PubMed Health⌕ Search

Biomedical subjects

E L Forker

Publications and source records attributed to E L Forker.

At least 19 recordsLinked to original sources
PubMed1995

Intralobular zonal heterogeneity and hepatic indicator dilution curves.

Conventional interpretation of hepatic indicator dilution curves rests on the assumption, among others, that every hepatocyte operates with the same rate constants. When this assumption is false, owing to intralobular zonal variation in surface-to-volume ratios and/or to zonal differences in permeability, the apparent rate constants recoverable from outflow transients are wrong estimates of average liver performance. We develop the theoretical basis for this conclusion and illustrate by example how it can confuse the interpretation of experimental data. The analysis proceeds from vascular and extracellular reference curves recorded from perfused rat livers and from a simple model of intralobular architecture in which highly arborized periportal sinusoids have a larger surface-to-volume ratio than the less-branched vasculature around the central vein. The experimental data and the model, applied to a wide range of hypothetical solutes, are used to compare the true average rate constants for uptake, efflux, and intracellular removal with the apparent values recoverable from outflow curves. When zonal differences in surface-to-volume ratios are the sole source of heterogeneity, the wrong estimates prove of little practical importance. By contrast, assigning larger regional variations in permeability leads to substantial errors. The confusion arising from such errors may be qualitative as well as quantitative. The presence of heterogeneity and thus the risk of interpretive error appears unrecognizable from outflow curves.

Animals↗
PubMed1993

A parameter identification problem arising from a model of canalicular bile formation.

We develop a simple mathematical model for bile formation and analyze some features of the model that suggest the design for future physiological experiments. The mathematical model results in a boundary value problem for a system of functional differential equations depending on several physical parameters. From the observability of the boundary values we can identify, both qualitatively and quantitatively, some of these physical parameters. This identification then suggests physical experiments from which one could infer some of the bile transport phenomena that are not, at present, directly observable. The mathematical parameter identification problem is solved by converting the boundary value problem to a transition time problem for a quadratic system of ordinary differential equations on the plane where we are able to employ some special properties of quadratic systems in order to obtain a solution.

Animals↗
PubMed1992

On the design and interpretation of experiments to elucidate albumin-dependent hepatic uptake.

The liver's apparently anomalous extraction of organic anions tightly bound to albumin continues to provoke controversy and confusion. Decisive experiments have proved difficult to design, and mathematical models have usually been constructed to defend one or another putative mechanism to the exclusion of others. To stimulate more decisive experiments and as an aid to interpreting those already reported, we discuss a general formulation of the problem that predicts the clearance pattern to be expected when facilitated dissociation and codiffusion are joint determinants of the uptake flux. The results provide an approach to modeling the various mechanisms by which the concentration of bound ligand at the cell surface could be a driving force for uptake. Further we present new calculations to clarify the interpretation of net ligand clearance when the removal of free ligand is the result of bidirectional fluxes into and out of an unstirred sink. Applied to a previously published comparison of the uptake performances of hepatocytes and polyethylene, the new calculations support the inference that facilitated dissociation of albumin-palmitate complexes occurs at or near the hepatocyte surface.

Animals↗
PubMed1990

Beta-lactoglobulin enhances the uptake of free palmitate by hepatocyte monolayers: the relative importance of diffusion and facilitated dissociation.

We compared the uptake of bound palmitate by rat hepatocytes to its uptake by polyethylene using beta-lactoglobulin (BLG) as the binding protein. The experiments were designed to supply a direct measure of the protein-dependent change in the diffusive conductance of extracellular fluid without determining the diffusion coefficients for free and bound fatty acid or the off-rate constant for protein binding. Rate-limiting dissociation in the stirred phase of extracellular fluid was excluded. The results obtained with BLG are strikingly similar to those previously obtained with albumin and provide additional circumstantial evidence that when the free fraction is small, palmitate uptake is partially driven by the concentration of bound fatty acid. Because this phenomenon is not specific for the binding protein, it may reflect direct exchange of ligand between the binding protein in extracellular fluid and the putative transport protein in the hepatocyte plasma membrane.

Animals↗
PubMed1989

Palmitate uptake by cultured hepatocytes: albumin binding and stagnant layer phenomena.

