Surface tension gradients: feasible model for gliding motility of Myxococcus xanthus.

We propose that surface tension is the driving force for the gliding motility of Myxococcus xanthus. Our model requires that the cell be able to excrete surfactant in a polar and reversible fashion. We present calculations that (i) estimate the surface tension difference across a cell necessary to move the cell at the observed rate, which is less than 10(-5) dyn/cm, an extremely small value; (ii) estimate the rate of surfactant excretion necessary to produce the required surface tension difference, a rate that we conclude to be metabolically reasonable; (iii) predict the behavior of cells moving in close apposition to each other, and show that the model is consistent with observed behavior; and (iv) predict the behavior of cells moving in dense swarms. In an accompanying paper we present experimental evidence to support the surface tension model.     

Arguments against and alternatives for an extracellular surfactant layer in the alveoli of mammalian lung

It is generally believed that lung alveoli contain an extracellular aqueous layer of surfactant material, which is allegedly required to prevent alveolar collapse at small lung volume; the surfactant's major constituent is a fully saturated phospholipid, referred to as dipalmitoyl lecithin or DPL. I herein demonstrate that the surfactant hypothesis of alveolar stability is fundamentally wrong. Although DPL is synthesized inside type II epithelial cells and stored in the typical inclusion bodies therein and lowers surface tension to zero in the surface balance, there is no evidence to the effect that type II cells secrete the DPL surfactant into the aqueous intra-alveolar layer which is shown by electron microscopy in support of the surfactant theory. To the contrary, all the evidence indicates that, when seen, such an extracellular layer is an artifact. This is probably upon the damage glutaraldehyde inflicts onto alveolar structures during fixation of air-inflated lung tissue. Furthermore, several cogent arguments invalidate the belief that an extracellular layer of DPL and serum proteins is present in the alveoli of normal lung. In light of these arguments, a surface tension role of DPL in alveolar stability is excluded. Three hypotheses for an alternative role of DPL in respiration mechanics are proposed. They are: (a) alveolar clearance by viscolytic and surfactant action (bubble or foam formation) on the aqueous systems which are present in lung alveoli during edema and in prenatal life and which would otherwise be impervious to air; (b) homeostasis of blood palmitate in normal lung; (c) modulation of the elasticity of terminal lung tissue by the intact inclusion bodies and parts thereof inside type II cells in normal lung.     

Surface tension influences cell shape and phagocytosis in alveolar macrophages

The effect of surface tension on alveolar macrophage shape and phagocytosis was assessed in vivo and in vitro. Surface tension was regulated in vivo by conditionally expressing surfactant protein (SP)-B in Sftpb-/- mice. Increased surface tension and respiratory distress were produced by depletion of SP-B and were readily reversed by repletion of SP-B in vivo. Electron microscopy was used to demonstrate that alveolar macrophages were usually located beneath the surfactant film on the alveolar surfaces. Reduction of SP-B increased surface tension and resulted in flattening of alveolar macrophages on epithelial surfaces in vivo. Phagocytosis of intratracheally injected fluorescent microbeads by alveolar macrophages was decreased during SP-B deficiency and was restored by repletion of SP-B in vivo. Incubation of MH-S cells, a mouse macrophage cell line, with inactive surfactant caused cell flattening and decreased phagocytosis in vitro, findings that were reversed by the addition of sheep surfactant or phospholipid containing SP-B. SP-B controls surface tension by forming a surfactant phospholipid film that regulates shape and nonspecific phagocytic activity of alveolar macrophages on the alveolar surface.     

