Binding of Griffonia simplicifolia I lectin to rat pulmonary alveolar macrophages and its use in purifying type II alveolar epithelial cells

Visualizing the beta interferon response in mice during infection with influenza A viruses expressing or lacking nonstructural protein 1

The innate host defense against influenza virus is largely dependent on the type I interferon (IFN) system. However, surprisingly little is known about the cellular source of IFN in the infected lung. To clarify this question, we employed a reporter mouse that contains the firefly luciferase gene in place of the IFN-β-coding region. IFN-β-producing cells were identified either by simultaneous immunostaining of lungs for luciferase and cellular markers or by generating conditional reporter mice that express luciferase exclusively in defined cell types. Two different strains of influenza A virus were employed that either do or do not code for nonstructural protein 1 (NS1), which strongly suppresses innate immune responses of infected cells. We found that epithelial cells and lung macrophages, which represent the prime host cells for influenza viruses, showed vigorous IFN-β responses which, however, were severely reduced and delayed if the infecting virus was able to produce NS1. Interestingly, CD11c(+) cell populations that were either expressing or lacking macrophage markers produced the bulk of IFN-β at 48 h after infection with wild-type influenza A virus. Our results demonstrate that the virus-encoded IFN-antagonistic factor NS1 disarms specifically epithelial cells and lung macrophages, which otherwise would serve as main mediators of the early response against infection by influenza virus.

Efficient targeting to alveolar macrophages by intratracheal administration of mannosylated liposomes in rats

The success of targeting systems to alveolar macrophages critically depends on internalization into these cells for pharmacological intervention. Direct respiratory delivery via inhalation of mannose modified liposomal carriers to alveolar macrophages is of great interest. To evaluate the targeting efficiency to alveolar macrophages by intratracheal administration of mannosylated liposomes (Man-liposomes), Man-liposomes with various ratio of mannosylated cholesterol derivatives, cholesten-5-yloxy-N-(4-((1-imino-2-D-thiomannosylethyl)amino)alkyl)formamide (Man-C4-Chol) as mannose receptor ligand were investigated with regard to their in vitro uptake in primary cultured alveolar macrophages and in vivo intratracheal administration in rats. The in vitro uptake of Man-liposomes took place in a concentration-dependent manner. The internalization of Man-liposomes with 7.5% (Man-7.5-liposomes) and 5.0% (Man-5.0-liposomes) Man-C4-Chol was considerably higher than that of Man-liposomes with 2.5% of Man-C4-Chol (Man-2.5-liposomes) and Bare-liposomes and significantly inhibited by an excess of mannan, suggesting mannose receptor-mediated endocytosis. After intratracheal administration of Man-7.5 and Man-5.0-liposomes in rats, a significantly high internalization and selective targeting to alveolar macrophages was observed. The enhanced cellular uptake in alveolar macrophages related to the mannose density of Man-liposomes was also confirmed both in vitro and in vivo confocal microscopy studies. These results demonstrate the efficient targeting to alveolar macrophages by the intratracheally administered Man-liposomes via mannose receptor-mediated endocytosis.

GM-CSF and the impaired pulmonary innate immune response following hyperoxic stress

We have previously demonstrated that mice exposed to sublethal hyperoxia (an atmosphere of >95% oxygen for 4 days, followed by return to room air) have significantly impaired pulmonary innate immune response. Alveolar macrophages (AM) from hyperoxia-exposed mice exhibit significantly diminished antimicrobial activity and markedly reduced production of inflammatory cytokines in response to stimulation with LPS compared with AM from control mice in normoxia. As a consequence of these defects, mice exposed to sublethal hyperoxia are more susceptible to lethal pneumonia with Klebsiella pneumoniae than control mice. Granulocyte/macrophage colony-stimulating factor (GM-CSF) is a growth factor produced by normal pulmonary alveolar epithelial cells that is critically involved in maintenance of normal AM function. We now report that sublethal hyperoxia in vivo leads to greatly reduced alveolar epithelial cell GM-CSF expression. Systemic treatment of mice with recombinant murine GM-CSF during hyperoxia exposure preserved AM function, as indicated by cell surface Toll-like receptor 4 expression and by inflammatory cytokine secretion following stimulation with LPS ex vivo. Treatment of hyperoxic mice with GM-CSF significantly reduced lung bacterial burden following intratracheal inoculation with K. pneumoniae, returning lung bacterial colony-forming units to the level of normoxic controls. These data point to a critical role for continuous GM-CSF activity in the lung in maintenance of normal AM function and demonstrate that lung injury due to hyperoxic stress results in significant impairment in pulmonary innate immunity through suppression of alveolar epithelial cell GM-CSF expression.

