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.     

Liquid plug flow in straight and bifurcating tubes.

A finite-length liquid plug may be present in an airway due to disease, airway closure, or by direct instillation for medical therapy. Air forced by ventilation propagates the plug through the airways, where it deposits fluid onto the airway walls. The plug may encounter single or bifurcating airways, an airway surface liquid, and other liquid plugs in nearby airways. In order to understand how these flow situations influence plug transport, benchtop experiments are performed for liquid plug flow in: Case (i) straight dry tubes, Case (ii) straight pre-wetted tubes, Case (iii) bifurcating dry tubes, and Case (iv) bifurcating tubes with a liquid blockage in one daughter. Data are obtainedfor the trailing film thickness and plug splitting ratio as a function of capillary number and plug volumes. For Case (i), the finite length plug in a dry tube has similar behavior to a semi-infinite plug. For Case (ii), the trailing film thickness is dependent upon the plug capillary number (Ca) and not the precursor film thickness, although the shortening or lengthening of the liquid plug is influenced by the precursor film. For Case (iii), the plug splits evenly between the two daughters and the deposited film thickness depends on the local plug Ca, except for a small discrepancy that may be due to an entrance effect or from curvature of the tubes. For Case (iv), a plug passing from the parent to daughters will deliver more liquid to the unblocked daughter (nearly double, consistently) and then the plug will then travel at greater Ca in the unblocked daughter as the blocked. The flow asymmetry is enhanced for a larger blockage volume and diminished for a larger parent plug volume and parent-Ca.     

Kinetics of Growth and Substrate Uptake in a Biological Film System

The rates of growth and substrate uptake in a biological film continuous-flow reactor were studied. The experiments were performed with high fluid velocities to bring the reactor operation to the reaction-controlled regime, thus avoiding external diffusional resistances. The glucose uptake experiments were performed with small film thicknesses so that full substrate penetration within the entire film thickness could be obtained. In this way, the catalyst effectiveness factor was 1.0 and the observed rate was the true, or intrinsic, rate. The results of the experiments indicate that both the intrinsic rate of substrate uptake and the rate of film growth are independent of the substrate concentration remaining in the reactor (zero-order reactions). However, the value of the initial substrate concentration when the film is in the early stages of growth defines the magnitude of both the rate of uptake and growth. This effect of the initial substrate concentration follows a saturation-function pattern.     

Surfactant-spreading and surface-compression disturbance on a thin viscous film

Spreading of a new surfactant in the presence of a pre-existing surfactant distribution is investigated both experimentally and theoretically for a thin viscous substrate. The experiments are designed to provide a better understanding of the fundamental interfacial and fluid dynamics for spreading of surfactants instilled into the lung. Quantitative measurements of spreading rates were conducted using a fluorescent new surfactant that was excited by argon laser light as it spread on an air-glycerin interface in a petri dish. It is found that pre-existing surfactant impedes surfactant spreading. However, fluorescent microspheres used as surface markers show that pre-existing surfactant facilitates the propagation of a surface-compression disturbance, which travels faster than the leading edge of the new surfactant. The experimental results compare well with the theory developed using lubrication approximations. An effective diffusivity of the thin film system is found to be Deff = (E*gamma)/(mu/H), which indicates that the surface-compression disturbance propagates faster for larger background surfactant concentration, gamma, larger constant slope of the sigma*-gamma* relation, -E*, and smaller viscous resistance, mu/H. Note that sigma* and gamma* are the dimensional surface tension and concentration, respectively, mu is fluid viscosity, and H is the unperturbed film thickness.     

