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PubMed1994

Comparison of two otitis media models for the study of middle ear antimicrobial pharmacokinetics.

We compared two models of acute otitis media that estimate middle ear antimicrobial pharmacokinetics. Using a crossover study design, we compared a systemic drug administration model with a diffusion model we devised that measures the disappearance of antimicrobials from the middle ear. We induced acute otitis media in 14 chinchillas by inoculating S. pneumoniae into the middle ear, then administered 3 antimicrobials: amoxicillin, trimethoprim, and sulfamethoxazole. Next we collected middle ear fluid samples to analyze drug concentrations and compare rate constants for the systemic and diffusion models by analysis of variance. We found that amoxicillin K values were not affected by model testing sequence (p = 0.827) or model type (systemic versus diffusion, p = 0.310), nor were sulfamethoxazole K values: model testing sequence (p = 0.917), model type (p = 0.963). Trimethoprim K values were also not affected by model testing sequence (p = 0/760), but were by model type (p = 0.0001). Trimethoprim elimination from the diffusion model was faster (K = 0.33 +/- 0.17 versus 0.57 +/- 0.09 hr-1) than from the systemic model, although it appears this was caused by sampling before drug distribution into the middle ear was complete. In conclusion, it appears K values derived from either systemic antimicrobial administration or direct middle ear instillation are similar for assessing middle ear antimicrobial pharmacokinetics, and these models can be used interchangeably to study factors affecting otitis media treatment response.

Amoxicillin↗
PubMed2004

Role of the water matric potential (psi(M)) and of equilibrium water content (EWC) on the water self-diffusion coefficient and on the oxygen permeability in hydrogel contact lenses.

This paper illustrates a new thermodynamic and kinetic model that describes the relationship between the water self-diffusion coefficient, D*(w/g), in hydrogel contact lenses, in terms of water matric potential (psi(M)) and equilibrium water content (EWC). Experimental measurements on commercial contact lenses yield water thermodynamic activity values ranging between 0.996 and 0.999. The corresponding psi(M) matric potential is, respectively, between -8 and -2J/mol at temperature 35 degrees C. Comparison between water self-diffusion coefficients derived in this paper and those suggested by other authors shows that our values are greater (25%-50%) than the previous ones. The impact of this model on the nature of the oxygen permeability, pi, in the lenses has been evaluated and the changes of pi with psi(M) and EWC are predicted and compared with direct experimental measurements. For the contact lenses investigated, the oxygen permeability turns out to be only a quadratic function of equilibrium water content, despite the fact that the fraction of the "free" water molecules can be as high as 50%.

Absorption↗
PubMed1982

Dural permeability to narcotics: in vitro determination and application to extradural administration.

The permeability of cranial and lumbar dura to various substances including a number of narcotic analgesics was measured in vitro. Preliminary data On human postmortem material is reported. Permeability had a linear relation to the inverse of the square root of molecular weight. This is the expected relationship for a diffusion process dependent upon molecular weight. The differential mass selectivity coefficients for lumbar and cranial dura were calculated; they were similar at 0.8 and 0.9. This was greater than for diffusion in simple liquids, but much less than that for biological lipid membranes. This suggests that the low rates of diffusion are a property of the thickness of the dura rather than any inherent impermeability. A simple model for the dural transfer of drugs is described, and applied to narcotics. Its purposes were to suggest: the factors involved in the dural transfer of drugs; the physicochemical properties of drugs relevant to their dural transfer; worthwhile measurements in future studies. The model indicates that drug molecular weight and rate of absorption are important determinants of the efficiency of dural transfer. Low molecular weight and slow absorption produce high dural transfers. When applied to narcotics, these factors could produce a difference of up to an order of magnitude in the amount transferred directly across the dura.

Absorption↗
PubMed1985

Determinants of CO2 generation and maintenance in the renal cortex: role of metabolic CO2 production and diffusive CO2 transfer.

