Dextran reduces surfactant inhibition by meconium

Exposure to polymers reverses inhibition of pulmonary surfactant by serum, meconium, or cholesterol in the captive bubble surfactometer

Dysfunction of pulmonary surfactant in the lungs is associated with respiratory pathologies such as acute respiratory distress syndrome or meconium aspiration syndrome. Serum, cholesterol, and meconium have been described as inhibitory agents of surfactant's interfacial activity once these substances appear in alveolar spaces during lung injury and inflammation. The deleterious action of these agents has been only partly evaluated under physiologically relevant conditions. We have optimized a protocol to assess surfactant inhibition by serum, cholesterol, or meconium in the captive bubble surfactometer. Specific measures of surface activity before and after native surfactant was exposed to inhibitors included i), film formation, ii), readsorption of material from surface-associated reservoirs, and iii), interfacial film dynamics during compression-expansion cycling. Results show that serum creates a steric barrier that impedes surfactant reaching the interface. A mechanical perturbation of this barrier allows native surfactant to compete efficiently with serum to form a highly surface-active film. Exposure of native surfactant to cholesterol or meconium, on the other hand, modifies the compressibility of surfactant films though optimal compressibility properties recover on repetitive compression-expansion cycling. Addition of polymers like dextran or hyaluronic acid to surfactant fully reverses inhibition by serum. These polymers also prevent surfactant inhibition by cholesterol or meconium, suggesting that the protective action of polymers goes beyond the mere enhancement of interfacial adsorption as described by depletion force theories.

Overcoming rapid inactivation of lung surfactant: analogies between competitive adsorption and colloid stability

Lung surfactant (LS) is a mixture of lipids and proteins that line the alveolar air-liquid interface, lowering the interfacial tension to levels that make breathing possible. In acute respiratory distress syndrome (ARDS), inactivation of LS is believed to play an important role in the development and severity of the disease. This review examines the competitive adsorption of LS and surface-active contaminants, such as serum proteins, present in the alveolar fluids of ARDS patients, and how this competitive adsorption can cause normal amounts of otherwise normal LS to be ineffective in lowering the interfacial tension. LS and serum proteins compete for the air-water interface when both are present in solution either in the alveolar fluids or in a Langmuir trough. Equilibrium favors LS as it has the lower equilibrium surface pressure, but the smaller proteins are kinetically favored over multi-micron LS bilayer aggregates by faster diffusion. If albumin reaches the interface, it creates an energy barrier to subsequent LS adsorption that slows or prevents the adsorption of the necessary amounts of LS required to lower surface tension. This process can be understood in terms of classic colloid stability theory in which an energy barrier to diffusion stabilizes colloidal suspensions against aggregation. This analogy provides qualitative and quantitative predictions regarding the origin of surfactant inactivation. An important corollary is that any additive that promotes colloid coagulation, such as increased electrolyte concentration, multivalent ions, hydrophilic non-adsorbing polymers such as PEG, dextran, etc. added to LS, or polyelectrolytes such as chitosan, also promotes LS adsorption in the presence of serum proteins and helps reverse surfactant inactivation. The theory provides quantitative tools to determine the optimal concentration of these additives and suggests that multiple additives may have a synergistic effect. A variety of physical and chemical techniques including isotherms, fluorescence microscopy, electron microscopy and X-ray diffraction show that LS adsorption is enhanced by this mechanism without substantially altering the structure or properties of the LS monolayer.

Kinematic viscosity of therapeutic pulmonary surfactants with added polymers

The addition of various polymers to pulmonary surfactants improves surface activity in experiments both in vitro and in vivo. Although the viscosity of surfactants has been investigated, the viscosity of surfactant polymer mixtures has not. In this study, we have measured the viscosities of Survanta and Infasurf with and without the addition of polyethylene glycol, dextran or hyaluronan. The measurements were carried out over a range of surfactant concentrations using two concentrations of polymers at two temperatures. Our results indicate that at lower surfactant concentrations, the addition of any polymers increased the viscosity. However, the addition of polyethylene glycol and dextran to surfactants at clinically used concentrations can substantially lower viscosity. Addition of hyaluronan at clinical surfactant concentrations slightly increased Infasurf viscosity and produced little change in Survanta viscosity. Effects of polymers on viscosity correlate with changes in size and distribution of surfactant aggregates and the apparent free volume of liquid as estimated by light microscopy. Aggregation of surfactant vesicles caused by polymers may therefore not only improve surface activity as previously shown, but may also affect viscosity in ways that could improve surfactant distribution in vivo.