Differential effects of chemoreceptor methylation-domain mutations on swarming and development in the social bacterium Myxococcus xanthus

The soil bacterium Myxococcus xanthus is a model organism for the study of multicellular behaviour and development in bacteria. M. xanthus cells move on solid surfaces by gliding motility, periodically reversing their direction of movement. Motility is co-ordinated to allow cells to effectively feed on macromolecules or prey bacteria when nutrients are plentiful and to form developmental fruiting bodies when nutrients are limiting. The Frz signal transduction pathway regulates cellular movements by modulating cell reversal frequency. Input to the Frz pathway is controlled by the cytoplasmic receptor, FrzCD, a methyl-accepting chemotaxis protein (MCP). FrzCD lacks the transmembrane and periplasmic domains common to MCPs but contains a unique N-terminal domain, the predicted ligand-binding domain. As deletion of the N-terminal domain of FrzCD only results in minor defects in motility, we investigated the possibility that the methylation of the conserved C-terminal domain of FrzCD plays a central role in regulating the pathway. For this study, each of the potential methylation sites of FrzCD were systematically modified by site-directed mutagenesis, substituting glutamine/glutamate pairs for alanines. Four of the seven mutations produced dramatic phenotypes; two of the mutations had a stimulatory effect on the pathway, as evidenced by cells hyper-reversing, whereas another two had an inhibitory effect, causing these cells to rarely reverse. These four mutants displayed defects in vegetative swarming and developmental aggregation. These results suggests a model in which the methylation domain can both activate and inhibit the Frz pathway depending on which residues are methylated. The diversity of phenotypes suggests that specific modifications of FrzCD act to differentially regulate motility and developmental aggregation in M. xanthus.     

The role of palmitic acid in pulmonary surfactant: enhancement of surface activity and prevention of inhibition by blood proteins

The surface activity of two surfactant preparations, Lipid Extract Surfactant (LES) and Survanta, was examined during adsorption and dynamic compression using a pulsating bubble surfactometer. At low surfactant phospholipid concentrations (1-2.5 mg/ml), Survanta reduces surface tension at minimum bubble radius faster than LES: however, with continued pulsation LES obtains a lower surface tension. Addition of surfactant-associated protein A (SP-A) to LES significantly reduces the time required to reduce surface tension. Survanta is completely unresponsive to the addition of SP-A in that no further reduction of surface tension is observed. Addition of various blood components has been previously shown to inactivate surfactants in vitro. Addition of fibrinogen to Survanta causes an increase in surface tension when measured in the absence of calcium. When assayed in the presence of calcium, inhibition by fibrinogen is not observed possibly due to aggregation of this protein. Albumin and alpha-globulin strongly inhibit Survanta at physiological serum concentrations both in the presence and absence of calcium. The surface activity of Survanta is also inhibited by lysophosphatidylcholine (lyso-PC). The role of palmitic acid in the surface activity of pulmonary surfactant was examined by adding palmitic acid to LES. At low phospholipid concentrations addition of palmitic acid (10% w/w of the surfactant phospholipid) greatly enhances the surface activity of LES. Maximal enhancement of surface activity and adsorption was observed at or above 7.5% added palmitic acid (w/w of surfactant lipid). LES supplemented with palmitic acid is more resistant to inhibition by fibrinogen, albumin, alpha-globulin and lyso-PC than LES alone, however, the counteraction of blood protein inhibition is not as pronounced as that observed with SP-A.     

Pulmonary surfactant function is abolished by an elevated proportion of cholesterol

A molecular film of pulmonary surfactant strongly reduces the surface tension of the lung epithelium-air interface. Human pulmonary surfactant contains 5-10% cholesterol by mass, among other lipids and surfactant specific proteins. An elevated proportion of cholesterol is found in surfactant, recovered from acutely injured lungs (ALI). The functional role of cholesterol in pulmonary surfactant has remained controversial. Cholesterol is excluded from most pulmonary surfactant replacement formulations, used clinically to treat conditions of surfactant deficiency. This is because cholesterol has been shown in vitro to impair the surface activity of surfactant even at a physiological level. In the current study, the functional role of cholesterol has been re-evaluated using an improved method of evaluating surface activity in vitro, the captive bubble surfactometer (CBS). Cholesterol was added to one of the clinically used therapeutic surfactants, BLES, a bovine lipid extract surfactant, and the surface activity evaluated, including the adsorption rate of the substance to the air-water interface, its ability to produce a surface tension close to zero and the area compression needed to obtain that low surface tension. No differences in the surface activity were found for BLES samples containing either none, 5 or 10% cholesterol by mass with respect to the minimal surface tension. Our findings therefore suggest that the earlier-described deleterious effects of physiological amounts of cholesterol are related to the experimental methodology. However, at 20%, cholesterol effectively abolished surfactant function and a surface tension below 15 mN/m was not obtained. Inhibition of surface activity by cholesterol may therefore partially or fully explain the impaired lung function in the case of ALI. We discuss a molecular mechanism that could explain why cholesterol does not prevent low surface tension of surfactant films at physiological levels but abolishes surfactant function at higher levels.     

