Alterations of myocardial capillary permeability by albumin in the isolated, perfused rabbit heart

Myocardial Oedema as a Consequence of Viral Infection and Persistence-A Narrative Review with Focus on COVID-19 and Post COVID Sequelae

Microvascular integrity is a critical factor in myocardial fluid homeostasis. The subtle equilibrium between capillary filtration and lymphatic fluid removal is disturbed during pathological processes leading to inflammation, but also in hypoxia or due to alterations in vascular perfusion and coagulability. The degradation of the glycocalyx as the main component of the endothelial filtration barrier as well as pericyte disintegration results in the accumulation of interstitial and intracellular water. Moreover, lymphatic dysfunction evokes an increase in metabolic waste products, cytokines and inflammatory cells in the interstitial space contributing to myocardial oedema formation. This leads to myocardial stiffness and impaired contractility, eventually resulting in cardiomyocyte apoptosis, myocardial remodelling and fibrosis. The following article reviews pathophysiological inflammatory processes leading to myocardial oedema including myocarditis, ischaemia-reperfusion injury and viral infections with a special focus on the pathomechanisms evoked by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. In addition, clinical implications including potential long-term effects due to viral persistence (long COVID), as well as treatment options, are discussed.

The Glomerular Endothelium Restricts Albumin Filtration

Inflammatory activation and/or dysfunction of the glomerular endothelium triggers proteinuria in many systemic and localized vascular disorders. Among them are the thrombotic microangiopathies, many forms of glomerulonephritis, and acute inflammatory episodes like sepsis and COVID-19 illness. Another example is the chronic endothelial dysfunction that develops in cardiovascular disease and in metabolic disorders like diabetes. While the glomerular endothelium is a porous sieve that filters prodigious amounts of water and small solutes, it also bars the bulk of albumin and large plasma proteins from passing into the glomerular filtrate. This endothelial barrier function is ascribed predominantly to the endothelial glycocalyx with its endothelial surface layer, that together form a relatively thick, mucinous coat composed of glycosaminoglycans, proteoglycans, glycolipids, sialomucins and other glycoproteins, as well as secreted and circulating proteins. The glycocalyx/endothelial surface layer not only covers the glomerular endothelium; it extends into the endothelial fenestrae. Some glycocalyx components span or are attached to the apical endothelial cell plasma membrane and form the formal glycocalyx. Other components, including small proteoglycans and circulating proteins like albumin and orosomucoid, form the endothelial surface layer and are bound to the glycocalyx due to weak intermolecular interactions. Indeed, bound plasma albumin is a major constituent of the endothelial surface layer and contributes to its barrier function. A role for glomerular endothelial cells in the barrier of the glomerular capillary wall to protein filtration has been demonstrated by many elegant studies. However, it can only be fully understood in the context of other components, including the glomerular basement membrane, the podocytes and reabsorption of proteins by tubule epithelial cells. Discovery of the precise mechanisms that lead to glycocalyx/endothelial surface layer disruption within glomerular capillaries will hopefully lead to pharmacological interventions that specifically target this important structure.

Myocardial Fluid Balance and Pathophysiology of Myocardial Edema in Coronary Artery Bypass Grafting

Myocardial edema is one of the most common complications of coronary artery bypass grafting (CABG) that is linearly related to many coronary artery diseases. Myocardial edema can cause several consequences including systolic dysfunction, diastolic dysfunction, arrhythmia, and cardiac tissue fibrosis that can increase mortality in CABG. Understanding myocardial fluid balance and tissue and systemic fluid regulation is crucial in order to ultimately link how coronary artery bypass grafting can cause myocardial edema in such a setting. The identification of susceptible patients by using imaging modalities is still challenging. Future studies about the technique of imaging modalities, examination protocols, prevention, and treatment of myocardial edema should be carried out, in order to limit myocardial edema occurrence and prevent complications.

Myocardial edema in paroxysmal permeability disorders: The paradigm of Clarkson's disease

PURPOSE

Paroxysmal Permeability Disorders (PPDs) comprise a variety of diseases characterized by recurrent and transitory increase of endothelial permeability. Idiopathic Systemic Capillary Leak Syndrome (ISCLS) is a rare PPD that leads to an abrupt massive shift of fluids and proteins from the intravascular to the interstitial compartment. In some cases, tissue edema may involve the myocardium, but its role in the development of shock has not been elucidated so far.