Effect of polycyclic aromatic hydrocarbons on generation and efflux of glutathione conjugates in primary cultured alveolar epithelial cells

This study was initiated to functionally characterize multidrug resistance associated protein (MRP)-mediated transport across the lung epithelium. Alveolar type II cells were isolated from rabbit lung and cultured on Transwell until forming a tight monolayers. After the cell monolayer was preloaded with monochlorobimane (mBCl) that is metabolized to a fluorescent glutathione conjugate (mBCl-SG), amount of mBCl-SG exported to apical and basal compartments were measured periodically. mBCl-SG was more preferentially exported in the apical direction than in the basolateral direction. Efflux of mBCl-SG from alveolar epithelial cells was significantly inhibited by a MRP inhibitor MK-571. Pharmacokinetic analysis of efflux profiles revealed that increased efflux of mBCl-SG by B[a]P is not due to enhanced MRP activity but simply due to an elevated level of mBCl-SG in the cells. Elevation of the intracellular level of mBCl-SG corresponded well to that of reduced GSH caused by B[a]P pretreatment.

Matrix metalloproteinases in acute inflammation: induction of MMP-3 and MMP-9 in fibroblasts and epithelial cells following exposure to pro-inflammatory mediators in vitro

Recent studies have demonstrated important pro-inflammatory roles for two matrix metalloproteinases (MMPs)-MMP-3 (stromelysin-1) and MMP-9 (gelatinase B)-in acute lung injury [Am. J. Respir. Cell Mol. Biol. 24 (2001) 1]. A role for MMP-3 in skin inflammation has also been demonstrated [Proc. Natl. Acad. Sci. U. S. A. 96 (1999) 6885]. While leukocytes (neutrophils and macrophages) are known to elaborate these tissue-destructive enzymes, parenchymal cells are also capable of synthesizing MMPs. In the present study, we examined the production of MMP-3 and MMP-9 by rodent lung fibroblasts, type II epithelial cells, and vascular endothelial cells. Dermal fibroblasts were also examined. Cells were examined under control conditions and in response to agonists that induce acute inflammatory tissue injury (IgG-containing immune complexes and lipopolysaccharide [LPS]). In the absence of stimulation, MMP-3 and MMP-9 were not detected or were present at low level. However, upon stimulation with either of the two pro-inflammatory agonists, production of both enzymes occurred in fibroblasts and epithelial cells (though not in endothelial cells). The observation that resident cells in the tissue parenchyma can elaborate MMPs in direct response to pro-inflammatory stimuli provides insight into possible mechanisms by which tissue damage occurs in acute inflammation.

ICAM-1 facilitates alveolar macrophage phagocytic activity through effects on migration over the AEC surface

We postulate that intercellular adhesion molecule-1 (ICAM-1) on type I alveolar epithelial cells (AEC) facilitates phagocytic activity of alveolar macrophages (AM) in the alveolus. When wild-type and ICAM-1-deficient mice were inoculated intratracheally with FITC-labeled microspheres, AM phagocytosis of beads (after 1 and 4 h) was significantly reduced in ICAM-1-/- mice compared with controls. To focus on ICAM-1-mediated interactions specifically involving AM and AEC, rat AM were placed in culture with rat AEC treated with neutralizing anti-ICAM-1 F(ab')(2) fragments. Blocking ICAM-1 significantly decreased the AM phagocytosis of beads. Planar chemotaxis of AM over the surface of AEC was also significantly impaired by neutralization of AEC ICAM-1. ICAM-1 in rat AEC is associated with the actin cytoskeleton. Planar chemotaxis of AM was also significantly reduced by pretreatment of the AEC monolayer with cytochalasin B to disrupt the actin cytoskeleton. These studies indicate that ICAM-1 on the AEC surface promotes mobility of AM in the alveolus and is critically important for the efficient phagocytosis of particulates by AM.

