An isolated perfused lung preparation for metabolic studies

Drug Absorption Parameters Obtained Using the Isolated Perfused Rat Lung Model Are Predictive of Rat In Vivo Lung Absorption

The ex vivo isolated perfused rat lung (IPL) model has been demonstrated to be a useful tool during drug development for studying pulmonary drug absorption. This study aims to investigate the potential use of IPL data to predict rat in vivo lung absorption. Absorption parameters determined from IPL data (ex vivo input parameters) in combination with intravenously determined pharmacokinetic data were used in a biopharmaceutics model to predict experimental rat in vivo plasma concentration-time profiles and lung amount after inhalation of five different inhalation compounds. The performance of simulations using ex vivo input parameters was compared with simulations using in vitro input parameters, to determine whether and to what extent predictability could be improved by using input parameters determined from the more complex ex vivo model. Simulations using ex vivo input parameters were within twofold average difference (AAFE < 2) from experimental in vivo data for all compounds except one. Furthermore, simulations using ex vivo input parameters performed significantly better than simulations using in vitro input parameters in predicting in vivo lung absorption. It could therefore be advantageous to base predictions of drug performance on IPL data rather than on in vitro data during drug development to increase mechanistic understanding of pulmonary drug absorption and to better understand how different substance properties and formulations might affect in vivo behavior of inhalation compounds.

Airway Retention and Pulmonary Absorption of Poly-α,β-[N(2-hydroxyethyl)-D, L-aspartamide]

Coarse aqueous sprays of the polymer, poly-α,β-[N(2-hydroxy ethyl)-D,L-aspartamide], (PHEA), containing a covalently bound fluorophore, ethyl carbonyl-6-aminofluorescein, were administered in doses ranging 2.2 through 3.6 mg to the airways of the isolated rat lung (IPRL). The polymer was characterized with number and weight averaged molecular weights of 5300 and 8600 Daltons, respectively. Transfer to the perfusate supplying the pul monary circulation was monitored with time in order to assess the polymer's systemic absorption potential and the transferred molecular weight distribu tions (MWD). The polymer was absorbed at an apparently constant rate during each experiment. The MWD of absorbed material was characterized by gel permeation chromatography and found to be shifted toward lower molecular weights when compared to that of the administered polymer. Two hours after dosing, absorbed material had mean values for weight mean molecular weight, Mw = 6670 ± 526 Daltons and number mean molecular weight, Mn = 4680 ± 640 Daltons where the ranges are standard deviations in 8 IPRL prep arations. PHEA was not metabolized in the 3 h duration of an experiment and there was some tendency for the median molecular weight of the absorbed material to increase with time after administration. Results are discussed in the context of macromolecular delivery to the systemic circulation via the lung.

Methods for the study of lung metabolism

The recognition of the non-respiratory functions of the lung has led to an increased interest in pulmonary metabolism. The practical methods used to study metabolism in other tissues have all been applied to the investigation of lung metabolism. Methods fall into two groups: those used in vivo and those used in vitro after disrupting the organ or isolating it from other tissues and perfusing its circulation. Some of the methods used in vitro disturb the tissue environment in such a way as to introduce experimental artifacts which make the interpretation of results difficult. There are advantages to the use of the isolated perfused organ technique which, although it has been established for many years, has recently been improved. This method lends itself particularly well to studies of the intermediary metabolism and uptake of drugs by lung tissue. These improvements are described and the application to pulmonary drug uptake is illustrated by reference to the drug mexiletine. The relative advantages of the various methods and their applications are discussed.

In vivo, in vitro and ex vivo models to assess pulmonary absorption and disposition of inhaled therapeutics for systemic delivery

Despite the interest in systemic delivery of therapeutic molecules including macromolecular proteins and peptides via the lung, the accurate assessment of their pulmonary biopharmaceutics is a challenging experimental task. This article reviews in vivo, in vitro and ex vivo models currently available for studying lung absorption and disposition for inhaled therapeutic molecules. The general methodologies are discussed with recent advances, current challenges and perspectives, especially in the context of their use in systemic pulmonary delivery research. In vivo approaches in small rodents continue to be the mainstay of assessment by virtue of the acquisition of direct pharmacokinetic data, more meaningful when attention is given to reproducible dosing and control of lung-regional distribution through use of more sophisticated lung-dosing methods, such as forced instillation, microspray, nebulization and aerosol puff. A variety of in vitro lung epithelial cell lines models and primary cultured alveolar epithelial (AE) cells when grown to monolayer status offer new opportunity to clarify the more detailed kinetics and mechanisms of transepithelial drug transport. While continuous cell lines, Calu-3 and 16HBE14o-, show potential, primary cultured AE cell models from rat and human origins may be of greater use, by virtue of their universally tight intercellular junctions that discriminate the transport kinetics of different therapeutic entities. Nevertheless, the relevance of using these reconstructed barriers to represent complex disposition of intact lung may still be debatable. Meanwhile, the intermediate ex vivo model of the isolated perfused lung (IPL) appears to resolve deficiencies of these in vivo and in vitro models. While controlling lung-regional distributions, the preparation alongside a novel kinetic modeling analysis enables separate determinations of kinetic descriptors for lung absorption and non-absorptive clearances, i.e., mucociliary clearance, phagocytosis and/or metabolism. This ex vivo model has been shown to be kinetically predictive of in vivo, with respect to macromolecular disposition, despite limitations concerning short viable periods of 2-3 h and likely absence of tracheobronchial circulation. Given the advantages and disadvantages of each model, scientists must make appropriate selection and timely exploitation of the best model at each stage of the research and development program, affording efficient progress toward clinical trials for future inhaled therapeutic entities for systemic delivery.

Interleukin-1beta and tumour necrosis factor-alpha increase microvascular leakage in the guinea pig trachea

OBJECTIVE

Airway microvascular leakage is considered to be an important component of airway inflammation in asthma. In the present study we examined the effect of interleukin-1beta (IL-1beta) and tumour necrosis factor-alpha (TNFalpha) on airway microvascular leakage in vivo.

