Periodic fluctuations in the pulmonary surfactant system in Gould's wattled bat (Chalinolobus gouldii)

The influence of meal size on the digestive energetics of Gould’s wattled bat, Chalinolobus gouldii

Although variation in meal size is known to have an impact on digestive energetics, there is limited information on how it influences metabolic rate and energy assimilation in insectivorous bats. We investigated the influence of meal size, representing 10% or 20% of an individual’s weight, on the digestive energetics of Gould’s wattled bat, Chalinolobus gouldii (n = 61 bats). Using open-flow respirometry, we recorded a median resting metabolic rate of 2.0 mL g–1 h–1 (n = 51, range = 0.4–4.8) at an air temperature of 32°C. Median postprandial metabolic rate peaked at 6.5 (range = 3.4–11.6, n = 4) and 8.2 (range = 3.8–10.6, n = 7), representing 3.3- and 4.1-fold increases from resting metabolic rate for the two meal sizes. Using bomb calorimetry, we calculated the calorific value of the two meal sizes, and the calories lost during digestion. Following gut passage times of 120 min (range = 103–172, n = 15) and 124 min (range = 106–147, n = 12), C. gouldii assimilated 88.0% (range = 84.6–93.8, n = 5) and 93.3% (range = 84.0–99.4, n = 10) of the kilojoules available from the 10% and 20% meal sizes, respectively. When fed ad libitum, C. gouldii consumed a mean of 23.2% of their body weight during a single sitting (n = 18, range = 6.3–34.1%). Overall, digestive energetics were not significantly different between 10% or 20% meal sizes. The ability to ingest small and large meals, without compromising the rate or efficiency of calorie intake, indicates that free-ranging C. gouldii are likely limited by food available in the environment, rather than the ability to assimilate energy.

Combined effect of synthetic protein, Mini-B, and cholesterol on a model lung surfactant mixture at the air-water interface

The overall goal of this work is to study the combined effects of Mini-B, a 34 residue synthetic analog of the lung surfactant protein SP-B, and cholesterol, a neutral lipid, on a model binary lipid mixture containing dipalmitolphosphatidylcholine (DPPC) and palmitoyl-oleoyl-phosphatidylglycerol (POPG), that is often used to mimic the primary phospholipid composition of lung surfactants. Using surface pressure vs. mean molecular area isotherms, fluorescence imaging and analysis of lipid domain size distributions; we report on changes in the structure, function and stability of the model lipid-protein films in the presence and absence of varying composition of cholesterol. Our results indicate that at low cholesterol concentrations, Mini-B can prevent cholesterol's tendency to lower the line tension between lipid domain boundaries, while maintaining Mini-B's ability to cause reversible collapse resulting in the formation of surface associated reservoirs. Our results also show that lowering the line tension between domains can adversely impact monolayer folding mechanisms. We propose that small amounts of cholesterol and synthetic protein Mini-B can together achieve the seemingly opposing requirements of efficient LS: fluid enough to flow at the air-water interface, while being rigid enough to oppose irreversible collapse at ultra-low surface tensions.

Evolution, Development, and Function of the Pulmonary Surfactant System in Normal and Perturbed Environments

Surfactant lipids and proteins form a surface active film at the air-liquid interface of internal gas exchange organs, including swim bladders and lungs. The system is uniquely positioned to meet both the physical challenges associated with a dynamically changing internal air-liquid interface, and the environmental challenges associated with the foreign pathogens and particles to which the internal surface is exposed. Lungs range from simple, transparent, bag-like units to complex, multilobed, compartmentalized structures. Despite this anatomical variability, the surfactant system is remarkably conserved. Here, we discuss the evolutionary origin of the surfactant system, which likely predates lungs. We describe the evolution of surfactant structure and function in invertebrates and vertebrates. We focus on changes in lipid and protein composition and surfactant function from its antiadhesive and innate immune to its alveolar stability and structural integrity functions. We discuss the biochemical, hormonal, autonomic, and mechanical factors that regulate normal surfactant secretion in mature animals. We present an analysis of the ontogeny of surfactant development among the vertebrates and the contribution of different regulatory mechanisms that control this development. We also discuss environmental (oxygen), hormonal and biochemical (glucocorticoids and glucose) and pollutant (maternal smoking, alcohol, and common "recreational" drugs) effects that impact surfactant development. On the adult surfactant system, we focus on environmental variables including temperature, pressure, and hypoxia that have shaped its evolution and we discuss the resultant biochemical, biophysical, and cellular adaptations. Finally, we discuss the effect of major modern gaseous and particulate pollutants on the lung and surfactant system.