A quantitative test of Zimm's model for the rotor-speed-department sedimentation of linear DNA molecules

In 1974, Zimm described a theory which predicts that the sedimentation coefficient of high-molecular-weight DNA will decrease as the rotor speed of measurement increases. In 1979, this theory was revised, and the new formula predicts speed-dependence effects that are substantially smaller than the predictions of the original version. This report describes the results of subjecting both the original and the revised versions of the theory to quantitative tests using a well-defined sucrose-gradient system and a DNA of known molecular weight (T4c DNA). T4c bacteriophage is a mutant, whose DNA contains the unmodified base cytosine, instead of the glucosylated hydroxymethylcytosine characteristic of the T-even bacteriophages, and has a molecular weight of 115 ± 3 × 10⁶. The DNA of the wild-type phage (T4D⁺) was also used in some experiments. In addition to the quantitative tests, the experiments test for an effect first observed by Rubenstein and Leighton, which showed that the sedimentation coefficient measured for T2 DNA depended on the composition of the centrifuge tube used for the measurement (tube composition effect). It can be inferred from this observation that an interaction occurs between particle and tube wall during sedimentation, and this leads to a reduction in sedimentation velocity independent of the reduction in S described by Zimm's theory. The results show that in the range of 25,000–50,000 rpm, the original but theoretically incorrect form of the theory quite accurately describes the sedimentation behavior of both T4c and T4D⁺ DNA, although T4D⁺ was a special case in some respects. The revised (corrected) form of the theory predicts much less of a speed-dependence effect than that actually observed. The discrepancy between corrected theory and observation suggests that other factors (perhaps arising from the use of the swinging bucket rotor geometry) are causing the additional observed reduction in S_20,w. However, the experiments show that the tube composition effect does not seem to be one of these.     

Simulation of growth and detachment in biofilm systems under defined hydrodynamic conditions

Detachment from biofilms was evaluated using a mixed culture biofilm grown on primary wastewater in a tube reactor. The growth of biofilms and the detachment of biomass from biofilms are strongly influenced by hydrodynamic conditions. In a long-term study, three biofilms were cultivated in a biofilm tube reactor. The conducted experiments of biofilm growth and detachment can be divided into three phases: 1) an exponential phase with a rapid increase of the biofilm thickness, 2) a quasi-steady-state with spontaneous fluctuation of the biofilm thickness between 500 and 1,200 microm in the investigated biofilm systems, and 3) a washout experiment with increased shear stress in three to four steps after several weeks of quasi-steady-state. Whereas the biofilm thickness during the homogeneous growth phase can be regarded constant throughout the reactor, it was found to be very heterogeneous during the quasi-steady-state and the washout experiments. Growth and detachment during all three phases was simulated with the same one-dimensional biofilm model. For each of the three phases, a different detachment rate model was used. During the homogeneous growth phase, detachment was modeled proportional to the biofilm growth rate. During the quasi-steady-state phase, detachment was described by random detachment events assuming a base biofilm thickness. Finally, the washout experiment was simulated with detachment being a function of the biofilm thickness before the increase of the shear stress.     

Effect of sodium dodecylbenzene sulfonate on subtilisin Carlsberg proteolysis of an immobilized ovalbumin film

Enzymatic degradation of immobilized ovalbumin multilayer films by subtilisin Carlsberg was investigated using in situ ellipsometry. Changes in the substrate cleavage rate in the presence of an anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), were assessed. Exposure of the protein film to SDBS prior to introduction of the enzyme increased the measured proteolysis rate threefold. Surfactant increased the measured film thickness, absorbing into the protein film and causing swelling. Surfactant-induced film swelling was reversible upon aqueous rinsing. Nevertheless, exposure of enzyme to the surfactant-rinsed film increased the proteolysis rate, most likely due to irreversible conformational changes induced in the substrate film by the surfactant. Simultaneous addition of SDBS with enzyme after the initial surfactant exposure did not produce additional protein-removal benefit.     

Vagal afferents and active upper airway closure during pulmonary edema in lambs.