METHODOLOGY

Tracheal Evans blue extravasation was examined in an isolated tracheal segment, in anaesthetized mechanically ventilated guinea pigs. Baseline tracheal microvascular leakage was measured in five animals. As a control group for aerosol challenge, the isolated tracheal segment (n = 5) underwent saline aerosol challenge. To test whether a combination of IL-1beta (10 ng/mL) and TNFalpha (100 ng/mL) induced Evans blue extravasation, the trachea was exposed to an aerosol of these cytokines (n = 5). As a positive control the tracheal segment was challenged with histamine aerosol (5 x 10(-2) mol) (n = 3). All aerosol challenges were for 1 min.

RESULTS

TNFalpha and IL-1beta aerosol challenge significantly increased Evans blue extravasation (28.9 +/- 1.6 microg/g wet tissue, mean +/- SE) compared to saline challenge (13.8 +/- 3.0 microg/g; P < 0.05). Tracheal dye extravasation without aerosol challenge, was not significantly different from saline-challenged animals (17.5 +/- 2.9 and 13.8 +/- 3.0 microg/g, respectively). Histamine significantly increased Evans blue extravasation (50.1 +/- 4.8 microg/g; P < 0.05) compared to saline challenge.

CONCLUSION

Pro-inflammatory cytokines, TNFalpha and IL-1beta are able to induce significant microvascular leakage in the guinea pig trachea.

Establishment and validation of an isolated rat lung model for pulmonary metabolism studies

An isolated rat lung model was established and validated for use in pulmonary metabolism studies. During the establishment phase of the study, several problems were encountered and overcome in order to maintain the lungs in physiological condition. In the validation phase of the study, the lungs were removed, ventilated and perfused from 34 male Fischer 344 rats. After an equilibration period, lungs were ventilated and perfused for up to 4 h. Morphological, biochemical and functional parameters were evaluated to validate the physiological condition of the lungs. Morphological parameters included wet/dry lung weight ratios and gross and histological scoring for edema. Biochemical parameters included assays for tissue ATP and reduced glutathione content, glutathione reductase activity and glucose utilization. Functional parameters included changes in lung tidal volume, dynamic compliance and airway resistance. Results indicated that edema formation was only detected histologically, that lungs remained nearly biochemically normal for 210 min and that pulmonary function declined to about 80-90% of normal. Overall, these findings indicated that the isolated, perfused rat lung remained in acceptable physiological condition for ca. 210 min. This period of time should be adequate for conducting pulmonary metabolism studies with a variety of exogenous compounds.

Measuring the weight of the isolated perfused rat lung during negative pressure ventilation

Weight measurement represents a means of quantitating edema formation in isolated lungs and following its time course. During ventilation by negative pressure measurements with commonly used weight transducers are impossible because the pressure changes inside the artificial thoracic chamber affect the weight reading. Therefore, we have developed a weight transducer that can be used during negative pressure ventilation. Its design and applicability for assessing weight gain induced by various agents in the negative-pressure-ventilated isolated perfused rat lung is presented here.

An improved setup for the isolated perfused rat lung

This article will describe a method for perfusing and ventilating rat lungs which allows assessment of lung mechanics, segmental vascular resistance, gas exchange, and vascular permeability. Isolated perfused rat lungs (IPL) were ventilated by negative pressure ventilation and were perfused at constant pressure with recirculating, albumin-containing Krebs-Henseleit buffer. A newly constructed respirator pump facilitated well-defined ventilation of the lungs and permitted switching between negative and positive pressure ventilation. Using this setup lungs were ventilated with positive pressure during both surgery and measurement of lung weight. Once the lungs were prepared, electronic control circuits ensured stability of the perfusate pH and perfusate pressure. Such preparations were stable for at least 3 hr. Breathing mechanics, for example, tidal volume, pulmonary compliance, and pulmonary resistance, perfusate characteristics, such as pulmonary vascular resistance, pulmonary pre- and postcapillary resistance, perfusate pH, PO2, and PCO2, and the capillary filtration coefficient were determined. All measured parameters were within normal physiological range, and the lungs were biochemically active, and responded to various stimuli.

Studies of human airways in vitro: a review of the methodology

The pathophysiology of human airway narrowing is only partly understood. In order to gain more insight in the mechanisms of human lung diseases and potential beneficial therapeutic agents, adequate models are needed. Animal airway models are of limited value since lung diseases such as asthma and chronic obstructive pulmonary disease (COPD) are unique to humans and because the mechanisms of airway narrowing differ between species. Therefore, it is important to perform studies on human isolated airways. We describe the models that have been developed to study airway function in vitro, emphasizing human airway preparations. The easily prepared airway strip and ring preparations are described first. The potential damage during preparation and the interference with airway structure are important drawbacks in these preparations. Lung parenchymal strips, described next, were designed in order to study responsiveness of small airways. However, parenchymal strips are anatomically complex, and responsiveness is determined by the relative amounts of airway and vascular smooth muscle. The lack of reproducibility between species and even within one animal limits their usefulness. Airway tube preparations, in which luminal and serosal stimulation can be separated, enable us to study the modulatory role of the airways epithelium in vitro. Furthermore, airway compliance can be measured. In the isolated perfused lung preparation, relationships between the airways and the vascular system are preserved and the interaction between these two systems can be studied. Weight gain due to fluid extravasation is a problem in this model which has not been used yet to study human lungs in vitro. Next, methodological aspects such as tissue handling and storage, recording of responses, removal of the epithelium, and electrical field stimulation are discussed in some detail. Although animal airways tissue can be studied immediately after removal, human tissue is often obtained with some delay. However, this seems tenable since electron microscopy of lung tissue obtained at autopsy showed that recovery of the preparation occurs during incubation of carbogenated Krebs-Henseleit (K-H) buffer. Dissected airways can be stored overnight in cooled K-H buffer until up to 55 hr after resection without losing viability. Commonly used physiological salt solutions which bath the tissue contain osmotic molecules, ions important for contractility, glucose as a substrate, and a bicarbonate-carbon dioxide buffer.(ABSTRACT TRUNCATED AT 400 WORDS)