Impaired functional activity of alveolar macrophages from GM-CSF-deficient mice

We hypothesized that pulmonary granulocyte-macrophage colony-stimulating factor (GM-CSF) is critically involved in determining the functional capabilities of alveolar macrophages (AM) for host defense. To test this hypothesis, cells were collected by lung lavage from GM-CSF mutant mice [GM(-/-)] and C57BL/6 wild-type mice. GM(-/-) mice yielded almost 4-fold more AM than wild-type mice. The percentage of cells positive for the beta(2)-integrins CD11a and CD11c was reduced significantly in GM(-/-) AM compared with wild-type cells, whereas expression of CD11b was similar in the two groups. The phagocytic activity of GM(-/-) AM for FITC-labeled microspheres was impaired significantly compared with that of wild-type AM both in vitro and in vivo (after intratracheal inoculation with FITC-labeled beads). Stimulated secretion of tumor necrosis factor-alpha (TNF-alpha) and leukotrienes by AM from the GM(-/-) mice was greatly reduced compared with wild-type AM, whereas secretion of monocyte chemoattractant protein-1 was increased. Transgenic expression of GM-CSF exclusively in the lungs of GM(-/-) mice resulted in AM with normal or supranormal expression of CD11a and CD11c, phagocytic activity, and TNF-alpha secretion. Thus, in the absence of GM-CSF, AM functional capabilities for host defense were significantly impaired but were restored by lung-specific expression of GM-CSF.

Organic cation transport in rabbit alveolar epithelial cell monolayers

PURPOSE

To characterize organic cation (OC) transport in primary cultured rabbit alveolar epithelial cell monolayers, using [14C]-guanidine as a model substrate.

METHODS

Type II alveolar epithelial cells from the rabbit lung were isolated by elastase digestion and cultured on permeable filters precoated with fibronectin and collagen. Uptake and transport studies of [14C]-guanidine were conducted in cell monolayers of 5 to 6 days in culture.

RESULTS

The cultured alveolar epithelial cell monolayers exhibited the characteristics of a tight barrier. [14C]-Guanidine uptake was temperature dependent, saturable, and inhibited by OC compounds such as amiloride, cimetidine, clonidine, procainamide, propranolol, tetraethylammonium, and verapamil. Apical guanidine uptake (Km = 129 +/- 41 microM, Vmax = 718 +/- 72 pmol/mg protein/5 min) was kinetically different from basolateral uptake (Km = 580 +/- 125 microM, Vmax = 1,600 +/- 160 pmol/mg protein/5 min). [14C]-Guanidine transport across the alveolar epithelial cell monolayer in the apical to basolateral direction revealed a permeability coefficient (Papp) of (7.3 +/- 0.4) x 10(-7) cm/sec, about seven times higher than that for the paracellular marker [14C]-mannitol.

CONCLUSIONS

Our findings are consistent with the existence of carrier-mediated OC transport in cultured rabbit alveolar epithelial cells.

Role of matrix metalloproteinases in models of macrophage-dependent acute lung injury. Evidence for alveolar macrophage as source of proteinases

Matrix metalloproteinases (MMPs) have been implicated in the tissue injury seen in neutrophil-dependent models of acute lung injury. However, the role of MMPs in macrophage-dependent models of lung injury is unknown. To address this issue, the macrophage-dependent immunoglobulin A immune complex-induced lung injury model and the macrophage-dependent portion of the lipopolysaccharide-induced acute lung injury model in the rat were assessed for MMP involvement and for the source of these activities. In both models, injury was inhibited by the recombinant human tissue inhibitor of metalloproteinases-2. Bronchoalveolar lavage fluids (BALFs) from injured animals in both models showed increased levels of MMPs. Characterization of MMP production by isolated lung fibroblasts, endothelial cells, type II epithelial cells, and alveolar macrophages revealed that only the macrophage had the same spectrum of MMP activity as seen in the BALF. Further, isolated alveolar macrophages from injured lungs showed evidence of in vivo activation with the release of the same spectrum of MMP activities. Together these studies show that MMPs are produced during macrophage-dependent lung injury, that these MMPs play a role in the development of the lung injury, and that the alveolar macrophage is the likely source of these MMPs.