Lysophospholipid and fatty acid inhibition of pulmonary surfactant: non-enzymatic models of phospholipase A2 surfactant hydrolysis

Secretory A(2) phospholipases (sPLA(2)) hydrolyze surfactant phospholipids cause surfactant dysfunction and are elevated in lung inflammation. Phospholipase-mediated surfactant hydrolysis may disrupt surfactant function by generation of lysophospholipids and free fatty acids and/or depletion of native phospholipids. In this study, we quantitatively assessed multiple mechanisms of sPLA(2)-mediated surfactant dysfunction using non-enzymatic models including supplementation of surfactants with exogenous lysophospholipids and free fatty acids. Our data demonstrated lysophospholipids at levels >or=10 mol% of total phospholipid (i.e., >or=10% hydrolysis) led to a significant increase in minimum surface tension and increased the time to achieve a normal minimum surface tension. Lysophospholipid inhibition of surfactant function was independent of the lysophospholipid head group or total phospholipid concentration. Free fatty acids (palmitic acid, oleic acid) alone had little effect on minimum surface tension, but did increase the maximum surface tension and the time to achieve normal minimum surface tension. The combined effect of equimolar free fatty acids and lysophospholipids was not different from the effect of lysophospholipids alone for any measurement of surfactant function. Surfactant proteins did not change the percent lysophospholipids required to increase minimum surface tension. As a mechanism that causes surfactant dysfunction, depletion of native phospholipids required much greater change (equivalent to >80% hydrolysis) than generation of lysophospholipids. In summary, generation of lysophospholipids is the principal mechanism of phospholipase-mediated surfactant injury in our non-enzymatic models. These models and findings will assist in understanding more complex in vitro and in vivo studies of phospholipase-mediated surfactant injury.     

A comparative study of mechanisms of surfactant inhibition

Pulmonary surfactant spreads to the hydrated air-lung interface and reduces the surface tension to a very small value. This function fails in acute respiratory distress syndrome (ARDS) and the surface tension stays high. Dysfunction has been attributed to competition for the air-lung interface between plasma proteins and surfactant or, alternatively, to ARDS-specific alterations of the molecular profile of surfactant. Here, we compared the two mechanisms in vitro, to assess their potential role in causing respiratory distress. Albumin and fibrinogen exposure at or above blood level concentrations served as the models for testing competitive adsorption. An elevated level of cholesterol was chosen as a known adverse change in the molecular profile of surfactant in ARDS. Bovine lipid extract surfactant (BLES) was spread from a small bolus of a concentrated suspension (27 mg/ml) to the air-water interface in a captive bubble surfactometer (CBS) and the bubble volume was cyclically reduced and increased to assess surface activity of the spread material. Concentrations of inhibitors and the concentration and spreading method of pulmonary surfactant were chosen in an attempt to reproduce the exposure of surfactant to inhibitors in the lung. Under these conditions, neither serum albumin nor fibrinogen was persistently inhibitory and normal near-zero minimum surface tension values were obtained after a small number of cycles. In contrast, inhibition by an increased level of cholesterol persisted even after extensive cycling. These results suggest that in ARDS, competitive adsorption may not sufficiently explain high surface tension, and that disruption of the surfactant film needs to be given causal consideration.     

