Myocardial fatty acid oxidation: evidence for an albumin-receptor-mediated membrane transfer of fatty acids

Use of the Isolated Perfused Heart for Evaluation of Cardiac Toxicity

The isolated perfused rat heart model can be used to evaluate cardiotoxicity, and is especially useful in distinguishing direct vs indirect cardiac injury. Various perfusion systems can be used to characterize the pathophysiologic as well as morphologic changes induced by drugs or chemicals of interest. The isolated perfused heart was used in the studies described herein to characterize the mechanism of allylamine cardiotoxicity. Rat hearts were perfused with Krebs-Henseleit buffer containing 10 mm allylamine and a latex balloon was inserted into the left ventricle to monitor pressure. Coronary flow in hearts perfused with 10 mm allylamine was similar to control hearts at 5, 10, and 30 min, but was reduced by 1 hr (11.5 ± 0.6 ml/min/g wet heart weight vs 16.0 ± 0.7, p < 0.01). Peak left ventricular systolic pressure increased in hearts perfused with allylamine for 5 min (156 ± 8 mm Hg vs 103 ± 9, p < 0.01), but by 2 hr was decreased compared to controls (89 ± 6 vs 105 ± 5, p < 0.05). End diastolic pressure was markedly increased at 2 hr (58 ± 3 vs 4 ± 0.8, p < 0.01). Morphologically, allylamine perfused hearts exhibited significant contraction band changes as well as numerous cells with marked swelling of the sarcoplasmic reticulum. The findings in this study suggest that allylamine produces direct myocardial damage that appears to be independent of coronary flow. These studies demonstrate that the isolated perfused rat heart model can be used to evaluate mechanisms of acute cardiotoxicity.

The Presence of a Transporter-Induced Protein Binding Shift: A New Explanation for Protein-Facilitated Uptake and Improvement for In Vitro-In Vivo Extrapolation

Accurately predicting hepatic clearance is an integral part of the drug-development process, and yet current in vitro to in vivo (IVIVE) extrapolation methods yield poor predictions, particularly for highly protein-bound transporter substrates. Explanations for error include inaccuracies in protein-binding measurements and the lack of recognition of protein-facilitated uptake, where both unbound and bound drug may be cleared, violating the principles of the widely accepted free drug theory. A new explanation for protein-facilitated uptake is proposed here, called a transporter-induced protein binding shift High-affinity binding to cell-membrane proteins may change the equilibrium of the nonspecific binding between drugs and plasma proteins, leading to greater cellular uptake and clearance than currently predicted. The uptake of two lower protein-binding organic anion transporting polypeptide substrates (pravastatin and rosuvastatin) and two higher binding substrates (atorvastatin and pitavastatin) were measured in rat hepatocytes in incubations with protein-free buffer versus 100% plasma. Decreased unbound K _m values and increased intrinsic clearance values were seen in the plasma incubations for the highly bound compounds, supporting the new hypothesis and mitigating the IVIVE underprediction previously seen for highly bound transporter substrates.

An examination of protein binding and protein-facilitated uptake relating to in vitro-in vivo extrapolation

As explained by the free drug theory, the unbound fraction of drug has long been thought to drive the efficacy of a molecule. Thus, the fraction unbound term, or f_u, appears in equations for fundamental pharmacokinetic parameters such as clearance, and is used when attempting in vitro to in vivo extrapolation (IVIVE). In recent years though, it has been noted that IVIVE does not always yield accurate predictions, and that some highly protein bound ligands have more efficient uptake than can be explained by their unbound fractions. This review explores the evolution of f_u terms included when implementing IVIVE, the concept of protein-facilitated uptake, and the mechanisms that have been proposed to account for facilitated uptake.