Characterization of matrix metalloproteinases produced by rat alveolar macrophages

Evidence presented in the accompanying article (Gibbs, D. F., T. P. Shanley, R. L. Warner, H. S. Murphy, J. Varani, and K. J. Johnson. 1999. Role of matrix metalloproteinases in models of macrophage-dependent acute lung injury: evidence for alveolar macrophage as source of proteinases. Am. J. Respir. Cell Mol. Biol. 20:1145-1154) implicates alveolar macrophage matrix metalloproteinases (MMPs) in two models of acute lung inflammation in the rat. As a prerequisite to understanding which specific MMPs might be involved in the injury and how they might function, it was necessary to know the spectrum of enzymes present. To this end, alveolar macrophages were obtained from normal rat lungs by bronchoalveolar lavage, placed in culture with and without various agonists, and assessed by a variety of techniques for MMPs. The identification process involved characterization by gelatin, beta-casein, and kappa-elastin zymography, with confirmation of identity by Western blot/immunoprecipitation. Message levels of detected MMPs were assessed by Northern blot. Rat alveolar macrophages were found to produce a low constitutive level of MMP-2 (72-kD gelatinase A) that was only modestly upregulated following stimulation with phorbol myristate acetate, bacterial lipopolysaccharide, or immunoglobulin A-containing immune complexes. Although control cells were found to produce little or no MMP-9 (92-kD gelatinase B) or MMP-12 (metalloelastase), both enzymes were markedly upregulated upon stimulation. In the same stimulated macrophages there was little activity against type I collagen (associated with MMP-13 [collagenase-3] on the basis of Western blotting), no activity suggestive of stromelysin or matrilysin, and no measurable secretion of the serine proteinases, elastase and cathepsin G. These data demonstrate the ability of rat alveolar macrophages to elaborate certain MMPs under proinflammatory conditions, consistent with their possible involvement in the progression of acute inflammation.

Bacterial toxin-induced pulmonary epithelial cytotoxicity and the protective effect of dibutyryl-cAMP

Bacterial infection is the most common cause of the adult respiratory distress syndrome which, in turn is associated with endothelial capillary permeability and alveolar oedema. Previously, we have demonstrated the direct cytotoxicity of the bacterial toxins Pseudomonas aeruginosa exotoxin A (Exo A) and Salmonella enteritidis lipopolysaccharide (LPS) on pulmonary endothelial cells. The purpose of this study was to investigate the effect of Exo A and LPS on pulmonary epithelial cells in vitro. We also tested the protective effect of dibutyryl cyclic adenosine monophosphate (db-cAMP) on Exo A-induced cytotoxicity. In cultured rat alveolar epithelial cells (RAEC) Exo A caused cytotoxicity as measured by 51Cr release from these cells. LPS did not injure RAEC's. Pretreatment of RAEC with db-cAMP (1 mM) attenuated Exo A induced cytotoxicity. We conclude that (1) Exo A directly injures epithelial lung cells and may contribute to lung injury in cases of bacterial infection; (2) db-cAMP protects alveolar epithelial cells against Exo A-induced cytotoxicity and (3) alveolar epithelial cells in this model are resistant to LPS induced injury.

Effector mechanisms responsible for gamma interferon-mediated host resistance to Legionella pneumophila lung infection: the role of endogenous nitric oxide differs in susceptible and resistant murine hosts

To facilitate identification of the effector mechanism(s) responsible for gamma interferon (IFN-gamma)-mediated host resistance to Legionella pneumophila, a murine model of legionellosis in BALB/c mice with a targeted disruption in the IFN-gamma gene (gamma knockout [GKO] mice) was developed. Immunocompetent BALB/c mice and GKO mice were inoculated intratracheally with virulent L. pneumophila (10(6) bacteria per mouse), and bacterial clearance and the pulmonary inflammatory response were assessed. L. pneumophila did not replicate in, and was rapidly cleared from, the lungs of immunocompetent BALB/c mice, demonstrating that immunocompetent BALB/c mice are resistant to replicative L. pneumophila pulmonary infections. In contrast, similarly infected GKO mice developed persistent, replicative intrapulmonary L. pneumophila infections with extrapulmonary dissemination of the bacteria to the spleen. Histopathologic and flow cytometric analysis of L. pneumophila-infected lung tissue demonstrated that while immunocompetent BALB/c mice develop multifocal pneumonitis which resolves, similarly infected GKO mice develop diffuse pneumonitis with persistent neutrophil recruitment into the lung. Intratracheal administration of exogenous IFN-gamma to L. pneumophila-infected GKO mice facilitated intrapulmonary clearance of the bacteria, confirming the pivotal role of IFN-gamma in innate host defenses to L. pneumophila lung infection in this murine host. The potential role of endogenous reactive nitrogen intermediates, including nitric oxide (NO), in IFN-gamma-mediated resistance to L. pneumophila pulmonary infections in immunocompetent BALB/c mice was subsequently assessed. Macrophage inducible nitric oxide synthetase (an enzyme responsible for the production of NO) was induced in alveolar cells from L. pneumophila-infected immunocompetent BALB/c mice (with maximal expression at 48 h postinfection) but was not induced in similarly infected GKO mice. However, administration of the NO synthetase inhibitor N-monomethyl-L-arginine did not significantly inhibit clearance of L. pneumophila from the lung of immunocompetent BALB/c mice (compared with that in similarly infected mice not administered N-monomethyl-L-arginine). In contrast, we have previously demonstrated that IFN-gamma-induced host resistance to replicative L. pneumophila lung infections in a susceptible murine host (A/J mice) is mediated, in part, by endogenous NO. Taken together, these studies identify a differing role of endogenous NO in IFN-gamma-mediated resistance to L. pneumophila pulmonary infection in susceptible and resistant murine hosts.