MATERIALS AND METHODS

Assessment of cardiac involvement during ten life-threatening ISCLS episodes admitted to ICU.

RESULTS

Transthoracic echocardiographic examination was performed in eight episodes, whereas a poor acoustic window prevented cardiac ultrasound assessment in two episodes. Myocardial edema was detected by echocardiography in eight episodes and marked pericardial effusion in one-episode. Cardiac magnetic resonance showed diffuse myocardial edema in another episode. In one case, myocardial edema caused fulminant left ventricular dysfunction, which required extracorporeal life support. The mean septum thickness was higher during the shock phase compared to the recovery phase [15.5 mm (13.1-21 mm) vs. 9.9 mm (9-11.3 mm), p = .0003]. Myocardial edema resolved within 72 h.

CONCLUSIONS

During early phases of ISCLS, myocardial edema commonly occurs and can induce transient myocardial dysfunction, potentially contributing to the pathogenesis of shock.

Extended quantitative dynamic contrast-enhanced cardiac perfusion imaging in mice using accelerated data acquisition and spatially distributed, two-compartment exchange modeling

The objective of the present work was to improve data acquisition and quantification of dynamic contrast-enhanced perfusion imaging in the in vivo murine heart. Four-fold undersampled data were acquired in 14 mice and reconstructed using k-t SPARSE. A two-compartment exchange model was employed to provide additional characterization of myocardial tissue based on compartment volumes and the permeability surface area product. The feasibility of the proposed method was tested using compartment-based analysis of contrast-enhanced perfusion data acquired with intravascular and extracellular contrast agents. A significantly different permeability surface area product was measured for the intravascular versus extracellular contrast agent (0.13-0.15 ml/g/min vs 0.86-0.88 ml/g/min). The reduced extravasation also resulted in significantly smaller interstitial volumes of the intravascular versus extracellular agent (9.8-11% vs 45-47%). No difference was found for myocardial blood flow (6.5-7.2 ml/g/min vs 6.0-7.0 ml/g/min). The results presented here show that two-compartment exchange modeling in the in vivo murine heart is feasible and gives access to tissue parameters beyond myocardial blood flow.

Myocardial microvascular permeability, interstitial oedema, and compromised cardiac function

The heart, perhaps more than any other organ, is exquisitely sensitive to increases in microvascular permeability and the accumulation of myocardial interstitial oedema fluid. Whereas some organs can cope with profound increases in the interstitial fluid volume or oedema formation without a compromise in function, heart function is significantly compromised with only a few percent increase in the interstitial fluid volume. This would be of little consequence if myocardial oedema were an uncommon pathology. On the contrary, myocardial oedema forms in response to many disease states as well as clinical interventions such as cardiopulmonary bypass and cardioplegic arrest common to many cardiothoracic surgical procedures. The heart's inability to function effectively in the presence of myocardial oedema is further confounded by the perplexing fact that the resolution of myocardial oedema does not restore normal cardiac function. We will attempt to provide some insight as to how microvascular permeability and myocardial oedema formation compromise cardiac function and discuss the acute changes that might take place in the myocardium to perpetuate compromised cardiac function following oedema resolution. We will also discuss compensatory changes in the interstitial matrix of the heart in response to chronic myocardial oedema and the role they play to optimize myocardial function during chronic oedemagenic disease.

An integrative model of coupled water and solute exchange in the heart

Physiologists have devised many models for interpreting water and solute exchange data in whole organs, but the models have typically neglected key aspects of the underlying physiology to present the simplest possible model for a given experimental situation. We have developed a physiologically realistic model of microcirculatory water and solute exchange and applied it to diverse observations of water and solute exchange in the heart. Model simulations are consistent with the results of osmotic weight transient, tracer indicator dilution, and steady-state lymph sampling experiments. The key model features that permit this unification are the use of an axially distributed blood-tissue exchange region, inclusion of a lymphatic drain in the interstitium, and the independent computation of transcapillary solute and solvent fluxes through three different pathways.