Transport of Free Fatty Acids from Plasma to the Endothelium of Cardiac Muscle: A Theoretical Study

Fatty acids are transported in a multistep process from the plasma to the mitochondria, where they are oxidized in order to meet energy requirements of the myocardium. Some of those steps, mainly the crossing of the involved cells' membranes are far from being understood. Here, by means of mathematical modeling we address the problem of the fatty acid transport from the microvascular compartment to the endothelium. Values of parameters that are incorporated in the model are deduced from relevant experimental work. Concentration profiles are established as solutions of diffusion-reaction equations both numerically and using an analytical asymptotic approximation. The analytical solution accurately determines the fatty acid flux for any set of parameter values in contrast to off-the-shelf numerical solvers that fail under quite a few circumstances due to the stiffness of the differential equation system. Sensitivity analysis indicates that in spite of few uncertain parameter values, most of our conclusions are expected to be valid throughout the physiological range of operation. We find that in order to have an adequate fatty acid uptake rate it is essential for the luminal endothelial membrane to have very fast fatty acid transporters and/or specific sites that interact with the albumin-fatty acids complex.

Update on the cellular and molecular basis of capillary permeability

The attenuated layer of endothelial cells lining the blood vessels forms the critical barrier controlling the exchange of molecules from the blood to the interstitial fluid. The interactions of normally circulating blood molecules with the endothelial glycocalyx can either restrict transcapillary exchange in general or selectively increase transendothelial transport of a specific group of ligands. Investigations into the mechanisms responsible for the effects of serum have identified specific receptors, some of which appear to be involved in receptor-mediated transcytosis and endocytosis via noncoated plasmalemma vesicles (also known as caveolae). Such studies suggest that regional differences in endothelial expression of cell surface glycoproteins can be exploited for the development of tissue-directed drug therapies.

Delivery of nano-objects to functional sub-domains of healthy and failing cardiac myocytes

Cardiovascular disease, including heart failure, is one of the leading causes of mortality in the world. Delivery of nano-objects as carriers for markers, drugs or therapeutic genes to cellular organelles has the potential to sharply increase the efficiency of diagnostic and treatment protocols for heart failure. However, cardiac cells present special problems to the delivery of nano-objects, and the number of papers devoted to this important area is remarkably small. The present review discusses fundamental aspects, problems and perspectives in the delivery of nano-objects to functional sub-domains of failing cardiomyocytes. What size nano-objects can reach cellular sub-domains in failing hearts? What are the mechanisms for their permeation through the sarcolemma? How can we improve the delivery of nano-objects to the sub-domains? Answering these questions is fundamental to identifying cellular targets within the failing heart and the development of nanocarriers for heart-failure therapy at the cellular level.

Potential role for nonesterified fatty acids in β-adrenoceptor-induced increases in brain tryptophan

We tested the hypothesis that beta2- and beta3-adrenergic receptor-mediated increases in brain tryptophan are due to the liberation of fatty acids, which in turn displace tryptophan from its albumin-binding site and thus facilitate its entry into the brain. Male CD-1 mice were injected with subtype-selective beta-adrenergic agonists 1h before brain samples were collected for analysis of tryptophan content by HPLC with electrochemical detection, and blood samples were collected for analysis of total and free tryptophan and nonesterified fatty acid (NEFA) concentrations. The beta2-selective agonist, clenbuterol (0.1 mg/kg), increased concentrations of tryptophan in all brain regions studied and decreased plasma total tryptophan, but had no effect on plasma free tryptophan or NEFAs. The beta3-selective agonists, BRL 37344 (0.2 mg/kg) or CL 316243 (0.01 mg/kg), increased brain tryptophan, plasma NEFAs and free tryptophan. Pretreatment with nicotinic acid (500 mg/kg), an inhibitor of lipolysis, almost completely prevented the increase in plasma free tryptophan and NEFAs, and attenuated the increase in brain tryptophan induced by CL 316243. These results suggest that beta2- and beta3-adrenergic agonists increase brain tryptophan by a mechanism other than the liberation of NEFAs. Nonetheless, beta3-adrenergic agonists appear to increase brain tryptophan by a mechanism that may depend partially on elevations of plasma NEFAs.

