First pass uptake of 14C-propranolol by the lung

Molecular imaging of the pulmonary circulation in health and disease

The pulmonary circulation, at the unique crossroads between the left and the right heart, is submitted to large physiologic hemodynamic variations and possesses numerous important metabolic functions mediated through its vast endothelial surface. There are many pathologic conditions that can directly or indirectly affect the pulmonary vasculature and modify its physiology and functions. Pulmonary hypertension, the end result of many of these affections, is unfortunately diagnosed too late in the disease process, meaning that there is a crying need for earlier diagnosis and surrogate markers of disease progression and regression. By targeting endothelial, medial and adventitial targets of the pulmonary vasculature, novel molecular imaging agents could provide early detection of physiologic and biologic perturbation in the pulmonary circulation. This review provides the rationale for the development of molecular imaging agents for the diagnosis and follow-up of disorders of the pulmonary circulation and discusses promising targets for SPECT and positron emission tomographic imaging.

Kinetic aspects of drug disposition in the lungs

1. The pharmacokinetic role of the lungs has been extensively studied using in vitro preparations, but this information has not been well integrated into many systemic pharmacokinetic models. 2. The lung is characterized by short diffusion distances, extremely high relative perfusion and heterogeneous cell types. Anionic and neutral lipophilic drugs have relatively small distribution volumes in the lungs due to their low lipid content. Cationic lipophilic drugs can accumulate in the lungs, probably due to trapping in mitochondria and lysosomes, forming very slowly eluting pools. 3. Drug metabolism in the lungs is possible, but not universal. The lung, generally, has a low activity for many of the metabolic enzymes found in the liver, although this activity is relatively more inducible. The resultant drug extraction would be 'enzyme limited', variable and flow dependent. 4. Double indicator studies of first-pass lung kinetics can characterize short-term distribution in the lungs, but not longer-term distribution or metabolism; the converse applies for studies of drug concentration gradients across the lungs. No single study or model has adequately defined the short- and long-term kinetics of drugs in the lungs. 5. Drug clearance in the lungs can contribute to an apparent total body clearance in excess of hepatic blood flow and cardiac output. The lung is a first pass filter for any drug administered on the venous side of the circulation and can act as a 'capacitor' that damps the first-pass concentration peak in the blood after intravenous bolus injection.

Pharmacokinetics and kinetic-dynamic modelling of aminophenones as methaemoglobin formers

Methaemoglobin, the oxidized form of haemoglobin, can be formed by a variety of agents, most of which act to oxidize haemoglobin directly or indirectly. Cyanide has a higher affinity for methaemoglobin than for mitochondrial cytochromes, making methaemoglobin formation a basis for the treatment of cyanide poisoning. We used the beagle dog model to investigate the relationship between drug concentration and methaemoglobin levels for two candidate anti-cyanide compounds. The compounds studied were the aminophenones p-aminopropiophenone (PAPP) and p-aminoheptylphenone (PAHP). Both PAPP and PAHP were given as intravenous boluses and as two different oral formulations. The kinetics of both compounds appeared to follow a three-compartment open model for intravenous bolus administration and a two-compartment open model for oral administration. The first distribution phase seen with the intravenous administration was obscured by the absorption phase during oral administration. Bioavailability for all formulations varied between 20 and 47%. For both compounds there was a delay between the appearance of drug in the plasma and the appearance of methaemoglobin (counter-clockwise hysteresis) which is suggestive of an active metabolite causing methaemoglobin formation. The pharmacodynamics were fit with an effect-compartment kinetic-dynamic model linked to a sigmoid Emax pharmacodynamic model. Maximum amounts of methaemoglobin occurred between 2 and 4 h for PAHP and between 1 and 3 h for PAPP. When administered intravenously estimates of EC50 were lower than the estimates of EC50 from oral administration for both compounds. This might be because of oral first-pass inactivation or a 'first-pass' activation through the lungs contributing to the formation of an active metabolite. The phenones as a class appear to have the drug cleared and methaemoglobin return to near baseline within 12 h. Both compounds seem to produce sufficient methaemoglobin to treat acute cyanide poisoning and to serve as prophylactic agents against acute cyanide poisoning in a military setting.