Enhanced glomerular permeability to macromolecules in the Nagase analbuminemic rat

Plasma albumin restricts capillary water filtration. Accordingly, the glomerular ultrafiltration coefficient is higher in Nagase analbuminemic rats (NAR) than in Sprague-Dawley controls. We investigated whether the glomerular permeability to macromolecules is also enhanced in NAR. SDS-PAGE fractionation of urine proteins showed several bands with molecular masses between 60 and 90 kDa in NAR only. Acute administration of BSA to NAR led to nearly complete disappearance of these proteins from urine, an effect partially reversed when most of the exogenous albumin was cleared from circulation. The fractional clearance of 70-kDa dextran was increased in NAR, indicating a size defect. Binding of cationized ferritin to the glomerular basement membrane was decreased in NAR, suggesting associated depletion of fixed anions. The magnitude of cationic ferritin binding correlated negatively with the fractional clearance of 70-kDa dextran, suggesting that the two abnormalities may share a common pathogenic mechanism. Collectively, these results suggest enhanced glomerular permeability to macromolecules in NAR. Albumin may be necessary to maintain the normal glomerular permselectivity properties.

Myocardial fluid balance

Fluid accumulation in the cardiac interstitium or myocardial edema is a common manifestation of many clinical states. Specifically, cardiac surgery includes various interventions and pathophysiological conditions that cause or worsen myocardial edema including cardiopulmonary bypass and cardioplegic arrest. Myocardial edema should be a concern for clinicians as it has been demonstrated to produce cardiac dysfunction. This article will briefly discuss the factors governing myocardial fluid balance and review the evidence of myocardial edema in various pathological conditions. In particular, myocardial microvascular, interstitial, and lymphatic interactions relevant to the field of cardiac surgery will be emphasized.

Effects of plasma proteins on sieving of tracer macromolecules in glomerular basement membrane

It was found previously that the sieving coefficients of Ficoll and Ficoll sulfate across isolated glomerular basement membrane (GBM) were greatly elevated when BSA was present at physiological levels, and it was suggested that most of this increase might have been the result of steric interactions between BSA and the tracers (5). To test this hypothesis, we extended the theory for the sieving of macromolecular tracers to account for the presence of a second, abundant solute. Increasing the concentration of an abundant solute is predicted to increase the equilibrium partition coefficient of a tracer in a porous or fibrous membrane, thereby increasing the sieving coefficient. The magnitude of this partitioning effect depends on solute size and membrane structure. The osmotic reduction in filtrate velocity caused by an abundant, mostly retained solute will also tend to elevate the tracer sieving coefficient. The osmotic effect alone explained only about one-third of the observed increase in the sieving coefficients of Ficoll and Ficoll sulfate, whereas the effect of BSA on tracer partitioning was sufficient to account for the remainder. At physiological concentrations, predictions for tracer sieving in the presence of BSA were found to be insensitive to the assumed shape of the protein (sphere or prolate spheroid). For protein mixtures, the theoretical effect of 6 g/dl BSA on the partitioning of spherical tracers was indistinguishable from that of 3 g/dl BSA and 3 g/dl IgG. This suggests that for partitioning and sieving studies in vitro, a good experimental model for plasma is a BSA solution with a mass concentration matching that of total plasma protein. The effect of plasma proteins on tracer partitioning is expected to influence sieving not only in isolated GBM but also in intact glomerular capillaries in vivo.

Role of a glycocalyx on coronary arteriole permeability to proteins: evidence from enzyme treatments