The isolated heart-lung preparation in the cat. An in situ model to study the role of the lungs in the disposition of drugs

In the search for drugs with an extreme short time course of action, compounds should be developed that are rapidly distributed to and temporarily stored in well-perfused organs. Since the lungs receive the complete cardiac output and have the ability to temporarily store drugs, we have developed an in situ, isolated lung preparation in the cat to study the contribution of the lungs to the disposition of drugs. The cat's own heart perfuses the lung in situ with autologous blood. The circulation between the left ventricle and the right atrium is short-circuited via an aorta-caval shunt. The right forelimb is added to study pharmacodynamics simultaneously (only for muscle relaxants). Validation of the model for 180 min of perfusion showed complete isolation of the organs without major biochemical changes or edema and a stable muscle response. In pilot experiments with two structurally related muscle relaxants, initial muscle relaxation was followed by spontaneous recovery of neuromuscular function and a gradually decreasing plasma concentration, indicating partial disposition by the lungs. This was confirmed by direct concentration measurements in the lung. The present model may provide a powerful experimental tool to elucidate the role of the lungs in the disposition of drugs.

Albumin protects against capsaicin- and adenosine-induced bronchoconstriction and reduces overflow of calcitonin gene-related peptide from guinea-pig lung

In the guinea-pig isolated, perfused lung, the effect of albumin on oedema formation and bronchoconstriction as well as on capsaicin-induced overflow of calcitonin gene-related peptide-like (CGRP-LI) immunoreactivity has been examined. CGRP was used as an indicator of sensory nerve activation since it is more stable than the tachykinins substance P and neurokinin A. As expected, the lung water content was significantly (P less than 0.001) higher in lungs perfused with albumin-free buffer than when the buffer contained 4.5% albumin. Also, in albumin-free buffer the baseline airway resistance (RL) was increased and dynamic compliance (CDYN) reduced (P less than 0.001). Capsaicin (1 x 10(-6) M) was about 100 times less potent as a bronchoconstrictor when preincubated with albumin for 45 min, and the associated overflow of CGRP-LI was inhibited (from 221.0 +/- 63.4 fmol to 8 fmol fraction-1). When CGRP (50-200 pM) was incubated for 60 min with albumin, the recovery of CGRP-LI was 48% lower (P less than 0.01) than in the absence of albumin, corresponding to a loss rate of about 1% min-1. Catabolism or binding of neuropeptides can therefore hardly explain the diminished bronchoconstrictor potency of capsaicin. Capsaicin was also less effective as a constrictor in isolated bronchi after preincubation with albumin, suggesting that capsaicin itself may be bound or absorbed to this macromolecule. The bronchoconstrictor response to adenosine was also diminished in the presence of albumin. Adenosine was about 1000 times less potent as a bronchoconstrictor if dissolved in albumin 45 min before infusion, but only 10 times less potent when administered as bolus doses to albumin buffer-perfused lungs. Metabolism of adenosine may be the reason for the decreased potency of adenosine. The enzymatic activity may have been associated with impurities in the albumin preparation used (bovine serum albumin fraction V is greater than or equal to 96% pure) or contained in the protein itself. Since the bronchoconstrictor effect of acetylcholine was not reduced in the presence of albumin, it is not likely that albumin affects directly the contractility of the smooth muscle. These data demonstrate the importance of studying the influence of albumin on the in-vitro actions of pharmacological agents. The absence or presence of albumin products in nutrient buffer solutions may mean dramatic differences in potencies of certain drugs. Furthermore, bolus injections of agents are preferable, and preincubation together with albumin should be avoided.

Solute absorption from the airways of the isolated rat lung. I. The use of absorption data to quantify drug dissolution or release in the respiratory tract

Coprecipitates of fluorescein and magnesium hydroxide demonstrate delayed absorption relative to fluorescein solutions when administered to the airways of the isolated perfused rat lung (IPRL). Perfusate concentration vs time profiles showed that dissolution and not epithelial permeability was the rate-controlling factor in the airway-to-perfusate transfer process. A simple data deconvolution method was developed to determine the fluorescein release from the microparticulate coprecipitates in the airways. The deconvolution technique is generally applicable and provides values for undissolved solute remaining in the airways as a function of time provided that (a) significant binding and/or metabolism does not occur, (b) absorption from solution is apparent first order, and (c) all solid or dissolved material reaching perfused regions is absorbed within the lifetime of the preparation. Increased release rates of fluorescein occurred from precipitates containing greater starting concentrations of the dye. Dissolution profiles were similar to those that occur for log-normally distributed powders. The analysis of two unusual time profiles implied that the regional distribution of solid and dissolved material, between perfused areas and nonperfused areas, could be nonhomogeneous despite the use of a standardized dosing technique. The studies describe a method of using the IPRL with the potential to screen aerosol formulations for extended dissolution in the respiratory tract.

A novel dosing method for drug administration to the airways of the isolated perfused rat lung

A novel method is described for the reproducible administration of known liquid quantities to the peripheral airways of the isolated perfused rat lung. The basis of the technique was to use a 25-microL metered dose of fluorocarbon propellant to expel liquid (as a coarse spray) from an intratracheal dosing cartridge into the airways, while simultaneously inflating the lungs with a fixed volume of gas. The methodology is illustrated by administration of 100-microL volumes of aqueous disodium fluorescein solutions to a series of lung preparations. The reproducibility and regional distribution of dosing were determined by dissection, homogenization, and fluorimetric assay. Even though the dye was distributed nonuniformly between the lung lobes, in a series of preparations, 65.9 +/- 4.8% of the recovered dose was still deposited in the lung periphery, the site from which absorption is believed to occur. The method will enable the study of airway-to-perfusate transfer kinetics for compounds administered in a variety of different liquid formulations.

