxycholate (DOC). Cell viability studies confirmed this observation. Unexpectedly, the ND vehicle restored the integrity of a membrane compromised by prior exposure to AMB. An alternative formulation of ND-AMB containing a high load of AMB per ND was not protective, suggesting that ND with a low ratio of AMB to PL can sequester additional AMB from membranes. ND-AMB also protected HepG2 cells from the cytotoxicity of AMB, as determined by cellular viability and lactate dehydrogenase (LDH) levels. This study suggests that ND-AMB may be safe for administration via inhalation and reveals a unique activity whereby ND-AMB protects lung epithelial membranes from AMB toxicity.

Impact of mechanical stress on ion transport in native lung epithelium (Xenopus laevis): short-term activation of Na+, Cl (-) and K+ channels

Epithelia, in general, and the lung epithelium, in particular, are exposed to mechanical forces, but little is known about their impact on pulmonary ion transport. In our present study, we employed transepithelial ion transport measurements on Xenopus lung preparations using custom-built Ussing chambers. Tissues were exposed to mechanical stress by increasing the water column (5 cm) at one side of the tissues. Apical exposure to hydrostatic pressure significantly decreased the short circuit current (I (SC): 24 +/- 1%, n = 152), slightly decreased the transepithelial resistance (R (T): 7 +/- 2%, n = 152), but increased the apical membrane capacitance (C (M): 16 +/- 6%, n = 9). The pressure-induced effect was sensitive to Na+ (amiloride), Cl(-) (DIDS, NFA, NPPB) and K+ channel blockers (Ba2+), glibenclamide). Further on, it was accompanied by increased extracellular ATP levels. The results show that mechanical stress leads to an activation of Na+, Cl(-), and K+ conductances in a native pulmonary epithelium resulting in a net decrease of ion absorption. This could be of considerable interest, since an altered ion transport may contribute to pathophysiological conditions, e.g., the formation of pulmonary edema during artificial ventilation.

Spironolactone improves lung diffusion in chronic heart failure

AIMS

To evaluate whether anti-aldosteronic treatment influences lung diffusion (DLCO) in chronic heart failure (HF) patients. Spironolactone improves clinical conditions and prognosis in chronic HF and reduces connective tissue matrix turnover; DLCO abnormalities in chronic HF are related to increase in fibrosis and connective tissue derangement.

METHODS AND RESULTS

Thirty stable chronic HF patients, with reduced DLCO (<80% of predicted), were randomly assigned to active treatment (25 mg spironolactone daily) or placebo in addition to conventional anti-failure treatment. They were evaluated by quality of life questionnaire, laboratory investigations, cardiopulmonary exercise test, and pulmonary function test, which included DLCO and membrane diffusing capacity (DM). The evaluation was done before treatment and 6 months after. Quality of life score and standard pulmonary function tests were not significantly affected by spironolactone, while active treatment increased DLCO due to an increase of DM (DLCO: 18.3+/-3.9 vs. 19.9+/-5.5 mL/min/mmHg; DM: 28.1+/-7.7 vs. 33.3+/-8.6 mL/min/mmHg) and peak oxygen consumption (peak VO2 16.8+/-1.9 vs.18.6+/-2.2 mL/min/kg). Increments of DLCO and peak VO2 were linearly related (R=0.849, P<0.001).

CONCLUSION

These data show a positive effect of spironolactone on gas diffusion and exercise capacity suggesting a novel mechanism by which anti-aldosteronic drugs improve HF clinical condition and prognosis.

Rapid and non-genomic reduction of intracellular [Ca(2+)] induced by aldosterone in human bronchial epithelium

1. Using a Ca(2+) imaging system and fura-2 AM (5 microM) we showed that exposure of polarised monolayers of human bronchial epithelial cells (16HBE14o- cell line) to aldosterone produced a fast intracellular [Ca(2+)] ([Ca(2+)](i)) decrease, in 70 % of cells. Exposure to aldosterone (1 nM) reduced the [Ca(2+)](i) by 39 +/- 9 nM (n = 282, P < 0.0001) within 10 min, from a basal [Ca(2+)](i) of 131 +/- 19 nM (n = 282). 2. The effect of aldosterone on [Ca(2+)](i) was not affected by inhibitors of the classical genomic pathway, cycloheximide (1 microM) or spironolactone (10 microM). The aldosterone-induced [Ca(2+)](i) decrease was inhibited by thapsigargin (1 microM), pertussis toxin (24 h at 200 ng ml(-1)), the adenylate cyclase inhibitors 2',3'-dideoxyadenosine (200 microM) and MDL-12,330A hydrochloride (500 microM), and the protein kinase A inhibitor R(P)-adenosine 3',5'-cyclic monophosphorothioate (200 microM). In addition, treatment of 16HBE14o- monolayers with aldosterone (1 nM) inhibited by approximately 30 % the large and transient [Ca(2+)](i) increase induced by apical exposure to uridine triphosphate (UTP, 0.1 mM), a known secretagogue in airway epithelia. 3. Our results demonstrate for the first time that in human bronchial epithelial cells, aldosterone decreases [Ca(2+)](i) levels via a non-genomic mechanism. The hormone-induced changes to [Ca(2+)](i) involve stimulation of thapsigargin-sensitive Ca(2+)-ATPase, via G-protein-, adenylate cyclase- and protein kinase A-coupled signalling pathways.