The present study was undertaken to gain further insight into the mechanisms responsible for the sustained active expiratory upper airway closure previously observed during high-permeability pulmonary edema in lambs. The experiments were conducted in nonsedated lambs, in which airflow and thyroarytenoid and inferior pharyngeal constrictor muscle electromyographic activity were recorded. We first studied the consequences of hemodynamic pulmonary edema (induced by impeding pulmonary venous return) on upper airway dynamics in five lambs; under this condition, a sustained expiratory upper airway closure consistently appeared. We then tested whether expiratory upper airway closure was related to vagal afferent activity from bronchopulmonary receptors. Five bivagotomized lambs underwent high-permeability pulmonary edema: no sustained expiratory upper airway closure was observed. Finally, we studied whether a sustained decrease in lung volume induced a sustained expiratory upper airway closure. Five lambs underwent a 250-ml pleural infusion: no sustained expiratory upper airway closure was observed. We conclude that 1) the sustained expiratory upper airway closure observed during pulmonary edema in nonsedated lambs is related to stimulation of vagal afferents by an increase in lung water and 2) a decrease in lung volume does not seem to be the causal factor.     

有花植物触敏柱头的闭合行为及其适应意义

有花植物触敏柱头的闭合行为一直是传粉生态学和进化生物学研究的热点问题之一。植物花的多裂片柱头在传粉者接触后迅速闭合的现象主要存在于玄参目的一些类群中。早在上个世纪初,植物学家就对花的柱头闭合现象进行过初步的研究,认为柱头闭合主要是由于花粉在柱头上萌发时吸收水分导致柱头细胞膨压降低而引起的;并对柱头闭合的适应性提出了一些假说。近年来,许多学者研究证实柱头闭合能促进花粉萌发及花粉管伸长,或减少花粉散出与柱头接受花粉间的相互干扰,提高传粉者访问过程中花粉的散播量,并且柱头的状态会直接影响传粉者的取食行为。到目前为止,对柱头闭合的机制及其避免自交的假说还存在许多争议。关于触敏柱头的闭合行为,尤其是其适应机制及其生态学意义还有待更加系统和深入的研究。本文概述了触敏柱头的研究进展,并对研究中尚存在的一些问题进行了讨论。     

Surfactant transport over airway liquid lining of nonuniform depth

Numerous effects (e.g., airway wall buckling, gravity, airway curvature, capillary instabilities) give rise to nonuniformities in the depth of the liquid lining of peripheral lung airways. The effects of such thickness variations on the unsteady spreading of a surfactant monolayer along an airway are explored theoretically here. Flow-induced film deformations are shown to have only a modest influence on spreading rates, motivating the use of a simplified model in which the liquid-lining depth is prescribed and the monolayer concentration satisfies a spatially inhomogeneous nonlinear diffusion equation. Two generic situations are considered: spreading along a continuous annular liquid lining of nonuniform depth, and spreading along a rivulet that wets the airway wall with zero contact angle. In both cases, transverse averaging at large times yields a one-dimensional approximation of axial spreading that is valid for the majority of the monolayer. However, a localized monolayer remains persistently two dimensional in a region at its leading edge having axial length scales comparable to the length scale of transverse depth variation. It is also shown how the transverse spreading of a monolayer may be arrested as it approaches a static contact line at the edge of a rivulet. Implications for Surfactant Replacement Therapy are discussed.     

A coarse-grained biophysical model of E. coli and its application to perturbation of the rRNA operon copy number