The accumulation and localisation of putrescine, spermidine, spermine and paraquat in the rat lung. In vitro and in vivo studies

Putrescine was accumulated into the isolated perfused rat lung by a temperature dependent process. The uptake obeyed saturation kinetics for which an apparent Km of 14 microM and Vmax of 48 nmol/g wet wt/hr was derived. After rats were dosed subcutaneously with [14C]putrescine, it was accumulated in the lung to concentrations greater than that in the plasma with the highest amount found between 3 and 12 hr. From 3 hr after dosing until 24 hr, there was a progressive increase in 14C label incorporated into spermidine, indicating that putrescine was converted to spermidine. Using autoradiographic techniques in lung slices the [3H]oligoamines were found in the alveolar epithelial type II. Clara and very probably the alveolar type I cells. With [3H]paraquat, the presence was detected only in the alveolar type II cells. Likewise, in the isolated perfused rat lung or following s.c. dosing of rats with [3H]putrescine the radiolabel was located only in the alveolar type II cell. We have suggested that the most likely explanation for the differences in localisation of label between in vitro and in vivo studies resulted from the use of [3H] label of different specific activity. Consequently we have concluded that the cell types with the ability of accumulate paraquat and oligoamines were the alveolar epithelial type I and type II cells and Clara cells.

Adenosine-induced bronchoconstriction in the guinea-pig isolated lung: interaction with theophylline and enprofylline

Effects of purine nucleosides and asthma mediators on airway tone have been examined in the guinea-pig isolated perfused lung preparation. Acetylcholine (10 pmol-0.3 nmol), histamine (1-10 nmol), adenosine (10 nmol-0.3 mumol), ATP (10 nmol-0.3 mumol) and inosine (10 mumol-0.1 mmol) all produced a dose dependent increase in lung resistance (RL) and a decrease in dynamic compliance (CDYN). ATP was equipotent with adenosine whereas inosine was about 500 times less potent. The adenosine-induced bronchoconstriction was affected neither by disodium cromoglycate (150 microM) nor by the histamine H1-receptor antagonist, mepyramine (1 microM) suggesting that histamine is not involved in this response. Furthermore, it was studied whether the xanthines theophylline and enprofylline specifically interacted with the adenosine induced bronchoconstriction. Theophylline significantly (P less than 0.01-0.001) and concentration dependently prevented both acetylcholine and adenosine-induced increase in RL. The response to 0.1 nmol acetylcholine was reduced by 32.8 +/- 8.4% (mean +/- SEM) and 58.1 +/- 4.0%, respectively, by 75 and 150 microM theophylline. Theophylline, 75 and 150 microM, also inhibited the increase in RL caused by 0.1 mumol of adenosine by 61.4 +/- 9.6% and 83.4 +/- 5.2%, respectively. Theophylline, was significantly (P less than 0.05-0.01) more potent in preventing the RL increase produced by adenosine than that by acetylcholine. Enprofylline, 30 microM, equally well as 75 microM theophylline reduced the acetylcholine-induced bronchoconstriction by 41.8 +/- 7.6% (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Lung mechanics of the guinea-pig isolated perfused lung

In the isolated perfused guinea-pig lung, lung resistance (RL) and dynamic compliance (CDYN) as well as flow, pH, PO2 and PCO2 of the perfusate were recorded. The baseline values were stable up to 2.5 h. Mean values (+/- SEM) for RL and CDYN were 0.36 cm H2O ml-1 s-1 +/- 0.04 and 0.27 ml cm-1 H2O +/- 0.02, respectively, which agree with in vivo values reported for unanaesthetized guinea-pigs. The pH was always slightly raised and the PCO2 always lowered in the effluent compared to the inflowing medium (P less than 0.001), indicating that the lung had an operative ventilation. Intravascularly administered acetylcholine, histamine and adenosine caused reproducible dose-dependent bronchoconstrictions recorded as increased RL and decreased CDYN. It is noteworthy that adenosine in this model and in vivo consistently produced bronchoconstriction which is in contrast to findings in other in vitro airway preparations. We conclude that this isolated perfused guinea-pig lung preparation has stable in vivo-like characteristics offering interesting possibilities for combining studies of respiratory effects with, for example, metabolism, pharmacokinetic and vascular reactivity studies.

An isolated perfused rat lung preparation for the study of aerosolized drug deposition and absorption

The isolated, perfused rat lung preparation was modified to allow characterized solid aerosol delivery. Deposition and airway-to-perfusate transfer of disodium fluorescein from 3-4 micron dae solid aerosols were studied under different ventilatory regimes. The lungs inhaled from an aerosol stream of constant concentration via a tracheal cannula. Air displacement from a sealed artificial thorax housing the lungs provided the driving force for inhalation. The lungs were suspended in a physiologically normal position and both left and right sides of the heart were cannulated for constant rate perfusate flow. Fractional deposition was inversely proportional to respiratory frequency implying that sedimentation was the primary deposition mechanism. Increasing tidal volumes similarly enhanced the ratio of amount deposited/amount administered. Fluorescein transfer to the perfusate occurred from the lung regions containing intact vasculature, was apparent first-order, and independent of perfusate flow. The average rate constant for transfer was 0.057 +/- 0.02 min-1 (t1/2 = 12.2 +/- 4.2 min-1). The ratio of transferable amount/amount deposited appeared to indicate the depth of aerosol penetration. This increased at high respiratory frequency and tidal volume, while decreasing with increasing aerosol particle size. Potential applications of the model are discussed in the light of these results.