Adaptation to low body temperature influences pulmonary surfactant composition thereby increasing fluidity while maintaining appropriately ordered membrane structure and surface activity

The interfacial surface tension of the lung is regulated by phospholipid-rich pulmonary surfactant films. Small changes in temperature affect surfactant structure and function in vitro. We compared the compositional, thermodynamic and functional properties of surfactant from hibernating and summer-active 13-lined ground squirrels (Ictidomys tridecemlineatus) with porcine surfactant to understand structure-function relationships in surfactant membranes and films. Hibernating squirrels had more surfactant large aggregates with more fluid monounsaturated molecular species than summer-active animals. The latter had more unsaturated species than porcine surfactant. Cold-adapted surfactant membranes displayed gel-to-fluid transitions at lower phase transition temperatures with reduced enthalpy. Both hibernating and summer-active squirrel surfactants exhibited lower enthalpy than porcine surfactant. LAURDAN fluorescence and DPH anisotropy revealed that surfactant bilayers from both groups of squirrels possessed similar ordered phase characteristics at low temperatures. While ground squirrel surfactants functioned well during dynamic cycling at 3, 25, and 37 degrees C, porcine surfactant demonstrated poorer activity at 3 degrees C but was superior at 37 degrees C. Consequently the surfactant composition of ground squirrels confers a greater thermal flexibility relative to homeothermic mammals, while retaining tight lipid packing at low body temperatures. This may represent the most critical feature contributing to sustained stability of the respiratory interface at low lung volumes. Thus, while less effective than porcine surfactant at 37 degrees C, summer-active surfactant functions adequately at both 37 degrees C and 3 degrees C allowing these animals to enter hibernation. Here further compositional alterations occur which improve function at low temperatures by maintaining adequate stability at low lung volumes and when temperature increases during arousal from hibernation.

Methyl-beta-cyclodextrin restores the structure and function of pulmonary surfactant films impaired by cholesterol

Pulmonary surfactant, a defined mixture of lipids and proteins, imparts very low surface tension to the lung-air interface by forming an incompressible film. In acute respiratory distress syndrome and other respiratory conditions, this function is impaired by a number of factors, among which is an increase of cholesterol in surfactant. The current study shows in vitro that cholesterol can be extracted from surfactant and function subsequently restored to dysfunctional surfactant films in a dose-dependent manner by methyl-beta-cyclodextrin (MbetaCD). Bovine lipid extract surfactant was supplemented with cholesterol to serve as a model of dysfunctional surfactant. Likewise, when cholesterol in a complex with MbetaCD ("water-soluble cholesterol") was added in aqueous solution, surfactant films were rendered dysfunctional. Atomic force microscopy showed recovery of function by MbetaCD is accompanied by the re-establishment of the native film structure of a lipid monolayer with scattered areas of lipid bilayer stacks, whereas dysfunctional films lacked bilayers. The current study expands upon a recent perspective of surfactant inactivation in disease and suggests a potential treatment.

Effect of cholesterol on the biophysical and physiological properties of a clinical pulmonary surfactant

Pulmonary surfactant is a complex mixture of lipids and proteins that forms a surface-active film at the air-water interface of alveoli capable of reducing surface tension to near 0 mN/m. The role of cholesterol, the major neutral lipid component of pulmonary surfactant, remains uncertain. We studied the physiological effect of cholesterol by monitoring blood oxygenation levels of surfactant-deficient rats treated or not treated with bovine lipid extract surfactant (BLES) containing zero or physiological amounts of cholesterol. Our results indicate no significant difference between BLES and BLES containing cholesterol immediately after treatment; however, during ventilation, BLES-treated animals maintained higher PaO2 values compared to BLES+cholesterol-treated animals. We used a captive bubble tensiometer to show that physiological amounts of cholesterol do not have a detrimental effect on the surface activity of BLES at 37 degrees C. The effect of cholesterol on topography and lateral organization of BLES Langmuir-Blodgett films was also investigated using atomic force microscopy. Our data indicate that cholesterol induces the formation of domains within liquid-ordered domains (Lo). We used time-of-flight-secondary ion mass spectrometry and principal component analysis to show that cholesterol is concentrated in the Lo phase, where it induces structural changes.