Pulmonary distribution of alfentanil and sufentanil studied with system dynamics analysis

We applied a system dynamics approach to the study of the pulmonary distribution of alfentanil and sufentanil in anesthetized pigs and patients, respectively. This method allows estimation of the mean transit time through the lungs and calculation of the volume of distribution of alfentanil in the lungs. In the first part of the study the pulmonary distribution of alfentanil was studied in six anesthetized pigs during three hemodynamic states (control, partial clamping of the inferior vena cave, and complete clamping of the right pulmonary artery). In the second part of the study the pulmonary distribution of sufentanil was studied in 10 patients, scheduled for elective CABG, during and after a constant rate infusion of 10 min. Pulmonary passage of the opioids was characterized by functions of transit times, derived from the pulmonary arterial and systemic arterial concentration curves. Pulmonary distribution volumes were calculated from the mean transit time and pulmonary blood flow. Pulmonary distribution volume of alfentanil during the control measurement was significantly higher (486 +/- 88 ml) than during either partial caval clamping (346 +/- 89 ml, p < 0.05) or right pulmonary artery clamping (336 +/- 56 ml, p < 0.05). There was no change in the extravascular volume of distribution of alfentanil with each hemodynamic state. Pulmonary volume of distribution of sufentanil in the patients was 22.6 (10.9) L. Pulmonary distribution of opioids can be studied using system dynamics analysis, both after bolus injection and during and after infusion. This method can be used for periods beyond the initial passage of the drug through the lungs.

First-pass metabolism of diltiazem in anesthetized rabbits: role of extrahepatic organs

PURPOSE

The aim of this study was to assess in vivo which organs contribute to the first-pass metabolism of diltiazem.

METHODS

Anaesthetized rabbits received diltiazem into the thoracic aorta (TA) ( 1mg/kg), jugular vein (JV) (2 mg/kg), portal vein (PV) (4 mg/kg) or small intestine (SI) (5 mg/kg). Serial blood samples were withdrawn from the abnormal aorta to assay diltiazem, N-demethyl-diltiazem (MA) and deacetyldiltiazem (M1).

RESULTS

The area under diltiazem plasma concentration curve/time (AUC0-infinity) normalized by the dose was AUCTA approximately equal to AUCJV > AUCPV > AUCSI: Intestinal and hepatic diltiazem availability was 43 and 33%, respectively. The systemic availability of oral diltiazem was 12%. Diltiazem given into the SI and PV generated primarily MA, and injected into the JV and TA produced mainly M1.

CONCLUSIONS

In rabbits, the intestine and the liver contribute to the first-pass metabolism of diltiazem, and the amount and species of metabolites generated depend upon the route of administration of diltiazem.

Myocardial and pulmonary uptake of S-1'-[18F]fluorocarazolol in intact rats reflects radioligand binding to beta-adrenoceptors

The biodistribution of S-(-)-4-(2-hydroxy-3-(1'-[18F]fluoroisopropyl)- aminopropoxy)carbazole ([18F]S-fluorocarazolol, a non-selective beta-adrenoceptor antagonist) was studied in rats (60 min after 18F injection when specific binding in peripheral organs was maximal). 18F uptake in brain, erythrocytes, heart and lung appeared to be linked to beta-adrenoceptors. CGP-20712A and ICI-89,406 inhibited 18F uptake in heart (predominantly beta 1-adrenoceptors) more potently than in lungs (predominantly beta 2-adrenoceptors). In contrast, ICI-118,551 and procaterol were more potent in the lungs than in the heart. ICI-118,551 inhibited 18F uptake in cerebellum (predominantly beta 2-adrenoceptors) more potently than in cerebral cortex (predominantly beta 1-adrenoceptors). Stereoselectivity of the in vivo binding was demonstrated since S-(-)-propranolol inhibited uptake in target tissues more effectively than R-(+)-propranolol. Myocardial and cerebral imaging may be hampered by poor heart-to-lung contrast and low signal-to-noise ratios, but [18F]S-fluorocarazolol seems suitable for positron emission tomography (PET) of pulmonary beta-adrenoceptors.