We compared uptake of palmitate by hepatocyte monolayers with uptake by polyethylene membranes under conditions of identical binding and stirring. Hepatocytes and polyethylene display similar clearances when the fatty acid is free, reflecting partial rate limitation by diffusion across the unstirred water layer. When palmitate is 99.8% bound to albumin, however, hepatocytes clear free fatty acid about seven times faster than does polyethylene. We analyzed the uptake of palmitate by polyethylene at two different pHs to isolate the diffusive resistance of the unstirred layer and to show that codiffusion of bound and free palmitate to the hepatocyte surface accounts for only approximately 20% of the albumin-dependent increment in the clearance of free palmitate. The clearance data are supported by independent measurements of the stagnant layer thickness obtained from indicator dilution data and by an electrochemical method. The findings suggest that hepatocytes facilitate the dissociation of albumin-palmitate complexes. Alternatively, albumin may modulate the uptake capacities of hepatocytes and/or polyethylene.

Analysis of Variance↗
PubMed1987

Protein binding of palmitate measured by transmembrane diffusion through polyethylene.

Thin polyethylene membranes permit ready diffusion of protonated long-chain fatty acids but are impermeable to protein and ions. This circumstance recommends polyethylene for measuring the free fraction of fatty acids in the presence of a binding protein and for estimating the ionization constant with which to compute the equilibrium constant for the binding of fatty acid anions. As an example of this approach we report the binding of tracer palmitate to bovine albumin and bovine beta-lactoglobulin. We find a binding constant for the high-affinity site on albumin that is close to that calculated by others from heptane:H2O partition ratios. Our procedure is simpler, however, and free of the theoretical objection that heptane may alter the binding characteristics of the protein. Our estimate of the pKa for palmitic acid is 4.9, a finding that conforms to the widely predicted but heretofore unconfirmed expectation that long-chain fatty acids should have a pKa of about 4.8. Unidirectional flux measurements exclude direct exchange of palmitate between albumin and polyethylene.

Albumins↗
PubMed1986

Only free bile acid drives ileal absorption of taurocholate.

We compared the intestinal absorption of monomeric taurocholate bound to albumin with the absorption of the free form. In the presence of albumin the apparent uptake coefficient is about three times greater than that in its absence. This result resembles the one reported previously for the liver. In the gut, however, this phenomenon is attributable to the diffusive resistance of an unstirred intervillous fluid layer, whereas in the liver this explanation has been excluded. We conclude that only free taurocholate engages the bile acid transport system in ileal mucosa, whereas in the liver bile acid uptake is driven by albumin-taurocholate complexes, as well as by free taurocholate.

Albumins↗
PubMed1986

Palmitate uptake by hepatocyte monolayers. Effect of albumin binding.

The uptake of 14C-palmitate by rat liver cell monolayers is depressed by binding of the fatty acid to albumin. When the uptake flux is divided by the concentration of free palmitate in the bathing medium, however, the resulting clearance is approximately 14 times greater in the presence of albumin than in its absence. These findings are not accounted for by the different diffusion rates of free and bound palmitate across an unstirred fluid layer, nor attributable to nonequilibrium binding. Instead we argue that the most plausible explanation is accelerated dissociation of albumin-palmitate complexes mediated by the cell surface--an interpretation that also explains the uptake kinetics of other albumin-bound organic anions by perfused rat liver.

Adsorption↗
PubMed1985

Hepatic transport and binding of rose bengal in the presence of albumin and gamma globulin.

Gamma globulin and albumin are compared with respect to their effects on the hepatic transport of rose bengal and with respect to the rates and affinities with which they bind this dye. The apparent intrinsic clearance of rose bengal is greater in the presence of albumin than in the presence of gamma globulin, and this difference increases with the protein concentration. Because the binding affinities of these proteins also differ, however, it cannot be concluded decisively that the mechanisms of dye removal are distinct. For both proteins the binding reaction rates as measured by stopped-flow spectrophotometry are much faster than the rate of convection along the sinusoid or the rate of removal of free dye by liver cells. The transport data and the binding rate constants are the basis for an extended theoretical model developed and analyzed in an accompanying report.

Animals↗
PubMed1985

Effects of unstirred Disse fluid, nonequilibrium binding, and surface-mediated dissociation on hepatic removal of albumin-bound organic anions.

Proceeding from the observation that organic anions bound to albumin have hepatic extraction fractions that are unexpectedly high, we have studied a distributed model that accounts for this phenomenon by invoking sites on the cell surface that catalyze the dissociation of albumin-anion complexes. The present report extends this model to include nonequilibrium binding and rate-limiting diffusion of bound anion to the cell surface. Simulation analysis of the extended model provides an unambiguous basis for interpreting the apparent intrinsic clearance of free anion. The model is fully consistent with the transport data that we obtained with rose bengal [Am. J. Physiol. 248 (Gastrointest. Liver Physiol. 11): G702-G708, 1985] but is suited to estimating only selected components of the unknown parameter vector. Uncertainties inherent in an alternate approach are discussed and illustrated.