Anion exchange in type II pneumocytes freshly isolated from adult guinea-pig lung

We have studied chloride influx and efflux in a highly purified preparation of type II cells freshly isolated from adult guinea-pig lung using 36Cl-. Chloride uptake was time-dependent, saturable (Km < 10 mM) and was inhibited by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; Ki approximately 80 microM). In the absence of external chloride (substituted by gluconate), 36Cl- uptake exhibited an overshoot above equilibrium. The rate of 36Cl- entry was strongly inhibited by addition of external nitrate; sulphate was a weaker inhibitor. 36Cl- efflux was stimulated by external bromide > bicarbonate > or = chloride > or = citrate; and was inhibited by propionate > acetate > oxalate. Although the "chloride channel blocker" 4-nitro-2-(3-phenylpropylamino)benzoate (0.14 mM) caused an inhibition, 36Cl- influx did not appear to be electrogenic. These data are compatible with the existence of a substantial electroneutral anion-exchange pathway for chloride transport in freshly isolated adult type II pneumocytes.

Alterations in eicosanoid production by rat alveolar type II cells isolated after silica-induced lung injury

Although alveolar type II cells in primary culture have been shown to produce eicosanoids and exposure of type II cells to silica in vitro alters eicosanoid production, the production of eicosanoids by alveolar type II cells isolated after acute lung injury in vivo has not been evaluated. Therefore, we investigated the production of arachidonic acid (AA) metabolites by alveolar type II cells isolated after silica-induced lung injury. Alveolar type II cells were isolated from rats 14 days after intratracheal silica instillation and from untreated animals. Type II cells were separated into normotrophic and hypertrophic populations by centrifugal elutriation, and secreted eicosanoids were determined under basal and stimulated conditions by enzyme immunoassay on the day of isolation and after 1 day in culture. Under basal conditions, freshly isolated type II cells from silica-treated animals produced more prostaglandin (PG) E2 than 6-keto-PGF1 alpha or thromboxane B2 (TxB2). Production of all three prostanoids increased with increasing cell size. The calcium ionophore A23187 stimulated a less than 2-fold increase in PGE2 and 6-keto-PGF1 alpha production in all groups of cells. In contrast, this calcium ionophore greatly enhanced TxB2 and leukotriene C4 (LTC4) production by normotrophic type II cells from both untreated and silica-treated animals. Incubation with exogenous AA suggested that the increased capability of the hypertrophic cells to synthesize PGE2 and TxB2 was due primarily to an increase in arachidonate availability. The hypertrophic type II cells also appear to have increased prostacyclin synthase activity. There were no differences in the catabolism of PGE2 between the normotrophic and the hypertrophic type II cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydrogen ion currents in rat alveolar epithelial cells

Alveolar epithelial cells isolated from rats and maintained in primary culture were studied using the whole-cell configuration of the "patch-clamp" technique. After other ionic conductances were eliminated by replacing permeant ions with N-methyl-D-glucamine methanesulfonate, large voltage-activated hydrogen-selective currents were observed. Like H+ currents in snail neurons and axolotl oocytes, those in alveolar epithelium are activated by depolarization, deactivate upon repolarization, and are inhibited by Cd2+ and Zn2+. Activation of H+ currents is slower in alveolar epithelium than in other tissues, and often has a sigmoid time course. Activation occurs at more positive potentials when external pH is decreased. Saturation of the currents suggests that diffusion limitation may occur; increasing the pipette buffer concentration from 5 to 120 mM at a constant pH of 5.5 increased the maximum current density from 8.7 to 27.3 pA/pF, indicating that the current amplitude can be limited in 5 mM buffer solutions by the rate at which buffer molecules can supply H+ to the membrane. These data indicate that voltage-dependent H+ currents exist in mammalian cells.

