Transport of sodium into apical membrane vesicles prepared from fetal sheep alveolar type II cells

Identification of Li+ binding sites and the effect of Li+ treatment on phospholipid composition in human neuroblastoma cells: a 7Li and 31P NMR study

Li(+) binding in subcellular fractions of human neuroblastoma SH-SY 5 Y cells was investigated using (7)Li NMR spin-lattice (T(1)) and spin-spin (T(2)) relaxation measurements, as the T(1)/T(2) ratio is a sensitive parameter of Li(+) binding. The majority of Li(+) binding occurred in the plasma membrane, microsomes, and nuclear membrane fractions as demonstrated by the Li(+) binding constants and the values of the T(1)/T(2) ratios, which were drastically larger than those observed in the cytosol, nuclei, and mitochondria. We also investigated by (31)P NMR spectroscopy the effects of chronic Li(+) treatment for 4--6 weeks on the phospholipid composition of the plasma membrane and the cell homogenate and found that the levels of phosphatidylinositol and phosphatidylserine were significantly increased and decreased, respectively, in both fractions. From these observations, we propose that Li(+) binding occurs predominantly to membrane domains, and that chronic Li(+) treatment alters the phospholipid composition at these membrane sites. These findings support those from clinical studies that have indicated that Li(+) treatment of bipolar patients results in irregularities in Li(+) binding and phospholipid metabolism. Implications of our observations on putative mechanisms of Li(+) action, including the cell membrane abnormality, the inositol depletion and the G-protein hypotheses, are discussed.

Single dexamethasone injection increases alveolar fluid clearance in adult rats

OBJECTIVE

Epithelial Na+ channels and Na+/K+-adenosine triphosphatase (ATPase) in alveolar epithelium have a very important role in the absorption of excessive fluid from the alveolar space. We examined whether single dexamethasone injection at therapeutic doses would modulate lung epithelial Na+ channels and Na+/K+-ATPase and increase alveolar fluid clearance in adult rats.

DESIGN

Controlled laboratory study.

SETTING

University research laboratory.

SUBJECTS

Adult male Sprague-Dawley rats (n = 138).

INTERVENTIONS

Rats were intraperitoneally injected with dexamethasone at a dose ranging from 0.02 to 2.0 mg/kg, and allowed free access to food and water.

MEASUREMENTS AND MAIN RESULTS

Alveolar fluid clearance was determined by measuring the increase in albumin concentration in the lung instillate solution. We discovered a significant increase in alveolar fluid clearance at 48 and 72 hrs after dexamethasone treatment. The effect of dexamethasone was dose dependent. In addition, increased alveolar fluid clearance was associated with a faster recover from hypoxemia, which was induced by filling the alveolar space with instillate solution. The dexamethasone-induced increase in alveolar fluid clearance was inhibited by amiloride and ouabain. Quantitative reverse transcriptase-polymerase chain reaction showed that dexamethasone treatment increased lung beta-epithelial Na+ channel mRNA levels. The expression of gamma-epithelial Na+ channel mRNA was also increased slightly. In contrast, alpha-epithelial Na+ channel mRNA levels did not differ from control levels. There was no change in alpha1- or beta1-Na+/K+-ATPase mRNA levels over 72 hrs after dexamethasone treatment. However, we found that lung Na+/K+-ATPase hydrolytic activity, determined by monitoring the ouabain-sensitive ATPase hydrolysis, was increased at 48 and 72 hrs after dexamethasone treatment.

CONCLUSIONS

Single dexamethasone injection at therapeutic doses is capable of modulating lung epithelial Na+ channels and Na+/K+-ATPase and increase alveolar fluid clearance, thereby accelerating recovery from pulmonary edema.