We propose a biophysical model of Escherichia coli that predicts growth rate and an effective cellular composition from an effective, coarse-grained representation of its genome. We assume that E. coli is in a state of balanced exponential steady-state growth, growing in a temporally and spatially constant environment, rich in resources. We apply this model to a series of past measurements, where the growth rate and rRNA-to-protein ratio have been measured for seven E. coli strains with an rRNA operon copy number ranging from one to seven (the wild-type copy number). These experiments show that growth rate markedly decreases for strains with fewer than six copies. Using the model, we were able to reproduce these measurements. We show that the model that best fits these data suggests that the volume fraction of macromolecules inside E. coli is not fixed when the rRNA operon copy number is varied. Moreover, the model predicts that increasing the copy number beyond seven results in a cytoplasm densely packed with ribosomes and proteins. Assuming that under such overcrowded conditions prolonged diffusion times tend to weaken binding affinities, the model predicts that growth rate will not increase substantially beyond the wild-type growth rate, as indicated by other experiments. Our model therefore suggests that changing the rRNA operon copy number of wild-type E. coli cells growing in a constant rich environment does not substantially increase their growth rate. Other observations regarding strains with an altered rRNA operon copy number, such as nucleoid compaction and the rRNA operon feedback response, appear to be qualitatively consistent with this model. In addition, we discuss possible design principles suggested by the model and propose further experiments to test its validity.     

A viscoelastic model of blood capillary extension and regression: derivation, analysis, and simulation

This work studies a fundamental problem in blood capillary growth: how the cell proliferation or death induces the stress response and the capillary extension or regression. We develop a one-dimensional viscoelastic model of blood capillary extension/regression under nonlinear friction with surroundings, analyze its solution properties, and simulate various growth patterns in angiogenesis. The mathematical model treats the cell density as the growth pressure eliciting a viscoelastic response from the cells, which again induces extension or regression of the capillary. Nonlinear analysis captures two cases when the biologically meaningful solution exists: (1) the cell density decreases from root to tip, which may occur in vessel regression; (2) the cell density is time-independent and is of small variation along the capillary, which may occur in capillary extension without proliferation. The linear analysis with perturbation in cell density due to proliferation or death predicts the global biological solution exists provided the change in cell density is sufficiently slow in time. Examples with blow-ups are captured by numerical approximations and the global solutions are recovered by slow growth processes, which validate the linear analysis theory. Numerical simulations demonstrate this model can reproduce angiogenesis experiments under several biological conditions including blood vessel extension without proliferation and blood vessel regression.     

Life-history evolution in guppies viii: the demographics of density regulation in guppies (poecilia reticulata)

In prior research, we found the way guppy life histories evolve in response to living in environments with a high or low risk of predation is consistent with life-history theory that assumes no density dependence. We later found that guppies from high-predation environments experience higher mortality rates than those from low-predation environments, but the increased risk was evenly distributed across all age/size classes. Life-history theory that assumes density-independent population growth predicts that life histories will not evolve under such circumstances, yet we have shown with field introduction experiments that they do evolve. However, theory that incorporates density regulation predicts this pattern of mortality can result in the patterns of life-history evolution we had observed. Here we report on density manipulation experiments performed in populations of guppies from low-predation environments to ask whether natural populations normally experience density regulation and, if so, to characterize the short-term demographic changes that underlie density regulation. Our experiments reveal that these populations are density regulated. Decreased density resulted in higher juvenile growth, decreased juvenile mortality rates, and increased reproductive investment by adult females. Increased density causes reduced offspring size, decreased fat storage by adult females, and increased adult mortality.     

An assessment of the role of physiological adaptation in the transient response of bacterial cultures

The RNA-limiting theory of transient response states that the primary physiological adaptation which occurs when microbial cultures are grown at specific rates less than their maximum is a decrease in the cellular level of RNA. It predicts that, as a result of this decrease, the response of the culture to a shift-up in growth rate will be limited by its RNA level. In order to test the RNA-limiting theory and to investigate the role physiological adaptation in transient response, experiments were performed in which steady-state chemostat cultures of Pseudomonasputida grown at various specific rates were transferred to batch reactors containing sufficient carbon source (L-lysine) and nutrients to remove all external growth restrictions. Samples were collected during the subsequent transient period for determination of the macromolecular composition and the maximum instantaneous oxygen uptake rate. The results indicated that, while decreases in the RNA level did significantly affect the nature of the transient response, other unidentified components varied with the steady-state specific growth rate at which the culture had been grown prior to the shift-up and that the levels of those components affected the nature of the subsequent transient response. This implies that the RNA-limiting theory is inadequate for describing the transient responses of cultures grown over a wide range of specific growth rates.     

