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At least 127 records · Page 7Linked to original sources
PubMed1977

The respiratory gas exchange of sea turtle nests (Chelonia, Caretta).

Sea turtles lay about 100 leathery-shelled eggs in a 25 cm diameter chamber carefully excavated about 50 cm deep in a nesting beach, where the eggs exchange gases (at approximately 28 degrees C) during their 60-day incubation period. The sand surrounding the spherical nest chamber restricts the diffusion of gases into and out of the nest so that as embryonic development progresses, PO2 decreases and PCO2 increases in the gas inside the nest. PO2 falls to 80-100 torr and PCO2 rises to 40-60 torr inside 100-egg man-made Chelonia and Caretta nests. The change in gas tensions in the nest during development is very similar to that seen in the air cell of the chicken egg. Gas tensions inside the turtle nest and in the sand surrounding the nest can be described by a radial steady-state diffusion model. The rate of diffusion of gases in the sand is 30-50% of the rate found in the nest and 6-12% of the rate found in an equal volume of air. The sand surrounding the turtle nest appears to determine the gas exchange of the eggs in the nest and is functionally analogous to the shell surrounding the chicken embryo. The female sea turtle may construct her nest so as the maximize its gas exchange and minimize gas partial pressure gradients inside the nest.

Animals↗
PubMed1990

Exact solution of a model of diffusion in an infinite chain or monolayer of cells coupled by gap junctions.

Analytic solutions are found for an infinite chain of cells coupled by gap junctions under two initial conditions: (a) One inner cell initially filled uniformly to a fixed concentration and (b) inner cell maintained indefinitely at constant concentration. The solution can be extended by the product method (Carslaw and Jaeger. 1959. Conduction of Heat in Solids. Oxford University Press.) to monolayers. We can also incorporate leakage through the plasma membrane by the product method. We demonstrate the utility of these results by fitting diffusion data from the septate axon of earthworm and by plots of theoretical profiles from monolayers of cells. Use of these analytic solutions enables one to overcome the limitations of methods that lump the effects of cytoplasmic diffusion and junctional permeability into an effective diffusion coefficient.

Animals↗
PubMed1989

The binding of G-protein to rod outer segment phospholipids at the nitrogen-water interface.

In the visual process, one photoexcited rhodopsin (R*) catalyzes the activation of hundreds of G-proteins. It remains to be determined whether G-protein and R* find one another by membrane surface diffusion of these components (diffusion model) or by diffusion of G-protein through the aqueous phase (hopping model). A monolayer of each main rod outer segment (ROS) phospholipid interacting with a subphase containing G-protein, has been used to simulate the interaction of G-protein with the cytoplasmic surface of discal membranes. The possible diffusion of G-protein through the aqueous phase was then measured by observing its adsorption-desorption in the monolayer of each main ROS phospholipid. From examination of surface pressure and ellipsometric isotherms at the nitrogen-water interface, we have determined that once incorporated into the monolayer, the G-protein remains associated, independent of surface pressure, thus providing evidence against the hopping model.

Animals↗
PubMed1991

Convergence of one-dimensional diffusion processes to a jump process related to population genetics.

A conjecture on the convergence of diffusion models in population genetics to a simple Markov chain model is proved. The notion of bi-generalized diffusion processes and their limit theorems are used systematically to prove the conjecture. Three limits; strong selection-weak mutation limit, moderate selection-weak mutation limit, weak selection-weak mutation limit are considered for typical diffusion models in population genetics.

Genetics, Population↗
PubMed1988

Effects of Tween 80 and liposomes on the corneal permeability of anti-inflammatory steroids.

The effects of Tween 80 and liposomes on the corneal permeability of dexamethasone (DM) and dexamethasone valerate (DV) were investigated in vitro. A model based on diffusion theory could successfully be applied to analyzing the process by which DM penetrates the cornea from DM preparations. The penetration rate increased according to the concentration of free DM. Although the penetration rate was increased by pretreatment with Tween 80, it was not affected by pretreatment with liposomes. The steroid that penetrated through the cornea from DV preparations had been metabolized to DM. The transfer of DM across the cornea from DV preparations tended to slow down to some extent during the penetration process. Diffusion models were not applicable to this process, in contrast to the corneal penetration of DM from DM preparations. When liposome preparations of DV were applied, the penetration rate of DM across the cornea depended the concentration of free DV in these preparations. When DV aqueous preparations containing different concentrations of Tween 80 were applied, the penetration rate increased as the concentration of the surfactant was increased, even though the concentration of free DV in the suspensions was kept constant. These results suggest that the corneal permeability of anti-inflammatory steroids is not affected by liposomes, but is accelerated by Tween 80.

Animals↗
PubMed1992

Sarcolemmal calcium binding sites in heart: II. Mathematical model for diffusion of calcium released from the sarcoplasmic reticulum into the diadic region.