A comparative study of mechanisms of surfactant inhibition

Pulmonary surfactant spreads to the hydrated air-lung interface and reduces the surface tension to a very small value. This function fails in acute respiratory distress syndrome (ARDS) and the surface tension stays high. Dysfunction has been attributed to competition for the air-lung interface between plasma proteins and surfactant or, alternatively, to ARDS-specific alterations of the molecular profile of surfactant. Here, we compared the two mechanisms in vitro, to assess their potential role in causing respiratory distress. Albumin and fibrinogen exposure at or above blood level concentrations served as the models for testing competitive adsorption. An elevated level of cholesterol was chosen as a known adverse change in the molecular profile of surfactant in ARDS. Bovine lipid extract surfactant (BLES) was spread from a small bolus of a concentrated suspension (27 mg/ml) to the air-water interface in a captive bubble surfactometer (CBS) and the bubble volume was cyclically reduced and increased to assess surface activity of the spread material. Concentrations of inhibitors and the concentration and spreading method of pulmonary surfactant were chosen in an attempt to reproduce the exposure of surfactant to inhibitors in the lung. Under these conditions, neither serum albumin nor fibrinogen was persistently inhibitory and normal near-zero minimum surface tension values were obtained after a small number of cycles. In contrast, inhibition by an increased level of cholesterol persisted even after extensive cycling. These results suggest that in ARDS, competitive adsorption may not sufficiently explain high surface tension, and that disruption of the surfactant film needs to be given causal consideration.     

Effects of fixatives on function of pulmonary surfactant.

The structure of pulmonary surfactant films remains ill defined. Although plausible film fragments have been imaged by electron microscopy, questions about the significance of the findings and even about the true fixability of surfactant films by the usual fixatives glutaraldehyde (GA), osmium tetroxide (OsO(4)), and uranyl acetate (UA) have not been settled. We exposed functioning natural surfactant films to fixatives within a captive bubble surfactometer and analyzed the effect of fixatives on surfactant function. The capacity of surfactant to reach near-zero minimum surface tension on film compression was barely impaired after exposure to GA or OsO(4). Although neither GA nor OsO(4) prevented the surfactant from forming a surface active film, GA increased the equilibrium surface tension to above 30 mN/m, and both GA and OsO(4) decreased film stability as seen in the slowly rising minimum surface tension from 1 to ~5 mN/m in 10 min. In contrast, the effect of UA seriously impaired surface activity in that both adsorption and minimum surface tension were substantially increased. In conclusion, the fixatives tested in this study are not suitable to fix, i.e., to solidify, surfactant films. Evidently, however, OsO(4) and UA may serve as staining agents.     

Binding properties of myxobacterial hemagglutinin

The nature of the receptor for myxobacterial hemagglutinin (MBHA) on the outer surface of Myxococcus xanthus was investigated by studying the binding of 125I-MBHA to vegetative and developmental cells. The amount of binding and hence the number of binding sites/cell appeared to increase 4-fold during development to 2.1 X 10(4) sites/cell. Furthermore, the apparent association constant (Ka) for MBHA increased 3-fold to 3 X 10(7) M-1. Fetuin, a glycoprotein which binds MBHA, blocked the binding of 125I-MBHA to vegetative cells but not developmental cells. Thus, the MBHA binding sites from developmental cells clearly differ from the vegetative binding sites. The Ka for MBHA binding to sheep erythrocytes (3.5 X 10(6) M-1) was an order of magnitude lower than that of developmental M. xanthus cells. The erythrocyte binding sites are also much more sensitive to concanavalin A inhibition than the M. xanthus sites.     