Isolated perfused rabbit lung as a model to study the absorption of organic aerosols

An improved technique for perfused lung preparations adapted for absorption studies with organic aerosols is described. The model developed by Niemeier and Bingham (1972) was modified to place greater emphasis on the respiratory functions of the lung as an index of viability. Perfusion of this preparation for 2 hr showed that tidal volume, pulmonary blood flow, and visual appearance were the most sensitive indicators of physiological change. The importance of relevant criteria for assessment of viability is discussed. In addition, a simple technique for on-line computerized measurement of tidal volume and respiratory frequency is described.

The pharmacokinetics of benzo[alpha]pyrene in the isolated perfused rabbit lung: the influence of benzo[alpha]pyrene, n-dodecane, particulate, or sulfur dioxide

The pharmacokinetics of benzo[a]pyrene (BaP) in the isolated perfused rabbit lung (IPL) following pretreatment of the whole animal or simultaneous administration to the IPL with n-dodecane, ferric oxide, crude airborne particulate (CAP), fly ash or sulfur dioxide have been investigated using a one compartment model. The rate constant for the appearance (ka) of BaP in the blood, the clearance of BaP from the blood, and the rate of appearance of BaP metabolites (RAM) were the kinetic parameters determined. BaP entered the blood rapidly with an average half-life of 11 min in experiments in which the IPLs received only BaP on perfusion. The logarithms of the clearances from these experiments were linearly correlated with the RAMs. In these experiments, pretreatment of the whole animal with BaP produced a 48-55-fold increase in BaP clearance while pretreatment with n-dodecane increased the clearance 4-fold in comparison with no pretreatment. Pretreatment with ferric oxide or ferric oxide and BaP increased the clearance by factors of 5.5 and 1.5, respectively, over those of unpretreated and BaP pretreated experiments.

The isolated perfused lung

The unique nonrespiratory functions of the lungs have become more apparent in recent years. The isolated perfused lung model offers many advantages over other methods for the study of pulmonary metabolism, xenobiotic disposition and the influence of interactions among agents of different physical forms. Detailed descriptions of the experimental preparation are elements in evaluating and comparing data from various sources but these are frequently neglected. A discussion and critique of the following elements are provided in this review in order to elucidate the typical problems one might encounter in evaluating data: perfusate type, perfusion method, construction materials, ventilation method, temperature control, surgical procedure, microbiological contamination and evaluation criteria of the preparation. Examples are given where the IPL method has been applied and suggestions are made for future research efforts.

The effect of in vivo hyperoxic exposure on the release of endogenous histamine from the rat isolated perfused lung

Female rats were exposed to either 1 atm air or 100% O2 for 12, 24, or 48 hr. The rats were killed, the lungs were removed, and an isolated perfused lung (IPL) system was prepared. The isolated lung preparation was perfused with a modified Krebs-Henseleit buffer in a recirculating system, and the effect of the O2 exposures on histamine release from the IPL was determined. The effect of these O2 exposures on malondialdehyde formation in the IPL also was examined. Maximal release of histamine occurred after 20 min of perfusion. A linear relationship was found between the maximal histamine concentration released into the perfusate and the length of time the rats were exposed to normobaric hyperoxia. Malondialdehyde in lung perfusate also increased in a linear manner with increasing O2 exposure time. Addition of the H1-receptor antagonist, d-chlorpheniramine, to the perfusate completely inhibited basal histamine release from the IPL of both air- and O2-exposed rats, while addition of the H2-receptor antagonist, metamide, potentiated the release process. There was no significant effect demonstrated when an equimolar concentration of atropine was added to the perfusate. Arterial plasma histamine from rats exposed to 100% O2 for 24 hr increased 40% when compared to air-exposed controls, while histamine release from the IPL increased 75%. In conclusion, exposure of rats to normobaric hyperoxia caused both histamine release and malondialdehyde formation. Histamine release probably occurred as a result of a free radical-induced peroxidation of the lipid membrane of histamine-containing mast cells. Release of histamine from the IPL may be an early biochemical marker of damage by O2.

The effects of a cocarcinogen, ferric oxide, on the metabolism of benzo[a]pyrene in the isolated perfused lung

An isolated perfused New Zealand rabbit lung preparation was used to investigate the effects of a cocarcinogen, ferric oxide (Fe2O3), on the metabolism of benzo[a]pyrene (BaP), a ubiquitous potent carcinogen that has been associated with the increased incidence of human bronchiogenic carcinoma in occupational and urban settings. [14C]-BaP was administered intratracheally to an isolated perfused lung (IPL) preparation with and without Fe2O3 after intraperitoneal pretreatment of the whole animal with BaP or intratracheal pretreatment of the whole animal with Fe2O3 and/or BaP. BaP and its metabolites were isolated from serial blood samples up to 180 min after administration of [14C]BaP to the IPL. BaP and its metabolites were also isolated from lung tissue, washout fluid, macrophage, and trachea bronchi at the end of the perfusion at 180 min. Patterns of BaP metabolites were determined by chromatographic techniques and liquid scintillation counting. Fe2O3 pretreatment to the whole animal or administration of Fe2O3 to the IPL altered BaP metabolism by the perfused lung. Fe2O3 pretreatment to the whole animal resulted in an increase in the total rate of appearance of metabolites of BaP in the blood (ng/g lung X h), while Fe2O3 administration to the IPL resulted in a decrease in the total rate of appearance of BaP metabolites in the blood and inhibited the effect of pretreatment. Administration of Fe2O3 with BaP to the IPL with or without Fe2O3 pretreatment to the whole animal, or BaP administration to the IPL preceded by Fe2O3 pretreatment to the whole animal, enhanced dihydrodiol formation and depressed formation of water-soluble metabolites. Since dihydrodiol formation is considered to be the active pathway of BaP metabolism, these data suggest that pulmonary exposure to a known cocarcinogen, Fe2O3, in the presence of BaP results in increased production of dihydrodiols of BaP, which may be further metabolized to the ultimate carcinogenic form(s) of BaP. Therefore, Fe2O3 can enhance the metabolic activation of BaP by the lung, as well as act as a carrier for penetration and retention of BaP in the lung.