Whereas the glycocalyx of endothelial cells has been shown to influence solute flux from capillary microvessels, little is known about its contribution to the movement of macromolecules across the walls of other microvessels. We evaluated the hypothesis that a glycocalyx contributes resistance to protein flux measured in coronary arterioles. Apparent solute permeability (P(s)) to two proteins of different size and similar charge, alpha-lactalbumin (alpha-lactalb) and porcine serum albumin (PSA), was determined in arterioles isolated from the hearts of 43 female Yucatan miniature swine. P(s) was assessed in arterioles with an "intact" glycocalyx under control conditions and again after suffusion with adenosine (Ado, 10(-5) M, n = 42 arterioles, N = 29 pigs). In a second set of experiments (n = 21 arterioles, N = 21 pigs) arteriolar P(s) was determined before and after perfusion with enzyme (pronase or heparinase), which was used to digest the glycocalyx. P(s) was assessed a third time on those microvessels after exposure to Ado. Consistent with the hypothesis, P(s) for PSA (P(PSA)(s)) and P(s) for alpha-lactalb (P(alpha-lactalb)(s)) increased from basal levels following enzyme treatment. Subsequent suffusion with Ado, a significant metabolite known to alter coronary vascular smooth muscle tone and permeability, resulted in a significant reduction of basal P(alpha-lactalb)(s) in both untreated and enzyme-treated arterioles. Furthermore, in untreated arterioles, P(PSA)(s) was unchanged by Ado suffusion, whereas Ado induced a pronounced reduction in P(PSA)(s) of enzyme-treated vessels. These data demonstrate that in intact coronary arterioles an enzyme-sensitive layer, most likely at the endothelial cell surface, contributes significantly to net barrier resistance to solute flux.

Effects of albumin on the disposition of morphine and morphine-3-glucuronide in the rat isolated perfused liver

1. The effect of albumin on the disposition of morphine and hepatically generated morphine-3-glucuronide (M3G) was investigated in the single-pass rat isolated perfused liver. 2. Using a balanced cross-over design, each of 10 livers was perfused at 30 mL/min with medium containing 2.7 mumol/L morphine in the presence and absence of 10 g/L bovine serum albumin (BSA). 3. Both bile flow rate and hepatic oxygen consumption were significantly higher (P < 0.005) when BSA was present in the perfusion medium, suggestive of a change in the functional performance of the perfused liver. 4. The binding of morphine and M3G was negligible in both BSA-free and -containing perfusate. 5. Outflow perfusate contained both morphine and M3G, while the metabolite but not morphine was found in bile. The recovery of the administered morphine was approximately 100% and was not altered (P > 0.05) by the presence or absence of BSA. 6. The fraction of morphine escaping heptic extraction in the absence of BSA (mean +/- SD; 0.41 +/- 0.14) was not altered significantly (P > 0.05) by the presence of the protein in perfusate (0.35 +/- 0.13), indicating no change in the intrinsic clearance or morphine despite the difference in oxygen consumption. 7. The fraction of hepatically generated M3G excreted in bile was significantly higher (P < 0.005) when BSA was present in the perfusate than when it was not (0.44 +/- 0.14 vs 0.38 +/- 0.16, respectively). 8. The results are consistent with the concept that BSA modifies the ability of solutes, including M3G, to move through the paracellular pathway from the canalicular to the vascular space. 9. It is concluded that because albumin may modify not only the unbound fraction of a ligand in perfusate, but also the functional performance of the liver, care is needed in the interpretation of studies examining the influence of the protein on the hepatic disposition of drugs and their metabolites.

Permeability of myocardial capillaries to hydrophilic drugs: the paracellular pathway

1. As the majority of drug molecules are relatively small and lipophilic, capillary uptake in those regions with a continuous endothelium, such as the heart, has generally been regarded as taking place by flow-limited transcellular capillary transport. Little attention has been paid to myocardial paracellular capillary transport of hydrophilic drugs, despite efficient transport of hydrophilic solutes, such as sucrose, inulin and EDTA, by this route. 2. The paracellular pathway is formed by the cleft between adjacent endothelial cells and its permeability properties are determined by the tight junction, a region of restricted diameter within the cleft. Paracellular capillary permeability is modulated by circulating macromolecules, such as albumin, by an unknown mechanism thought to involve the intra-endothelial cell Ca2+ concentration. 3. Studies in perfused rat heart have shown that capillary permeability of quinidine does not vary when perfusate pH is varied from 7.0-8.0, suggesting that capillary transport involves the ionized as well as unionized moiety. Addition of albumin to the perfusion medium reduced quinidine capillary permeability, which is consistent with paracellular transport of quinidine ions. 4. Paracellular transport increases in capillary inflammation. Therefore, if there is significant paracellular transport of hydrophilic drugs, an increase in uptake of such drugs in areas of inflamed myocardium associated with acute myocardial infarction would be expected to result.