Effect of surfactant concentration, compression ratio and compression rate on the surface activity and dynamic properties of a lung surfactant

This paper reports dynamic surface tension experiments of a lung surfactant preparation, BLES, for a wide range of concentrations, compression ratios and compression rates. These experiments were performed using Axisymmetric Drop Shape Analysis-Constrained Sessile Drop (ADSA-CSD). The main purpose of the paper is to interpret the results in terms of physical parameters using the recently developed Compression-Relaxation Model (CRM). In the past, only the minimum surface tension was used generally for the characterization of lung surfactant films; however, this minimum value is not a physical parameter and depends on the compression protocol. CRM is based on the assumption that the dynamic surface tension response is governed by surface elasticities, adsorption and desorption of components of the lung surfactant. The ability of CRM to fit the surface tension response closely for a wide variety of parameters (compression ratio, compression rate and surfactant concentration) and produce sensible values for the elastic and kinetic parameters supports the validity of CRM.     

Influence of liquid-layer thickness on pulmonary surfactant spreading and collapse

Pulmonary surfactant spreads on the thin ( approximately 0.1 microm) liquid layer that lines the alveoli, forming a film that reduces surface tension and allows normal respiration. Pulmonary surfactant deposited in vitro on liquid layers that are several orders of magnitude thicker, however, does not reach the low surface tensions ( approximately 0.001 N/m) achieved in the lungs during exhalation when the surfactant film compresses. This is due to collapse, a surface phase transition during which the surfactant film, rather than decreasing surface tension by increasing its surface density, becomes thicker at constant surface tension ( approximately 0.024 N/m). Formation of the collapse phase requires transport of surfactant to collapse sites, and this transport can be hindered in thinner liquid layers by viscous resistance to motion. Our objective is to determine the effect of the liquid-layer thickness on surfactant transport, which might affect surfactant collapse. To this end, we developed a mathematical model that accounts for the effect of the liquid-layer thickness on surfactant transport, and focused on surfactant spreading and collapse. Model simulations showed a marked decrease in collapse rates for thinner liquid layers, but this decrease was not enough to completely explain differences in surfactant film behavior between in vitro and in situ experiments.     

Jamming dynamics of stretch-induced surfactant release by alveolar type II cells

Secretion of pulmonary surfactant by alveolar epithelial type II cells is vital for the reduction of interfacial surface tension, thus preventing lung collapse. To study secretion dynamics, rat alveolar epithelial type II cells were cultured on elastic membranes and cyclically stretched. The amounts of phosphatidylcholine, the primary lipid component of surfactant, inside and outside the cells, were measured using radiolabeled choline. During and immediately after stretch, cells secreted less surfactant than unstretched cells; however, stretched cells secreted significantly more surfactant than unstretched cells after an extended lag period. We developed a model based on the hypothesis that stretching leads to jamming of surfactant traffic escaping the cell, similar to vehicular traffic jams. In the model, stretch increases surfactant transport from the interior to the exterior of the cell. This transport is mediated by a surface layer with a finite capacity due to the limited number of fusion pores through which secretion occurs. When the amount of surfactant in the surface layer approaches this capacity, interference among lamellar bodies carrying surfactant reduces the rate of secretion, effectively creating a jam. When the stretch stops, the jam takes an extended time to clear, and subsequently the amount of secreted surfactant increases. We solved the model analytically and show that its dynamics are consistent with experimental observations, implying that surfactant secretion is a fundamentally nonlinear process with memory representing collective behavior at the level of single cells. Our results thus highlight the importance of a jamming dynamics in stretch-induced cellular secretory processes.     

Purification and properties of Myxococcus xanthus cell surface antigen 1604.