A Coarse-Grained Biophysical Model of E. coli and Its Application to Perturbation of the rRNA Operon Copy Number

We propose a biophysical model of Escherichia coli that predicts growth rate and an effective cellular composition from an effective, coarse-grained representation of its genome. We assume that E. coli is in a state of balanced exponential steady-state growth, growing in a temporally and spatially constant environment, rich in resources. We apply this model to a series of past measurements, where the growth rate and rRNA-to-protein ratio have been measured for seven E. coli strains with an rRNA operon copy number ranging from one to seven (the wild-type copy number). These experiments show that growth rate markedly decreases for strains with fewer than six copies. Using the model, we were able to reproduce these measurements. We show that the model that best fits these data suggests that the volume fraction of macromolecules inside E. coli is not fixed when the rRNA operon copy number is varied. Moreover, the model predicts that increasing the copy number beyond seven results in a cytoplasm densely packed with ribosomes and proteins. Assuming that under such overcrowded conditions prolonged diffusion times tend to weaken binding affinities, the model predicts that growth rate will not increase substantially beyond the wild-type growth rate, as indicated by other experiments. Our model therefore suggests that changing the rRNA operon copy number of wild-type E. coli cells growing in a constant rich environment does not substantially increase their growth rate. Other observations regarding strains with an altered rRNA operon copy number, such as nucleoid compaction and the rRNA operon feedback response, appear to be qualitatively consistent with this model. In addition, we discuss possible design principles suggested by the model and propose further experiments to test its validity.     

Ribonucleotide reductase subunit R1: a gene conferring sensitivity to valproic acid-induced neural tube defects in mice

Neural tube defects (NTDs), although prevalent and easily diagnosed, are etiologically heterogeneous, rendering mechanistic interpretation problematic. To date, there is evidence that mammalian neural tube closure (NTC) initiates and fuses intermittently at four discrete locations. Disruption of this process at any of these four sites may lead to a region-specific NTDs, possibly arising through closure site-specific genetic mechanisms. Although recent efforts have focused on elucidating the genetic components of NTDs, a void persists regarding gene identification in closure site-specific neural tissue. To this end, experiments were conducted to identify neural tube closure site-specific genes that might confer regional sensitivity to teratogen-induced NTDs. Using an inbred mouse strain (SWV/Fnn) with a high susceptibility to VPA- induced NTDs that specifically targets and disrupts NTC between the prosencephalon and mesencephalon region (future fore/midbrain; neural tube closure site II), we identified a VPA-sensitive closure site II-specific clone. Sequencing of this clone from an SWV neural tube cDNA library confirmed that it encodes the r1 subunit of the cell cycle enzyme ribonucleotide reductase (RNR). The abundance of rnr-r1 mRNA was significantly increased in response to VPA drug treatment. This upregulated expression was accompanied by a significant decrease in cellular proliferation in the closure site II neural tube region of the embryos, as determined by ELISA cellular proliferation assays performed on BrdU-pulsed neuroepithelial cells in vivo. We hypothesize that rnr-r1 plays a critical role in the development of VPA-induced exencephaly.     

Resilience and local stability in a nutrient-limited resource-consumer system

The “paradox of enrichment” predicts that increasing the growth rate of the resource in a resource-consumer dynamic system, by nutrient enrichment, for example, can lead to local instability of the system—that is, to a Hopf bifurcation. The approach to the Hopf bifurcation is accompanied by a decrease in resilience (rate of return to equilibrium). On the other hand, studies of nutrient cycling in food webs indicate that an increase in the nutrient input rate usually results in increased resilience. Here these two apparently conflicting theoretical results are reconciled with a model of a nutrient-limited resource-consumer system in which the tightly recycled limiting nutrient is explicitly modelled. It is shown that increasing nutrient input may at first lead to increased resilience and that resilience decreases sharply only immediately before the Hopf bifurcation is reached.     