Rapid activation of KATP channels by aldosterone in principal cells of frog skin

1. In epithelial cells of frog skin, potassium ions are recycled across the basolateral membrane via an inward-rectifier, ATP-sensitive K+ channel (KATP channel). In this study, we show that aldosterone has a stimulatory effect on KATP channel activity and we have investigated the involvement of Na+-H+ exchange and intracellular pH (pHi) in this phenomenon. 2. Aldosterone (10 nM) produced an increase in the open probability of the KATP channel within 15 min from 0.21 +/- 0.05 to 0.93 +/- 0.10 (n = 8), measured in cell-attached patches. Aldosterone also increased the tolbutamide-sensitive K+ current across the basolateral membrane within 30 min from 17.2 +/- 1.9 to 30.3 +/- 1.6 microA cm-2 (n = 8) in nystatin-permeabilized whole skins. 3. The KATP channel is very sensitive to variations in cytosolic pH within the physiological range 7.0-7.4. 4. The intracellular pH of principal cells is regulated by Na+-H+ exchange, and the stimulatory effect of aldosterone on KATP channel activity was abolished by amiloride (100 microM) added on the basolateral side of the epithelium either before or after aldosterone treatment. 5. We propose that aldosterone activates the KATP channels via stimulation of Na+-H+ exchange. The rapidity of aldosterone activation of KATP channels is presented as evidence for a novel non-genomic steroid hormone effect on epithelial ion transport.

Quinidine-sensitive K+ channels in the basolateral membrane of embryonic coprodeum epithelium: regulation by aldosterone and thyroxine

Basolateral K+ channels and their regulation during aldosterone- and thyroxine-stimulated Na+ transport were studied in the lower intestinal epithelium (coprodeum) of embryonic chicken in vitro. Isolated tissues of the coprodeum were mounted in Ussing chambers and investigated under voltage-clamped conditions. Simultaneous stimulation with aldosterone (1 mumol.l-1) and thyroxine (1 mumol.l-1) raised short-circuit current after a 1- to 2-h latent period. Maximal values were reached after 6-7 h of hormonal treatment, at which time transepithelial Na+ absorption was more than tripled (77 +/- 11 microA.cm-2) compared to control (24 +/- 8 microA.cm-2). K+ currents across the basolateral membrane were investigated after permeabilizing the apical membrane with the pore-forming antibiotic amphotericin B and application of a mucosal-to-serosal K+ gradient. This K+ current could be dose dependently depressed by the K+ channel blocker quinidine. Fluctuation analysis of the short-circuit current revealed a spontaneous and a blocker-induced Lorentzian noise component in the power density spectra. The Lorentzian corner frequencies increased linearly with the applied blocker concentration. This enabled the calculation of single K+ channel current and K+ channel density. Single K+ channel current was not affected by stimulation, whereas the number of quinidine-sensitive K+ channels in the basolateral membrane increased from 11 to 26.10(6).cm-2 in parallel to the hormonal stimulation transepithelial Na+ transport. This suggests that the basolateral membrane is a physiological target during synergistic aldosterone and thyroxine regulation of transepithelial Na+ transport for maintaining intracellular K+ homeostasis.

K+ recirculation in A6 cells at increased Na+ transport rates

Homocellular regulation of K+ at increased transcellular Na+ transport implies an increase in K+ exit to match the intracellular K+ load. Increased K+ conductance, gK, was suggested to account for this gain. We tested whether such a mechanism is operational in A6 monolayers. Na+ transport was increased from 5.1 +/- 1.0 microA/cm2 to 20.7 +/- 1.3 microA/cm2 by preincubation with 0.1 mumol/l dexamethasone for 24 h. Basolateral K+ conductances were derived from transference numbers of K+, tK, and basolateral membrane conductances, gb, using conventional microelectrodes and circuit analysis with application of amiloride. Activation of Na+ transport induced an increase in gb from 0.333 +/- 0.067 mS/cm2 to 1.160 +/- 0.196 mS/cm2 and tK was reduced to 0.22 +/- 0.01 from a value of 0.70 +/- 0.05 in untreated control tissues. As a result, gK remained virtually unchanged at increased Na+ transport rates. The increase in gb after dexamethasone was due to activation of a conductive leak pathway presumably for Cl-. Increased K+ efflux, IK, was a consequence of the larger driving force for K+ exit due to depolarization at an elevated Na+ transport rate. The relationship between calculated K+ fluxes and Na+ transport rate, measured as the Isc, is described by the linear function IK = 0.624 x INa -0.079, which conforms with a stoichiometry 2:3 for the fluxes of K+ and Na+ in the Na+/K(+)-ATPase pathway. Our data show that homocellular regulation of K+ in A6 cells is not due to up-regulation of gK.