High gene flow due to pelagic larval dispersal among South Pacific archipelagos in two amphidromous gastropods (Neritomorpha: Neritidae)

The freshwater stream fauna of tropical oceanic islands is dominated by amphidromous species, whose larvae are transported to the ocean and develop in the plankton before recruiting back to freshwater habitat as juveniles. Because stream habitat is relatively scarce and unstable on oceanic islands, this life history would seem to favor either the retention of larvae to their natal streams, or the ability to delay metamorphosis until new habitat is encountered. To distinguish between these hypotheses, we used population genetic methods to estimate larval dispersal among five South Pacific archipelagos in two amphidromous species of Neritid gastropod (Neritina canalis and Neripteron dilatatus). Sequence data from mitochondrial cytochrome oxidase I (COI) revealed that neither species is genetically structured throughout the Western Pacific, suggesting that their larvae have a pelagic larval duration (PLD) of at least 8 weeks, longer than many marine species. In addition, the two species have recently colonized isolated Central Pacific archipelagos in three independent events. Since colonization, there has been little or no gene flow between the Western and Central Pacific archipelagos in N. canalis, and high levels of gene flow across the same region in N. dilatatus. Both species show departures from neutrality and recent dates for colonization of the Central Pacific archipelagos, which is consistent with frequent extinction and recolonization of stream populations in this area. Similar results from other amphidromous species suggest that unstable freshwater habitats promote long-distance dispersal capabilities.

Phenotype-dependent synthesis of transferrin receptor in rat alveolar epithelial cell monolayers

The iron carrier protein transferrin plays a prominent antioxidant and anti-bacterial role in the lower respiratory tract and is present at elevated concentrations in lung epithelial lining fluid relative to plasma. The level of transferrin receptor synthesis in primary cultures of rat alveolar epithelial cells (AECs) was investigated. Transferrin receptor was found to be synthesized early in AEC cultures with the alveolar type II cell-like phenotype. Cell-surface receptor localization was attenuated upon apparent transdifferentiation to the alveolar type I cell-like phenotype later in culture. Binding of (125)I-labeled transferrin to the receptor indicated that surface and total cellular transferrin receptor levels were decreased in the type I-like cells. Inclusion of keratinocyte growth factor (KGF) in culture media (10 ng/ml) resulted in retention of transferrin receptor localized to the basolateral surface. Transferrin-receptor-specific internalization of (59)Fe-transferrin was also limited to the basolateral surface of KGF-treated monolayers. These data suggest that alveolar type II (but not type I) cells express functional transferrin receptor in adult rat alveolar epithelium.

Vesicular stomatitis virus G-pseudotyped lentivirus vectors mediate efficient apical transduction of polarized quiescent primary alveolar epithelial cells

We investigated the use of lentivirus vectors for gene transfer to quiescent alveolar epithelial cells. Primary rat alveolar epithelial cells (AEC) grown on plastic or as polarized monolayers on tissue culture-treated polycarbonate semipermeable supports were transduced with a replication-defective human immunodeficiency virus-based lentivirus vector pseudotyped with the vesicular stomatitis virus G (VSV-G) protein and encoding an enhanced green fluorescent protein reporter gene. Transduction efficiency, evaluated by confocal microscopy and quantified by fluorescence-activated cell sorting, was dependent on the dose of vector, ranging from 4% at a multiplicity of infection (MOI) of 0.1 to 99% at an MOI of 50 for AEC grown on plastic. At a comparable titer and MOI, transduction of these cells by a similarly pseudotyped murine leukemia virus vector was approximately 30-fold less than by the lentivirus vector. Importantly, comparison of lentivirus-mediated gene transfer from the apical or basolateral surface of confluent AEC monolayers (R(t) > 2 kOmega. cm(2); MOI = 10) revealed efficient transduction only when VSV-G-pseudotyped lentivirus was applied apically. Furthermore, treatment with EGTA to increase access to the basolateral surface did not increase transduction of apically applied virus, indicating that transduction was primarily via the apical membrane domain. In contrast, differentiated tracheal epithelial cells were transduced by apically applied lentivirus only in the presence of EGTA and at a much lower overall efficiency (approximately 15-fold) than was observed for AEC. Efficient transduction of AEC from the apical cell surface supports the feasibility of using VSV-G-pseudotyped lentivirus vectors for gene transfer to the alveolar epithelium and suggests that differences exist between upper and lower airways in the polarity of available receptors for the VSV-G protein.

Re-evaluating the Na(+) conductance of adult rat alveolar type II pneumocytes: evidence for the involvement of cGMP-activated cation channels

1. Alveolar epithelial type II pneumocytes were isolated and purified from adult rat lung by elastase digestion and differential adhesion, and cultured in serum-free medium for approximately 2 days on glass coverslips for subsequent patch-clamp studies employing symmetrical sodium isethionate solutions. 2. Whole-cell Na(+) currents exhibited essentially linear current-voltage relationships which were mildly inhibited (by approximately 25 %) by 10 microM amiloride. In contrast, 1 mM Zn(2+) inhibited the currents by approximately 55 % with an IC(50) of approximately 134 microM and maximal blockade achieved between 5 and 10 mM. The effects of Zn(2+) and amiloride were additive, and independent of the order of blocker addition. 3. Gd(2+), Zn(2+) and La(3+) at 10 mM were all effective at rapidly, reversibly and significantly blocking the amiloride-insensitive currents by approximately 60%. in contrast, Ni(2+) was a very weak inhibitor (30 % inhibition at 10 mM). 4. Pimozide (10 microM) caused inhibition of whole-cell cation conductance by approximately 55 %. The inhibitory effect of pimozide was concentration dependent with an IC(50) of approximately 1 microM and was maximally effective between 10 and 30 microM. Sequential addition of Zn(2+) and pimozide, in either order, revealed no overlapping inhibitory effect on the amiloride-insensitive conductance, and supported the notion that the Zn(2+)- and pimozide-sensitive currents are identical. 5. The amiloride-insensitive, Zn(2+)-blockable conductance was characterised by a Na(+)/K(+) permeability ratio (P(Na)/P(K)) of 0.73 +/- 0.02. 6. 8Br-cGMP (100 microM), a membrane-permeable analogue of cGMP, evoked a robust activation of whole-cell cation conductance to 220 % of control. This activation was apparent in either the absence or the presence of 10 microM amiloride, but was completely abolished in the presence of Zn(2+). 7. These data support the in vivo and in situ observations of a substantial amiloride-resistant Na(+) conductance, demonstrate directly that cyclic nucleotide-gated non-selective cation channels are functionally expressed in alveolar epithelial type II cells, and suggest that these channels may contribute to the fluid-reabsorptive driving force in adult lung.

Differential regulation of rat aquaporin-5 promoter/enhancer activities in lung and salivary epithelial cells

Aquaporin-5 (AQP5) is a water channel protein that is selectively expressed in respiratory, salivary, and lacrimal tissues. In order to establish the tissue-specific transcriptional programs that underlie its lung- and salivary-specific expression, a 4.5-kilobase pair DNA fragment encompassing the 5'-flanking region of the rat AQP5 gene has been characterized in detail. A major transcription start site utilized in lung and salivary glands has been localized downstream of a TATAA-like motif. Transient transfection assays of -4.3- and -1.7-AQP5-luciferase constructs in AQP5-expressing lung (MLE-15) and salivary (Pa-4) cells and nonexpressing fibroblast (NIH3T3) and epithelial (HeLa) cells demonstrate preferential transcriptional enhancement of reporter activities in MLE-15 and Pa-4 cells. Transient transfection assays of a series of 5' --> 3' deletion constructs of -4.3-AQP5-luciferase suggest that a common salivary and lung enhancer is located between nucleotides -274 and -139, and a lung-specific enhancer is located between nucleotides -894 and -710. There is one putative lung-specific repressor located in the region of nucleotides -1003/-894 and a common lung and salivary repressor located at nucleotides -503/-385. Moreover, 3' --> 5' deletions up to -171 and -127 base pairs almost abolish transcriptional activation in salivary and lung cells, respectively. Together, our findings indicate that the combination of enhancer/repressor elements within the proximal 5'-flanking region of rat AQP5 gene dictates its restricted expression in both lung and salivary cells.