Fentanyl and propofol uptake by the lung: effect of time between injections

The effect of time between the administrations of fentanyl and propofol on the first pass uptake of propofol in the cat lung was studied using double indicator dilution technique. The pulmonary first pass uptake of propofol (mean +/- s.e. mean) was 58 +/- 6% in six cats (control group) that had received no fentanyl prior to propofol (1 mg/kg) administration. The uptake was significantly reduced to 32 +/- 3% in animals pretreated with fentanyl (1 microgram/kg) 30 seconds before propofol administration (n = 6). However, when fentanyl was administered 3 minutes (n = 6) or 10 minutes (n = 6) prior to propofol, the pulmonary uptake of propofol (45 +/- 5%, 50 +/- 7% respectively) was not significantly reduced. The results demonstrate that the ability of fentanyl to inhibit pulmonary removal of propofol depends on its time of administration prior to propofol. These data may have clinical implication with respect to timing of the preinduction opiate injection.

Extrahepatic metabolism of drugs in humans

Although the liver plays the major role in drug metabolism [e.g. by oxidative cytochrome P450 (CYP)-dependent phase I and conjugation or phase II reactions], drug metabolising enzymes are also present at other sites. Depending on the particular drug and enzymes involved, these extrahepatic organs and/or tissues can contribute to the elimination of drugs and, thus, should be considered in any discussion of drug disposition. By the use of relatively new techniques in molecular biology, e.g. immunoblotting with antibodies directed to various CYP isoenzymes, the tissue and organ distribution of these drug metabolising enzymes can be determined. In addition, microsomal and cytosolic enzyme activity and capacity can be directly assessed in vitro by incubation of the enzymes with the drugs of interest. Both approaches have demonstrated the presence of 3 CYP families at different extrahepatic sites, such as the mucosa of the gastrointestinal tract, kidney, lung, brain or skin. Enzymes including epoxide hydrolases, hydrolysing enzymes, glutathione S-transferases, UDP-glucuronosyltransferases, sulphotransferases, N-acetyltransferases, and methyltransferases are discussed. Indirect evidence of extrahepatic drug metabolism can be generated from pharmacokinetic studies whenever total body clearance exceeds liver blood flow, or when severe liver dysfunction or anhepatic conditions do not affect metabolic clearance. Indeed, extrahepatic metabolism has been demonstrated for numerous drugs. Therefore, the metabolic profile and sites of enzymatic reactions for each drug should be determined.

N-isopropyl-p-iodoamphetamine receptors in normal and cancerous tissue of the human lung

N-Isopropyl-p-iodoamphetamine (IMP) receptors in normal human lung tissue were characterized using a radioligand binding assay with iodine-125 IMP as the ligand. Saturation binding studies revealed the presence of two binding sites with dissociation constant (Kd) values of 53 +/- 2 and 4687 +/- 124 nM and maximum binding capacity (Bmax) values of 7 +/- 1 and 133 +/- 27 pmol/mg protein (n = 5) respectively. The IC50 values of various amines were as follows: IMP, 9 x 10(-5) M; propranolol, 5 x 10(-4) M; haloperidol, 6 x 10(-4) M; ketamine, 9 x 10(-3) M; dopamine, 1 x 10(-2) M. The IMP receptors of cancerous tissue obtained from human lung also had two binding sites with Kd values of 54 +/- 2 and 5277 +/- 652 nM and Bmax values of 7 +/- 1 and 103 +/- 21 pmol/mg protein (n = 3) respectively. There was no significant difference in binding parameters between normal and cancerous lung tissue. These results demonstrate the existence of IMP receptors and suggest that cancer does not affect the nature of IMP receptors in human lung tissue.