Palmitic acid incorporation into intramuscular acylglycerols depends on both total and unbound to albumin palmitic acid concentration

Palmitic acid incorporation into the intramuscular acylglycerols in rat skeletal muscles of different fiber types was investigated at various total and unbound to albumin concentrations by means of the hind-limb perfusion technique. It was found that at simultaneously increasing total and unbound to albumin palmitic acid concentrations in the perfusion medium the incorporation of palmitic acid into acylglycerols increased. However, when the concentration of palmitic acid not bound to albumin was kept constant and the total palmitic acid concentration was increased, the incorporation also increased although markedly less than under former conditions. The increase was most apparent in the muscles composed of slow-twitch oxidative and fast-twitch oxidative-glycolytic fibers where fatty acid uptake is the greatest. These findings suggest that fatty acid incorporation into intramuscular acylglycerols depends not only on the unbound to albumin fatty acid concentration but also, to some extent, on the total fatty acid concentration.

Transcytosis of plasma macromolecules in endothelial cells: a cell biological survey

The modern exploration of endothelial cell biology is a largely interdisciplinary exercise. Cell biological, physiological, and more recently molecular biology approaches were used to study the pathways and the organelles involved in transcytosis of macromolecules in endothelial cell (EC). Here we discuss mainly the cell biological findings that revealed that EC have the attributes to fulfill the transport function. They are polarized cells, heterogeneous, and, thus, structurally and functionally adapted to the vascular bed in which they reside. The structural heterogeneity involves the number and distribution of plasmalemmal vesicles (caveolae), their generated channels, and the organization of intercellular junctions. The closely related functional heterogeneity comprises the degree of permeability for plasma molecules that vary as a function of organ. The EC are endowed with the cellular machinery to perform (1) endocytosis, that is to take up plasma proteins and the molecules they carry to be used for themselves (cholesterol-carrying low density lipoproteins, fatty acid carrying albumin, iron carrying transferrin, etc.), and (2) transcytosis, which implies to transport plasma proteins to the subjacent cells and tissues. The possible pathways for transport of molecules are transcellular, via caveolae and channels, and paracellular via intercellular junctions. Most of the results obtained, so far, indicate that transcytosis of albumin, low-density lipoproteins, metaloproteases, and insulin, is performed by cargo-vesicles and their generated channels. The paracellular pathway can be used for water and ions; in postcapillary venules, at the level of which approximately 30% of junctions are open to a space of 6 nm, small molecules may take this route. Recent data obtained by molecular biology techniques revealed that caveolae are endowed with the molecular machinery for fusion/fission, docking, and movement across cells. Moreover, the various and numerous molecules that have been detected in the caveolae membrane and the different functions assumed by this differentiated microdomain strengthen the postulate that there are at least two or more types of vesicles molecularly tailored for the local physiological requirements.

Palmitic acid uptake by the rat soleus muscle in vitro

The rate of fatty acid uptake, oxidation, and deposition in skeletal muscles in relation to total and unbound to albumin fatty acids concentration in the medium were investigated in the incubated rat soleus muscle. An immunohistochemical technique was applied to demonstrate whether the albumin-bound fatty acid complex from the medium penetrates well within all areas of the muscle strips. It was found that the percentage of incorporation of palmitic acid into intramuscular lipids was fairly constant, independently of the fatty acid concentration in the medium, and amounted to 63–72% for triacylglycerols, 7–12% for diacylglycerols-monoacylglycerols, and 19–26% for phospholipids. Both palmitic acid incorporation into the muscle triacylglycerol stores and its oxidation to CO2closely correlated with an increase in both total and unbound to albumin fatty acid concentrations in the incubation medium. Under conditions of increased total but constant unbound to albumin palmitic acid concentrations, the incorporation of palmitic acid into triacylglycerols and its oxidation to CO2were also increased, but to a lower extent. This supports the hypothesis that the cellular fatty acid metabolism depends not only on the availability of fatty acids unbound to albumin, but also on the availability of fatty acids complexed to albumin.Key words: skeletal muscle, fatty acids, triacylglycerols, phospholipids.