Rates of protein transport across rat alveolar epithelial cell monolayers

The transport of model proteins, ranging from 12,300 to 150,000 Da, across tight rat alveolar epithelial cell monolayers (> 2000omegacm2) grown on polycarbonate filters, was studied. Model proteins were 14C-cytochrome c, 14C-ovalbumin, granulocyte-colony stimulating factor (G-CSF), 14C-bovine serum albumin (BSA), 125I-transferrin, and 14C-immunoglobulin G. Cytochrome c was extensively metabolized, as indicated by < 10% of the dose being translocated in intact form. This contrasts with 20-80% for the other model proteins studied. The flux of cytochrome c and G-CSF was symmetric in the apical-to-basolateral (ab) and basolateral-to-apical (ba) directions. By contrast, the flux of intact ovalbumin, BSA, transferrin and immunoglobulin G showed asymmetry, with the ab flux being higher by 2-5 times. There was no relationship between ab or ba fluxes and the molecular weights of these four model proteins. Since some of the proteins were translocated at much greater rates than are consistent with restricted diffusion or pinocytosis, receptor-mediated or adsorptive transcytosis may be involved.

Epidermal growth factor regulation in adult rat alveolar type II cells of amiloride-sensitive cation channels

Using the patch-clamp technique, we studied the effects of epidermal growth factor (EGF) on whole cell and single channel currents in adult rat alveolar epithelial type II cells in primary culture in the presence or absence of EGF for 48 h. In symmetrical sodium isethionate solutions, EGF exposure caused a significant increase in the type II cell whole cell conductance. Amiloride (10 microM) produced approximately 20-30% inhibition of the whole cell conductance in both the presence and absence of EGF, such that EGF caused the magnitude of the amiloride-sensitive component to more than double. Northern analysis showed that alpha-, beta- and gamma-subunits of rat epithelial Na(+) channel (rENaC) steady-state mRNA levels were all significantly decreased by EGF. At the single channel level, all active inside-out patches demonstrated only 25-pS channels that were amiloride sensitive and relatively nonselective for cations (P(Na(+))/P(K(+)) approximately 1.0:0.48). Although the biophysical characteristics (conductance, open-state probability, and selectivity) of the channels from EGF-treated and untreated cells were essentially identical, channel density was increased by EGF; the modal channel per patch was increased from 1 to 2. These findings indicate that EGF increases expression of nonselective, amiloride-sensitive cation channels in adult alveolar epithelial type II cells. The contribution of rENaC to the total EGF-dependent cation current under these conditions is quantitatively less important than that of the nonselective cation channels in these cells.

Mechanisms of EGF-induced stimulation of sodium reabsorption by alveolar epithelial cells

We investigated the effects of epidermal growth factor (EGF) on active Na+ absorption by alveolar epithelium. Rat alveolar epithelial cells (AEC) were isolated and cultivated in serum-free medium on tissue culture-treated polycarbonate filters. mRNA for rat epithelial Na+ channel (rENaC) alpha-, beta-, and gamma-subunits and Na+ pump alpha1- and beta1-subunits were detected in day 4 monolayers by Northern analysis and were unchanged in abundance in day 5 monolayers in the absence of EGF. Monolayers cultivated in the presence of EGF (20 ng/ml) for 24 h from day 4 to day 5 showed an increase in both alpha1 and beta1 Na+ pump subunit mRNA but no increase in rENaC subunit mRNA. EGF-treated monolayers showed parallel increases in Na+ pump alpha1- and beta1-subunit protein by immunoblot relative to untreated monolayers. Fixed AEC monolayers demonstrated predominantly membrane-associated immunofluorescent labeling with anti-Na+ pump alpha1- and beta1-subunit antibodies, with increased intensity of cell labeling for both subunits seen at 24 h following exposure to EGF. These changes in Na+ pump mRNA and protein preceded a delayed (>12 h) increase in short-current circuit (measure of active transepithelial Na+ transport) across monolayers treated with EGF compared with untreated monolayers. We conclude that EGF increases active Na+ resorption across AEC monolayers primarily via direct effects on Na+ pump subunit mRNA expression and protein synthesis, leading to increased numbers of functional Na+ pumps in the basolateral membranes.

Modulation of t1alpha expression with alveolar epithelial cell phenotype in vitro

T1alpha is a recently identified gene expressed in the adult rat lung by alveolar type I (AT1) epithelial cells but not by alveolar type II (AT2) epithelial cells. We evaluated the effects of modulating alveolar epithelial cell (AEC) phenotype in vitro on T1alpha expression using either soluble factors or changes in cell shape to influence phenotype. For studies on the effects of soluble factors on T1alpha expression, rat AT2 cells were grown on polycarbonate filters in serum-free medium (MDSF) or in MDSF supplemented with either bovine serum (BS, 10%), rat serum (RS, 5%), or keratinocyte growth factor (KGF, 10 ng/ml) from either day 0 or day 4 through day 8 in culture. For studies on the effects of cell shape on T1alpha expression, AT2 cells were plated on thick collagen gels in MDSF supplemented with BS. Gels were detached on either day 1 (DG1) or day 4 (DG4) or were left attached until day 8. RNA and protein were harvested at intervals between days 1 and 8 in culture, and T1alpha expression was quantified by Northern and Western blotting, respectively. Expression of T1alpha progressively increases in AEC grown in MDSF +/- BS between day 1 and day 8 in culture, consistent with transition toward an AT1 cell phenotype. Exposure to RS or KGF from day 0 prevents the increase in T1alpha expression on day 8, whereas addition of either factor from day 4 through day 8 reverses the increase. AEC cultured on attached gels express high levels of T1alpha on days 4 and 8. T1alpha expression is markedly inhibited in both DG1 and DG4 cultures, consistent with both inhibition and reversal of the transition toward the AT1 cell phenotype. These results demonstrate that both soluble factors and alterations in cell shape modulate T1alpha expression in parallel with AEC phenotype and provide further support for the concept that transdifferentiation between AT2 and AT1 cell phenotypes is at least partially reversible.

Keratinocyte growth factor modulates alveolar epithelial cell phenotype in vitro: expression of aquaporin 5

We investigated the role of keratinocyte growth factor (KGF) in regulation of alveolar epithelial cell (AEC) phenotype in vitro. Effects of KGF on cell morphology, expression of surfactant apoproteins A, B, and C (SP-A, -B, and -C), and expression of aquaporin 5 (AQP5), a water channel present in situ on the apical surface of alveolar type I (AT1) cells but not expressed in alveolar type II (AT2) cells, were evaluated in AECs grown in primary culture. Observations were made on AEC monolayers grown in serum-free medium without KGF (control) or grown continuously in the presence of KGF (10 ng/ml) from either Day 0 (i.e., the time of plating) or Day 4 or 6 through Day 8 in culture. AECs monolayers express AQP5 only on their apical surfaces as determined by cell surface biotinylation studies. Control AECs grown in the absence of KGF through Day 8 express increasing levels of AQP5, consistent with transition toward the AT1 cell phenotype. Exposure of AECs to KGF from Day 0 results in decreased AQP5 expression, retention of a cuboidal morphology, and greater numbers of lamellar bodies relative to control on Day 8 in culture. AECs treated with KGF from Day 4 or 6 exhibit a decrease in AQP5 expression through subsequent days in culture, as well as an increase in expression of surfactant apoproteins. These data, showing that KGF both prevents and reverses the increase in AQP5 (and decrease in surfactant apoprotein) expression that accompanies progression of the AT2 toward the AT1 cell phenotype, support the concepts that transdifferentiation between AT2 and AT1 cell phenotypes is at least partially reversible and that KGF may play a major role in modulating AEC phenotype.