Purifying selection drives the evolution of surfactant protein C (SP-C) independently of body temperature regulation in mammals

The pulmonary surfactant system of heterothermic mammals must be capable of dealing with the effect of low body temperatures on the physical state of the lipid components. We have shown previously that there is a modest increase in surfactant cholesterol during periods of torpor, however these changes do not fully explain the capacity of surfactant to function under the wide range of physical conditions imposed by torpor. Here we examine indirectly the role of surfactant protein C (SP-C) in adapting to variable body temperatures by testing for the presence of positive (adaptive) selection during evolutionary transitions between heterothermy and homeothermy. We sequenced SP-C from genomic DNA of 32 mammalian species from groups of closely related heterothermic and homeothermic species (contrasts). We used phylogenetic analysis by maximum likelihood estimates of rates of non-synonymous to synonymous substitutions and fully Bayesian inference of these sequences to determine whether the mode of body temperature regulation exerts a selection pressure driving the molecular adaptation of SP-C. The protein sequence of SP-C is highly conserved with synonymous or highly conservative amino acid substitutions being predominant. The evolution of SP-C among mammals is characterised by high codon usage bias and high rates of transition/transversion. The only contrast to show evidence of positive selection was that of the bears (Ursus americanus and U. maritimus). The significance of this result is unclear. We show that SP-C is under strong evolutionary constraints, driven by purifying selection, presumably to maintain protein function despite variation in the mode of body temperature regulation.

The evolution of a physiological system: the pulmonary surfactant system in diving mammals

Pulmonary surfactant lines the alveolar air-water interface, varying surface tension with lung volume to increase compliance and prevent adhesion of respiratory surfaces. We examined whether the surfactant system of diving mammals exhibits adaptations for more efficient lung function during diving, to complement other respiratory adaptations. Here we review adaptations at the molecular, compositional, functional and cellular levels and during development for animals beginning life on land and progressing to an aquatic environment. Molecular adaptations to diving were examined in surfactant protein C (SP-C) from terrestrial, semi-aquatic and diving mammals using phylogenetic analyses. Diving species exhibited sites under positive selection in the polar N-terminal domain. These amino acid substitutions may lead to stronger binding of SP-C to the phospholipid film and increased adsorption to the air-liquid interface. The concentration of shorter chain phospholipid molecular species was greater and SP-B levels were lower in diving than terrestrial mammals. This may lead to a greater fluidity and explain the relatively poor surface activity of diving mammal surfactant. There were no consistent differences in cholesterol between diving and terrestrial mammals. Surfactant from newborn California sea lions was similar to that of terrestrial mammals. Secretory activity of alveolar type II epithelial cells of sea lions demonstrated an insensitivity to pressure relative to sheep cells. The poor surface activity of diving mammal surfactant is consistent with the hypothesis that it has an anti-adhesive function that develops after the first entry into the water, with a surfactant film that is better suited to repeated collapse and respreading.

The composition of pulmonary surfactant from diving mammals

Maintaining a functional pulmonary surfactant system at depth is critical for diving mammals to ensure that inspiration is possible upon re-emergence. The lipid and protein composition of lavage extracts from three pinniped species (California sea lion, Northern elephant seal and Ringed seal) were compared to several terrestrial species. Lavage samples were purified using a NaBr discontinuous gradient. Concentrations of phospholipid classes and molecular species were measured using electrospray ionisation mass spectrometry, cholesterol was measured using high-performance liquid chromatography, surfactant protein A (SP-A) and SP-B were measured using enzyme-linked immunosorbent assays. There were small differences in phospholipid classes, with a lower level of anionic surfactant phospholipids, PG and PI, between diving and terrestrial mammals. There were no differences in PL saturation or SP-A levels between species. PC16:0/14:0, PC16:0/16:1, PC16:0/16:0, long chain PI species and the total concentrations of alkyl-acyl species of PC and PG as a ratio of diacyl species were increased in diving mammals, whereas concentrations of PC16:0/18:1, PG16:0/16:0 and PG16:0/18:1 were decreased. Cholesterol levels were very variable between species and SP-B was very low in diving mammals. These differences may explain the very poor surface activity of pinniped surfactant that we have previously described [Miller, N.J., Daniels, C.B., Schürch, S., Schoel, W.M., Orgeig, S., 2005. The surface activity of pulmonary surfactant from diving mammals. Respir. Physiol. Neurobiol. 150 (2006) 220-232], supporting the hypothesis that pinniped surfactant has primarily an anti-adhesive function to meet the challenges of regularly collapsing lungs.