Nonlinear theory of the excitation of azimuthal surface waves during dissipative instability

A theoretical study is made of the surface electromagnetic eigenmodes that are excited by an annular charged-particle beam due to dissipative instability and propagate across the external axial magnetic field in a cylindrical metal waveguide partially filled with plasma. A self-consistent set of differential equations for a cold low-density charged-particle beam moving above the plasma surface is constructed in the single-mode approximation and is solved numerically. It is shown that the larger the dissipation, the slower the instability growth rate and the larger the wave amplitude in the saturation stage of the instability. An increase in the transverse dimensions of a charged-particle beam results in a slower growth of the dissipative instability, in which case, however, the beam transfers a larger fraction of its kinetic energy to the wave.     

Regulatory control and the costs and benefits of biochemical noise

Experiments in recent years have vividly demonstrated that gene expression can be highly stochastic. How protein concentration fluctuations affect the growth rate of a population of cells is, however, a wide-open question. We present a mathematical model that makes it possible to quantify the effect of protein concentration fluctuations on the growth rate of a population of genetically identical cells. The model predicts that the population's growth rate depends on how the growth rate of a single cell varies with protein concentration, the variance of the protein concentration fluctuations, and the correlation time of these fluctuations. The model also predicts that when the average concentration of a protein is close to the value that maximizes the growth rate, fluctuations in its concentration always reduce the growth rate. However, when the average protein concentration deviates sufficiently from the optimal level, fluctuations can enhance the growth rate of the population, even when the growth rate of a cell depends linearly on the protein concentration. The model also shows that the ensemble or population average of a quantity, such as the average protein expression level or its variance, is in general not equal to its time average as obtained from tracing a single cell and its descendants. We apply our model to perform a cost-benefit analysis of gene regulatory control. Our analysis predicts that the optimal expression level of a gene regulatory protein is determined by the trade-off between the cost of synthesizing the regulatory protein and the benefit of minimizing the fluctuations in the expression of its target gene. We discuss possible experiments that could test our predictions.     

Inhibition of proteasome activity strongly affects kiwifruit pollen germination. Involvement of the ubiquitin/proteasome pathway as a major regulator.

The 26S proteasome is a multicatalytic complex that acts as primary protease of the ubiquitin-mediated proteolytic pathway in eukaryotes. We provide here the first evidence that the proteasome plays a key role in regulating pollen tube growth. Immunoblotting experiments revealed the presence of high levels of free ubiquitin and ubiquitin conjugates in rehydrated and germinating pollen of kiwifruit [Actinidia deliciosa var. deliciosa (A. Chev) C. F. Liang et A. R. Ferguson]. Proteasome activity, assayed fluorometrically, accompanied the progression of germination. Specific inhibitors of proteasome function such as benzyloxycarbonyl-leucinyl-leucinyl-leucinal (MG-132), clasto-lactacystin beta-lactone, and epoxomicin significantly decreased tube growth or altered tube morphology. High-molecular mass, ubiquitinated proteins accumulated in MG-132- and beta-lactone-treated pollen, indicating that proteasome function was effectively impaired. The inhibitors were also able to decrease in vitro proteasome activity in pollen extracts. Because MG-132 can inhibit calpains, as well as the proteasome, trans-epoxy succinyl-L-leucylamido-(4-guanidino) butane (E-64), an inhibitor of cysteine proteases, was investigated. Some reduction in tube growth rate was observed, but only at 80 microM E-64, and no abnormal tubes were produced. Furthermore, no inhibition of tube growth was observed when another inhibitor of cysteine proteases, leupeptin, or inhibitors of serine and aspartic proteases (phenylmethylsulfonyl fluoride and pepstatin) were used. Our results indicate that protein turnover during tube organization and elongation in kiwifruit pollen is important, and our results also implicate the ubiquitin/26S proteasome as the major proteolytic pathway involved.