Ultrastructural study on the interaction of insulin-albumin-gold complex with mouse brain microvascular endothelial cells

The interaction between the brain microvascular endothelium and bovine serum albumin complexed with insulin and colloidal gold (insulin-BSA-gold) was studied in adult and newborn mice. The results suggest: (a) the modification of albumin enhances its binding to the luminal front of the endothelial cells, as compared to unmodified albumin used in previous studies from this laboratory; (b) the binding density of insulin-BSA-gold complex to blood-brain barrier microvessels is approximately 2.5 times higher in newborn than in adult mice; (c) in adult mice, fenestrated endothelia of the median eminence and choroid plexus demonstrate the highest binding capacity (over five and two times higher, respectively, than in blood-brain barrier endothelia); (d) in the median eminence only, the gold-labelled tracer particles may be transported across the vessel wall. Our observations offer new ultrastructural evidence that: (1) the modification of BSA molecules by complexing with insulin does not enhance the transport of BSA across the blood-brain barrier in mouse brain, and (2) insulin-BSA-gold complex appears adequate for ultrastructural localization of blood-brain barrier insulin receptors but is of questionable value as a tracer for demonstration of increased transendothelial transport in blood-brain barrier microvasculature.

Effect of plasma protein binding on kinetics of capillary uptake and efflux

We examined the effect of plasma protein binding on the kinetics of organ accumulation and washout of drugs using the single-pass Kety-Renkin-Crone capillary model. An equation relating the accumulation and washout rate constant (k) with the plasma unbound fraction (fu) was derived. Simulations showed that k was highly dependent on fu if capillary permeability was high but was independent of fu if permeability was low. The effect of plasma protein binding was to increase the rate of tissue accumulation and washout of drug but to decrease the equilibrium amount of drug taken up by the tissue, both effects mediated via a decrease in the volume of distribution. This model was used to analyze published data on the effect of plasma protein binding on the kinetics of accumulation and washout of isradipine and propafenone in the isolated perfused heart preparation. The relationship between k and fu and the directly measured volume of distribution were in accordance with the model. Although more complex models relating k and fu could be proposed, taking into account unequal flows in capillaries, slow dissociation of ligand from protein, and unstirred layer constraints, this simple model appears adequate for describing the effect of fu on myocardial accumulation and washout of isradipine and propafenone.

Subarachnoidal macrophages share a common epitope with resident non-cerebral macrophages and show receptor-mediated endocytosis of albumin-gold and IgG-gold complexes

Cerebral macrophages are supposed to exploit a pivotal role in scavenger functions of the central nervous system. We have examined the in vivo uptake of serum albumin and IgG conjugated with colloidal gold by subarachnoidal macrophages. These serum-borne proteins are endocytosed by receptor-mediated endocytosis by varying kinetics. Albumin-gold conjugates were found to be associated to a significant amount with coated pits and coated vesicles 1 min after superfusion of the cerebral surface. Within 25 min the major fraction of albumin-gold was transferred to the lysosomal compartment. IgG-conjugates revealed a less pronounced uptake. The uptake of albumin-gold could be competed by saturating of the 'receptor sites' with free albumin. It is suggested that the described receptor-mediated uptake of serum-borne proteins by subarachnoidal macrophages serves a cleansing function following blood-brain barrier disruption during acute or subacute inflammatory reactions.

Synthesis and biodistribution of a new class of 99mTc-labeled fatty acid analogs for myocardial imaging

A series of 99mTc-bis(aminoethanethiol)-fatty acid (99mTc-BAT-fatty acid) analogs were synthesized and evaluated as potential tracers of myocardial metabolism. The BAT-fatty acid precursors were prepared using a new synthetic route that avoids the use of strong reducing agents such as lithium aluminum hydride. Biodistribution studies of the no-carrier-added 99mTc-complexes were conducted in rats using [125I]IPPA as an internal standard. The myocardial uptake of the 99mTc-BAT-fatty acid analogs was significantly less than that of [125I]IPPA and indicates the 99mTc analogs are not suitable candidates for SPECT-based myocardial imaging studies.