Alveolar epithelial cells block lymphocyte proliferation in vitro without inhibiting activation

In the face of constant exposure to inhaled antigens, precise local regulation of immune responses in the pulmonary alveolar space is essential to achieve a delicate balance between host defense and excessive immune responses that are incompatible with the primary physiologic function of the lung. We postulated that the cells of the alveolar epithelium may have an immunoregulatory role in the lung. Therefore, we have examined the effects of primary cultures of rat type II alveolar epithelial cells on lymphocyte proliferation and on the expression of a number of markers of T-cell activation. Monolayers of alveolar epithelial cells suppressed proliferation and DNA synthesis by concanavalin A-stimulated rat splenocytes. Suppression of [3H]thymidine incorporation was independent of the dose of mitogen and was also apparent when lymphocytes were stimulated with phorbol esters and calcium ionophore, suggesting that the effect was independent of cell surface binding of the lectin. Suppression was reversed 48 h after lectin-stimulated splenocytes were removed from co-culture with alveolar epithelial cells. Despite inhibition of lymphocyte proliferation, other markers of T-cell activation were induced normally in lymphocytes cultured with alveolar epithelial cells. Culture with alveolar epithelial cells did not inhibit the the production of interleukin-2 by stimulated lymphocytes. Furthermore, by fluorescence-activated cell sorter analysis, equal proportions of stimulated lymphocytes in culture alone or with alveolar epithelial cell monolayers were induced to express receptors for interleukin-2 and for transferrin.(ABSTRACT TRUNCATED AT 250 WORDS)

Whole-cell K+ currents in type II pneumocytes freshly isolated from rat lung: pharmacological evidence for two subpopulations of cells

The patch clamp technique was used to record whole cell K+ currents in type II pneumocytes freshly isolated from adult rats. Depolarizing voltage steps evoked outward K+ currents which were distinguished into low and high threshold types, only one type being apparent in any one cell. Low-threshold (LT) currents were activated at test potentials of -40 mV to -20 mV and were reduced in amplitude by 20 mM tetraethylammonium (TEA). High-threshold (HT) currents were activated only at test potentials positive to -20 mV and current noise was always greater than for LT currents. HT currents were also significantly more sensitive than were LT currents to block by TEA. Quinine (1 mM) blocked LT currents reversibly at all activating test potentials. HT currents were also reversibly blocked by 1 mM quinine, but in a voltage-dependent manner, the degree of block increasing with increasing test potential. 4-Aminopyridine (2 mM) further distinguished the two current types: it was virtually without effect on HT currents but caused large reductions in LT current amplitudes, apparently by acting on the open channels underlying this current. These data clearly distinguish type II pneumocytes into two subpopulations and suggest that they may play separate roles in the functioning of the intact alveolar epithelium.

State-dependent inactivation of K+ currents in rat type II alveolar epithelial cells

Inactivation of K+ channels responsible for delayed rectification in rat type II alveolar epithelial cells was studied in Ringer, 160 mM K-Ringer, and 20 mM Ca-Ringer. Inactivation is slower and less complete when the extracellular K+ concentration is increased from 4.5 to 160 mM. Inactivation is faster and more complete when the extracellular Ca2+ concentration is increased from 2 to 20 mM. Several observations suggest that inactivation is state-dependent. In each of these solutions depolarization to potentials near threshold results in slow and partial inactivation, whereas depolarization to potentials at which the K+ conductance, gK, is fully activated results in maximal inactivation, suggesting that open channels inactivate more readily than closed channels. The time constant of current inactivation during depolarizing pulses is clearly voltage-dependent only at potentials where activation is incomplete, a result consistent with coupling of inactivation to activation. Additional evidence for state-dependent inactivation includes cumulative inactivation and nonmonotonic from inactivation. A model like that proposed by C.M. Armstrong (1969. J. Gen. Physiol. 54: 553-575) for K+ channel block by internal quaternary ammonium ions accounts for most of these properties. The fundamental assumptions are: (a) inactivation is strictly coupled to activation (channels must open before inactivating, and recovery from inactivation requires passage through the open state); (b) the rate of inactivation is voltage-independent. Experimental data support this coupled model over models in which inactivation of closed channels is more rapid than that of open channels (e.g., Aldrich, R.W. 1981. Biophys. J. 36:519-532). No inactivation results from repeated depolarizing pulses that are too brief to open K+ channels. Inactivation is proportional to the total time that channels are open during both a depolarizing pulse and the tail current upon repolarization; repolarizing to more negative potentials at which the tail current decays faster results in less inactivation. Implications of the coupled model are discussed, as well as additional states needed to explain some details of inactivation kinetics.