We present a model for predicting the temporal and spatial dependence of [Ca] in the cardiac subsarcolemmal diadic region (cleft), following Ca release from the "feet" of the sarcoplasmic reticulum. This region is modeled as a disc 10 nm thick, 430 nm in radius, with or without Ca binding sites and open at its periphery to the cytosol. [Ca] is computed for three diffusion coefficients (100, 20 and 4% of aqueous diffusion), following release of a 20-msec square pulse sufficient to produce 50% maximal contractile force, or repetitive release (400/min) of such pulses. Numerical solutions are obtained for the general diffusion/binding problem and analytic solutions for the case of no binding sites. For the middle value of diffusion coefficient, and in the absence of binding sites, [Ca] rises to approximately 1.5 mM in 20-msec and then falls to approximately 0.1 microM in less than 3 msec. Adding binding sites reduces peak [Ca] to approximately 0.6 mM but prolongs its decline, requiring approximately 200 msec to reach 20 microM. For repetitive release [Ca] is greater than 100 microM for roughly half of each cycle. Two major implications of the predicted [Ca] are: (i) The effect of Ca binding sites on [Ca] will cause Ca efflux from the cleft via the Na-Ca exchanger (Km(Ca) approximately 20 microM) to continue at a significant level for greater than 200 msec. (ii) The time constant for inactivation of release from the "feet" must be much greater than for activation if Ca-induced Ca release is to continue for greater than 1-2 msec.

Animals↗
PubMed1986

Kinetics of sorption of ionizable solutes by plastic infusion bags.

Aqueous solutions of several ionizable substances were stored in plastic infusion bags and the sorption of the substances monitored with time. The substances used were p-nitrophenol, p-toluidine, warfarin sodium [3-(alpha-acetonylbenzyl)-4-hydroxycoumarin sodium salt] and trifluoperazine hydrochloride (10-[3-(4-methyl-1-piperazinyl)propyl]-2-(trifluoromethyl)phenothiazine dihydrochloride). The rate and extent of sorption for each substance varied with pH and was consistent with a preferential uptake of the un-ionized species. The uptake of p-nitrophenol and p-toluidine was adequately described by a diffusion model derived assuming that sorption is rate-controlled by the diffusivity of the solute in the plastic matrix, and that only the un-ionized species was sorbed by the plastic matrix. However, the uptake of warfarin sodium and trifluoperazine hydrochloride was described more accurately by a diffusion model in which the diffusional resistance of the plastic matrix and of an interfacial resistance barrier both contributed to the diffusional resistance encountered in the sorption process. It appeared that the rate of uptake of the un-ionized form of these solutes was diminished due to the influence of interfacial or aqueous diffusional barriers. Solute lipophilicity and degree of ionization appeared to be important factors determining the relative contribution of the respective barriers to the overall diffusional resistance.

Absorption↗
PubMed1985

Comparison of models describing the sorption of nitroglycerin and diazepam by plastic infusion systems: diffusion and compartment models.

The time course of sorption of diazepam and nitroglycerin from aqueous solutions into plastic materials has been represented by the diffusion and compartmental models for a variety of storage conditions. The diffusion model seemed to be the more satisfactory model in respect to both description and prediction of the drug uptake for all conditions. The compartment model appeared to be useful for describing the drug uptake at earlier times, giving a satisfactory fit to the data and reliable final parameter estimates. However, that model was not able to describe the loss as equilibrium was approached or accurately predict the disappearance profiles for these solutes with alterations in solution volume or infusion bag size. Approximations of the diffusion model gave parameter estimates consistent with those obtained by nonlinear regression using the full equations.

Adsorption↗
PubMed1988

A diffusion-adsorption model for the computation of the amount of hormone around a secreting cell, detected by the reverse hemolytic plaque assay.

The reverse hemolytic plaque assay enables the detection of secretion products from individual cells in cultures by visualizing the plaques formed after complement-mediated hemolysis around the secreting cells. However, the precise quantitation of the amount of secretion remains problematic. In this study we propose a computation model for estimating the spreading of the secreted molecules, based on the underlying processes of diffusion and antigen adsorption by immobilized antibodies. The translational diffusion coefficient of rat prolactin at 37 degrees C, determined by laser light scattering, was 9.89 x 10(-7) cm2/s. The time-dependent concentration distribution around a constantly secreting cell in a flat quasi infinite layer, was derived from the diffusion equation, using an analytical approach based on Laplace transformation. The relations between plaque size, incubation time and secretion level were expressed as a function of the threshold concentration of secretion product that can be detected and the effective diffusion coefficient, taking antigen adsorption into account. We obtained very good agreement between observed and predicted results for plaque formation by dispersed prolactin secreting cells of 14-day-old female rat pituitaries. This study confirms the validity of the assumptions underlying the reverse hemolytic plaque assay, provided that the cell density is low, the incubation time is moderately long and the concentration of specific antiserum is sufficiently high.

Adsorption↗
PubMed1990

Fluid flow rates in human peritumoural oedema.

Five patients with various types of brain tumours were infused with x-ray contrast material in a schedule designed to maintain a constant plasma concentration of tracer over a period of 3 hours. CT scans from an equatorial section of the tumour were taken at frequent intervals the first hour; then at 2 and 3 hours, and when possible up to 14 hours. Two different mathematical models-1. simple diffusion, and 2. transport by bulk flow plus diffusion were used to analyze the changes in tracer amount along profiles placed radially from the tumour center into the oedematous white matter. We found that the simple diffusion model could not account for the spread of contrast material in 3 cases. Adding bulk flow transport gave a very good fit to the measurements, also for the late scans. This model gave bulk flow rates of 0.0005 to 0.005 ml cm-2 min-1 for the extratumoural tissue close to the tumour, and values from 0.25 to 0.55 for the extracellular space in this region. We conclude that the peritumoural tissue is "perfused" by oedema fluid at relatively high flow rates and that this flow transports tracer and other components of plasma into the extracellular space.

Body Fluids↗