The effect of respiratory disorders on clinical pharmacokinetic variables

Respiratory disorders induce several pathophysiological changes involving gas exchange and acid-base balance, regional haemodynamics, and alterations of the alveolocapillary membrane. The consequences for the absorption, distribution and elimination of drugs are evaluated. Drug absorption after inhalation is not significantly impaired in patients. With drugs administered by this route, an average of 10% of the dose reaches the lungs. It is not completely clear whether changes in pulmonary endothelium in respiratory failure enhance lung absorption. The effects of changes in blood pH on plasma protein binding and volume of distribution are discussed, but relevant data are not available to explain the distribution changes observed in acutely ill patients. Lung diffusion of some antimicrobial agents is enhanced in patients with pulmonary infections. Decreased cardiac output and hepatic blood flow in patients under mechanical ventilation cause an increase in the plasma concentration of drugs with a high hepatic extraction ratio, such as lidocaine (lignocaine). On a theoretical basis, hypoxia should lead to decreased biotransformation of drugs with a low hepatic extraction ratio, but in vivo data with phenazone (antipyrine) or theophylline are conflicting. The effects of disease on the lung clearance of drugs are discussed but clinically relevant data are lacking. The pharmacokinetics of drugs in patients with asthma or chronic obstructive pulmonary disease are reviewed. Stable asthma and chronic obstructive pulmonary disease do not appear to affect the disposition of theophylline or beta 2-agonists such as salbutamol (albuterol) or terbutaline. Important variations in theophylline pharmacokinetics have been reported in critically ill patients, the causes of which are more likely to be linked to the poor condition of the patients than to a direct effect of hypoxia or hypercapnia. Little is known regarding the pharmacokinetics of cromoglycate, ipratropium, corticoids or antimicrobial agents in pulmonary disease. In patients under mechanical ventilation, the half-life of midazolam, a new benzodiazepine used as a sedative, has been found to be lengthened but the underlying mechanism is not well understood. Pulmonary absorption of pentamidine was found to be increased in patients under mechanical ventilation. Pharmacokinetic impairment does occur in patients with severe pulmonary disease but more work is needed to understand the exact mechanisms and to propose proper dosage regimens.

Bronchoconstriction of the asthmatic airway by inhaled and ingested propranolol

Responsiveness to inhaled histamine and DL propranolol hydrochloride was measured in 31 adult asthmatics and compared with bronchoconstriction provoked by acute oral propranolol dosing (max 160 mg). Twelve asthmatics developed greater than or equal to 15% reduction in the forced expired volume in 1 s (FEV1), 2 h after less than or equal to 100 mg oral propranolol; cardiac beta-adrenoceptor blockade was confirmed by cycle exercise tests in the 19 without airway response. The provocative inhaled dose of each aerosol causing a 20% fall in FEV1 (PC20) was lower, histamine 0.43 mg.ml-1, propranolol 3.12 mg.ml-1, in the 12 with a positive oral test compared with the 19 with a negative test, PC20 histamine 1.65 mg.ml-1, PC20 propranolol 16.2 mg.ml-1 (P less than 0.001 for both aerosols). A correlation was demonstrated between the PC20 values for asthmatics with a negative oral test (r = 0.72, P less than 0.001, n = 19) but not for the remainder (r = 0.14, P greater than 0.05, n = 12). Plasma propranolol concentrations (CL, ng.ml-1) after the final oral dose did not correlate with the % delta FEV1 (26.3) (r = -0.28) when an airway response was provoked or with the reduction in exercise tachycardia (25.9%) (r = 0.31) when no bronchoconstriction occurred. CL exceeded the limit of detection after the final inhaled propranolol dose (7.5 ng.ml-1) and was weakly related to the PC20 propranolol value (r = 0.53, P = 0.01, n = 27). The prevalence of a positive oral challenge was low in this group (39%).(ABSTRACT TRUNCATED AT 250 WORDS)

Clearance of vasoactive intestinal polypeptide (VIP) in the porcine pulmonary circulation