A cell surface antigen complex from Zwittergent-solubilized Myxococcus xanthus has been purified by immunoaffinity chromatography with monoclonal antibody (MAb) 1604 and by subsequent gel filtration. We propose that the cell surface antigen (CSA) 1604 complex participates in intercellular interactions. The apparent total molecular mass of the CSA 1604 complex is 200 kilodaltons (kDa), as determined by gel filtration and by electrophoresis and Western immunoblot probing with MAb 1604. The antigen epitope recognized by MAb 1604 is on a 51-kDa polypeptide. The CSA complex also contains 14% neutral carbohydrate and a 23-kDa polypeptide that lacks the 1604 epitope. The carbohydrate is most likely part of a lipopolysaccharide (LPS) associated with the CSA, because an MAb recognizing an O antigen epitope from the LPS of M. xanthus also reacted with CSA 1604 on Western immunoblots. Thus, the 200-kDa CSA complex consists of 97 +/- 6 kDa of protein and many associated LPS molecules. The LPS evidently produces the multiplicity of bands observed on Western immunoblots between 100 and 200 kDa. The association with LPS may contribute to the negative charge of the CSA 1604 complex, which has a pI of 4.3. The CSA was clustered on the surface of intact M. xanthus cells after labeling with MAb 1604 and immunogold. Furthermore, fractionation studies indicated that cells grown on a plastic surface had 50% of their total CSA 1604 in the cytosol, 39% in the membrane fraction, and 8% in the periplasm. Saturable binding studies with 125I-MAb 1604 indicated that there were 2,400 CSA 1604 sites per cell. The Kd for MAb 1604 binding to the cell was 9 nM.     

Pulmonary surfactant

Pulmonary surfactant reduces the surface tension of the alveolar air-liquid interface, thereby providing mechanical stability and preventing alveolar atelectasis. More than 50% of surfactant is dipalmitoyl phosphatidylcholine, a material that is capable of reducing the surface tension of the alveolar interface to uniquely low values. The functions of the remaining 25% unsaturated phosphatidylcholines, 5-10% phosphatidylglycerol, 5% cholesterol, and 8-10% protein are unknown. Surfactant is synthesized by alveolar epithelial type II cells and is probably secreted as a lipoprotein complex. Lamellar bodies, which distinguish type II cells, are likely to be intracellular sites of transport of processing. The catabolism of surfactant after it is secreted into the alveolar lumen is complicated and involves different turnover times for the phosphatidylcholines, phosphatidylglycerol, and the proteins. The metabolic events are under hormonal control and may involve an interplay between beta-adrenergic agonists cAMP, and prostaglandins. In disease, such as the neonatal and adult respiratory distress syndromes, derangements in the metabolic processes may produce surfactant that is abnormal with respect to its chemical and physical properties.     

Thermodynamic effects of the hydrophobic surfactant proteins on the early adsorption of pulmonary surfactant

We determined the influence of the two hydrophobic proteins, SP-B and SP-C, on the thermodynamic barriers that limit adsorption of pulmonary surfactant to the air-water interface. We compared the temperature and concentration dependence of adsorption, measured by monitoring surface tension, between calf lung surfactant extract (CLSE) and the complete set of neutral and phospholipids (N&PL) without the proteins. Three stages generally characterized the various adsorption isotherms: an initial delay during which surface tension remained constant, a fall in surface tension at decreasing rates, and, for experiments that reached approximately 40 mN/m, a late acceleration of the fall in surface tension to approximately 25 mN/m. For the initial change in surface tension, the surfactant proteins accelerated adsorption for CLSE relative to N&PL by more than ten-fold, reducing the Gibbs free energy of transition (DeltaG(O)) from 119 to 112 kJ/mole. For the lipids alone in N&PL, the enthalpy of transition (DeltaH(O), 54 kJ/mole) and entropy (-T. DeltaS, 65 kJ/mole at 37 degrees C) made roughly equal contributions to DeltaG(O). The proteins in CLSE had little effect on -T. DeltaS(O) (68 kJ/mole), but lowered DeltaG(O) for CLSE by reducing DeltaH(O) (44 kJ/mole). Models of the detailed mechanisms by which the proteins facilitate adsorption must meet these thermodynamic constraints.     

Optical monitoring of bubble size and shape in a pulsating bubble surfactometer.