Endothelium-medicated control of the coronary circulation. Exercise training-induced vascular adaptations

This review discusses the role of the endothelium in the regulation of coronary vascular function. The role of endothelium-mediated mechanisms at rest, during exercise, in exercise training-induced adaptations of coronary function and in the presence of coronary heart disease (CHD) are examined. Mechanisms of control of coronary blood flow are briefly discussed with emphasis on endothelium-mediated control of vascular resistance. The concept that the relative importance of vascular control mechanisms differs as a function of position along the coronary arterial tree is developed and discussed. Metabolic, myogenic and endothelium-mediated control systems contribute in parallel to regulating coronary blood flow. The relative importance of these mechanisms varies throughout the coronary arterial tree. Endothelium-dependent vasodilation contributes to maintenance of resting coronary blood flow but the endothelium's role in dilation of small resistance arteries, thereby increasing coronary blood flow during exercise, remains in question. In contrast, the endothelium plays an essential role in dilation of the conduit coronary arteries during exercise. Atherosclerosis and CHD convert this exercise-induced dilation to a vasoconstriction, apparently due to endothelium dysfunction. Long term increases in physical activity and exercise training alter the control of coronary blood flow. Adaptations in endothelium-mediated control play a role in these changes. However, the effects of the mode, frequency, and intensity of exercise training bouts and duration of training on adaptive changes in endothelial function have not been established. The role of the endothelium in control of the permeability characteristics of the exchange vessels in the coronary circulation is discussed. Current evidence indicates that vascular permeability is a dynamic characteristic of the vessel wall that is controlled, at least in part, by endothelium-dependent phenomena. Also, preliminary results indicate that exercise training alters microvessel permeability and the control of permeability in the coronary circulation. Further research is needed to provide clarification of the effects of exercise training on coronary endothelial control of vascular resistance and vascular permeability in atherosclerosis and CHD.

Not trophoblast alone: a review of the contribution of the fetal microvasculature to transplacental exchange

The fetal microcirculation of the term human placenta offers an interesting microvascular model. A perfused placenta can be used for integrated studies of vascular permeability-structure relationships. The organization of the paracellular pathway in human placental microvessels closely resembles not only that of the guinea-pig placenta, but also that seen in typical continuous non-cerebral capillaries such as those of the myocardium. This uniformity of organization has allowed the development of a model of the organization of endothelial junctional complexes that allows testable predictions about the relationship between junctional organization and microvascular permeability. The key features of this model are: (1) molecular size restriction may be determined by a fibre matrix based on cadherin arrays in the zonula adhaerens. (2) The zonula occludens (tight junction) is discontinuous and so cannot act as a molecular sieve for solutes. It may serve as a shutter that limits the proportion of the paracellular cleft available for permeation. The main implication for placental function is that the human placental microcirculation is relatively tight and is an important restriction to diffusive permeation of the maternal-fetal barrier by large molecules.

Induction of HSP-70 after hyperosmotic opening of the blood-brain barrier in the rat

The cellular response resulting from breakdown of the blood-brain barrier was evaluated 24 h after hyperosomotic infusion of mannitol into the internal carotid artery in the rat. Heat shock protein (HSP-70), a marker of cellular stress and/or injury, was induced in scattered patches of neurons and glia in regions of barrier breakdown. These findings suggest that osmotically induced breakdown of the blood-brain barrier may result in cell injury.

The influence of hypoxia on transvascular leakage in the isolated rat heart: quantitative and ultrastructural studies

1. The multiple indicator dilution method was used to study the transvascular movement of gamma-globulin, bovine serum albumin, insulin and cyanocobalamin in the isolated rat heart. 2. Perfusion of the heart with well-oxygenated solution for 75 min (constant flow) did not produce a significant change either in the total area under the dilution curve or the 'leakage index' (an arbitrary measure of transvascular flux) for all the tracers. 3. Perfusion of the heart with hypoxic solution produced a significant increase in leakage of gamma-globulin of 38.6 +/- 18, 48.5 +/- 17.6, 60.5 +/- 24 and 58 +/- 20% after 15, 30, 45 and 60 min, respectively, compared with the well-oxygenated equilibration period. Permeability- surface area products (PS) for the smaller diffusible solutes, therefore, could not be estimated. 4. The flux of albumin, insulin and cyanocobalamin in response to hypoxia was similar to that of gamma-globulin. 5. Ultrastructural examination of well-oxygenated hearts revealed that Monastral Blue-labelled albumin remained within the lumen and that endothelial integrity remained intact. 6. Conversely biopsies from hypoxic hearts showed that the labelled albumin had passed to the interstitium through gaps (approximately 3 microns) in venular endothelium. 7. The results showed that, in intact hearts, hypoxia produced gaps in the endothelium of venules and that these gaps could be the possible route for transvascular leakage of macromolecules.