Uptake, incorporation and metabolism of (3H)triolein in the isolated perfused rabbit heart

The purpose of these experiments was to study the uptake and metabolism of exogenous triglyceride in the isolated perfused rabbit heart. When infused into the rabbit heart, [9,10-3H(N)]triolein was retained and incorporated into a lipid fraction that had the chromatographic mobility of authentic triolein. Incorporation of labeled triolein was not likely to be the result of a lipoprotein lipase-mediated lipolysis/resynthesis cycle, since: (i) The distribution of radioactivity following administration of [3H]oleic acid was markedly different from the distribution of radioactivity following the administration of [3H]triolein; (ii) heparin was administered to the rabbits at the time of sacrifice; and (iii) the hearts were perfused with a protein-free buffer for 20 min prior to the labelling period. When isoproterenol was administered to hearts labelled with [3H]triolein, there was an increased output of total radioactivity, composed of labelled free fatty acids, diacylglycerol and monoacylglycerol. In these same hearts, there was an increased output of glycerol in response to isoproterenol. However, following the administration of bradykinin or angiotensin II, neither the radioactivity nor the glycerol content of the perfusate was changed. These data suggest that [3H]triolein is selectively incorporated into the triglyceride pool of the isolated perfused rabbit heart. Furthermore, this [3H]triolein is available to hormonally-activated lipolytic enzymes.

Radiopharmaceuticals for studying cardiac metabolism

(1) Metabolism is the link between myocardial blood flow and physiological performance of the heart. (2) Metabolic myocardial radiopharmaceuticals have the potential to identify metabolic alterations unique to a given intrinsic cardiac disease (e.g. cardiomyopathies), to assess acute metabolic changes or in delineating a specific chronic metabolic defect (e.g. coronary artery disease). (3) Two approaches can be employed to evaluate in vivo myocardial utilization of subtracts: (a) use of radiolabeled "physiologic" substrates e.g. positron emitting 11C-palmitic acid was successfully employed for assessing the in vivo metabolic sequelae of myocardial ischemia, infarction and cardiomyopathies, and (b) use of modified tracers which enter known metabolic pathways. However, because of their unique structure, metabolism of the tracer stops at a certain state thus leaving the radiolabel trapped in the cell, e.g. [18F]FDG for measuring glucose metabolic rate in the human brain and myocardium. (4) Among the radiopharmaceuticals for planar and single photon tomography, the para and the ortho isomers of 123I-phenyl iodoheptadecanoic acids and their beta-methyl derivatives are the most promising tracers for myocardial metabolic studies. (5) Ortho-(123I-phenyl)-pentadecanoic acid (o-IPPA) human myocardial uptake was rapidly and markedly elevated in well perfused segments; myocardial turnover was strikingly prolonged, suggesting some "trapping" phenomenon, resulting in excellent scintigrams. This is in contrast to the relatively shorter clearance of the para isomer from the myocardium. (6) 11C-Palmitic acid and [18F]FDG are the most widely used for PET scanning for following myocardial metabolism. The most important clinical application of these agents is predicting viability of ischemic myocardium. (7) A significant proportion of fixed perfusion defects seen on thallium studies can be demonstrated to be viable myocardium on PET scans using metabolic agents. If the markers of perfusion alone are relied on to assess tissue viability, the extent of salvageable myocardium may be underestimated. The demonstration of myocardial viability is crucial in the decision of the optimal treatment of the disease.