Metabolism of isosorbide dinitrate in the isolated perfused rabbit lung

The uptake and metabolism of isosorbide dinitrate was investigated in the recirculating isolated perfused rabbit lung and in lung homogenate 9000 X g supernatant. Concentration versus time profiles from the isolated lung experiments indicate rapid metabolism of isosorbide dinitrate and corresponding increases in the metabolites 5-isosorbide mononitrate, 2-isosorbide mononitrate, and isosorbide. The data suggest that the mononitrates formed in the lung tissue were converted to isosorbide at an extraordinarily high rate. Surprisingly, the rate of appearance of completely denitrated isosorbide was greater when isosorbide dinitrate was administered to the lung than when the mononitrate metabolites of isosorbide dinitrate were administered. The results suggest rapid metabolism of a substantial portion of the mononitrates formed endogenously from isosorbide dinitrate before partitioning of mononitrates into the perfusion medium could occur. The metabolism of isosorbide dinitrate in lung homogenate 9000 X g supernatant exhibited a metabolic scheme kinetically different from the intact lung studies, as isosorbide was formed slowly from a mononitrate intermediate and not by a near-simultaneous cleavage of both nitrate ester groups. Intravascular multiple-dose studies did not demonstrate any inhibition between isosorbide dinitrate and the mononitrates.

Influence of airborne particulate on the metabolism of benzo[a]pyrene in the isolated perfused lung

Benzo[a]pyrene (BaP), a ubiquitous potent carcinogen, has been associated with the increased incidence of human bronchiogenic carcinoma in occupational and urban settings. A detailed knowledge of the rate and pattern of metabolite formation and factors affecting their formation is essential for understanding the mechanism of action of BaP in the lung. An isolated perfused New Zealand rabbit lung preparation was used to investigate the effects of a crude airborne particulate mixture on the metabolism of BaP. [14C]BaP with and without crude air particulate (CAP) was administered intratracheally to an isolated perfused lung (IPL) preparation after intratracheal pretreatment of the whole animal with CAP and/or BaP, or intraperitoneal pretreatment of the whole animal with BaP. BaP and its metabolites were extracted from perfusing blood at 6 time points up to 180 min after administration of [14C]BaP to the IPL. BaP and its metabolites were also extracted from lung tissue, washout fluid, aveolar macrophages, and trachea bronchi at the end of the perfusion at 180 min. Patterns of BaP metabolites were determined by chromatographic techniques and liquid scintillation counting. Particulate pretreatment of the whole animal or administration of the particulate to the IPL altered BaP metabolism by the perfusing lung. Particulate pretreatment of the whole animal resulted in increases in the total rates of appearance of metabolites of BaP in the blood (ng/g lung . h), while particulate administration to the IPL resulted in decreases in the total rate of appearance of metabolites of BaP in the blood and negated the effects of pretreatments. Coadministration of particulate with BaP to the IPL with and without particulate pretreatment of the whole animal, or BaP administration to the IPL preceded by particulate pretreatment of the whole animal, enhanced dihydrodiol formation and depressed formation of water-soluble materials. This is important because dihydrodiol formation is considered part of the active pathway of BaP carcinogenicity. These data suggest that pulmonary particulate exposure in the presence of BaP results in the initial increased production of dihydrodiols of BaP that may be further metabolized to compounds believed to be the ultimate carcinogenic form(s) of BaP.

The effect of ethanol on the absorption, accumulation and biotransformation of xenobiotics by the isolated perfused rabbit lung

The effects of acutely administered ethanol on absorption, accumulation and biotransformation of several model compounds were examined in the isolated perfused lung (IPL). To determine pulmonary accumulation imipramine and paraquat were studied. To determine the effect of ethanol on the absorption of substances from the airways into the blood sulfanilic acid was used as an example of a relatively lipid soluble compound while mannitol was chosen as a non-lipid soluble compound. Aldrin was chosen to demonstrate the effects of circulating ethanol on pulmonary biotransformation. The results obtained with imipramine and paraquat indicate that after a 60-min perfusion period 93% and 20% of the original amount added are respectively lost from the blood perfusate. Ethanol at an initial concentration of 300 mg/100 ml has no apparent effect either on the rate or the quantities accumulated by the lungs. When sulfanilic or mannitol were administered into the airways both compounds appeared in the perfusate and ethanol had no effect on this absorption. Finally we obtained evidence that aldrin is converted to dieldrin by the IPL but that the quantities metabolized are not modified by the presence of ethanol.

Mutagenicity of benzo(a)pyrene metabolites generated on the isolated perfused lung following particulate exposure

The isolated perfused rabbit lung (IPL) is being used to study the effects of particulate exposure on the pulmonary metabolism of benzo(a)pyrene (BaP). Pasturealla-free New Zealand white rabbits were treated intraperitoneally with BaP prior to kill. The isolated lungs were then administered either 14C-labeled BaP alone or BaP plus Fe2O3 or fly ash by intratracheal injection. Rates of appearance of BaP metabolites in the perfusing blood were determined. The extent of metabolism, distribution of metabolites, and types of metabolites produced were quantified for various lung tissue types by high-performance liquid chromatography and liquid scintillation spectrometry. Procedures were developed to apply the Salmonella/microsome test in the assay of mutagenicity of lung tissue and blood extracts as an indicator of their biologic activity. With few exceptions, blood extracts from IPL receiving BaP only were not mutagenic. Lung, trachea-bronchi, and macrophage extracts, by contrast, were mutagenic. A part of this activity could be attributed to BaP metabolites rather than to parent compound remaining in extracts. When lungs were exposed to Fe2O3 or to fly ash, only macrophage extracts were consistently mutagenic. This activity was due to significant amounts of unmetabolized BaP.