Size-dependent dextran transport across rat alveolar epithelial cell monolayers

The transport of dextrans (approximately 4 to approximately 150 kDa) across an in vitro model of the alveolar epithelial barrier was studied to determine the effects of molecular size on pulmonary absorption of macromolecular drugs. Fluorescein isothiocyanate (FITC)-labeled dextrans (FDs) with average molecular weights (all in kDa) of 3.86 (FD4), 9 (FD10), 19.8 (FD20), 40.5 (FD40), 71.6 (FD70), and 156.9 (FD150) were utilized as model macromolecular drugs. Unidirectional fluxes of FDs at 37 and 4 degrees C were measured from the appearance rates of FD in the receiver fluid of open-circuited monolayers (>2000 omega-cm2) of rat alveolar epithelial cells. Apparent permeability coefficients (P(app)) were estimated from the observed flux and the corresponding concentration gradient of FD. Results showed that FD fluxes were the same in both apical-to-basolateral (AB) and opposite (BA) directions at each molecular weight studied. The P(app) was not significantly different at 0.5 and 1.0 mg/mL FD40 donor concentrations. The FD P(app) (x 10(-8)cm/s) decreased gradually from 1.35 for FD4 to 0.32 for FD40, indicating an apparent inverse relationship between P(app) and molecular weight of FD. By contrast, P(app) was about the same at 0.13 for both FD70 and FD150. When experimental temperature was lowered to 4 degrees C, P(app) decreased by approximately 40% for FDs of 4 through 40 kDa, whereas the decrease in P(app) was by approximately 80% for larger FDs of both 70 and 150 kDa. Moreover, these FDs were found to be relatively intact (approximately 90%) in either receiver fluid after 5-h flux experiments without detectable levels of metabolites in the respective donor fluid, suggesting that alveolar epithelial cells allow translocation of FDs intact across the barrier. Equivalent pore analysis, assuming restricted diffusion of FDs of 4-40 kDa via cylindrical, water-filled pores across the cell monolayer revealed a population of large equivalent pores with approximately 5.6 nm radius. These data suggest that smaller macromolecules (radius <5 nm) traverse the alveolar epithelial barrier via paracellular pathways, and that larger (i.e., radius > or = 6 nm) macromolecules likely cross the barrier via other pathways (e.g., pinocytosis).

Effects of Mycobacterium tuberculosis on the bioelectric properties of the alveolar epithelium

To investigate the hypothesis that Mycobacterium tuberculosis penetrates the alveolar epithelium by downregulating its barrier properties, we evaluated the interactions between M. tuberculosis and rat alveolar epithelial cell monolayers that are believed to share electrophysiologic properties of the human alveolar epithelium. Nonproteinaceous components of M. tuberculosis caused marked declines in electrical resistance and equivalent short-circuit current of the alveolar epithelial cell monolayers, indicating a reduction in the capacity to maintain tight intercellular junctions and to actively reabsorb sodium. M. tuberculosis elicited production of TNF-alpha mRNA and protein by alveolar epithelial cells, and the effects of recombinant TNF-alpha on the bioelectric properties of the alveolar epithelial paralleled those of M. tuberculosis. Furthermore, the effects of M. tuberculosis on alveolar epithelial resistance were abrogated by neutralizing anti-TNF-alpha antibodies. These results indicate that M. tuberculosis elicits production of TNF-alpha, which in turn reduces the bioelectric barrier properties of the alveolar epithelium. These findings provide insight into potential mechanisms by which M. tuberculosis establishes infection and disease in the lung.

Horseradish peroxidase transport across rat alveolar epithelial cell monolayers

PURPOSE

To evaluate the transport characteristics of horseradish peroxidase (HRP, a nonspecific fluid-phase endocytosis marker) across an in vitro model of tight (> 2,000 ohm-cm2) rat alveolar epithelial cell monolayers grown on tissue culture-treated polycarbonate filters.

METHODS

Unidirectional HRP fluxes were estimated from the appearance rate of HRP in the receiver fluid following instillation in the donor fluid as a function of donor [HRP] and temperature. Molecular species present in either bathing fluid were determined at the end of flux experiments using fluorescein isothiocyanate (FITC)-labeled HRP by gel permeation chromatography. Cell-associated HRP activity at the end of the transport experiment was determined, as were the rates of recycling and transcellular movement of HRP. An enzymatic assay was uses to quantify HRP activity in the bathing fluid and cells.

RESULTS

Unidirectional HRP fluxes were symmetric and increased linearly with up to 50 microM donor [HRP]. The apparent permeability coefficient of HRP was reduced by 3.5 times upon lowering the temperature from 37 to 4 degrees C. About 50% of the FITC-labeled species present in either receiver fluid was intact HRP. Cell-associated HRP estimated from apical HRP incubation was about 4 times greater than that from basolateral incubation. Recycling into apical fluid of cell-associated HRP following apical incubation occurred rapidly with a half-time (T1/2) of approximately 5 min, reaching a plateau at approximately 67% of the initial cell-associated HRP, while transcellular movement of HRP (into basolateral fluid) took place with a T1/2 of approximately 20 min, attaining a steady-state at approximately 13% of the initial cell-associated HRP. Basolateral recycling of HRP was also rapid (T1/2 = approximately 5 min) reaching a steady-state at approximately 35% of the initial basolaterally-bound HRP. Transcellular movement of HRP following basolateral incubation was slower (T1/2 = approximately 70 min), leveling off at 50% of the initial cell-associated HRP.

CONCLUSIONS

HRP appears to be transported relatively intact (approximately 50%) across rat alveolar epithelial barrier via nonspecific fluid-phase endocytosis. The transepithelial pinocytotic rate of alveolar epithelial cells is estimated to be about 25 nL/cm2/h.

Effects of EGF on alveolar epithelial junctional permeability and active sodium transport

We evaluated the effects of epidermal growth factor (EGF) on transepithelial resistance (Rt) and active ion transport by alveolar epithelial cell (AEC) monolayers on tissue culture-treated polycarbonate filters. Rat type II cells were cultured in completely defined serum-free medium (MDSF) or MDSF supplemented with EGF. The addition of EGF from either day 0 (chronic) or day 4 (subacute) resulted in significant increases in Rt and short-circuit current (ISC) on day 5. After subacute exposure, these effects were delayed in onset by 6-12 h and sustained for > 24 h. Basolateral (but not apical) EGF was responsible for these effects, which were prevented by preincubation with tyrphostin RG-50864, a reversible specific inhibitor of the EGF receptor tyrosine kinase. ISC decreased, with a sensitivity to apical inhibitors of sodium transport in the order benzamil > amiloride > 5-(N-ethyl-N-isopropyl) amiloride in MDSF +/- EGF, and was completely inhibited by the addition of basolateral ouabain. Net sodium flux and Na+, K+ -ATPase activity both increased approximately 50% in the presence of EGF. These results indicate that 1) EGF decreases tight junctional permeability and increases active sodium transport by AEC monolayers via basolaterally located EGF receptors, and 2) the pathways for AEC sodium entry and exit (+/- EGF) are apical high amiloride affinity sodium channels and basolateral sodium pumps.

Reversible transdifferentiation of alveolar epithelial cells

Alveolar epithelial type II (AT2) cells have been thought to be the progenitors of terminally differentiated type I (AT1) cells in the adult animal in vivo. In this study, we used an AT1 cell-specific monoclonal antibody (mAb VIII B2) to investigate expression of the AT1 cell phenotype accompanying reversible changes in expression of the AT2 cell phenotype. AT2 cells were isolated and cultured either on attached collagen gels or on gels detached 1 or 4 days after plating and maintained thereafter as floating gels. Monolayers on both attached and floating gels were harvested on days 4 and 8 and analyzed by electron microscopy for changes in morphology and binding of mAb VIII B2. Results indicate that: (1) alveolar epithelial cells (AEC) on attached gels develop characteristics of the AT1 cell phenotype, (2) AEC on gels detached on day 1 maintain features of the AT2 cell phenotype (and do not react with mAb VIII B2), and (3) the expression of AT1 cell phenotypic traits seen by day 4 on attached gels is reversed after detachment. We conclude that commitment to the AT1 and AT2 cell lineages requires continuous regulatory input to maintain the differentiated states, and that transdifferentiation between AT2 and AT1 cells may be reversible.