The pulsating bubble surfactometer (PBS) is often used for in vitro characterization of exogenous lung surfactant replacements and lung surfactant components. However, the commercially available PBS is not able to dynamically track bubble size and shape. The PBS therefore does not account for bubble growth or elliptical bubble shape that frequently occur during device use. More importantly, the oscillatory volume changes of the pulsating bubble are different than those assumed by the software of the commercial unit. This leads to errors in both surface area and surface tension measurements. We have modified a commercial PBS through the addition of an image-acquisition system, allowing real-time determination of bubble size and shape and hence the accurate tracking of surface area and surface tension. Compression-expansion loops obtained with the commercially available PBS software were compared with those provided by the image-analysis system for dipalmitoylphosphatidylcholine, Infasurf, and Tanaka lipids (dipalmitoylphosphatidylcholine-palmitoyloleoylphosphatidyl-glycerol-palmitic acid, 68:22:9) at concentrations of 0.1 and 1.0 mg/ml and at frequencies of 1 and 20 cycles/min. Whereas minimum surface tension as determined by the image-analysis system is similar to that measured by the commercially available software, the maximum surface tension and the shapes of the interfacial area-surface tension loops are quite different. Differences are attributable to bubble drift, nonsinusoidal volume changes, and variable volume excursions seen with the modified system but neglected by the original system. Image analysis reveals that the extent of loop hysteresis is greatly overestimated by the commercial device and that an apparent, rapid increase in surface tension upon film expansion seen in PBS loops is not observed with the image-analysis system. The modified PBS system reveals new dynamic characteristics of lung surfactant preparations that have not previously been reported.     

A species comparison of alveolar size and surface forces

The independent roles of alveolar size and surface tension in relation to lung stability were investigated in 11 different mammalian species whose body weight ranged from 0.03 to 50 kg. This range in species provided a wide variation in subgross anatomy as well as a fourfold range in alveolar diameter. Alveolar diameter was estimated from the mean linear intercept (Lm) of fixed lungs. Quasi-static pressure-volume curves were determined in excised lungs and the percent volume remaining on deflation from total lung capacity at 30 cmH2O to 10 cmH2O (%V10) provided an index of deflation stability related to functional surfactant. Surface tension of lung extract was measured in the Wilhelmy balance, and the minimum surface tension measured provided an index of surface tension lowering capacity of surfactant. Relationships of %V10 with alveolar diameter and surface tension with alveolar diameter were examined for correlations. Our results indicated that despite a range in Lm between 31 and 133 micron (mouse to pig), %V10 did not change in proportion with Lm across species. Similarly, minimum surface tension was about the same (6.1 to 8.8 dyn/cm) across a threefold difference in alveolar diameter. These results suggest that a stable alveolar configuration is maintained by both surface and tissue forces in a complex manner yet to be analyzed.     

Effects of hydrogen sulfide exposure on surface properties of lung surfactant.

Hydrogen sulfide is an irritant and chemical asphyxiant gas that exerts its primary toxic effects on the respiratory and neurological systems. Exposure to hydrogen sulfide above a threshold value of 200-300 ppm is characterized by the sudden onset of hemorrhagic pulmonary edema. The purpose of this study was to determine whether this response is associated with changes in the surface properties of pulmonary surfactant. Bronchoalveolar lavage fluid was retrieved from the lungs of Fischer 344 rats exposed to two concentrations of hydrogen sulfide or fresh air for 4 h. Surface tension-lowering properties were assayed using a captive bubble surface tensiometer. Lung injury was assessed by histopathology and measurements of total protein and lactate dehydrogenase activity in the lavagate. Marked abnormalities in surfactant activity were demonstrated in the lavagates from rats exposed to the highest concentration (300 ppm) of hydrogen sulfide. These involved the properties of adsorption to the air-water interface and surface tension lowering under quasi-static interfacial compression. Exposure to 200 ppm hydrogen sulfide had no effect on minimum surface tension despite a significant increase in protein and lactate dehydrogenase in the lavagate. This would suggest a threshold-type response for the inhibition of surfactant activity by hydrogen sulfide. In vitro studies using normal rat surfactant showed that the abnormalities in surfactant activity were due to inhibitors in the edema fluid and not to a direct effect of sulfide on surfactant. The pathophysiological consequences of increased alveolar surface tension after hydrogen sulfide exposure may need to be considered in the clinical setting.