We have developed a mathematical model utilizing steady-state mass balance considerations in a compartmental analysis of bicarbonate reabsorption in the superficial proximal tubule and associated vasculature in order to describe quantitatively the determinants of the elevated PCO2 in the renal cortex. This analysis demonstrated for the first time that diffusive transfer of CO2 between vascular segments coupled with metabolic CO2 production could account for the PCO2 measured experimentally. In the present study the predictive value of the model was extended by comparing model-predicted and measured values for PCO2 before and after administration of metabolic and transport inhibitors. In control conditions diffusive CO2 transfer (FCO2) in combination with accepted levels of metabolic CO2 production (MCO2) can account for an elevation in PCO2 to 60 mm Hg. During rotenone and DNP administration the model predicts values for PCO2 which approximate closely the value measured in vivo. In addition, the model demonstrates that the reduction in PCO2 after rotenone and increase in PCO2 after DNP can be accounted for by parallel changes in MCO2 alone as expected. In each case a significant contribution from FCO2 is necessary, however. Therefore, this study demonstrates close agreement in both the magnitude and direction of predicted and determined values in cortical PCO2 in a variety of experimental conditions, thus, supporting a dual role for the major determinants: metabolic CO2 production and diffusive vascular to vascular CO2 transfer.

Animals↗
PubMed2006

Directional sensing in eukaryotic chemotaxis: a balanced inactivation model.

Many eukaryotic cells, including Dictyostelium discoideum amoebae, fibroblasts, and neutrophils, are able to respond to chemoattractant gradients with high sensitivity. Recent studies have demonstrated that, after the introduction of a chemoattractant gradient, several chemotaxis pathway components exhibit a subcellular reorganization that cannot be described as a simple amplification of the external gradient. Instead, this reorganization has the characteristics of a switch, leading to a well defined front and back. Here, we propose a directional sensing mechanism in which two second messengers are produced at equal rates. The diffusion of one of them, coupled with an inactivation scheme, ensures a switch-like response to external gradients for a large range of gradient steepness and average concentration. Furthermore, our model is able to reverse the subcellular organization rapidly, and its response to multiple simultaneous chemoattractant sources is in good agreement with recent experimental results. Finally, we propose that the dynamics of a heterotrimeric G protein might allow for a specific biochemical realization of our model.

Animals↗
PubMed1981

Mechanism of bicarbonate action on photosynthetic electron transport in broken chloroplasts.

In CO2-depleted chloroplasts electron transport between the Photosystem II electron acceptor Q and plastoquinone is largely suppressed. In the presence of a high concentration of sodium formate (greater than 10 mM), which probably binds to the bicarbonate site, addition of bicarbonate restores the ferricyanide Hill reaction only after incubation in the dark. With lower formate concentrations bicarbonate is able to restore electron transport in the light. The Hill reaction rate in CO2-depleted chloroplasts after bicarbonate addition, divided by the rate in CO2-depleted chloroplasts before bicarbonate addition, shows a sharp optimum at pH 6.5. Furthermore, the rate-limiting step in bicarbonate action is probably diffusion. The results are explained in terms of a hypothetical model: the bicarbonate-binding site is located at the outer side of the thylakoid membrane, but not directly accessible from the "bulk". To reach the site from the bulk, the molecule has to pass a channel with negatively charge groups on its side walls. In the light these groups are more negatively charged than in the dark. Therefore, the formate ion cannot exchange for bicarbonate in the light, and a dark period is necessary to enable exchange of formate for bicarbonate.

Bicarbonates↗
PubMed2026

Glymphatic dysfunction mediates inflammation-driven vascular burden and cognitive decline in cerebral small vessel disease.

BACKGROUND: Cerebral small vessel disease (CSVD) is increasingly recognized as a disorder involving microvascular dysfunction, impaired perivascular clearance, and inflammatory processes. However, how systemic inflammatory burden, neurovascular coupling (NVC), glymphatic MRI markers, vascular lesion burden, and cognition are interrelated remains unclear. MATERIALS AND METHODS: In this prospective study, 155 patients with CSVD and 70 healthy controls (HCs) underwent multimodal MRI. NVC was quantified using the cerebral blood flow/fractional amplitude of low-frequency fluctuations ratio. Glymphatic function was assessed via the diffusion tensor image analysis along the perivascular space (ALPS) index, choroid plexus volume (CPV), and perivascular space (PVS) fractions. Structural equation modeling (SEM) was employed to evaluate the direct and indirect effects of inflammatory markers on vascular burden and cognitive performance. RESULTS: Patients with CSVD exhibited significantly diminished NVC (specifically in the right median cingulate and left frontal gyri) and impaired glymphatic function (lower ALPS-index; higher CPV and PVS fractions) compared to HCs. SEM revealed that inflammatory biomarkers exerted both a direct effect on vascular burden and a substantial indirect effect (accounting for 66.3% of the total effect) mediated through two pathways: a single-mediation path via glymphatic function (42.8%) and a serial-mediation path via NVC and glymphatic function (23.5%). Increased vascular burden was significantly associated with poorer cognitive performance. CONCLUSION: Inflammation drives CSVD progression and cognitive decline primarily through the disruption of NVC and glymphatic clearance mechanisms. These findings highlight glymphatic dysfunction as a critical mediator of inflammation-related structural brain damage.