Effect of perfusate pH on reduction of quinidine capillary permeability by albumin in isolated perfused rat heart

It has been suggested that albumin reduces quinidine capillary permeability (PS) in the single-pass perfused heart preparation by reducing paracellular transport of quinidine ions. Using this preparation, we examined the effect of albumin (0.1 per cent) on quinidine PS at perfusate pH's of 7.1 and 7.9 during uptake of quinidine (19 microM) and also during washout of the drug using a randomized design. Quinidine PS was approximately 16 ml/min/g heart at pH 7.9 and was not altered by the presence of albumin in perfusate. At pH 7.1, in the absence of albumin, quinidine PS was also 16 ml/min/g, but in the presence of albumin (0.1 per cent) PS was reduced significantly to approximately 5 ml/min/g (P < 0.001). In the absence of albumin PS was the same at pH 7.1 and 7.9 in spite of a greater degree of ionisation of quinidine at pH 7.1. This suggests that there is significant uptake of ionised quinidine at pH 7.1. The greater effect of albumin on PS at pH 7.1 supports the hypothesis that albumin reduces paracellular transport of quinidine ions.

Effect of alpha 1-acidglycoprotein on myocardial uptake and pharmacodynamics of quinidine in perfused rat heart

The myocardial uptake and pharmacodynamics of quinidine were examined in the isolated perfused rat heart preparation under conditions of varying concentrations of bovine alpha 1-acidglycoprotein (AAG) in the perfusate. Three hearts were perfused for five consecutive 35 min phases with buffer containing quinidine and AAG in concentrations of 0, 0.1, 0.5, 1.5 and 0 g/L, in that order, with a 55 min washout period between each phase. The equilibration rate constant for the quinidine output concentration increased with increasing AAG concentration, but not as much as predicted by the conventional pharmacokinetic uptake model, which assumes constant capillary permeability among the phases. Estimates of the permeability surface product for the two zero AAG phases (17.7 +/- 1.91 and 19.1 +/- 0.82 mL/min/g) were significantly greater than those for the three AAG phases (8.94 +/- 0.99, 8.70 +/- 0.26, 9.01 +/- 0.26 mL/min/g; P < 0.05). This effect of AAG is the same as that observed previously by us with bovine serum albumin in this same experimental preparation. This suggests that the mechanism of reduced capillary permeability is the same for both proteins, i.e., the formation of a steric barrier to paracellular transport rather than an electrostatic barrier. There was a direct, linear relationship between lengthening of the QT interval of the electrocardiogram and total and unbound quinidine concentrations, but the relationship for unbound concentration was independent of quinidine unbound fraction. Therefore, the electrocardiogram effect of quinidine was directly related to the circulating unbound rather than total drug concentration.

Permeability of the fetal villous microvasculature in the isolated perfused term human placenta