Basolateral localization of Na(+)-HCO3- cotransporter activity in alveolar epithelial cells

We investigated the polarized distribution of Na(+)- and HCO3(-)-dependent recovery from intracellular acidification in alveolar epithelial cell monolayers. Rat alveolar type II cells were grown in primary culture on detachable tissue culture-treated Nuclepore filters. Each filter was mounted in a cuvette containing two fluid compartments (apical and basolateral) separated by the monolayer. Cells were loaded with the pH-sensitive dye BCECF and intracellular pH (pHi) measured spectrofluorometrically. Monolayers were studied at ambient temperature on days 3-4 in culture, coincident with the development of high tissue resistance (RT > or = 1000 omega.cm2). After the cells were acidified by NH3 prepulse, pHi recovered to baseline when Na+ was present in the basolateral fluid, but did not recover when Na+ was present only in the apical fluid. This basolateral Na(+)-dependent pHi recovery in the presence of HCO3-/CO2 was reduced, but present, in experiments where dimethylamiloride (DMA, 100 microM) or the stilbene derivative DIDS (500 microM) was in basolateral fluid. However, recovery was completely inhibited when both DMA and DIDS were present basolaterally. pHi recovery was not inhibited under Cl(-)-free conditions, indicating that cytoplasmic realkalinization was not effected by Na(+)-dependent Cl-HCO3- exchange. These data indicate that alveolar epithelial cells express a basolateral Na(+)- and HCO3(-)-dependent, DIDS-sensitive, Cl(-)-independent pHi recovery process that probably represents Na(+)-HCO3(-)-cotransport (symport). Basolateral Na(+)-HCO3- cotransport modulates pHi in alveolar epithelial cells, may contribute to regulation of intracellular volume and osmolarity, and may participate in signal transduction by hormones and growth factors.

Cl(-)-HCO3- exchanger isoform AE2 is restricted to the basolateral surface of alveolar epithelial cell monolayers

We investigated the polarized distribution and isoform specificity of anion exchange (Cl(-)-HCO3- exchange) in alveolar epithelial cell monolayers. Rat alveolar type II epithelial cell monolayers were grown in primary culture on detachable tissue culture-treated nuclepore filters. Each filter was mounted in a cuvette containing two fluid compartments (apical and basolateral) separated by the monolayer, the cells loaded with pH-sensitive dye, and intracellular pH (pHi) measured spectrofluorometrically. To assay for Cl(-)-HCO3- exchange, monolayers were incubated in medium containing 24 mM HCO3-/5% CO2 and 140 mM NaCl at pH 7.4 and acutely alkalinized by replacement of the fluid by HCO3(-)-free buffer containing Hepes (6 mM) at pH 7.4. Monolayers exhibited basolateral (but not apical) Cl(-)-dependent, Na(+)-independent recovery from an alkaline load that was abolished when Cl- was substituted by equimolar gluconate in the basolateral fluid, or if DIDS (500 microM) was present basolaterally. Substitution of gluconate for Cl- in the basolateral fluid, but not the apical fluid, resulted in a rise in steady-state pHi that was reversible on replacement of the basolateral fluid with Cl(-)-containing buffer, which occurred in HCO3(-)- but not Hepes-buffered medium. These data indicate that alveolar epithelial cells express basolateral membrane domain of these cells. Northern analysis of alveolar epithelial cell mRNA using anion exchanger (AE) isoform-specific cDNA probes indicates that alveolar epithelial cells express the AE2 isoform predominantly, if not exclusively, and do not express detectable AE1 (i.e., band-3 protein) or AE3.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat serum inhibits progression of alveolar epithelial cells toward the type I cell phenotype in vitro

Serum contains a number of polypeptide growth factors, hormones, and soluble matrix components and may influence the state of differentiation of epithelial cells in general and of alveolar epithelial cells (AEC) in particular. To evaluate the influence of sera on the transition from the type II toward the type I cell phenotype, we compared the effects of newborn bovine serum (NBS) and rat serum (RS) on morphologic changes and expression of a type I cell-specific epitope in AEC monolayers with time in primary culture. Rat type II AEC were harvested and cultured in defined serum-free medium (MDSF), MDSF + RS (5%), or MDSF + NBS (10%). Monolayer integrity was monitored by measuring transepithelial resistance (approximately 2,000 omega.cm2) and short-circuit current (approximately 4 microA/cm2). Binding of the type I cell-specific monoclonal antibody VIIIB2 was assessed between day 1 and day 11 by cell-based enzyme-linked immunosorbent assay (ELISA) and by immunoelectron microscopy (IEM). By ELISA, in MDSF and MDSF + NBS, VIIIB2 binding increased markedly after day 2, rising approximately 4-fold by day 8 (compared with day 1). In dramatic contrast, there was essentially no increase in VIIIB2 binding through day 11 in MDSF + RS. Results from IEM for apical surface binding of VIIIB2 were similar to those obtained by ELISA. Some morphologic differences were also noted, with cells in MDSF + RS being somewhat less spread at later times than those in MDSF or MDSF + NBS. These data indicate that the rate of rat type II AEC differentiation toward the type I cell phenotype is significantly modulated by soluble factor(s) present in rat serum.

Late appearance of a type I alveolar epithelial cell marker during fetal rat lung development

Recent studies in fetal lung using immunological and molecular probes have revealed type I and type II cell phenotypic markers in primordial lung epithelial cells prior to the morphogenesis of these cell types. We have recently developed monoclonal antibodies specific for adult type I cells. To evaluate further the temporal appearance of the type I cell phenotype during alveolar epithelial cell ontogeny, we analyzed fetal lung development using one of our monoclonal antibodies (mAb VIII B2). The epitope recognized by mAb VIII B2 first appears in the canalicular stage of fetal lung development, at approx. embryonic day 19 (E19), in occasional, faintly stained tubules. Staining with this type I cell probe becomes more intense and more widespread with increasing gestational age, during which time the pattern of staining changes. Initially, all cells of the distal epithelial tubules are uniformly labelled along their apical and basolateral surfaces. As morphological differentiation of the alveolar epithelium proceeds, type I cell immunoreactivity appears to become restricted to the apical surface of the primitive type I cells in a pattern approaching that seen in the mature lung. We concurrently analyzed developing fetal lung with an antiserum to surfactant apoprotein-A (alpha-SP-A). Consistent with the findings of others, labeling of SP-A was first detectable in scattered cuboidal cells at E18. Careful examination of the double-labeled specimens suggested that some cells were reactive with both the VIII B2 and SP-A antibodies, particularly at E20. Confocal microscopic analysis of such sections from E20 lung confirmed this impression. Three populations of cells were detected: cells labeled only with alpha-SP-A, cells labeled only with mAb VIII B2, and a smaller subset of cells labeled by both.(ABSTRACT TRUNCATED AT 250 WORDS)

Polarized distribution of Na(+)-H+ antiport activity in rat alveolar epithelial cells

In this study, we investigated the polarized distribution of Na(+)-H+ antiport activity in alveolar epithelial cell monolayers. Rat alveolar type II pneumocytes were grown on detachable tissue culture-treated Nuclepore filters. The membrane filters, with their adherent intact alveolar epithelial cell monolayers, were mounted in a cuvette designed to contain two fluid compartments separated by the monolayer. Cells were loaded with the pH-sensitive dye 2',7'-biscarboxyethyl-5,6-carboxylfluorescein and intracellular pH (pHi) measured spectrofluorometrically. Monolayers were studied at ambient temperature on days 3-4 in culture, coincident with the development of high tissue resistance (RT > or = 2000 omega.cm2). Cells were incubated in HCO(3-)-free Na+ buffer [(in mM) 140 NaCl, 6 HEPES, pH 7.4] and acidified by NH3 prepulse. Rates of realkalinization (JH+) were calculated as the product of the initial rate of recovery (dpHi/dt) and the intracellular buffer capacity (beta i). Under control conditions, recovery occurred with an initial JH+ of 28.4 mM/min. When 100 microM dimethylamiloride (DMA), an amiloride analogue with enhanced specificity for inhibiting the Na(+)-H+ antiporter, was present in the basolateral fluid, recovery was inhibited by > 90%. Conversely, when the monolayers were acidified in Na+ buffer containing DMA (100 microM) in the apical fluid, acidification and recovery were identical to control. Recovery from acidification was inhibited by basolateral DMA with a one-half maximal inhibitory concentration (IC50) of 100 nm and by basolateral amiloride with an IC50 of 10 microns. Recovery was completely inhibited by omission of Na+ from the basolateral fluid, but omission of Na+ from apical fluid had no effect. We conclude that Na(+)-H+ antiport activity is located exclusively on the basolateral surface of these alveolar epithelial cell monolayers, where it most likely represents the high-amiloride affinity isoform of the Na(+)-H+ antiporter, NHE-1. The Na(+)-H+ antiporter, asymmetrically distributed to the basolateral surface of the polarized alveolar epithelium, contributes to intracellular homeostasis in alveolar pneumocytes and may also play a role in signal transduction in these cells.