Resistance of rat pulmonary alveolar epithelial cells to neutrophil- and oxidant-induced injury

We have previously reported that rat pulmonary alveolar epithelial cells are resistant to neutrophil-generated oxidants in contrast to the situation described for endothelial cells. In the present study, we investigated the roles of intracellular catalase and glutathione-dependent reactions in providing protection against cytotoxic concentrations of H2O2 and stimulated neutrophils. Catalase was found to be instrumental in protecting epithelial cells because when inhibited by either azide or 3-amino-1,2,4-triazole, there was an increase in the cytotoxic effect of exogenous H2O2 and stimulated neutrophils. Associated with this potentiation of injury was a reduction in epithelial cell clearance of H2O2. Partial inhibition of glutathione-dependent reactions by depleting intracellular glutathione with buthionine sulfoximine or by inhibiting the enzyme glutathione reductase with 1,3-bis(2-chloroethyl)-1-nitrosourea also augmented the cytotoxic effect of both H2O2 and stimulated neutrophils. This increase in neutrophil-induced cytotoxicity was caused by the addition of an oxidant-dependent mechanism of killing on top of the previously described oxidant-independent pathway. Importantly, the increased susceptibility to injury caused by inhibition of glutathione-dependent reactions was not associated with a reduction in epithelial cell consumption of exogenous H2O2, contrary to the case with catalase. This suggests that there are glutathione-dependent reactions that protect epithelial cells in ways separate from reducing the total burden of exogenous H2O2 on the cells.

Griffonia simplicifolia I: fluorescent tracer for microcirculatory vessels in nonperfused thin muscles and sectioned muscle

Previous studies on mice have revealed that the Griffonia simplicifolia I (GSI) lectin selectively binds to capillaries in a number of microvascular beds. These observations suggest that the lectin might be a suitable microvascular marker for physiological studies of skeletal muscle, particularly when fluorescent visualization of vessels is desired independently of their perfusion status. Since species and strain heterogeneity has been demonstrated for certain lectins associated with the microcirculatory vessels, lectin binding was studied in a number of muscles taken from the major species of mammals used for experimental purposes. Staining of cryostat sections confirmed the utility of GSI as a marker for capillaries from muscle of mice, rats, hamsters, rabbits, dogs, and monkeys. Differential staining of arterioles and veins was revealed by double labeling with GSI and antisera to Factor VIII-related antigen. Double labeling for GSI binding and alkaline phosphatase activity revealed that the GSI method detects many more capillaries and terminal arterioles than does the alkaline phosphatase method. GSI binding to unfixed whole mounts of thin skeletal muscles (hamster cheek pouch, mouse diaphragm, and rat cremaster) was studied to determine whether the GSI lectin would be a suitable marker for intravital studies. An extensive microvascular bed, including terminal arterioles, venules, and capillaries, was revealed which could be visualized in the complete absence of perfusion with fluorescent markers. These observations suggest that the GSI lectin may be extremely useful as a probe for the microcirculation of skeletal muscle in many types of physiological experiments.

Rat alveolar macrophages synthesize leukotriene B4 and 12-hydroxyeicosatetraenoic acid from alveolar epithelial cell-derived arachidonic acid

Although recent reports have described arachidonic acid metabolism of individual types of lung cells, the actual profile of eicosanoids that are produced in the lung may reflect interactions between different cell types. Because macrophages and epithelial cells are in close physical contact within the alveolus, we measured the eicosanoids produced by combined cultures of these cells. We found that the [14C]arachidonic acid that was released from previously labeled epithelial cells following A23187 stimulation was metabolized by alveolar macrophages to leukotriene B4 and 12-hydroxyeicosatetraenoic acid, which are products not normally produced by these epithelial cells. Simultaneously, there was a decrease in 6-keto-prostaglandin F1 alpha, the end product of prostacyclin metabolism and a major product of epithelial cell arachidonate metabolism but not macrophage arachidonate metabolism. A net increase in leukotriene B4 and a net decrease in 6-keto-prostaglandin F1 alpha were demonstrated by radioimmunoassay. Thus, the interaction of stimulated alveolar macrophages and epithelial cells alters the eicosanoid profile produced by each cell type alone in a manner that would tend to accentuate inflammatory processes within the alveolus.