A freeze-fracture transmission electron microscopy and small angle x-ray diffraction study of the effects of albumin, serum, and polymers on clinical lung surfactant microstructure

Freeze-fracture transmission electron microscopy shows significant differences in the bilayer organization and fraction of water within the bilayer aggregates of clinical lung surfactants, which increases from Survanta to Curosurf to Infasurf. Albumin and serum inactivate all three clinical surfactants in vitro; addition of the nonionic polymers polyethylene glycol, dextran, or hyaluronic acid also reduces inactivation in all three. Freeze-fracture transmission electron microscopy shows that polyethylene glycol, hyaluronic acid, and albumin do not adsorb to the surfactant aggregates, nor do these macromolecules penetrate the interior water compartments of the surfactant aggregates. This results in an osmotic pressure difference that dehydrates the bilayer aggregates, causing a decrease in the bilayer spacing as shown by small angle x-ray scattering and an increase in the ordering of the bilayers as shown by freeze-fracture electron microscopy. Small angle x-ray diffraction shows that the relationship between the bilayer spacing and the imposed osmotic pressure for Curosurf is a screened electrostatic interaction with a Debye length consistent with the ionic strength of the solution. The variation in surface tension due to surfactant adsorption measured by the pulsating bubble method shows that the extent of surfactant aggregate reorganization does not correlate with the maximum or minimum surface tension achieved with or without serum in the subphase. Albumin, polymers, and their mixtures alter the surfactant aggregate microstructure in the same manner; hence, neither inhibition reversal due to added polymer nor inactivation due to albumin is caused by alterations in surfactant microstructure.

Enhanced surfactant adsorption via polymer depletion forces: a simple model for reversing surfactant inhibition in acute respiratory distress syndrome

Lung surfactant adsorption to an air-water interface is strongly inhibited by an energy barrier imposed by the competitive adsorption of albumin and other surface-active serum proteins that are present in the lung during acute respiratory distress syndrome. This reduction in surfactant adsorption results in an increased surface tension in the lung and an increase in the work of breathing. The reduction in surfactant adsorption is quantitatively described using a variation of the classical Smolukowski analysis of colloid stability. Albumin adsorbed to the interface induces an energy barrier to surfactant diffusion of order 5 k(B)T, leading to a reduction in adsorption equivalent to reducing the surfactant concentration by a factor of 100. Adding hydrophilic, nonadsorbing polymers such as polyethylene glycol to the subphase provides a depletion attraction between the surfactant aggregates and the interface that eliminates the energy barrier. Surfactant adsorption increases exponentially with polymer concentration as predicted by the simple Asakura and Oosawa model of depletion attraction. Depletion forces can likely be used to overcome barriers to adsorption at a variety of liquid-vapor and solid-liquid interfaces.

Morphological alterations of exogenous surfactant inhibited by meconium can be prevented by dextran

Background

Surfactant dysfunction due to inhibition is involved in the pathophysiology of meconium aspiration syndrome. Dextran addition has been shown to reverse exogenous surfactant inactivation by meconium, but the precise mechanisms and the morphological correlate of this effect are yet unknown. Morphological surfactant analysis by transmission electron microscopy (TEM) and stereology allows the differentiation of active (large aggregates = LA) and inactive (small aggregates = SA) subtypes.

Methods

To determine the in vitro effects of meconium and dextran addition on the morphology of a modified porcine natural surfactant (Curosurf), Curosurf samples were either incubated alone or together with meconium or with meconium and dextran, fixed and processed for TEM. Volume fractions of surfactant subtypes [lamellar body-like forms (LBL), multilamellar vesicles (MV), unilamellar vesicles (UV)] were determined stereologically.

Results

All preparations contained LBL and MV (corresponding to LA) as well as UV (corresponding to SA). The volume fraction of UV increased with addition of meconium and decreased with further addition of dextran. Correspondingly, the UV/(LBL+MV) ratio (resembling the SA/LA ratio) increased when meconium was added and decreased when dextran was added to the surfactant-meconium mixture.

Conclusion

Meconium causes alterations in the ultrastructural composition of Curosurf that can be visualized and analyzed by TEM and stereology. These alterations resemble an increase in the SA/LA ratio and are paralleled by an increase in minimum surface tension. Dextran prevents these effects and may therefore be a useful additive to exogenous surfactant preparations to preserve their structural and functional integrity, thereby improving their resistance to inactivation.

Biophysical and Physiological Properties of Porcine Surfactant Enriched with Polymyxin B

OBJECTIVE

We examined whether the biophysical and physiological properties of Curosurf were improved by the cyclic amphipathic decapeptide polymyxin B (PxB).

METHODS

Curosurf was diluted to 1-5 mg/ml with PxB added at 1, 2 or 3% (w/w). Albumin was added at 40 mg/ml. Minimum surface tension (gammamin) during surface compression was determined for each mixture with pulsating bubble. Immature newborn rabbits were treated with 2.5 ml/kg of Curosurf 80 mg/ml, or Curosurf 32 mg/ml with or without 2% PxB and ventilated for up to 5 h.