Thin and strong! The bioengineering dilemma in the structural and functional design of the blood-gas barrier

In gas exchangers, the tissue barrier, the partition that separates the respiratory media (water/air and hemolymph/blood), is exceptional for its remarkable thinness, striking strength, and vast surface area. These properties formed to meet conflicting roles: thinness was essential for efficient flux of oxygen by passive diffusion, and strength was crucial for maintaining structural integrity. What we have designated as "three-ply" or "laminated tripartite" architecture of the barrier appeared very early in the evolution of the vertebrate gas exchanger. The design is conspicuous in the water-blood barrier of the fish gills through the lungs of air-breathing vertebrates, where the plan first appeared in lungfishes (Dipnoi) some 400 million years ago. The similarity of the structural design of the barrier in respiratory organs of animals that remarkably differ phylogenetically, behaviorally, and ecologically shows that the construction has been highly conserved both vertically and horizontally, i.e., along and across the evolutionary continuum. It is conceivable that the blueprint may have been the only practical construction that could simultaneously grant satisfactory strength and promote gas exchange. In view of the very narrow allometric range of the thickness of the blood-gas barrier in the lungs of different-sized vertebrate groups, the measurement has seemingly been optimized. There is convincing, though indirect, evidence that the extracellular matrix and particularly the type IV collagen in the lamina densa of the basement membrane is the main stress-bearing component of the blood-gas barrier. Under extreme conditions of operation and in some disease states, the barrier fails with serious consequences. The lamina densa which in many parts of the blood-gas barrier is <50 nm thin is a lifeline in the true sense of the word.

Dipalmitoylphosphatidylcholine is not the major surfactant phospholipid species in all mammals

Pulmonary surfactant, a complex mixture of lipids and proteins, lowers the surface tension in terminal air spaces and is crucial for lung function. Within an animal species, surfactant composition can be influenced by development, disease, respiratory rate, and/or body temperature. Here, we analyzed the composition of surfactant in three heterothermic mammals (dunnart, bat, squirrel), displaying different torpor patterns, to determine: 1) whether increases in surfactant cholesterol (Chol) and phospholipid (PL) saturation occur during long-term torpor in squirrels, as in bats and dunnarts; 2) whether surfactant proteins change during torpor; and 3) whether PL molecular species (molsp) composition is altered. In addition, we analyzed the molsp composition of a further nine mammals (including placental/marsupial and hetero-/homeothermic contrasts) to determine whether phylogeny or thermal behavior determines molsp composition in mammals. We discovered that like bats and dunnarts, surfactant Chol increases during torpor in squirrels. However, changes in PL saturation during torpor may not be universal. Torpor was accompanied by a decrease in surfactant protein A in dunnarts and squirrels, but not in bats, whereas surfactant protein B did not change in any species. Phosphatidylcholine (PC)16:0/16:0 is highly variable between mammals and is not the major PL in the wombat, dunnart, shrew, or Tasmanian devil. An inverse relationship exists between PC16:0/16:0 and two of the major fluidizing components, PC16:0/16:1 and PC16:0/14:0. The PL molsp profile of an animal species is not determined by phylogeny or thermal behavior. We conclude that there is no single PL molsp composition that functions optimally in all mammals; rather, surfactant from each animal is unique and tailored to the biology of that animal.

The pattern of surfactant cholesterol during vertebrate evolution and development: does ontogeny recapitulate phylogeny?