Defined medium for primary culture de novo of adult rat alveolar epithelial cells

Isolated type II pneumocytes grown in serum on tissue culture-treated polycarbonate filters form monolayers with characteristic bioelectric properties, and change morphologically with time in culture to resemble type I cells. Concurrently, the cells express type I cell surface epitopes, making this a potentially useful in vitro model with which to study regulation of alveolar epithelial cell function and differentiation. To define specific soluble growth factors and matrix substances that may regulate these processes, it would be preferable to culture isolated pneumocytes de novo under completely defined, serum-free conditions. In this study, we developed a completely defined serum-free medium that is capable of supporting alveolar epithelial cells in primary culture, allowing the formation of monolayers with characteristic bioelectric and phenotypic properties. Freshly isolated rat type II cells were resuspended in completely defined serum-free medium and plated de novo on polycarbonate filters. Plating efficiency, bioelectric properties, morphology, and binding of a type I cell-specific monoclonal antibody were determined as functions of time. Plating efficiency plateaus at about 14% by Day 3 in culture. Transepithelial resistance rises to high levels, peaking at 1.76 +/- 0.14 K omega-cm2 by Day 5 in culture. Short-circuit current peaks on Day 3 in culture at 2.71 +/- 0.35 microA/cm2. With time, the cells gradually become flattened with protuberant nuclei and long cytoplasmic extensions, more closely resembling type I cells, and begin to express a type I cell surface epitope.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of solute fluxes in isolated rat lungs

Most previous studies in isolated perfused lungs have utilized measurements of solute flow from alveolar to vascular space to characterize the barrier and transport properties of the alveolar epithelium. In this study, we measured flux of a series of nonionic hydrophilic solutes and sodium across the alveolar epithelium of the isolated rat lung from perfusate to airspace (P-->A), as well as from airspace to perfusate (A-->P). Apparent permeability-surface area products (PS) were calculated from the rates of isotope appearance downstream in either the airspace or the perfusate. Equivalent pore analysis of data for P-->A solute flow demonstrated a small pore population with radius 0.6 nm occupying 85% of the total pore area and a large pore population with radius 3.8 nm occupying 15% of the total area. Similar analysis of A-->P solute flux demonstrated a small pore population of 0.6 nm occupying 86% of the total pore area and a large pore population with radius 2.9 nm occupying 14% of total pore area. The ratio (R) of PSP-->A divided by PSA-->P was 0.8 for the nonionic hydrophilic solutes, while R for sodium was 0.5. In the presence of amiloride and ouabain, R for sucrose was unchanged while R for sodium increased to 0.8 due to a fall in PSA-->P. The difference between R for sodium and R for the passively transported solutes, and the reduction in this difference in the presence of sodium transport inhibitors, are consistent with active sodium reabsorption by the intact alveolar epithelium. Differences in measured unidirectional passive solute fluxes probably result from unequal effective surface areas for diffusion from vascular space to airspace and vice versa in the anatomically complex mammalian lung.

Evidence for amiloride-sensitive sodium channels in alveolar epithelial cells

To maintain alveolar air spaces relatively fluid free, the alveolar epithelium appears capable of vectorial transport of water and solutes. Active transepithelial transport of sodium by alveolar epithelial cell monolayers has previously been demonstrated, indicating that alveolar pneumocytes must possess ion transport mechanisms by which sodium can enter the cells apically for subsequent extrusion via Na(+)-K(+)-adenosinetriphosphatase activity at the basolateral surface. In this study, sodium entry mechanisms were investigated by directly measuring 22Na uptake into rat alveolar epithelial cells grown in primary culture. Cells exhibited increasing 22Na uptake with time over a 30-min interval. Total sodium uptake was compared in the presence and absence of several sodium transport inhibitors. Uptake was inhibited by the sodium channel blockers amiloride and benzamil but was not affected by two amiloride analogues (bromohexamethylene amiloride and dimethylamiloride) with diminished specificity for blocking sodium channels and enhanced specificity for inhibiting the Na(+)-H+ antiporter. Uptake was also unaffected by the chloride transport inhibitor bumetanide or by the absence of glucose. These data suggest that sodium uptake occurs primarily via sodium channel and that Na(+)-H+ antiport, Na(+)-K(+)-2Cl- cotransport, and Na(+)-glucose cotransport do not contribute significantly to sodium uptake under these experimental conditions. The presence of sodium channels in the alveolar epithelial cell membrane may provide the major entry mechanism by which sodium enters these cells for subsequent active extrusion, thereby effecting net salt and water reabsorption from the alveolar spaces.

Reactivity of alveolar epithelial cells in primary culture with type I cell monoclonal antibodies

An understanding of the process of alveolar epithelial cell growth and differentiation requires the ability to trace and analyze the phenotypic transitions that the cells undergo. This analysis demands specific phenotypic probes to type II and, especially, type I pneumocytes. To this end, monoclonal antibodies have been generated to type I alveolar epithelial cells using an approach designed to enhance production of lung-specific clones from a crude lung membrane preparation. The monoclonal antibodies were screened by a combination of enzyme-linked immunosorbent assay and immunohistochemical techniques, with the determination of type I cell specificity resting primarily on immunoelectron microscopic localization. Two of these new markers of the type I pneumocyte phenotype (II F1 and VIII B2) were used to analyze primary cultures of type II cells growing on standard tissue culture plastic and on a variety of substrata reported to affect the morphology of these cells in culture. On tissue culture plastic, the antibodies fail to react with early (days 1 to 3) type II cell cultures. The cells become progressively more reactive with time in culture to a plateau of approximately 6 times background by day 8, with a maximum rate of increase between days 3 and 5. This finding is consistent with the hypothesis that type II cells in primary culture undergo at least partial differentiation into type I cells. Type II cells grown on laminin, which reportedly delays the loss of type II cell appearance, and on fibronectin, which has been reported to facilitate cell spreading and loss of type II cell features, develop the type I cell markers during cultivation in vitro with kinetics similar to those on uncoated tissue culture plastic. Cells on type I collagen and on tissue culture-treated Nuclepore filters, which have been reported to support monolayers with type I cell-like morphology, also increase their expression of the II F1 and VIII B2 epitopes around days 3 to 5. Taken together with available morphologic information, these data suggest that expression of different alveolar epithelial cell phenotypic markers by type II cells in primary culture may be independently regulated. The monoclonal antibody probes described in this report should prove useful in the continued investigation of the mechanisms and regulation of alveolar epithelial cell differentiation.

Regulation of intracellular pH in alveolar epithelial cells

Alveolar type II epithelial cells in adult mammalian lungs actively transport salt and water, secrete surfactant, and differentiate into type I cells under normal conditions and following lung injury. It has become increasingly apparent that, like all epithelial cells, alveolar pneumocytes have evolved specialized ion transport mechanisms by which they regulate their intracellular pH (pHi). pHi is an important biological parameter in all living cells whose regulation is necessary for normal cellular homeostasis. pHi, and the ion transport mechanisms by which it is regulated, may contribute to many cellular processes, including transcellular transport, cell volume and osmolarity regulation, and intracellular transport, cell volume and osmolarity regulation, and intracellular electrolyte composition. Moreover, changes in pHi may serve as intracellular signals for biological processes such as cell growth, proliferation, and differentiation. We review herein the general principles of pHi regulation in epithelia and describe the mechanisms and effects of pHi regulation in alveolar pneumocytes. Many of the critical issues in current pulmonary research involve processes that pHi is most likely to affect, including maintenance of alveolar epithelial barrier integrity, development and maintenance of epithelial polarity, epithelial proliferation and differentiation, and regulation of transepithelial transport with respect to alveolar fluid balance in normal individuals and in those with excess alveolar fluid (i.e., pulmonary edema). Investigations into the regulation of pHi in alveolar pneumocytes and the regulatory effects of pHi in turn on other cellular processes are likely to yield information important to the understanding of lung biology and pulmonary disease.