Albumin interacts specifically with a 60-kDa microvascular endothelial glycoprotein

Confluent monolayers of microvascular endothelial cells, derived from the rat epididymal fat pad and grown in culture, were radioiodinated by using the lactoper-oxidase method. Their radioiodinated surface polypeptides were detected by NaDodSO4/PAGE (followed by autoradiography) and were characterized by both lectin affinity chromatography and protease digestion to identify the proteins involved in albumin binding. All detected polypeptides were sensitive to Pronase digestion, whereas several polypeptides were resistant to trypsin. Pronase treatment of the cell monolayer significantly reduced the specific binding of radioiodinated rat serum albumin, but trypsin digestion did not. Limax flavus, Ricinus communis, and Triticum vulgaris agglutinins competed significantly with radioiodinated rat serum albumin binding, whereas other lectins did not. A single 60-kDa glyco-protein was precipitated in common by these three lectins and was trypsin-resistant and Pronase-sensitive. Rat serum albumin affinity chromatography columns weakly but specifically bound a 60-kDa polypeptide from cell lysates derived from radioiodinated cell monolayers. These findings indicate that the 60-kDa glycoprotein is directly involved in a specific interaction of albumin with the cultured microvascular endothelial cells used in these experiments.

Are isolated cardiomyocytes a suitable experimental model in all lines of investigation in basic cardiology?

Isolated cardiac myocytes of adult rats resemble the intact myocardium in many respects. Thus, use of isolated cells has been established in many lines of basic cardiological research. In electrophysiology, ionic channels can apparently be characterized more accurately than in intact tissue. The transport of metabolites across the sarcolemma can be studied independently of the influence of other types of cells and transport barriers. However, most reports about metabolism deal with quiescent cells, which obviously have a very low metabolic rate, provided they are intact, and their oxidative phosphorylation is not uncoupled. Thus, their application as a model of a working heart appears to be restricted. But using electrical stimulation, the metabolic activity of the cells can be gradually enhanced up to those values observed in beating hearts. In this case, the measurement of mechanical parameters as the myocytes respond to the electrical stimulation is of interest. The combination of the measurements of both metabolic and mechanical parameters in a physical model, led us to investigate the possibility of measuring inotropic effects as well as the relationship between mechanical changes and changes in oxygen consumption, e.g. as a result of the utilization of different substrates. This expands the application of the model to pharmacology, in which the influence of the mechanical action of the heart and its oxygen consumption is of major interest. If the model of isolated cardiomyocytes is employed in screening studies, a reduction in the number of experimental animals required for this line of research will inevitably result.

Identification of albumin-binding proteins in capillary endothelial cells

Isolated fat tissue microvessels and lung, whose capillary endothelia express in situ specific binding sites for albumin, were homogenized and subjected to SDS-gel electrophoresis and electroblotting. The nitrocellulose strips were incubated with either albumin-gold (Alb-Au) and directly visualized, or with [125I]albumin (monomeric or polymeric) and autoradiographed. The extracts of both microvascular endothelium and the lung express albumin-binding proteins (ABPs) represented by two pairs of polypeptides with major components of molecular mass 31 and 18 kD. The ABP peptides have pIs 8.05 to 8.75. Rabbit aortic endothelium, used as control, does not express detectable amounts of ABPs. The ABPs subjected to electrophoresis bind specifically and with high affinity (Kd = approximately 60 X 10(-9)M) both monomeric and polymeric albumin: the binding is saturable at approximately 80 nM concentration and 50% inhibition is reached at 5.5 micrograms/ml albumin concentration. Sulfhydryl-reducing agents beta-mercaptoethanol and dithiothreitol do not markedly affect the ABPs electrophoretic mobility and binding properties. As indicated by cell surface iodination of isolated capillary endothelium followed by electroblotting, autoradiography, and incubation with Alb-Au, the bands specifically stained by this ligand are also labeled with radioiodine.

Blood binding and tissue uptake of drugs. Recent advances and perspectives

The free drug hypothesis, which states that only the unbound moiety of drug in blood is available for tissue diffusion, is discussed according to recent investigations. In some experimental conditions, it must be assumed that part of the protein-bound drug in plasma is extracted during a single passage through the organ studied. The mechanisms underlying these observations are not unequivocal and remain hypothetical. In the liver, high-affinity binding sites for serum albumin have been demonstrated, and they would explain the high extraction by liver of endogenous and exogenous compounds. However, these experiments measure the unidirectional transfer of a drug from the vascular to the extravascular space in non-steady-state conditions. Hence, in steady-state conditions, the free drug hypothesis cannot be ruled out because it is supported by numerous pharmacokinetic studies.