Pulmonary surfactant is a complex mixture of phospholipids (PLs), neutral lipids and proteins that lines the inner surface of the lung. Here it modulates surface tension, thereby increasing lung compliance and preventing the transudation of fluid. In humans, pulmonary surfactant is comprised of approximately 80% PLs, 12% neutral lipids and 8% protein. In most eutherian (i.e. placental) mammals, cholesterol (Chol) comprises approximately 8-10% by weight or 14-20 mol% of both alveolar and lamellar body surfactant. It is regarded as an integral component of pulmonary surfactant, yet few studies have concentrated on its function or control. The lipid composition is highly conserved within the vertebrates, except that surfactant of teleost fish is dominated by cholesterol, whereas tetrapod pulmonary surfactant contains a high proportion of disaturated phospholipids (DSPs). The primitive Australian dipnoan lungfish Neoceratodus forsterii demonstrates a 'fish-type' surfactant profile, whereas the other derived dipnoans demonstrate a surfactant profile similar to that of tetrapods. Homology of the surfactant proteins within the vertebrates points to a single evolutionary origin for the system and indicates that fish surfactant is a 'protosurfactant'. Among the terrestrial tetrapods, the relative proportions of DSPs and cholesterol vary in response to lung structure, habitat and body temperature (Tb), but not in relation to phylogeny. The cholesterol content of surfactant is elevated in species with simple saccular lungs or in aquatic species or in species with low Tb. The DSP content is highest in complex lungs, particularly of aquatic species or species with high Tb. Cholesterol is controlled separately from the PL component in surfactant. For example, in heterothermic mammals (i.e. mammals that vary their body temperature), the relative amount of cholesterol increases in cold animals. The rapid changes in the Chol to PL ratio in response to various physiological stimuli suggest that these two components have different turnover rates and may be packaged and processed differently. In mammals, the pulmonary surfactant system develops towards the end of gestation and is characterized by an increase in the saturation of PLs in lung washings and the appearance of surfactant proteins in amniotic fluid. In general, the pattern of surfactant development is highly conserved among the amniotes. This conservation of process is demonstrated by an increase in the amount and saturation of the surfactant PLs in the final stages (>75%) of development. Although the ratios of surfactant components (Chol, PL and DSP) are remarkably similar at the time of hatching/birth, the relative timing of the maturation of the lipid profiles differs dramatically between species. The uniformity of composition between species, despite differences in lung morphology, birthing strategy and relationship to each other, implies that the ratios are critical for the onset of pulmonary ventilation. The differences in the timing, on the other hand, appear to relate primarily to birthing strategy and the onset of air breathing. As the amount of cholesterol relative to the phospholipids is highly elevated in immature lungs, the pattern of cholesterol during development and evolution represents an example of ontogeny recapitulating phylogeny. The fact that cholesterol is an important component of respiratory structures that are primitive, when they are not in use or developing in an embryo, demonstrates that this substance has important and exciting roles in surfactant. These roles still remain to be explored.

Thermal acclimation of surfactant secretion and its regulation by adrenergic and cholinergic agonists in type II cells isolated from warm-active and torpid golden-mantled ground squirrels, Spermophilus lateralis

Homeothermic mammals experience pulmonary surfactant dysfunction with relatively small fluctuations in body temperature. However, ground squirrels survive dramatic changes in body temperature during hibernation, when body temperature drops from 37 degrees C to 0-5 degrees C during prolonged torpor bouts. Using type II cells isolated from both warm-active and torpid squirrels, we determined the effect of assay temperature, autonomic agonists and torpor on surfactant secretion. Basal secretion was significantly higher in type II cells isolated from torpid squirrels compared with warm-active squirrels when assayed at the body temperature of the animal from which they were isolated (4 degrees C and 37 degrees C, respectively). A change in assay temperature significantly decreased surfactant secretion. However, the change in secretory rate between 37 degrees C and 4 degrees C was less than expected if due to temperature alone (Q(10) range=0.8-1.2). Therefore, the surfactant secretory pathway in squirrel type II cells demonstrates some temperature insensitivity. When incubated at the body temperature of the animal from which the cells were isolated, the adrenergic agonist, isoproterenol, significantly increased surfactant secretion in both warm-active and torpid squirrel type II cells. However, the cholinergic agonist, carbamylcholine chloride, only increased secretion in torpid squirrel type II cells when incubated at 4 degrees C. Torpor did not affect basal cAMP production from isolated type II cells. However, the production of cAMP appears to be upregulated in response to isoproterenol in torpid squirrel type II cells. Thus, at the cellular level, both the secretory and regulatory pathways involved in surfactant secretion are thermally insensitive. Upregulating basal secretion and increasing the sensitivity of type II cells to cholinergic stimulation may be adaptative characteristics of torpor that enable type II cells to function effectively at 0-5 degrees C.