Contribution of active Na+ and Cl- fluxes to net ion transport by alveolar epithelium

Changes in bioelectric properties of alveolar epithelial cell monolayers due to pharmacological agents such as beta-agonists, amiloride and ouabain have recently been reported. In order to determine specifically which ionic species contribute to these changes, fluxes of Na+ and Cl- across primary cultured monolayers of rat type II pneumocytes were directly measured. Monolayers were mounted in modified flux chambers and short-circuited. Unidirectional fluxes of 22Na (or 36Cl) and [14C]-mannitol were measured simultaneously. Experimental maneuvers included apical (A) exposure to 10 microM amiloride, basolateral (B) exposure to 1 mM ouabain, or basolateral exposure to 20 microM terbutaline. Results show that baseline monolayers actively reabsorb Na+ (about 0.14 micro Eq.cm-2.h-1) from the apical fluid, while mannitol and Cl- appear to traverse the alveolar epithelium passively. Active Na+ reabsorption was abolished by amiloride or ouabain, while Cl- and mannitol fluxes were unaffected. Terbutaline, on the other hand, markedly increased net absorption of Na+ and caused active transport of Cl- in the A to B direction. Passive mannitol flow was somewhat increased with terbutaline. These data indicate that active Na+ reabsorption across alveolar epithelial monolayers is dependent on intact Na+,K(+)-ATPase activity and cell Na+ entry (probably via Na+ channels), and can be stimulated by beta-agonists. Beta-agonists also cause active reabsorption of Cl- (passive under other conditions).

Na(+)-HCO3- symport modulates intracellular pH in alveolar epithelial cells

We investigated Na(+)-HCO3- cotransport as a mechanism for regulation of intracellular pH (pHi) in rat alveolar pneumocytes grown in primary culture. pHi was monitored using the fluorescent pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Cells incubated in 6 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) medium at pH 7.4 were subjected to rapid acidification by CO2 pulse. pHi recovered in the presence of Na+ with an initial rate (dpHi/dt) of 0.15 min-1, which was reduced by 67% when Na+ was replaced by choline, unaffected by substitution of gluconate for Cl-, reduced 40% in the presence of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 500 microM), and unchanged by amiloride (1 mM). In parallel experiments, cells were incubated at pH 7.4 with 20 mM HCO3- and pHi acutely lowered by NH3 prepulse. dpHi/dt in these experiments was 0.14 min-1 in the presence of Na+ and HCO3-, and reduced 79% under Na(+)-free conditions. These data indicate the presence of a Na(+)-dependent, Cl(-)-independent, DIDS-sensitive and amiloride-insensitive mechanism of recovery from acute intracellular acidification in alveolar pneumocytes, most consistent with Na(+)-HCO3- cotransport (symport) effecting acid extrusion under these experimental conditions. This ion transport mechanism may contribute to regulation of pHi in alveolar pneumocytes, transepithelial transport of acid-base equivalents across the alveolar epithelium, and modulation of pH of alveolar fluid in adult mammalian lungs.

NO2 decreases paracellular resistance to ion and solute flow in alveolar epithelial monolayers

Primary cultured monolayers of rat alveolar epithelial cells grown on tissue culture-treated Nuclepore filters were exposed to 2.5 ppm nitrogen dioxide (NO2) for 2-20 min. Changes in monolayer bioelectric properties and solute permeabilities were subsequently measured. Exposure to NO2 produced a dose-dependent decrease in monolayer transepithelial electrical resistance (Rt), whereas monolayer short-circuit current was unaffected. Post-exposure monolayer permeability to 14C-sucrose (which primarily crosses alveolar epithelium via the paracellular pathway) increased markedly. That for 3H-glycerol (which permeates through both paracellular and transcellular pathways) increased to a lesser extent. Partial recovery of Rt and solute permeabilities was noted by 48-h post-exposure. The time courses of the decrease in Rt and increase in solute permeabilities were similar. These results suggest that NO2 primarily impairs passive alveolar epithelial barrier functions in vitro, probably by altering intercellular junctions, and does not appear to directly affect cell membrane active ion transport processes. When correlated with results obtained from experimental approaches, studies of in vitro alveolar epithelial monolayers may facilitate investigations of dosimetry, sites, and mechanisms of oxidant injury in the lung.

Evidence for active H+ secretion by rat alveolar epithelial cells

A plasma membrane proton-translocating adenosinetriphosphatase (ATPase) has been identified in rat alveolar pneumocytes in primary culture using the pH-sensitive fluorescent probe 2',7'-biscarboxyethyl-5,6-carboxyfluorescein. Intracellular pH (pHi) was acutely lowered by NH3 prepulse in HCO3(-)-free medium buffered with 6 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid, and its recovery was measured thereafter under control conditions, in the presence of amiloride to inhibit Na(+)-H+ antiport, and in the presence of N-ethylmaleimide (NEM), a plasma membrane H(+)-ATPase inhibitor. Initial rate of pHi recovery was reduced by 67% in the presence of amiloride, 52% in the presence of NEM, and 96% in the presence of both. Recovery was decreased but not abolished in Na(+)-free buffer, was essentially abolished when NEM was present in the absence of Na+, and was also abolished by addition of the metabolic inhibitor KCN in glucose- and Na(+)-free medium. These data suggest that alveolar epithelial cells possess a plasma membrane H(+)-ATPase. In Na(+)-containing buffer at pH 7.4, steady-state pHi was 7.50. This value was unaffected by amiloride but decreased to 7.01 in the presence of NEM, suggesting active H(+)-ATPase and inactive Na(+)-H+ antiport at steady-state pHi. We conclude that this plasma membrane proton-translocating ATPase in alveolar pneumocytes may be an important mechanism contributing to regulation of steady-state pHi, recovery from acute intracellular acidification, and modulation of extracellular alveolar fluid pH.

Type I cell-like morphology in tight alveolar epithelial monolayers

The pulmonary alveolar epithelium separates air spaces from a fluid-filled interstitium and might be expected to exhibit high resistance to fluid and solute movement. Previous studies of alveolar epithelial barrier properties have been limited due to the complex anatomy of adult mammalian lung. In this study, we characterized a model of isolated alveolar epithelium with respect to barrier transport properties and cell morphology. Alveolar epithelial cells were isolated from rat lungs and grown as monolayers on tissue culture-treated Nuclepore filters. On Days 2-6 in primary culture, monolayers were analyzed for transepithelial resistance (Rt) and processed for electron microscopy. Mean cell surface area and arithmetic mean thickness (AMT) were determined using morphometric techniques. By Day 5, alveolar epithelial cells in vitro exhibited morphologic characteristics of type I alveolar pneumocytes, with thin cytoplasmic extensions and protruding nuclei. Morphometric data demonstrated that alveolar pneumocytes in vitro develop increased surface area and decreased cytoplasmic AMT similar to young type I cells in vivo. Concurrent with the appearance of type I cell-like morphology, monolayers exhibited high Rt (greater than 1000 omega.cm2), consistent with the development of tight barrier properties. These monolayers of isolated alveolar epithelial cells may reflect the physiological and morphological properties of the alveolar epithelium in vivo.