Humans↗
PubMedDate not supplied

Laboratory study on the high-temperature capture of HCl gas by dry-injection of calcium-based sorbents.

This is a laboratory study on the reduction of combustion-generated hydrochloric acid (HCl) emissions by in-furnace dry-injection of calcium-based sorbents. HCl is a hazardous gaseous pollutant emitted in significant quantities by municipal and hazardous waste incinerators, coal-fired power plants, and other industrial furnaces. Experiments were conducted in a laboratory furnace at gas temperatures of 600-1000 degrees C. HCl gas diluted with N2, and sorbent powders fluidized in a stream of air were introduced into the furnace concurrently. Chlorination of the sorbents occurred in the hot zone of the furnace at gas residence times approximately 1 s. The sorbents chosen for these experiments were calcium formate (CF), calcium magnesium acetate (CMA), calcium propionate (CP), calcium oxide (CX), and calcium carbonate (CC). Upon release of organic volatiles, sorbents calcine to CaO at approximately 700 degrees C, and react with the HCl according to the reaction CaO + 2HCl <=> CaCl2 + H2O. At the lowest temperature case examined herein, 600 degrees C, direct reaction of HCl with CaCO3 may also be expected. The effectiveness of the sorbents to capture HCl was interpreted using the "pore tree" mathematical model for heterogeneous diffusion reactions. Results show that the thin-walled, highly porous cenospheres formed from the pyrolysis and calcination of CF, CMA, and CP exhibited high relative calcium utilization at the upper temperatures of this study. Relative utilizations under these conditions reached 80%. The less costly low-porosity sorbents, calcium carbonate and calcium oxide also performed well. Calcium carbonate reached a relative utilization of 54% in the mid-temperature range, while the calcium oxide reached an 80% relative utilization at the lowest temperature examined. The data matched theoretical predictions of sorbent utilization using the mathematical model, with activation energy and pre-exponential factors for the calcination reaction of 17,000 K and 300,000 (g gas/cm2/s/atm gas), respectively. Thus, the kinetics of the calcination reaction were found to be much faster (approximately 500 times) than those of the sulfation reaction examined previously in this laboratory.

Journal Article↗
PubMed1987

Inhibition of capillary endothelial cell growth by pericytes and smooth muscle cells.

Morphological studies of developing capillaries and observations of alterations in capillaries associated with pathologic neovascularization indicate that pericytes may act as suppressors of endothelial cell (EC) growth. We have developed systems that enable us to investigate this possibility in vitro. Two models were used: a co-culture system that allowed direct contact between pericytes and ECs and a co-culture system that prevented physical contact but allowed diffusion of soluble factors. For these studies, co-cultures were established between bovine capillary ECs and the following growth-arrested cells (hereafter referred to as modulating cells): pericytes, smooth muscle cells (SMCs), fibroblasts, epithelial cells, and 3T3 cells. The modulating cell type was growth arrested by treatment with mitomycin C before co-culture with ECs. In experiments where cells were co-cultured directly, the effect of co-culture on EC growth was determined by comparing the mean number of cells in the co-cultures to the mean for each cell type (EC and modulating cell) cultured separately. Since pericytes and other modulating cells were growth arrested, any cell number change in co-cultures was due to EC growth. In the co-cultures, pericytes inhibited all EC proliferation throughout the 14-d time course; similar levels of EC inhibition were observed in SMC-EC co-cultures. Co-culture of ECs with fibroblasts, epithelial cells, and 3T3 cells significantly stimulated EC growth over the same time course (30-192% as compared to EC cultured alone). To determine if cell contact was required for inhibition, cells were co-cultured using Millicell chambers (Millipore Corp., Bedford, MA), which separated the cell types by 1-2 mm but allowed the exchange of diffusible materials. There was no inhibition of EC proliferation by pericytes or SMCs in this co-culture system. The influence of the cell ratios on observed inhibition was assessed by co-culturing the cells at EC/pericyte ratios of 1:1, 2:1, 5:1, 10:1, and 20:1. Comparable levels of EC inhibition were observed at ratios from 1:1 to 10:1. When the cells were co-cultured at a ratio of 20 ECs to 1 pericyte, inhibition of EC growth at 3 d was similar to that observed at other ratios. However, at higher ratios, the inhibition diminished so that by the end of the time course the co-cultured ECs were growing at the same rate as the controls. These results suggest that pericytes and SMCs can modulate EC growth by a mechanism that requires contact or proximity. We postulate that similar interactions may operate to modulate vascular growth in vivo.