Alterations in surface activity of pulmonary surfactant in Gould's wattled bat during rapid arousal from torpor

The small microchiropteran bat, Chalinolobus gouldii, undergoes large daily fluctuations in metabolic rate, body temperature, and breathing pattern. These alterations are accompanied by changes in surfactant composition, predominantly an increase in cholesterol relative to phospholipid during torpor. Furthermore, the surface activity changes, such that the surfactant functions most effectively at that temperature which matches the animal's activity state. Here, we examine the surface activity of surfactant from bats during arousal from torpor. Bats were housed at 24 degrees C on an 8:16h light:dark cycle and their surfactant was collected during arousal (28<T(b)<32 degrees C). Surface activity was examined with a Captive Bubble Surfactometer at 24 and 37 degrees C. Surfactant from arousing bats was more active at 37 degrees C than at 24 degrees C, indicated by a lower ST(min) and reduced film area compression required to reach ST(min). It appears that the arousal-induced changes in surfactant composition, i.e., lower levels of cholesterol, inhibit adsorption of surfactant at 24 degrees C. Furthermore, the alterations in surfactant composition during arousal are very rapid, such that the mixture behaves more like surfactant from warm-active bats, and therefore, functions more effectively at 37 degrees C.

Torpor-associated fluctuations in surfactant activity in Gould's wattled bat

The primary function of pulmonary surfactant is to reduce the surface tension (ST) created at the air-liquid interface in the lung. Surfactant is a complex mixture of lipids and proteins and its function is influenced by physiological parameters such as metabolic rate, body temperature and breathing. In the microchiropteran bat Chalinolobus gouldii these parameters fluctuate throughout a 24 h period. Here we examine the surface activity of surfactant from warm-active and torpid bats at both 24 degrees C and 37 degrees C to establish whether alterations in surfactant composition correlate with changes in surface activity. Bats were housed in a specially constructed bat room at Adelaide University, at 24 degrees C and on a 8:16 h light:dark cycle. Surfactant was collected from bats sampled during torpor (25<T(b)<28 degrees C), and while active (T(b)>35 degrees C). Alterations in the lipid composition of surfactant occur with changes in the activity cycle. Most notable is an increase in surfactant cholesterol (Chol) with decreases in body temperature [Codd et al., Physiol. Biochem. Zool. 73 (2000) 605-612]. Surfactant from active bats was more surface active at higher temperatures, indicated by lower ST(min) and less film area compression required to reach ST(min) at 37 degrees C than at 24 degrees C. Conversely, surfactant from torpid bats was more active at lower temperatures, indicated by lower ST(min) and less area compression required to reach ST(min) at 24 degrees C than at 37 degrees C. Alterations in the Chol content of bat surfactant appear to be crucial to allow it to achieve low STs during torpor.

Development of the pulmonary surfactant system in the green sea turtle, Chelonia mydas

This study describes the developmental changes in pulmonary surfactant (PS) lipids throughout incubation in the sea turtle, Chelonia mydas. Total phospholipid (PL), disaturated phospholipid (DSP) and cholesterol (Chol) harvested from lung washings increased with advancing incubation, where secretion was maximal at pipping, coincident with the onset of pulmonary ventilation. The DSP/PL ratio increased, whereas the Chol/PL and the Chol/DSP ratio declined throughout development. The phospholipids, therefore, are independently regulated from Chol and their development matches that of mammals. To explore whether hypoxia could elicit an effect on the development of the PS system, embryos were exposed to a chronic dose of 17% O2 for the final approximately 40% of incubation. Hypoxia did not affect incubation time, absolute, nor relative abundance of the surfactant lipids, demonstrating that the development of the system is robust and that embryonic development continues unabated under mild hypoxia. Hypoxia-incubated hatchlings had lighter wet lung weights than those from normoxia, inferring that mild hypoxia facilitates lung clearance in this species.

The roles of cholesterol in pulmonary surfactant: insights from comparative and evolutionary studies