Hydrophilic solute transport across rat alveolar epithelium

Diffusional fluxes of a series of hydrophilic nonelectrolytes (molecular radii ranging from 0.15 to 0.57 nm) were measured across the alveolocapillary barrier in the isolated perfused fluid-filled rat lung. Radiolabeled solutes were lavaged into the distal air spaces of isolated Ringer-perfused lungs, and apparent permeability-surface area products were calculated from the rates of isotope appearance in the recirculating perfusate. These data were used to estimate theoretical equivalent pore radii in the alveolar epithelium, with the assumption of diffusive flow through water-filled cylindrical pores. The alveolar epithelium is best characterized by two pore populations, with small pores (radius 0.5 nm) occupying 98.7% of total pore area and larger pores (radius 3.4 nm) occupying 1.3% of total pore area. Net water flow out of the alveolar space was measured by including an impermeant solute (dextran) in the lavage fluid and measuring its concentration in the alveolar space as a function of time. Under control conditions, net water flow averaged 167 nl/s. When 24 microM terbutaline was added to the perfusate, net water flow increased significantly to 350 nl/s (P less than 0.001). Terbutaline had no effect on the fluxes of either glycerol (which traverses the small pore pathway) or sucrose (which traverses the large pore pathway). These findings indicate that the intact mammalian alveolar epithelium is complex and highly resistant to the flow of solutes and water.

Tight monolayers of rat alveolar epithelial cells: bioelectric properties and active sodium transport

Because the pulmonary alveolar epithelium separates air spaces from a fluid-filled compartment, it is expected that this barrier would be highly resistant to the flow of solutes and water. Investigation of alveolar epithelial resistance has been limited due to the complex anatomy of adult mammalian lung. Previous efforts to study isolated alveolar epithelium cultured on porous substrata yielded leaky monolayers. In this study, alveolar epithelial cells isolated from rat lungs and grown on tissue culture-treated Nucleopore filters resulted in tight monolayers with transepithelial resistance greater than 2,000 omega.cm2. Changes in bioelectric properties of these alveolar epithelial monolayers in response to ouabain, amiloride, and terbutaline are consistent with active sodium transport across a polarized barrier. 22Na flux measurements under short-circuit conditions directly confirm net transepithelial absorption of sodium by alveolar epithelial cells in the apical to basolateral direction, comparable to the observed short-circuit current (4.37 microA/cm2). The transport properties of these tight monolayers may be representative of the characteristics of the mammalian alveolar epithelial barrier in vivo.

Velocity of CO2 exchanges in the lungs

As outlined above, investigations over the past decade have provided further insight into the kinetics of many of the component processes that affect the overall velocity of CO2 exchanges in the lungs. The evolution of our understanding of the importance and role of these reactions and transport processes has not been entirely predictable. A variety of investigations into Roughton's original hypothesis regarding the mechanism of pH equilibration in capillary blood resulted in a renewed interest in carbonic anhydrase in the lung and other tissues. These latter studies have helped better define the location and kinetic properties of lung CA and its role in the overall velocity of CO2 exchange. Concurrently, a wealth of information has emerged regarding the transport properties of the red cell membrane. This has led to a better understanding of the mechanisms and characteristics of the band 3-mediated pathway for electroneutral anion exchange. Unfortunately, most of the kinetic data for this pathway have been obtained under nonphysiological conditions, making it difficult to utilize them directly in analyses of lung CO2 exchange kinetics. The potential detrimental effect of pharmacological agents in modifying lung CA and red cell CA activity and red cell anion exchange kinetics is among the more important factors to have emerged in the past decade. Ironically, the original question of whether a significant blood pH disequilibrium is present in the arterial circulation in man in vivo remains unresolved. However, the mechanisms underlying these phenomena are now recognized to be more complex than originally appreciated.

Sodium-dependent lysine flux across bullfrog alveolar epithelium

Amino acid transport across the alveolar epithelial barrier was studied by measuring radiolabeled lysine fluxes across bullfrog lungs in an Ussing chamber. In the absence of a transmural electrical gradient, L-[14C]lysine was instilled into the upstream reservoir and the rate of appearance of the radiolabel in the downstream reservoir was determined. Two lungs from the same animal were used simultaneously to determine tracer fluxes both into and out of the alveolar bath. Results showed that the radiolabel flux measured in the alveolar to the pleural direction was greater than that measured in the opposite direction in the presence of sodium in the bathing fluids. The net flux of L-[14C]lysine was saturable with [Na+], with an apparent transport coefficient (Kt) of 28 mM for Na+. Hill analysis of [14C]lysine flux vs. [Na+] indicated a coupling ratio of 1:1 between sodium and radiolabeled L-lysine. Total L-lysine flux as a function of [L-lysine] was also saturable, with Kt of 7.3 mM for L-lysine. Ouabain significantly decreased absorptive (alveolar-to-pleural) radiolabel flux, while slightly increasing the flux observed in the opposite direction. L-leucine completely inhibited absorptive net flux of L-[14C]lysine. alpha-Methylaminoisobutyric acid (MeAIB), on the other hand, only slightly reduced net flux of L-[14C]lysine from the control value. The presence of a net absorptive, Na+-dependent amino acid flux across the alveolar epithelial barrier indicates that the tissue is capable of removing amino acids and sodium from the alveolar fluid by a coupled cotransport mechanism, which may be important for both protein metabolism and fluid balance by alveolar epithelium.

Cl-/HCO3- exchange modulates intracellular pH in rat type II alveolar epithelial cells

The role of an anion exchange pathway in modulating intracellular pH (pHi) under steady-state and alkaline load conditions was investigated in confluent monolayers of rat type II alveolar epithelial cells using the pH-sensitive fluorescent probe 2'-7'-biscarboxy-ethyl-5,6-carboxylfluorescein. Under steady-state conditions in the presence of 25 mM HCO3-, 5% CO2 at pHo 7.4, pHi was 7.32 in a Na+-replete medium and 7.33 in the absence of Na+. Steady-state pHi was 7.19 in a nominally HCO3(-)-free medium at pHo 7.4, and 7.52 in a Cl(-)-free medium, with both values significantly different from that obtained in the presence of both HCO3- and Cl-. Monolayers in which pHi was rapidly elevated by removal of HCO3-/CO2 from the bathing medium demonstrated an absolute requirement for Cl- to recover toward base-line pHi. The Km of Cl- for the external site of the exchange pathway was 11 +/- 1 mM. Recovery of pHi from the alkaline load in the presence of Cl- was inhibited 60% by the stilbene derivative 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Removal of Cl- from the medium of cells bathed in HCO3-/CO2 resulted in a rapid increment in pHi which returned to base line when Cl- was reintroduced into the bathing medium. In contrast, pHi was not perturbed by removal or addition of Cl- to monolayers bathed in a 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid-buffered medium, indicating that HCO3- was the preferred species for transport. Recovery of pHi from an alkaline load was not affected by the presence or absence of Na+. These findings define the transport pathway as Na+-independent Cl-/HCO3- exchange. This pathway contributes importantly to determining resting pHi of pneumocytes and enables the cell to recover from an alkaline load.

Effects of culture conditions on susceptibility of alveolar epithelial cell monolayers to NO2

The effects of nitrogen dioxide (NO2) exposure on primary cultured monolayers of rat type II pneumocytes were investigated as a function of the isolation and culture conditions. Monolayers were cultured in Eagle's minimum essential medium (MEM) and in MEM supplemented with Ham's F-12; in some experiments, the initial cell suspension was also replated after 3 h. Both supplementation of the basal medium and replating increased the sensitivity of the monolayers to NO2, as measured by reduction in dome formation of plastic dishes 24 h post-exposure. These findings suggest that comparisons of in vitro toxicologic observations may be complicated by the effects of specific experimental conditions.