Identification and characterization of the pulmonary alveolar type II cell Maclura pomifera agglutinin-binding membrane glycoprotein

The lectin Maclura pomifera agglutinin (MPA) binds to the apical surface of pulmonary alveolar type II but not type I cells. We show that MPA binds to a single membrane glycoprotein in type II cells with a molecular mass of 230 kDa in the rabbit and 200 kDa in the rat. The glycoprotein has an abundance of terminal N-acetylgalactosamine residues. It is a hydrophilic integral membrane protein suggesting that it has an extensive extramembrane domain or is an ion channel. The glycoprotein is similar in rat and rabbit, with the exception that the rat glycoprotein is partially sialylated and is trypsin sensitive. The MPA-binding glycoprotein represents a new integral membrane marker of the apical domain of the pulmonary alveolar type II cell.

Potassium currents in rat type II alveolar epithelial cells

1. Type II alveolar epithelial cells isolated from adult rats and grown in primary culture were studied using the whole-cell configuration of the gigohm-seal voltage clamp technique. 2. The average specific capacitance of type II cells was 2.5 microF/cm2, suggesting that type II cell membranes in vitro are irregular, with an actual area more than twice the apparent area. 3. Most type II cells have time- and voltage-dependent outward currents carried by potassium ions. Potassium currents activate with a sigmoid time course upon membrane depolarization, and inactivate during maintained depolarization. The average maximum whole-cell K+ conductance was 1.6 nS. 4. Two distinct types of K+-selective channels underlie outward currents in type II cells. Most cells have currents resembling delayed rectifier K+ currents in skeletal muscle, nerve and immune cells. A few cells had a different type of K+ conductance which is more sensitive to block by tetraethylammonium ions, has faster 'tail currents', and activates at more positive potentials. 5. In some experiments, individual type II cells were identified by staining with phosphine, a fluorescent dye which is concentrated in lamellar bodies. Both types of K+ channels were seen in type II cells identified with this dye. 6. Phosphine added to the bathing solution reversibly reduced K+ currents and shifted K+ channel activation to more positive potentials. Excitation of phosphine to fluoresce reduced irreversibly K+ currents in type II cells. The usefulness of phosphine as a means of identifying cells for study is discussed.

Lung injury in acute pancreatitis: primary inhibition of pulmonary phospholipid synthesis

Alterations in the pulmonary surfactant system are partly responsible for the respiratory insufficiency seen with acute pancreatitis. In this model of cerulein-induced pancreatitis in rats, we utilized a new stable isotope metabolic tracer technique to examine one aspect of the pulmonary surfactant system and its relationship to associated lung injury. We have demonstrated primary, early depression of lung phospholipid synthesis reflected in both lung tissue and alveolar washings. We suggest that this quantitative change in pulmonary surfactant synthetic rate may partly explain the occurrence of respiratory failure with acute pancreatitis.

Neutrophil-induced injury of rat pulmonary alveolar epithelial cells

The damage to pulmonary alveolar epithelial cells that occurs in many inflammatory conditions is thought to be caused in part by phagocytic neutrophils. To investigate this process, we exposed monolayers of purified rat alveolar epithelial cells to stimulated human neutrophils and measured cytotoxicity using a 51Cr-release assay. We found that stimulated neutrophils killed epithelial cells by a process that did not require neutrophil-generated reactive oxygen metabolites. Pretreatment of neutrophils with an antibody (anti-Mo1) that reduced neutrophil adherence to epithelial cells limited killing. Although a variety of serine protease inhibitors partially inhibited cytotoxicity, we found that neutrophil cytoplasts, neutrophil lysates, neutrophil-conditioned medium, purified azurophilic or specific granule contents, and purified human neutrophil elastase did not duplicate the injury. We conclude that stimulated neutrophils can kill alveolar epithelial cells in an oxygen metabolite-independent manner. Tight adherence of stimulated neutrophils to epithelial cell monolayers appears to promote epithelial cell killing.