In most eutherian mammals, cholesterol (Chol) comprises approximately 8-10 wt.% or 14-20 mol.% of both alveolar and lamellar body surfactant. It is regarded as an integral component of pulmonary surfactant, yet few studies have concentrated on its function or control. Throughout the evolution of the vertebrates, the contribution of cholesterol relative to surfactant phospholipids decreases, while that of the disaturated phospholipids (DSP) increases. Chol generally appears to dominate in animals with primitive bag-like lungs that lack septation, in the saccular lung of snakes or swimbladders which are not used predominantly for respiration, and also in immature lungs. It is possible that in these systems, cholesterol represents a protosurfactant. Cholesterol is controlled separately from the phospholipid (PL) component in surfactant. For example, in heterothermic mammals such as the fat-tailed dunnart, Sminthopsis crassicaudata, and the microchiropteran bat, Chalinolobus gouldii, and also in the lizard, Ctenophorus nuchalis, the relative amount of Chol increases in cold animals. During the late stages of embryonic development in chickens and lizards, the Chol to PL and Chol to DSP ratios decrease dramatically. While in isolated lizard lungs, adrenaline and acetylcholine stimulate the secretion of surfactant PL, Chol secretion remains unaffected. This is also supported in isolated cell studies of lizards and dunnarts. The rapid changes in the Chol to PL ratio in response to various physiological stimuli suggest that these two components have different turnover rates and may be packaged and processed differently. Infusion of [3H]cholesterol into the rat tail vein resulted in a large increase in Chol specific activity within 30 min in the lamellar body (LB) fraction, but over a 48-h period, failed to appear in the alveolar surfactant fraction. Analysis of the limiting membrane of the lamellar bodies revealed a high (76%) concentration of LB cholesterol. The majority of lamellar body Chol is, therefore, not released into the alveolar compartment, as the limiting membrane fuses with the cell membrane upon exocytosis. It appears unlikely, therefore, that lamellar bodies are the major source of alveolar Chol. It is possible that the majority of alveolar Chol is synthesised endogenously within the lung and stored independently from surfactant phospholipids. The role of cholesterol in the limiting membrane of the lamellar body may be to enable fast and easy processing by maintaining the membrane in a relatively fluid state.

The comparative biology of pulmonary surfactant: past, present and future

Richard E. Pattle contributed enormously to the biology of the pulmonary surfactant system. However, Pattle can also be regarded as the founding father of comparative and evolutionary research of the surfactant system. He contributed eight seminal papers of the 167 publications we have located on this topic. In particular, Pattle produced a synthesis interpreting the evolution of the surfactant system that formed the foundation for the area. Prepared 25 years ago this synthesis spawned the three great discoveries in the comparative biology of the surfactant system: (1) that the surfactant system has been highly conserved throughout the enormous radiation of the air breathing vertebrates; (2) that temperature is the major selective condition that influences surfactant composition; (3) that acting as an anti-adhesive is one primitive and ubiquitous function of vertebrate surfactant. Here we review the literature and history of the comparative and evolutionary biology of the surfactant system and highlight the areas of comparative physiology that will contribute to our understanding of the surfactant system in the future. In our view the surfactant system is a neatly packaged system, located in a single cell and highly conserved, yet spectacularly complex. The surfactant system is one of the best systems we know to examine evolutionary processes in physiology as well as gain important insights into gas transfer by complex organisms.

Development of the pulmonary surfactant system in non-mammalian amniotes

Pulmonary surfactant (PS) is a complex mixture of phospholipids, neutral lipids and proteins that lines the inner surface of the lung. Here, it modulates surface tension thereby increasing lung compliance and preventing the transudation of fluid. In mammals, the PS system develops towards the end of gestation, characterized by an increase in the saturation of phospholipids in lung washings and the appearance of surfactant proteins in amniotic fluid. Birth, the transition from in utero to the external environment, is a rapid process. At this time, the PS system is important in opening and clearing the lung of fluid in order to initiate pulmonary ventilation. In oviparous vertebrates, escape from an egg can be a long and exhausting process. The young commence pulmonary ventilation and hatching by 'pipping' through the eggshell, where they remain for some time, presumably clearing their lungs. This paper relates changes in the development of the pulmonary surfactant system within the non-mammalian amniotes in response to birth strategy, lung morphology and phylogeny in order to determine the conservatism of this developmental process. Total phospholipid (PL), disaturated phospholipid (DSP) and cholesterol (Chol) were quantified from lung washings of embryonic and hatchling chickens, bearded dragons (oviparous), sleepy lizards (viviparous), snapping turtles and green sea turtles throughout the final stages of incubation and gestation. In all cases, the pattern of development of the pulmonary surfactant lipids was consistent with that of mammals. PL and DSP increased throughout the latter stages of development and Chol was differentially regulated from the PLs. Maximal secretion of both PL and DSP occurred at 'pipping' in oviparous reptiles, coincident with the onset of airbreathing. Similarly, the amount of DSP relative to total PL was maximal immediately after the initiation of airbreathing in chickens. The relative timing of the appearance of the lipids differed between groups. In the oviparous lizard, surfactant lipids were released over a relatively shorter time than that of the sleepy lizard, turtles, birds and mammals. Thus, despite temporal differences and vastly different lung morphologies, birth strategies and phylogenies, the overall development and maturation of the PS system is highly conserved amongst the amniotes.