Adrenal Cortex↗
PubMed2000

Transport in nonequilibrium systems with position-dependent mobility

We propose a transport mechanism for overdamped diffusing particles. The mechanism is based on a position-dependent mobility and on breaking detailed balance. Using a two-state model, we show that the transport direction is given by the phase shift between mobility and potential or mobility and transition rate. No symmetry breaking potential as employed in ratchet-type transport models is needed.

Journal Article↗
PubMed2001

Phase separation under two-dimensional Poiseuille flow.

The spinodal decomposition of a two-dimensional binary fluid under Poiseuille flow is studied by numerical simulation. We investigated time dependence of domain sizes in directions parallel and perpendicular to the flow. In an effective region of the flow, the power-law growth of a characteristic length in the direction parallel to the flow changes from the diffusive regime with the growth exponent alpha=1/3 to a new regime. The scaling invariance of the growth in the perpendicular direction is destroyed after the diffusive regime. A recurrent prevalence of thick and thin domains which determines log-time periodic oscillations has not been observed in our model. The growth exponents in the infinite system under two-dimensional Poiseuille flow are obtained by the renormalization group.

Journal Article↗
PubMed2003

Diffuse X-ray scattering from 4,4'-dimethoxybenzil, C16H14O4: analysis via automatic refinement of a Monte Carlo model.

A recently developed method for fitting a Monte Carlo computer-simulation model to observed single-crystal diffuse X-ray scattering has been used to study the diffuse scattering in 4,4'-dimethoxybenzil, C16H14O4. A model involving only nine parameters, consisting of seven intermolecular force constants and two intramolecular torsional force constants, was refined to give an agreement factor, omegaR = [sigma omega(deltaI)2/sigma omegaI2(obs)](1/2), of 18.1% for 118 918 data points in two sections of data. The model was purely thermal in nature. The analysis has shown that the most prominent features of the diffraction patterns, viz. diffuse streaks that occur normal to the [101] direction, are due to longitudinal displacement correlations along chains of molecules extending in this direction. These displacements are transmitted from molecule to molecule via contacts involving pairs of hydrogen bonds between adjacent methoxy groups. In contrast to an earlier study of benzil itself, it was not found to be possible to determine, with any degree of certainty, the torsional force constants for rotations about the single bonds in the molecule. It is supposed that this result may be due to the limited data available in the present study.

Journal Article↗
PubMed2000

Graphical evolution of the arnold web: from order to chaos

We represent graphically the evolution of the set of resonances of a quasi-integrable dynamical system, the so-called Arnold web, whose structure is crucial for the stability properties of the system. The basis of our representation is the use of an original numerical method, whose definition is directly related to the dynamics of orbits, and the careful choice of a model system. We also show the transition from the Nekhoroshev stability regime to the Chirikov diffusive one, which is a generic nontrivial phenomenon occurring in many physical processes, such as slow chaotic transport in the asteroid belt and beam-beam interaction.

Journal Article↗
PubMed1986

Pharmacokinetic studies of the disposition of acetaminophen in the sheep maternal-placental-fetal unit.