Stimulation of fatty acid uptake and triglyceride synthesis in human cultured skin fibroblasts and adipocytes by a serum protein

Lipoprotein deficient serum has been shown to enhance lipid synthesis in cultured normal human skin fibroblasts incubated in the presence of oleate-albumin. The factor responsible is nondialyzable and trypsin sensitive. The stimulation is proportional to the concentration of lipoprotein deficient serum in the media and is present at all oleate concentrations and incubation times assayed. The protein has been partially purified by column chromatography to yield a Peak II fraction which stimulates triglyceride synthesis in both fibroblasts and isolated human adipocytes. The stimulation is dependent on the concentration of protein fraction and increases to an apparent saturation level of 200% in fibroblasts. Triglyceride synthesis, however, increases to a much greater extent in adipocytes and did not demonstrate saturation at the maximum Peak II protein concentration assayed. These results suggest that human serum contains a protein which stimulates fatty acid uptake and esterification by adipose tissue.

Uptake of oleate from albumin solutions by rat liver. Failure to detect catalysis of the dissociation of oleate from albumin by an albumin receptor

The hepatic removal of albumin-bound substances from plasma requires that they dissociate from albumin. Using indirect methods, we and others have proposed that dissociation may be catalyzed by interaction of albumin with the liver cell surface. This study looked for direct evidence of catalysis by comparing the rate of dissociation of oleate from albumin in vitro with the rate observed within the sinusoids of perfused rat liver. No evidence for catalysis was found. The rate of hepatic oleate removal from dilute albumin solutions did not exceed but instead closely paralleled the rate predicted from the in vitro dissociation rate constant (0.14s-1). These results suggest that under some conditions the liver can remove unbound material from the sinusoids faster than it can be replenished by dissociation from albumin, resulting in dissociation-limited removal. However, dissociation of oleate does not appear to be catalyzed by the liver.

Specific binding sites for albumin restricted to plasmalemmal vesicles of continuous capillary endothelium: receptor-mediated transcytosis

The interaction of homologous and heterologous albumin-gold complex (Alb-Au) with capillary endothelium was investigated in the mouse lung, heart, and diaphragm. Perfusion of the tracer in situ for from 3 to 35 min was followed by washing with phosphate-buffered saline, fixation by perfusion, and processing for electron microscopy. From the earliest time examined, one and sometimes two rows of densely packed particles bound to some restricted plasma membrane microdomains that appeared as uncoated pits, and to plasmalemmal vesicles open on the luminal front. Morphometric analysis, using various albumin-gold concentrations, showed that the binding is saturable at a very low concentration of the ligand and short exposure. After 5 min, tracer-carrying vesicles appeared on the abluminal front, discharging their content into the subendothelial space. As a function of tracer concentration 1-10% of plasmalemmal vesicles contained Alb-Au particles in fluid phase; from 5 min on, multivesicular bodies were labeled by the tracer. Plasma membrane, coated pits, and coated vesicles were not significantly marked at any time interval. Heparin or high ionic strength did not displace the bound Alb-Au from vesicle membrane. No binding was obtained when Alb-Au was competed in situ with albumin or was injected in vivo. Gold complexes with fibrinogen, fibronectin, glucose oxidase, or polyethyleneglycol did not give a labeling comparable to that of albumin. These results suggest that on the capillary endothelia examined, the Alb-Au is adsorbed on specific binding sites restricted to uncoated pits and plasmalemmal vesicles. The tracer is transported in transcytotic vesicles across endothelium by receptor-mediated transcytosis, and to a lesser extent is taken up by pinocytotic vesicles. The existence of albumin receptors on these continuous capillary endothelia may provide a specific mechanism for the transport of albumin and other molecules carried by this protein.