The clearance of vasoactive intestinal polypeptide (VIP) in the lung was determined in pigs. To measure the first pass uptake, a bolus of VIP (0.9 pmol.kg-1 and 9 pmol.kg-1) with an inert intravascular marker, indocyanine green (ICG), was injected into the right atrium. The percent uptake of VIP after the higher bolus, as estimated by comparing the levels of VIP and ICG in the pulmonary artery and the aorta, was 36 +/- 6% during control infusion and 36 +/- 13% during continuous infusion of VIP (3 pmol.kg-1.min-1). The VIP concentrations in the pulmonary artery and the aorta were not different under baseline conditions, but during continuous VIP infusion the levels of plasma VIP in the pulmonary artery were higher than those in the aorta (24.3 +/- 1.6 pmol.l-1 and 20.4 +/- 1.3 pmol.l-1, resp. P less than 0.0001). These results indicate that the lung is not a source of plasma VIP, but the pulmonary circulation is a substantial contributor to the removal of VIP from plasma.

The potential role of lysosomes in tissue distribution of weak bases

The potential importance of lysosomes as a site of accumulation of weak bases in tissues is discussed. A simple mathematical treatment predicts the quantitative significance of lysosomal trapping for monoacidic and diacidic weak bases. The features which are characteristics of lysosomal trapping are discussed, particularly in comparison with active transport and intracellular binding mechanisms. These features include: linear accumulation at low concentrations; nonlinearity at higher concentrations; dependence on structural integrity of tissue; energy dependence and competition with other weak bases. Subcellular distribution studies have previously shown that weak bases accumulate extensively in membranes; however, the dependence of accumulation on the structural integrity of tissue suggests that this is not the only significant mechanism of accumulation. The results of a range of studies of tissue distribution of weak bases are discussed to illustrate that these findings are consistent with accumulation in lung and liver being attributable to a combination of lysosomal trapping and accumulation in membranes whereas, in muscle, accumulation in membranes is the predominant mechanism of accumulation. The possible pharmacokinetic significance of lysosomal trapping of weak bases is also discussed.

Pulmonary disposition of pethidine in postoperative patients

1. Two methods of pethidine administration, namely constant-rate infusion and single i.v. injection, were used to assess the pulmonary disposition of the drug in 10 postoperative patients. Using two sites of blood sampling, the pulmonary extraction ratio was determined. 2. Pronounced pulmonary uptake of pethidine was found in all patients (n = 10). On the other hand, there was no significant evidence of pulmonary clearance. 3. The mean total plasma clearance was 810 ml min-1 and the volume of distribution was 3.11 kg-1. 4. A flow model was used to describe the disposition of pethidine in man. The concentration-time profiles calculated by the model were in accordance with observed data. The data showed that both pulmonary uptake and pulmonary release of pethidine were rapid. 5. Constant-rate infusion was found advantageous in the determination of pulmonary extraction, with respect to the accuracy and precision of the results. The extraction obtained after a single injection may be overestimated on account of uptake of the drug by the lungs.

Selective lysosomal uptake and accumulation of the beta-adrenergic antagonist propranolol in cultured and isolated cell systems

The beta adrenoreceptor antagonist propranolol is rapidly taken up and accumulated in various cultured cell lines. When incubated in the presence of low concentrations of propranolol (10(-9) M), Hela (non-differentiated epithelia), BC3H1 (smooth muscle) and MDCK (differentiated kidney epithelia) cell cultures take up (t1/2 = 4-10 min) and accumulate the drug such that the intracellular concentration is over 1000 times that in the incubation medium. The release of propranolol from the cells was slower (t1/2 = 22 min) than the rate of uptake but the dissociation was stimulated by the addition of 1 microM propranolol to the external medium (t1/2 = 9 min). Uptake, which is non-stereoselective, is dependent on pH and is inhibited by the lysosomotropic agents, NH4Cl, methylamine and chloroquine. At higher concentrations (greater than 10(6) M), uptake is accompanied by a visual swelling of intracellular acidic vesicles staining with acridine orange. These results suggest that propranolol, a basic amphiphilic amine, is accumulated within the lysosomes of these cells. Uptake was confined to these cultured cell systems with no chloroquine-sensitive propranolol uptake, being found in isolated rabbit ventricular myocytes, red blood cells or blood platelets. Although alprenolol and cyanopindolol competed with propranolol for uptake, isoprenaline, adrenaline, noradrenaline, phenylephrine, atenolol, practolol and salbutamol were not effective inhibitors. The possible consequences of this uptake and accumulation of propranolol by certain tissues is discussed in relation to the known actions of the drug, particularly during or after abrupt withdrawal from chronic applications.