Isolated perfused rabbit lung as a model for intravascular and intrabronchial administration of bronchodilator drugs I:Isoproterenol

The absorption, uptake, and metabolism of isoproterenol was studied following intravascular, intrabronchial, and aerosol administration of the drug to the isolated perfused rabbit lung. Capacity-limited metabolism of isoproterenol was observed following the addition of five doses, ranging from 10(-7) to 10(-5) moles, directly into the circulation of the lung system. A physiologically based perfusion model was developed to describe the disposition of the drug and metabolite in the isolated lung preparation. This model was also used to analyze data collected following intrabronchial and aerosol administration of isoproterenol.

Isolated perfused rabbit lung as a model for intravascular and intrabronchial administration of bronchodilator drugs II:Isoproterenol prodrugs

The pulmonary disposition of two diester prodrugs of isoproterenol (di-p-toluoylisoproterenol and dipivaloylisoproterenol) was studied in the isolated perfused rabbit lung preparation. High-pressure liquid chromatographic methods were developed to measure diester, monoester, isoproterenol, and 3-O-methylisoproterenol from a single 1-ml perfusate sample. The prodrugs were administered directly into the circulating perfusion medium and by endotracheal instillation. Perfusate concentrations of diester, monoester, isoproterenol, and 3-O-methylisoproterenol were measured for 180 min. The diesters were rapidly eliminated from the perfusate with a subsequent increase in monoester concentrations. Isoproterenol levels were observed within minutes of prodrug administration, peaked at 60-80 min, and declined slowly thereafter. The prodrugs were rapidly absorbed following endotracheal administration with 30-50% of the diester being metabolized during the first pass through the lung.

Lung, aorta, and platelet metabolism of 14C-arachidonic acid in vitamin E deficient rats

14C-arachidonic acid metabolism was determined in aortas, platelets, and perfused lungs from rats pair fed a basal diet supplemented with 0 or 100 ppm vitamin E for 11 weeks. Spontaneous erythrocyte hemolysis tests showed 92% and 8% hemolysis for the 0 and 100 ppm vitamin E groups, respectively. Elevated lung homogenate levels of malonaldehyde in the 0 ppm group confirmed its deficient vitamin E status. Aortas from the vitamin E deficient group synthesized 54% less prostacyclin than aortas from the supplemented group (p less than 0.05). Although thromboxane generation by platelets from the vitamin E deficient group exhibited a 37% increase, this difference was not statistically significant compared to the supplemented animals. Greater amounts of PGE2, PGF2 alpha, TXB2, and 6-keto-PGF1 alpha were obtained in albumin buffer perfusates from lungs of vitamin E deficient rats than in those from supplemented rats. Significant differences (p less than 0.05) were noticed, however, only for PGE2 and PGF2 alpha. These studies indicate that vitamin E quantitatively alters arachidonic acid metabolism in aortic and lung tissue but its effect on thromboxane synthesis by platelets is less marked.

Influence of sulfur dioxide on metabolism and distribution of benzo[a]pyrene in isolated perfused rabbit lung

An isolated perfused lung [IPL) preparation was used to investigate the effects of SO2 (1-2 ppm) on the metabolism of benzo[a]pyrene (BaP), a ubiquitous potent carcinogen that has been associated with the increased incidence of a human bronchiogenic carcinoma in occupational and urban populations. [14C]BaP, with and without crude air particulate (CAP), was administered intratracheally to the IPL in conjunction with SO2 or after pretreatment of the whole animal with SO2. Metabolites were isolated from serial blood samples up to 3 h after the administration of [14C]BaP to the IPL. Metabolites were also isolated from lung tissue, washout fluid, macrophage, and trachea and bronchi at the end of the perfusion at 180 min. Patterns of BaP metabolites were determined by thin-layer and high-performance liquid chromatography and scintillation counting. SO2 given in conjunction with BaP on the IPL or given to the whole animal followed by BaP on the IPL, in comparison with BaP only on the IPL, resulted in a twofold increase in the total rate of appearance of metabolites of BaP in the blood with changes in the metabolic pattern. SO2 given in conjunction with BaP and CAP on the IPL, in comparison with BaP plus SO2 on the IPL, resulted in a threefold decrease in the total rate of appearance of metabolites of BaP in the blood with changes in the metabolic pattern.

Effect of chlorphentermine on the pulmonary disposition of norepinephrine in the isolated perfused rabbit lung

Chlorphentermine (CP) has been noted to cause primary pulmonary hypertension both clinically and experimentally. It was postulated that CP might affect the pulmonary clearance of endogenous vasoactive substances such as norepinephrine (NE). The uptake and metabolism of 14C-NE were followed in artificially ventilated isolated perfused rabbit lung preparations using a constituted perfusate with initial NE concentration of 5 micrograms/100 ml. Perfusate samples were analyzed for total radioactivity, metabolites, and parent compound. Preloading the lungs with 0.25 mM CP significantly increased the concentrations of total radioactivity, deaminated products, and decreased the concentration of normetanephrine in the perfusate. In addition, the accumulation of total radioactivity in the lung tissue after 60 min of perfusion was significantly decreased in CP-treated lungs. The proportion of deaminated metabolites in the lung tissue was slightly decreased while the percent of normetanephrine, and parent compound were significantly increased by the CP treatment. CP (0.1 mM) also inhibited the in vitro metabolism of NE by 79%. These results provide experimental evidence in support of a hindered pulmonary clearance of circulating NE by CP.

The isolated perfused lung--a critical evaluation

Recent refinement and application of the technique of perfusing isolated intact lung preparations from suitable experimental animal has enhanced our knowledge of the non-respiratory functions of the lung. The technique consists of perfusion with whole blood or constituted media via pulmonary artery or artificially ventilated lungs. Such preparations have been known to be especially useful for studies of uptake, metabolism and disposition of exogenous and endogenous substances. Recent advances in unilateral, split-lung perfusion in which left and right lungs are unilaterally perfused simultaneously have enabled investigators to maintain paired controls during perfusion. Such techniques are useful in the study of the interactions of endogenous and exogenous chemicals in the lung tissue. Differences between subcellular or lung slice preparations and intact lung perfusion are to be expected on the basis of distorting natural vascular and extra-vascular barriers in the case of in vitro preparations. Areas in which perfused lungs have not been extensively used include uptake and disposition of gases, solvents and vapors, effects of toxic chemicals on respiratory and non-respiratory functions of the lung and alteration of pulmonary mechanisms and hemodynamics in the presence of interacting chemical or physical stimuli. With suitable modifications, application of isolated perfused lung preparations for these investigations should be technically feasible in the future.