Pharmacokinetic studies of the disposition of acetaminophen were carried out in 12 chronically catheterized pregnant sheep. Acetaminophen was infused separately into the mother and the fetus on two occasions to steady state. The transplacental clearances from both sides of the placental membrane and the nontransplacental clearances in both the mother and the fetus were then determined using an indirect method, a two-compartment open pharmacokinetic body model. The placental clearance and the nontransplacental clearances were also determined independently by direct methods by measuring the extraction ratio and urinary clearances in both the mother and fetus. Model dependent (indirect) and model independent (direct) methods yielded similar results. Neither acetaminophen glucuronide nor acetaminophen sulfate was transferred across the placenta. However, acetaminophen was transferred in both directions exclusively via a passive diffusion mechanism with an extraction ratio of 0.12 across the placenta. Neither glucuronidation nor sulfation capacity was found in the sheep placenta. However, the conjugation pathways accounted for nearly all the nontransplacental elimination of acetaminophen in the mother. Both transplacental clearance and nontransplacental clearance of acetaminophen in the fetus increased with gestational age. The glucuronidation and sulfation activities were determined in the fetus in utero and found to be 16 and 74% of the respective activities in the pregnant ewe on a body weight basis. The two conjugation clearances did not increase significantly with the gestational age of the fetus during the third trimester.

Acetaminophen↗
PubMed2006

Natural history of Canavan disease revealed by proton magnetic resonance spectroscopy (1H-MRS) and diffusion-weighted MRI.

Canavan disease is a childhood leukodystrophy caused by mutations in the gene for human aspartoacylase ( ASPA), which leads to an abnormal accumulation of the substrate molecule N-acetyl-aspartate (NAA) in the brain. This study was designed to model the natural history of Canavan disease using MRI and proton magnetic resonance spectroscopy ( (1)H-MRS). NAA and various indices of brain structure (morphology, quantitative T1, fractional anisotropy, apparent diffusion coefficient) were measured in white and gray matter regions during the progression of Canavan disease. A mixed-effects statistical model was used to fit all outcome measures. Longitudinal data from 28 Canavan patients were directly compared in each brain region with reference data obtained from normal, age-matched pediatric subjects. The resultant model can be used to non-invasively monitor the natural history of Canavan disease or related leukodystrophies in future studies involving drug, gene therapy, or stem cell treatments.

Age Factors↗
PubMedDate not supplied

Determining ultrafiltration properties of the peritoneum.

Peritoneal solute reflection coefficients have previously been determined by simply comparing the time dependence of the solute concentration in the dialysis solution with that predicted by mathematical models. The present study theoretically examines the experimental conditions required to determine the solute reflection coefficient from the dependence of the dialysate concentration on time. Optimal experimental conditions are first derived using an approximate mathematical model that permits simple interpretation; these predictions are confirmed by numerical computation using a more rigorous model. It is demonstrated that the peritoneal solute reflection coefficient can be determined when solute transport is in the blood to dialysate direction only during a hypertonic exchange. When solute transport is in the dialysate to blood direction, the solute reflection coefficient can be determined using a hypotonic dialysis solution only for solutes that diffuse slowly across the peritoneum. Reliable determinations of the peritoneal solute reflection coefficient can only be obtained when using these experimental conditions.

Biological Transport↗
PubMed2002

[A model of a long functioning drainage catheter].

The model of a long functioning tube was used in 98 patients with abscesses, phlegmons, suppurating hematomas and diffuse purulent peritonitis. The tube had a purifying device in its lumen as a small bead used for permanent destruction of the fibrin clot in the tube lumen in the postoperative period. It allowed maintenance of its patency for the limitless duration. The tube in the purulent cavity had the through direction, i.e. its both ends were brought onto the skin surface. The duration of treatment of patients with purulent diffuse peritonitis was from 18 to 23 days, with the suppurating process of soft tissues--from 12 to 18 days. There were no complications associated with using the model of the tube.

Catheters, Indwelling↗
PubMed1990

Numerical investigation of a novel spiral wound membrane sandwich design for an implantable bioartificial pancreas.

A novel spiral wound membrane sandwich (SWMS) design for an implantable bioartificial pancreas is presented and is numerically evaluated using a comprehensive model of the glucose-insulin kinetics within the device. The spiral blood flow pattern in this design induces a convective flow of blood ultrafiltrate directly through the islet chamber. Use of ultrafiltration in addition to diffusion allows rapid transmission of blood glucose changes to the islet chamber and efficient transport of insulin from the islet chamber back to the blood stream. Simulation results suggest that the implantable bioartificial pancreas design presented in this paper may offer a means of improving the control of blood glucose levels in type I diabetics.

Blood Glucose↗