Impairment of transalveolar fluid transport and lung Na(+)-K(+)-ATPase function by hypoxia in rats

We examined whether hypoxic exposure in vivo would influence transalveolar fluid transport in rats. We found a significant decrease in alveolar fluid clearance of the rats exposed to 10% oxygen for 48 h. Terbutaline did not stimulate alveolar fluid clearance, and alveolar fluid cAMP levels were lower than those determined in normoxia experiment. Hypoxia did not influence the alveolar fluid lactate dehydrogenase levels, Evans blue dye fluid-to-serum concentration ratio, or lung wet-to-dry weight ratio, indicating no significant change in the permeability of alveolar-capillary barrier. Histological examination showed no significant fluid accumulation into the interstitium and the alveolar space. Hypoxia did not reduce lung ATP content; however, we found significant decrease in Na(+)-K(+)-ATPase hydrolytic activity in lung tissue preparations and isolated alveolar type II cells. Our data indicate that hypoxic exposure in vivo impairs transalveolar fluid transport, and this impairment is related to the decrease in alveolar epithelial Na(+)-K(+)-ATPase hydrolytic activity but is not secondary to the alteration of cellular energy source.

Transalveolar fluid absorption ability in rat lungs preserved with Euro-Collins solution and EP4 solution

BACKGROUND

Pulmonary reimplantation response, presenting lung edema, is a major obstacle of lung transplantation. Transalveolar fluid absorption mechanism, regulated by active transalveolar Na+ transport via Na+ channel and Na+-K+-ATPase, is considered to be essential for resolution of lung edema. We investigated the effect of lung preservation on this fluid transport mechanism.

METHODS

The rat lungs were flushed and preserved with either EP4 solution (EP4) or Euro-Collins solution (EC). First, we determined the basal transalveolar fluid movement by calculating alveolar fluid clearance (AFC) after pulmonary flushing, 24- and 72-hr preservation. Then, we assessed the effects of Na+ channel blocker, amiloride, and Na+-K+-ATPase inhibitor, ouabain, on AFC after 24-hr preservation. We further measured lung ATP content and Na+-K+-ATPase activity after 24-hr preservation to evaluate cellular metabolism and enzymatic activity during preservation.

RESULTS

We found that the lungs preserved with EC showed significantly lower AFC and less inhibitory effects of both blockers than with EP4 after 24-hr preservation. Na+-K+-ATPase activity was significantly lower with EC than with EP4, even though lung ATP content was not affected by preservation solution.

CONCLUSIONS

EP4 preservation provides a better environment for maintaining transalveolar fluid absorption mechanism than EC preservation. Therefore, lung preservation with EP4 may ensure more reliable ability in resolving pulmonary edema.

Conductive cation transport in apical membrane vesicles prepared from fetal lung

In order to characterise the apically-located conductive cation pathway of the type II pneumocyte, apical plasma membranes were prepared from mature fetal guinea pig lung. The protocol yielded purified apical membranes that enriched 19-fold with the brush border enzyme marker alkaline phosphatase; there was no significant contamination with other cellular membranes. A technique for imposing an outwardly-directed electrochemical Na+ gradient was used to amplify conductive 22Na+ uptake into vesicles. Uptake of 22Na+ was time-dependent, proportional to the magnitude of the Na+ gradient, specific and sensitive to the amiloride analogues phenamil and EIPA (apparent minimum Ki values of 50 nM and 10 microM, respectively, with maximum uptake inhibition of 42% and 39% at 100 microM). Uptake experiments in which the outwardly-directed Na+ gradient was replaced by outwardly-directed gradients of small monovalent cations and molecular cations were performed. The Na+/K+ permeability ratio was 1.2:1, and over the extended range of small monovalent cations, a permeability sequence of Na+ > K+ > Li+ > Rb+ > Cs+ was observed, indicating the presence of fixed negative charge in or spatially close to the pore. The molecular cation permeability sequence of NH4+ > methylamine+ > dimethylamine+ > choline+ > N-methyl-D-glucamine+ > tetraethylammonium+ > tetramethylammonium+, after transformation, gives an estimate of 8 A for the conducting pore diameter. These data are consistent with the presence in the apical membrane of fetal type II pneumocytes of a cation specific channel with low Na+ selectivity and amiloride sensitivity.