Influence of pretreatment with tobacco smoke condensate or 3-methylcholanthrene on [14C] benzo (a) pyrene metabolism in the isolated perfused rabbit lung

Pretreatment 24 h before sacrifice with i.p. tobacco smoke condensate (TSC) or 3-methylcholanthrene (3MC) increased the rate of disappearance of [14C]benzo(a)pyrene (BP) from an isolated perfused rabbit lung model. Both pretreatments significantly increased the amount of most metabolites formed. This study indicates that rabbits tend to resemble rats, mice and hamsters in that increased rates of pulmonary BP metabolism are a consequence of exposure to TSC.

Methods for analysis of the mutagenicity of indirect mutagens/carcinogens in eukaryotic cells

The review discusses the variety of methods for activation of indirect mutagens/carcinogens and testing them in cell cultures, especially in mammalian cell cultures. After the necessity for including metabolizing components in mutagenicity tests has been pointed out, the enzymes that transform foreign compounds metabolically, and the factors influencing them, are described. In the main section the various methods of activating indirect mutagens/carcinogens are presented. The methods of including in vivo metabolism in mutagenicity tests are: Analysis of cells from organisms contaminated with a chemical (III.l.a); body fluid-mediated mutagenesis (III.l.b); host-mediated assay (III.l.c). The following activation systems are suitable for including in vitro metabolism of test compounds in mutagenicity tests: Liver and lung perfusion (III.2.a.alpha); organ slices and homogenates (III.2.a.beta); subcellular fractions (III.2.a.gamma); cultivated cells (cell-mediated mutagenesis) (III.2.b); nonenzymatic activation systems (III.2.c). Finally the main factors that influence the metabolism of test substances are summarized. Two factors illustrate the mutagenicity tests with regard to the metabolism of mammalian livers and the methods of performing mutagenicity tests in man.

An isolated rat lung perfusion system for use in tobacco smoke studies

An isolated rat lung perfusion system has been developed for use in tobacco smoke studies. The lungs are ventilated by means of subatmospheric (negative) pressure produced through operation of an artificial thorax. The system enables standard respiratory conditions to be employed and so eliminates variations in animal breathing characteristics. The various tests of viability which have been carried out have shown that the preparations are viable for periods of at least 1 hr. It was also demonstrated that transfer of 14C-nicotine from smoke to perfusate was rapid and linear over the period of smoke exposure and that first-pass metabolism of nicotine was of little significance.

Acute tobacco smoke exposure alters the profile of metabolites produced from benzo[alpha]pyrene by the isolated perfused rabbit lung

The influence of whole tobacco smoke or the gas phase from smoke on the metabolism of [14C]benzo[alpha]pyrene was examined using the isolated perfused rabbit lung model. Fresh whole tobacco smoke mixed with the air ventilating the perfused lung produces an immediate and dose related decrease in the metabolism of [14C]benzo[alpha]pyrene. The metabolites of [14C]benzo[alpha]pyrene, diols, quinones, phenols and polar compounds are generally decreased in quantity. At the lowest level of smoke administered the percentage of BP-7,8-diol produced is increased dramatically. The results indicate that one of the factors contributing to the carcinogenicity of tobacco smoke may be its ability to produce an immediate alteration in the pulmonary metabolism of polycyclic aromatic hydrocarbons.

Absorption of inhaled antigen into the circulation of isolated lungs from normal and immunized rabbits

The purpose of this study was to determine whether the absorption of inhaled antigen (Ag) across the pulmonary air-blood barrier of the isolated perfused lung can be modulated by immunologic mechanisms. Lungs from immunized or nonimmunized rabbits were removed, ventilated, and perfused with autochthonous blood. Radioiodinated Ag (human serum albumin or ovalbumin) was introduced as an aerosol into the isolated lung for 15 min and blood samples were taken over a 4-h period. The results showed that radioactivity fom inhaled Ag entered the perfusing blood as two fractions. One fraction was precipitable by 5% trichloroacetic acid or antiserum. The TCA-soluble fraction chromatographed differently from iodide and may have represented metabolites of the Ag. Immunization specifically reduced the amount of antigenically intact protein entering the blood. On the other hand, the metabolite reached higher concentrations in the blood of immunized lungs. We conclude that the alveolar capillary barrier of the normal rabbit lung could provide a significant route of entry for inhaled antigen into the systemic circulation and that immunization reduces absorption via this route and enhances pulmonary metabolism of the Ag.

Absorption of paraquat and diquat from the airways of the perfused rat lung

The uptake of paraquat dichloride (bis-N-[14C]methyl-4,4'-bipyridilium chloride) and diquat dibromide (N,N-ethylene-[U-14C]2,2'-bipyridilium dibromide monohydrate) from the airways and by the vasculature of the isolated and perfused rat lung (IPL) were studied. A semilogarithmic plot of the percent unabsorbed with time revealed a bi-exponential decay, suggesting at least two phases of removal of paraquat and diquat from the airways. The rapid initial process was similar for both herbicides. The slow component had at t1/2 of 355.98 min for paraquat and 75.03 min for diquat. This second process may represent the storage pool associated with the pulmonary toxicity of paraquat. When paraquat or diquat was presented to the capillary side of the lung, long-term storage was not evident. Uptake by the lung occurred from the pulmonary circulation with similar velocity. These data suggest that the energy-dependent uptake observed with lung slices probably represents airway transport and may be associated with cell membranes lining the alveolus.