RESULTS

At surfactant concentration 2 mg/ml, gammamin was high (17 +/- 8.9 mN/m) but remained low (2.7 +/- 0.8 mN/m) when PxB was added. Albumin inactivated Curosurf at both 2 and 3.5 mg/ml; this inactivation was prevented by 2% PxB. Treatment of newborn rabbits with Curosurf 80 mg/kg + 2% PxB significantly decreased incidence of pneumothorax in comparison with controls but had no significant effect on lung-thorax compliance or alveolar expansion.

CONCLUSION

Addition of 2% PxB improves surface activity of Curosurf at low concentration, increases its resistance to inactivation by albumin, and reduces the incidence of pneumothorax in immature newborn rabbits undergoing prolonged ventilation.

Inhibition of pulmonary surfactant adsorption by serum and the mechanisms of reversal by hydrophilic polymers: theory

A theory based on the Smolukowski analysis of colloid stability shows that the presence of charged, surface-active serum proteins at the alveolar air-liquid interface can severely reduce or eliminate the adsorption of lung surfactant from the subphase to the interface, consistent with the observations reported in the companion article (pages 1769-1779). Adding nonadsorbing, hydrophilic polymers to the subphase provides a depletion attraction between the surfactant aggregates and the interface, which can overcome the steric and electrostatic resistance to adsorption induced by serum. The depletion force increases with polymer concentration as well as with polymer molecular weight. Increasing the surfactant concentration has a much smaller effect than adding polymer, as is observed. Natural hydrophilic polymers, like the SP-A present in native surfactant, or hyaluronan, normally present in the alveolar fluids, can enhance adsorption in the presence of serum to eliminate inactivation.

Dextran or Polyethylene Glycol Added to Curosurf for Treatment of Meconium Lung Injury in Rats

BACKGROUND

In experimental lung injuries, improvement of lung function after treatment with surfactant/polymer mixtures may depend on both type of polymer and the specific surfactant. In vitro studies suggest that dextran is more effective when mixed with Curosurf, and polyethylene glycol (PEG) is more effective when mixed with Survanta. We therefore wanted to find out whether these results held true in an animal model of acute lung injury.

OBJECTIVE

To compare the response to therapy of PEG vs. dextran when added to Curosurf after meconium lung injury.

METHODS

Lung injury was produced by intratracheal instillation of meconium (30 and 4 ml/kg). One hour after injury, Curosurf (35 mg/ml) with or without 5% dextran (68 kDa) or 5% PEG (10 kDa) was given. Arterial blood gases and peak inspiratory pressures were measured for 3 h after treatment while animals were supported by volume-regulated ventilation. Then animals were sacrificed and pressure volume relationships, lung wet/dry weights, and histology were assessed.

RESULTS

Initially, improved PaO2 and inspiratory pressure occurred for both Curosurf/PEG and Curosurf/dextran groups compared with Curosurf, but at three hours, peak inspiratory pressure and PaO2 remained significantly improved for the Curosurf/dextran but not for Curosurf/PEG groups when compared with Curosurf alone. Total lung capacity at the end of the experiment was also significantly increased in the Curosurf/dextran group, but not the Curosurf/PEG group when compared with Curosurf.

CONCLUSION

Under these experimental conditions, Curosurf/dextran mixtures provided a better therapeutic response than Curosurf/PEG or Curosurf.

Inactivation of pulmonary surfactant due to serum-inhibited adsorption and reversal by hydrophilic polymers: experimental

The rate of change of surface pressure, pi, in a Langmuir trough following the deposition of surfactant suspensions on subphases containing serum, with or without polymers, is used to model a likely cause of surfactant inactivation in vivo: inhibition of surfactant adsorption due to competitive adsorption of surface active serum proteins. Aqueous suspensions of native porcine surfactant, organic extracts of native surfactant, and the clinical surfactants Curosurf, Infasurf, and Survanta spread on buffered subphases increase the surface pressure, pi, to approximately 40 mN/m within 2 min. The variation with concentration, temperature, and mode of spreading confirmed Brewster angle microscopy observations that subphase to surface adsorption of surfactant is the dominant form of surfactant transport to the interface. However (with the exception of native porcine surfactant), similar rapid increases in pi did not occur when surfactants were applied to subphases containing serum. Components of serum are surface active and adsorb reversibly to the interface increasing pi up to a concentration-dependent saturation value, pi(max). When surfactants were applied to subphases containing serum, the increase in pi was significantly slowed or eliminated. Therefore, serum at the interface presents a barrier to surfactant adsorption. Addition of either hyaluronan (normally found in alveolar fluid) or polyethylene glycol to subphases containing serum reversed inhibition by restoring the rate of surfactant adsorption to that of the clean interface, thereby allowing surfactant to overcome the serum-induced barrier to adsorption.