Animals↗
PubMed1985

Effect of a transported ligand on the binding of albumin to rat liver cells.

Organic anions destined for hepatic uptake often bind to albumin in the circulation. Because albumin binds to liver cells but is not transported, we suggest that sites on the hepatocyte surface catalyze the dissociation of albumin-anion complexes, thus making more free anion available for transport than would otherwise occur. To learn whether liver cells distinguish between free albumin and albumin-anion complexes, we measured the binding of 125I-albumin to isolated rat hepatocytes in the presence and absence of rose bengal, a transported anion that binds extensively to albumin. Albumin binding to hepatocytes is reported as the albumin space corrected for extracellular fluid (14C-inulin space). Corrected albumin spaces are 2.95 and 2.83 microliter/mg cell protein with and without rose bengal, respectively. The mean difference and its 95% confidence interval computed from four comparisons in each of six rats is 0.12 +/- 0.67 microliter/mg cell protein. Inulin space is 32% of the uncorrected albumin space. Thus the affinities of albumin-rose bengal complexes and of free albumin for the hepatocyte surface differ by at most 28%. Accordingly, free albumin can compete with albumin-rose bengal complexes for cell surface sites, impairing the surface-mediated generation of free rose bengal for uptake. This finding explains the otherwise paradoxical observation that adding albumin to liver perfusate inhibits the uptake of rose bengal even when sufficient albumin is already present to bind 99.9% of this dye.

Animals↗
PubMed1984

Determining hepatic transport kinetics by mathematical modeling.

Three approaches to the measurement of hepatic transport kinetics by mathematical modeling are briefly reviewed. The so-called lumped, two-compartment model and the single-pass multiple-indicator dilution method enjoy a long history of wide application, but each is subject to important errors inherent in their underlying assumptions. An understanding of the reasons for these errors, as revealed by simulation analysis, suggests a third approach that can substantially improve the previous difficulties.

Animals↗
PubMed1983

Analyzing tracer disappearance curves to study hepatic transport kinetics.

This paper is devoted to a discussion of recent developments in the compartmental analysis of hepatic transport, especially the interpretation of tracer disappearance curves recorded from the reservoir of an isolated perfused liver preparation. The emphasis is on the advantages that this approach enjoys over other mathematical models, and a critical review of alternative methods is therefore included. The mathematical equations are largely suppressed, however, having appeared elsewhere in detail.

Animals↗
PubMed1983

Albumin-mediated transport of rose bengal by perfused rat liver. Kinetics of the reaction at the cell surface.

Rapid dissociation of organic anions from plasma albumin maximizes the presentation of free ligand to the cell surface and thus favors its efficient hepatic extraction. Even assuming these optimal conditions, however, taurocholate and rose bengal have hepatic extraction fractions that are higher than can be accounted for by spontaneous dissociation of their albumin-ligand complexes. In this study we developed a transport model that attributes this behavior to sites on the hepatocyte plasma membrane that bind the albumin-ligand complexes, promoting the transport of ligand into the hepatocyte. Fitting this model to rose bengal removal rates measured over a wide range of albumin concentrations yields estimates of the number of cell surface sites and their affinity for albumin. These estimates are in good agreement with those reported by Weisiger, Gollan, and Ockner for the binding of ligand-free albumin to isolated hepatocytes. We conclude that both experiments measure the same phenomenon and, accordingly, that the binding of albumin to the cell surface is the functional equivalent of albumin-mediated transport.

Animals↗
PubMed1982

Simulation and analysis of hepatic indicator dilution curves.

The "multiple-indicator dilution method" of measuring hepatic transport kinetics is subjected to simulation analysis. The objective is to examine the errors that may arise from treating the nonexchanging vasculature as a simple delay and to study the information content of simulated venous outflow curves. We find that the method cannot be counted on to provide consistently reliable estimates of either the transport rate constants or the sinusoidal volume. Estimates of the rate constant for irreversible solute removal from within liver cells are especially likely to be wrong. We suggest an alternative formulation of the governing differential equations that can substantially improve the estimates of the uptake parameter. These estimates may otherwise be subject to large systematic errors. Finally we discuss why the steady-state extraction fraction computed from the fitted parameters should be checked against a model-independent estimate obtained directly from the areas under the outflow curves. A method for making this comparison is at hand and should prove useful as a minimum criterion of internal consistency in animal experiments.

Animals↗