The diseased lung and drugs

Among the numerous ways by which drug-lung relationships can be discussed we chose to examine three questions: Is the lung capable of directly modulating drug pharmacokinetics? Knowing that the pulmonary circulation can exert very selective and efficient clearance and catabolism of biogenic molecules such as serotonin (5-HT), norepinephrine (N.E.), prostaglandins PGE2 and PGF2 alpha, and bradykinin, as well as activation of angiotensin (A)I to AII, a similar behavior has been sought for drugs. Uptake of circulating drugs occurs clearance of biogenic amines, the significance of which remains to be fully evaluated, and iatrogenic phospholipidosis localization of which in the lung might result from the elevated drug concentration in the tissue. Lung cells contain enzymes, among them mixed function oxidases (MFO), which enables them to metabolize xenobiotics even in the intact organ. Although blood supply to the lung is higher than that to the liver, the comparatively low MFO content of the former organ predicts only a marginal pulmonary metabolic contribution in vivo. On the otherhand, enzymatic activity upon drugs can generate instable reactive metabolites which, even in minute amounts are toxic and can damage cells; furthermore, it is hypothesized that the relatively high bronchiolar Clara cell MFO content explains the striking susceptibility of the long to xenobiotics. Do lung diseases indirectly influence drug pharmacokinetics? Hindrance to blood flow is common in respiratory failure (R.F.), from both reduced area and hypoxic vasoconstriction, resulting in altered flow distribution to the liver and kidney.

First-pass elimination. Basic concepts and clinical consequences

First-pass elimination takes place when a drug is metabolised between its site of administration and the site of sampling for measurement of drug concentration. Clinically, first-pass metabolism is important when the fraction of the dose administered that escapes metabolism is small and variable. The liver is usually assumed to be the major site of first-pass metabolism of a drug administered orally, but other potential sites are the gastrointestinal tract, blood, vascular endothelium, lungs, and the arm from which venous samples are taken. Bioavailability, defined as the ratio of the areas under the blood concentration-time curves, after extra- and intravascular drug administration (corrected for dosage if necessary), is often used as a measure of the extent of first-pass metabolism. When several sites of first-pass metabolism are in series, the bioavailability is the product of the fractions of drug entering the tissue that escape loss at each site. The extent of first-pass metabolism in the liver and intestinal wall depends on a number of physiological factors. The major factors are enzyme activity, plasma protein and blood cell binding, and gastrointestinal motility. Models that describe the dependence of bioavailability on changes in these physiological variables have been developed for drugs subject to first-pass metabolism only in the liver. Two that have been applied widely are the 'well-stirred' and 'parallel tube' models. Discrimination between the 2 models may be performed under linear conditions in which all pharmacokinetic parameters are independent of concentration and time. The predictions of the models are similar when bioavailability is large but differ dramatically when bioavailability is small. The 'parallel tube' model always predicts a much greater change in bioavailability than the 'well-stirred' model for a given change in drug-metabolising enzyme activity, blood flow, or fraction of drug unbound. Many clinically important drugs undergo considerable first-pass metabolism after an oral dose. Drugs in this category include alprenolol, amitriptyline, dihydroergotamine, 5-fluorouracil, hydralazine, isoprenaline (isoproterenol), lignocaine (lidocaine), lorcainide, pethidine (meperidine), mercaptopurine, metoprolol, morphine, neostigmine, nifedipine, pentazocine and propranolol. One major therapeutic implication of extensive first-pass metabolism is that much larger oral doses than intravenous doses are required to achieve equivalent plasma concentrations. For some drugs, extensive first-pass metabolism precludes their use as oral agents (e. g. lignocaine, naloxone and glyceryl trinitrate).(ABSTRACT TRUNCATED AT 400 WORDS)

Lung uptake of lidocaine in man as influenced by anaesthesia, mepivacaine infusion or lung insufficiency