Effects of nitrogen dioxide on alveolar epithelial barrier properties

This study analyzed the effects of nitrogen dioxide (NO2) on alveolar epithelial permeability and transport properties. Primary cultured monolayers of rat Type II pneumocytes, cultured on both nonporous and porous surfaces, were used as models of isolated alveolar epithelium for in vitro exposure to nitrogen dioxide. The effects of nitrogen dioxide exposure for monolayers cultured on nonporous substrata were monitored by observing the changes in the net volume of fluid under the monolayer; for cells cultured on porous substrata, alterations in tissue bioelectric properties were noted. As a first step, primary cultured monolayers of rat Type II pneumocytes plated on nonporous plastic Petri dishes were used to investigate the effects of nitrogen dioxide on alveolar epithelial barrier properties. Such monolayers form fluid filled domes that are thought to result from active solute transport from medium to substratum, with water following passively. We used dome formation as a transport marker. Five-day-old cultures were directly exposed to 30 ppm NO2 in 5 percent CO2 in air at 25 degrees C, by cyclically tilting culture plates from side to side, so that both halves of the monolayer were exposed during each cycle. Exposures consisted of 10 cycles of four minutes each (two minutes per side), for a cell exposure time of 20 minutes. Control plates were simultaneously exposed to 5 percent CO2 in air under identical conditions. One day after the exposure, nitrogen dioxide-exposed monolayers exhibited significant decreases in dome density and individual dome volume, compared to the controls. By 48 hours post-exposure, differences between nitrogen dioxide-exposed and control monolayers were less, but remained significant. These results showed that short-term sublethal exposures to nitrogen dioxide produce a decrease in dome formation in Type II alveolar epithelial cell monolayers. This finding is most likely due to a decrease in the active transepithelial sodium transport rate, or an increase in the permeability of cell membranes or tight junctions, or both. Addition of vitamin E-containing liposomes to the culture media 24 hours pre-exposure did not affect the nitrogen dioxide-induced decrease in dome formation, indicating that under these circumstances no protective effect was provided by the antioxidant.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for active sodium transport across alveolar epithelium of isolated rat lung

We have previously presented evidence that cultured alveolar epithelial cell monolayers actively transport sodium from medium to substratum, a process that can be inhibited by sodium transport blockers and stimulated by beta-agonists. In this study, the isolated perfused rat lung was utilized in order to investigate the presence of active sodium transport by intact adult mammalian alveolar epithelium. Radioactive tracers (22Na and [14C]sucrose) were instilled into the airways of isolated Ringer-perfused rat lungs whose weight was continuously monitored. The appearance of isotopes in the recirculated perfusate was measured, and fluxes and apparent permeability-surface area products were determined. A pharmacological agent (amiloride, ouabain, or terbutaline) was added to the perfusate during each experiment after a suitable control period. Amiloride and ouabain resulted in decreased 22Na fluxes and a faster rate of lung weight gain. Terbutaline resulted in increased 22Na flux and a more rapid rate of lung weight loss. [14C]sucrose fluxes were unchanged by the presence of these pharmacological agents. These data are most consistent with the presence of a regulable active component of sodium transport across intact mammalian alveolar epithelium that leads to removal of sodium from the alveolar space, with anions and water following passively. Regulation of the rate of sodium and fluid removal from the alveolar space may play an important role in the prevention and/or resolution of alveolar pulmonary edema.

Characterization of Na+-H+ antiport in type II alveolar epithelial cells

The presence of a Na+-H+ exchange pathway in the plasma membrane of type II alveolar epithelial cells was explored using the pH-sensitive fluorescent probe 2,7-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) to monitor changes in cytosolic pH. Freshly prepared pneumocytes suspended in medium at pH 7.4 had an intracellular pH of 7.07 +/- 0.07. Acid-loaded cells equilibrated in sodium-free buffer showed rapid cytoplasmic alkalinization when exposed to sodium. This response to sodium was inhibited greater than 90% by 10(-4) M amiloride. The presence of the K+ ionophore, valinomycin, had no effect on the rate of Na+-dependent alkalinization, indicating the electroneutrality of the system. Li+ partially supported the alkalinization process, but other monovalent cations, notably K+, Rb+, and Cs+, were without effect. Kinetic analysis for Na+ at the external binding site yielded KNat (dissociation constant) = 62 +/- 3 mM. Hill equation analysis of the data derived a Hill coefficient (n) = 1.2 +/- 0.1 for Na+, consistent with a 1:1 stoichiometry for Na+ and H+ for the transporter. The Ki for amiloride inhibition of proton efflux at the external locus was 0.45 microM. These findings define the transport pathway as Na+-H+ antiport, with kinetic parameters somewhat similar to those described for other cell types. Antiport activity was detected at intracellular pH (pHi) values of 6.8 or below, with no activity observed at pHi 7.0-7.2. It is suggested that Na+-H+ exchange is a major mechanism whereby pneumocytes recover from an acid load and that this transport pathway may play an important role in vectorial reabsorption of Na+ from the alveolar air spaces.

Effects of dextran-bound inhibitors on carbonic anhydrase activity in isolated rat lungs

Effects of macromolecular Prontosil-dextran inhibitors (PD) on carbonic anhydrase (CA) activity in isolated rat lungs were studied. Isolated lungs were perfused with Krebs-Ringer bicarbonate (KRB) solutions containing no inhibitor, PD 100,000 (mol wt 100,000), PD 5,000 (mol wt 5,000), or low-molecular-weight inhibitors (Prontosil or acetazolamide). The time course of effluent perfusate pH equilibration was measured in a stop-flow pH electrode apparatus. Pulmonary CO2 excretion (Vco2) was monitored by continuously recording expired CO2 concentration. The lungs were ventilated with room air and perfused at 37 degrees C with KRB prebubbled with 5% CO2- 20% O2- 75% N2. The results obtained show that both the low-molecular-weight inhibitors and PD's caused postcapillary pH disequilibria (delta pH) in effluent perfusate. However, only acetazolamide and Prontosil caused a reduction in Vco2. These results suggest that there is an intravascular CA, presumably associated with endothelial cell membranes, that is accessible to all inhibitors used and is responsible in part for equilibration of the CO2- HCO3- -H+ reactions in the perfusate but, under the conditions used, does not affect CO2 excretion; and there is an extravascular (possibly intracellular) CA that can be inhibited by low-molecular-weight inhibitors, is primarily responsible for enhanced CO2 transfer across the alveolar-capillary barrier (perhaps via facilitation of CO2 diffusion), and is in part responsible for pH equilibration.

Effects of terbutaline on sodium transport in isolated perfused rat lung

We have previously presented evidence that cultured alveolar epithelial cell monolayers actively transport sodium from medium to substratum, and that this process can be stimulated by beta-agonists. In this study the isolated perfused rat lung was utilized to investigate sodium transport across intact mammalian alveolar epithelium. Radioisotopic tracer(s) (22Na and/or [14C]sucrose) were instilled into the airways of isolated Ringer-perfused rat lungs. The appearance of isotope(s) in the recirculated perfusate was measured and a permeability-surface area product was calculated. Pharmacological agent(s) (terbutaline and/or propranolol) were present in the instillate or were added to the perfusate during the experiments. Terbutaline alone, whether in the instillate or perfusate, caused a significant increase in 22Na flux. This increase was prevented by the presence of propranolol. [14C]sucrose fluxes were unaffected by the presence of terbutaline. These data are consistent with the presence of an active component of sodium transport across intact mammalian alveolar epithelium that leads to removal of sodium from the alveolar space.

Sodium-amino acid cotransport by type II alveolar epithelial cells

Type II alveolar epithelial cell monolayers have been shown to actively transport sodium (Na+). Coupling to amino acid uptake could be an important mechanism for Na+ entry into these cells. This study demonstrates the presence of such a coupled cotransport mechanism in the plasma membrane of isolated type II cells by use of the nonmetabolizable amino acid analogue alpha-methylaminoisobutyric acid (MeAIB). Transport of MeAIB in 137 mM Na+ is saturable, with the uptake constant (Vmax) equaling 13.9 pmol X mg prot-1 X s-1 and the Michaelis-Menten constant (Km) equaling 0.13 mM. In the presence of Na+, MeAIB is accumulated against a concentration gradient. MeAIB uptake in the absence of Na+ is linear with MeAIB concentration, as expected for simple diffusion. The Hill coefficient for Na+-MeAIB cotransport is 1.11, suggesting a 1:1 stoichiometry. Proline inhibits Na+-MeAIB cotransport, with Ki equaling 0.5 mM. These findings suggest that Na+-amino acid cotransport may be an important pathway for Na+ (and/or amino acid) uptake into type II alveolar epithelial cells.