Sodium-coupled transport of glucose by plasma membranes of type II pneumocytes

Sodium-dependent absorption of alveolar fluid promotes efficient gas exchange. In animal models, alveolar glucose stimulates phlorizin-sensitive, Na(+)-dependent fluid absorption. It is hypothesized that Na+/glucose cotransporters are localized to apical membranes of type II pneumocytes. Enriched apical and basolateral plasma membrane vesicles were isolated from adult bovine type II pneumocytes. Uptakes of 22Na+ and [3H]glucose by enriched apical and basolateral vesicles were monitored over time. Following addition of external glucose (75 mM), 22Na+ uptake by mannitol-loaded, apically-enriched vesicles was significantly increased over controls. Substitution of interior-negative charge gradients for internally directed Na+ gradients increased glucose-dependent Na+ uptakes even greater. By contrast, external glucose did not significantly promote 22Na+ uptake by enriched basolateral vesicles. External Na+ (75 mM) significantly increased [3H]glucose uptakes by enriched apical vesicles with evidence of overshoot. Phlorizin (100 microM) inhibited both glucose-coupled 22Na+ uptakes and Na(+)-coupled [3H]glucose uptakes. These observations support localization of electrogenic, Na+/glucose cotransporters to enriched apical membranes of mature type II pneumocytes.

Sodium/proton transport by apical membranes of type-II pneumocytes

Recent studies fail to confirm the coexistence of Na+ channels and Na+/H+ exchange at the apical membranes of lower airway epithelia. Availability of plasma membrane vesicles simplifies the investigation of membrane transport processes. Apical and basolateral plasma membrane vesicles of disrupted type-II pneumocytes were fractionated upon nonlinear, continuous sucrose gradients. To investigate sodium transport, 22Na+ uptake by apical membrane vesicles was assayed in the presence and absence of transmembrane sodium diffusion potentials. Interior-negative sodium diffusion potentials promoted 22Na+ uptake 1.5-fold. Internally-directed H+ gradients or NH+4 gradients inhibited 22Na+ uptake 40-50%. Amiloride (1-1000 microM) inhibited uptake 10-79%. To investigate H+ transport, decay of transmembrane pH gradients was monitored with pH probe acridine orange. In the presence or absence of externally-directed H+ gradients, external sodium promoted internal alkalinization, except in the presence of external amiloride. These observations of amiloride-sensitive, electrogenic Na+ uptake and amiloride-sensitive, electroneutral, Na+/H+ coupling indicate coexistence of Na+ channels and Na+/H+ exchange at the apical membrane of type-II pneumocytes.

Epithelial ion transport in the fetal and perinatal lung

The lung is a complex organ whose intrauterine development depends on many factors, one of which is a continuous secretion of large volumes of Cl(-)-enriched fluid by the pulmonary epithelium. At birth this fluid must be cleared, and it is now known that this process depends in large part on active Na+ transport by the pulmonary epithelium. Only recently has it been possible to culture some of the different lung epithelial cells so that it is possible to investigate the role of individual epithelial cell types, their individual cellular transport mechanisms, and how these are affected by developmental lung maturity.

Sodium-proton exchange across the apical membrane of the alveolar type II cell of the fetal sheep

In order to detect and characterise Na(+)-H+ countertransport in the fetal lung epithelium we have studied under a variety of conditions the effect of an outward facing H+ gradient on Na+ uptake into purified apical membrane vesicles prepared from alveolar type II cells. Kinetic analysis of the data reveals both a diffusional and a saturable component of total Na+ uptake. Evidence for the presence of a Na(+)-H+ exchanger is demonstrated by (1) stimulation of Na+ uptake by proton loading of vesicles both in the presence and absence of chemical voltage clamping; (2) saturation kinetics with respect to external Na+ with a Km of 16 mM and a Vmax of 2.1 nmol/mg protein per min; (3) amiloride inhibition of Na+ uptake driven by pH gradient. We conclude that although diffusion may be the major component of total Na+ uptake at physiological external Na+ concentration, Na(+)-H+ countertransport provides a possible mechanism for the acidification of fetal lung liquid in-vivo in addition to its established role in intracellular pH and volume regulation.