Kinetics of uptake and metabolism by endothelial cell from indicator dilution data

A distributed parameter model of cerebral blood-tissue exchange with account of capillary transit time distribution

Quantitative estimates of physiological parameters associated with cerebral blood flow can be derived from the analysis of dynamic contrast-enhanced (DCE) images, using an appropriate model of the underlying tissue impulse residue function. The theoretical formulation of a distributed parameter model of tissue microcirculation, which accounts for the effects of capillary permeability and transit time distribution, is presented here. This model considers a statistical distribution of capillary-tissue units, each described by a distributed parameter model that accounts for convective transport within the capillary and transcapillary axial diffusion. Monte Carlo simulations were performed to study the confidence of the parameter estimates, and the model was used to analyze DCE CT images of patient study cases with metastatic cerebral tumors. The tumors were found to yield significantly higher estimates than normal tissues for the parameters associated with the extravasation of tracer and for the standard deviation of capillary transit times. The proposed model can be used with DCE imaging to study the microcirculatory characteristics of cerebral tumors.

Pulmonary fluid extraction and osmotic conductance, sigmaK, measured in vivo

The change in aortic blood density in an in vivo rabbit preparation was measured to assess fluid movement at the pulmonary capillaries caused by infusion of hypertonic solution (NaCl, urea, glucose, sucrose, or raffinose in isotonic saline) into the vena cava over 20 s. The hypertonic disturbance increased the plasma osmotic pressure by </=30 mosmol/l. The density change indicates that the fluid extraction from the lung tissue was completed within 10 s. It was followed by a fluid filtration into the lung tissue and then an extraction and filtration from peripheral organs. An exchange model with flow dispersion yields two equations to estimate the osmotic conductance (sigmaK; where sigma is the reflection coefficient of the test solute and K is the filtration coefficient including the total capillary surface area), and the tissue fluid volume from the area and first moment of the measured density change over the extraction phase. The values of sigmaK are 1.40 +/- 0.11, 1.00 +/- 0. 10, 1.71 +/- 0.10, 2.60 +/- 0.23, and 3.73 +/- 0.34 (SE) ml . h-1 . mosmol-1 . l . g-1 for NaCl, urea, glucose, sucrose, and raffinose, respectively. Consistent with the model prediction, the tissue fluid volume (0.28 +/- 0.04 ml/g wet lung tissue) was independent of the solute used. This value suggests that all fluid spaces in the alveolar septa participate in the process of fluid extraction due to an increase in plasma osmotic pressure.

Pulmonary disposition of lipophilic amine compounds in the isolated perfused rabbit lung

We measured the pulmonary venous concentration vs. time curves for [3H]alfentanil, [14C]lidocaine, and [3H]codeine after the bolus injection of each of these lipophilic amine compounds (LAC) and a vascular-reference indicator (fluorescein isothiocyanate-dextran) into the pulmonary artery of isolated perfused rabbit lungs. A range of flows and perfusate albumin concentrations was studied. To evaluate the information content of the data, we developed a kinetic model describing the pulmonary disposition of these LAC that was based on indicator dilution theory, and we sought a robust approach for interpreting the estimated model parameters. We found that the distribution of the kinetic model rate constants of the lipophilic amine-tissue interactions can be described by alpha, H, and psi, where alpha is a measure of the capacity of the rapidly equilibrating interactions between the lipophilic amine and the tissue; H is a measure of the equilibrium capacity of the slowly equilibrating interactions between the lipophilic amine and the tissue; and psi is the mean sojourn time. The values of alpha, H, and psi were 0.8 +/- 0.1 (SE), 0.6 +/- 0.1, and 1.6 +/- 0.5 s; 1.9 +/- 0.1, 5.3 +/- 0.4, and 5.6 +/- 0.5 s; and 1.1 +/- 0.1, 9.8 +/- 0.4, and 4.7 +/- 0.2 s for alfentanil, lidocaine, and codeine, respectively. These values for alpha, H, and psi reveal the relative dominance of the slowly equilibrating interactions for lidocaine and codeine in comparison with alfentanil. This approach to data analysis may have utility for the potential use of LAC to reveal and to quantify changes in lung tissue composition associated with lung disease.