Pulmonary uptake of lidocaine was investigated in patients before surgery, and aimed at elucidating the influence of general anaesthesia, the presence of another local anaesthetic agent in the blood, or the possible impact of lung insufficiency. When the lung uptake of lidocaine, injected as a bolus together with indocyanine green dye, was calculated as uptake at 95% pass of the dye, there were no statistically significant differences between the four groups. When the extraction in each of the arterial blood samples was calculated on the basis of the relation between relative concentrations, there were statistically significant differences, with a general tendency towards higher extraction of lidocaine in the awake, healthy volunteers, not given mepivacaine, compared to the other groups. In the group in whom mepivacaine was infused, the arterial concentration of mepivacaine increased transiently after the injection of lidocaine. This probably reflects a displacement of mepivacaine from binding sites for both agents. From this study, it is postulated that the ability of the pulmonary circulation to clear the blood of lidocaine is high, and that it is not affected markedly by those situations studied in the present investigation.

Dependence of lung uptake of lidocaine in vivo on blood pH

Lung uptake of lidocaine was studied in anaesthetized Swedish landrace pigs using the double indicator dilution method with indocyanine green dye as intravascular marker. The pigs were given infusions of sodium bicarbonate or hydrochloric acid to arterial blood pH in the range 7 to 8. Lung uptake of lidocaine was found to correlate statistically significant (P less than 0.05) with pH. Lung uptake in the first injection before the infusion of acid or base, was 42 +/- 4 (mean +/- S.E.M.)%. The uptake was not found to correlate to cardiac output. The conclusion from this work is therefore that lung uptake of xenobiotic amines in part is dependent on blood pH.

Uptake and displacement of [3H]dihydroalprenolol, [3H]epinephrine and [3H]clonidine in isolated perfused rabbit lung

Uptake and displacement of three adrenergic receptor ligands, [3H]dihydroalprenolol ([3H]DHA), [3H]epinephrine ([3H]EPI) and [3H]clonidine ([3H]CLON), were examined in isolated rabbit lungs by recirculating perfusion. Removal of [3H]DHA was the most extensive (85% uptake; 6.6 ml/min clearance), [3H]CLON removal was intermediate (50%; 3.8 ml/min), and [3H]EPI removal was the lowest (33%; 1.2 ml/min). Specific displacement of each radioligand from lung was attempted using several competing agents. Both (--)- and (+)-propranolol equally displaced [3H]DHA from lung. Phentolamine, (--)-phenylephrine and (--)-epinephrine were unable to displace 10 nM [3H]EPI from lung, although the latter two agents did produce concentration-dependent increases in perfusion pressure. High concentrations of (--)-epinephrine, which produced near maximal physiological responses, inconsistently displaced 30-4- nM [3H]EPI from lung. [3H]Clonidine was displaced by unlabeled clonidine at concentrations that caused increases in perfusion pressure. Pretreatment of lungs with either 10 microM phentolamine or phenoxybenzamine did not alter the total amount of [3H]CLON displaced by clonidine, suggesting that [3H]CLON was displaced predominantly from non-specific sites, perhaps preventing detection of [3H]CLON displacement from specific (receptor) sites. Alternatively, these results may be interpreted as inhibition of uptake of each radioligand. Thus, both (--)- and (+)-propranolol interfered with [3H]DHA removal, suggesting a common mechanism for uptake and/or retention for these two beta-adrenergic receptor antagonists. Inhibition of [3H]EPI removal was observed only at high concentrations of (--)-epinephrine which indicates that pulmonary removal of epinephrine occurs through a low affinity uptake system. [3H]Clonidine removal was effectively inhibited by the same (microM) concentrations of unlabeled clonidine that produced physiological responses. Neither phentolamine nor phenoxybenzamine was able to interfere with pulmonary removal of [3H]CLON. Therefore, uptake and displacement of these adrenergic receptor radioligands showed no correlation with pharmacological effects produced by these agents in isolated perfused rabbit lung. The results are more closely associated with inhibition of removal and/or non-specific retention of the radioligands examined.