Efficient numerical methods for nonlinear-facilitated transport and exchange in a blood-tissue exchange unit

The analysis of experimental data obtained by the multiple-indicator method requires complex mathematical models for which capillary blood-tissue exchange (BTEX) units are the building blocks. This study presents a new, nonlinear, two-region, axially distributed, single capillary, BTEX model. A facilitated transporter model is used to describe mass transfer between plasma and intracellular spaces. To provide fast and accurate solutions, numerical techniques suited to nonlinear convection-dominated problems are implemented. These techniques are the random choice method, an explicit Euler-Lagrange scheme, and the MacCormack method with and without flux correction. The accuracy of the numerical techniques is demonstrated, and their efficiencies are compared. The random choice, Euler-Lagrange and plain MacCormack method are the best numerical techniques for BTEX modeling. However, the random choice and Euler-Lagrange methods are preferred over the MacCormack method because they allow for the derivation of a heuristic criterion that makes the numerical methods stable without degrading their efficiency. Numerical solutions are also used to illustrate some nonlinear behaviors of the model and to show how the new BTEX model can be used to estimate parameters from experimental data.

An interpretation of 14C-urea and 14C-primidone extraction in isolated rabbit lungs

We measured the venous concentration versus time curves of 14C-urea and 14C-primidone after rapid bolus injections of a vascular reference indicator, fluorescein isothiocyanate dextran, and one of the two 14C-labeled indicators in isolated rabbit lungs perfused with Krebs-Ringer bicarbonate solution containing 4.5% bovine serum albumin at flow rates (F) of 6.67, 3.33, 1.67, and 0.83 ml/sec and with nearly constant microvascular pressure and total lung vascular volume. When we calculated the permeability-surface area product, PS, from the 14C-urea and 14C-primidone outflow curves using the Crone model, the estimates of the PS product were directly proportional to F. However, the fractional change in the PS with flow was different for the two indicators. We also estimated the PS from the same 14C-urea and 14C-primidone data using an alternative model that includes perfusion heterogeneity, estimated in a previous study, and flow-limited and barrier-limited extravascular volumes accessible to both urea and primidone. This model was able to fit the outflow curves of either 14C-urea or 14C-primidone at all four flows studied with one flow-independent PS for each indicator. The ability of the new model to explain the 14C-urea and 14C-primidone data with no flow-dependent change in PS suggests that a change in PS with F estimated using other models such as the Crone model is not sufficient for capillary surface area recruitment.

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

Blood-tissue exchange via transport and transformation by capillary endothelial cells

The escape of solutes from the blood during passage along capillaries in heart and skeletal muscle occurs via diffusion through clefts between endothelial cells and, for some solutes, via adsorption to or transport across the luminal plasmalemma of the endothelial cell. To quantitate the rates of permeation via these two routes of transport across capillary wall, we have developed a linear model for transendothelial transport and illustrated its suitability for the design and analysis of multiple simultaneous indicator dilution curves from an organ. Data should be obtained for at least three solutes: 1) an intravascular reference, albumin; 2) a solute transported by endothelial cells; and 3) another reference solute, of the same molecular size as solute 2, which neither binds nor traverses cell membranes. The capillary-tissue convection-permeation model is spatially distributed and accounts for axial variation in concentrations, transport through and around endothelial cells, accumulation and consumption within them, exchange with the interstitium and parenchymal cells, and heterogeneity of regional flows. The upslope of the dilution curves is highly sensitive to unidirectional rate of loss at the luminal endothelial surface. There is less sensitivity to transport across the antiluminal surface, except when endothelial retention is low. The model is useful for receptor kinetics using tracers during steady-state conditions and allows distinction between equilibrium binding and reaction rate limitations. Uptake rates at the luminal surface are readily estimated by fitting the model to the experimental dilution curves. For adenosine and fatty acids, endothelial transport accounts for 30-99% of the transcapillary extraction.

Kinetics of serotonin uptake in the intact lung

The pulmonary endothelium is capable of removing and metabolizing serotonin (5HT) carried in the venous blood. Thus the lungs can influence the arterial concentrations of 5HT. In addition, there is evidence that changes in the lung uptake of 5HT might portend more serious endothelial damage wherein the barrier function of the endothelium is compromised. This has been a stimulus for finding methods for evaluating these endothelial functions. These methods must be able to distinguish changes in whole organ function which result from changes in perfusion (e.g., cardiac output, redistribution of flow, etc.) from those resulting from changes in the function of the endothelial cells. When a bolus containing radio-labeled 5HT and an unmetabolizable indicator which is confined to the vascular space is injected into the pulmonary artery, the pulmonary venous or systemic arterial concentration curves contain information about both the convective transport and endothelial cell process involved. Some of this information can be interpreted quantitatively using a simple mathematical model.