1. Capillary permeability-surface area (PS) products for the low molecular weight radioactive tracers, 22Na, 51Cr-EDTA (relative molecular mass 357) and 57Co-cyanocobalamin (relative molecular mass 1353) were measured in the fetal circulation of isolated dually perfused lobules of normal term human placentae using the single circulation, multiple-tracer dilution technique. 2. In lobules perfused with M199 medium, containing dextran and 5 g l-1 bovine albumin, the extractions of all three tracers decreased as the flow was increased over the range of 2-8 ml min-1, and PS products for 51Cr-EDTA and 57Co-cyanocobalamin, but not for 22Na, reached constant values at flows above 0.1 ml min-1 g-1. 3. Flow-independent PS products in the presence of albumin were 0.025 +/- 0.002 ml min-1 g-1 (mean +/- S.E.M., n = 25) for 57Co-cyanocobalamin and 0.057 +/- 0.003 ml min-1 g-1 (n = 25) for 51Cr-EDTA. The ratio of PS values (51Cr-EDTA/57Co-cyanocobalamin) was 2.28, while the ratio of the corresponding free diffusion coefficients was 1.79, indicating substantial restriction to the diffusion of the 57Co-cyanocobalamin. 4. In another series of lobules perfused in the absence of albumin, extraction values for all three test tracers were constant over the same flow range. Values at high flow rates were therefore about twice those measured in the presence of albumin, and PS products for all three tracers failed to reach diffusion-limited values. 6. Lobules perfused with and without albumin were fixed using a glutaraldehyde fixative containing 1% Alcian Blue dye. An ultrastructural examination of the endothelium showed no significant changes in cell or cleft morphology, or in the glycocalyx, in the absence of albumin which might account for the observed permeability change. 7. These data are the first physiological measurements specifically characterizing fetal microvascular permeability in the human placenta. The results suggest that permeability resembles that found in skeletal muscle and, as such, the endothelium presents a significant barrier to the diffusion of large solutes. The observed 'protein effect' indicates that albumin can interact with elements of the solute pathway to increase its restrictiveness.

Capillary diffusion capacity for Cr-EDTA and cyanocobalamine in spontaneously beating rat hearts

In order to obtain a functional estimate of the diffusional capacity of the myocardial capillary bed, the permeability surface area product (PS) for Cr-EDTA (mol. wt = 341) and cyanocobalamine (vitamin B12, mol. wt = 135) was determined in spontaneously beating Langendorff-perfused rat hearts over a wide range of coronary flow rates (700-3000 ml min-1 100 g-1). PS was determined by a single injection, colorimetric indicator dilution technique, allowing multiple, rapid and accurate determinations to be made in the same heart. During maximal vasodilation with nitroprusside Na PS averaged 535 +/- 33 and 220 +/- 22 ml min-1 100 g-1 for Cr-EDTA and vitamin B12 respectively at the highest flow (2917 +/- 74 ml min-1 100 g-1). The vasculature of the heart was found to be highly heterogeneous, since PS increased with flow and there were marked variations of extraction over transit times. A functional estimate of 'equivalent pore radius' was obtained from the ratio PSCr-EDTA/PSB12, which was 2.61 +/- 0.15 demonstrating a marked restriction to diffusion corresponding to a pore radius of 51 (41-75) A. This value is similar to that from skeletal muscle determined by the same method while PS-values are 40-45 times higher in the heart (Haraldsson & Rippe 1986). Taken together with morphological estimations of capillary surface area and endothelial path depth, these data indicate a 3-fold increase in the density of pores available for diffusion in the myocardium, compared to skeletal muscle.

Plasma proteins modify the endothelial cell glycocalyx of frog mesenteric microvessels

1. We have investigated the interaction of plasma proteins with the endothelial cell using cationized ferritin as a marker of the cell surface glycocalyx. 2. Single microvessels of the frog mesentery were sequentially perfused using glass micropipettes with solutions containing cationized ferritin (CF, 6.7 mg ml-1) in 0.10 M-NaCl and then with either frog plasma or bovine serum albumin (BSA; 50 or 10 mg ml-1), or protein-free Ringer solution, before suffusion fixation in 2.5% glutaraldehyde. 3. A layer of CF, usually two to four molecules thick, was associated with the luminal endothelial cell surface. In vessels post-flushed with protein-free Ringer solution the CF layer was closely adherent to all regions of the luminal endothelium, including the plasma membrane, vesicle diaphragms, coated pits and the entrances to clefts. However, when plasma was present during fixation the CF layer was separated from the cell surface by up to 100 nm over all regions. In vessels post-flushed with BSA the CF layer was also separated from the membrane but the effect was less striking. 4. The association of cationized ferritin with the endothelial cell surface was assessed quantitatively using electron micrographs of transverse sections (approximately 50 nm thick) of the perfused vessels, and expressed in terms of the depth of the layer of CF associated with the endothelial cell surface, its separation from the plasma membrane of the luminal endothelium, and the concentration of CF in the layer. The mean (+/- S.D.) separation in the presence of plasma, 32.3 +/- 10.5 nm (n = 12), was significantly greater (P less than 0.01) than that with either protein-free Ringer solution, 3.0 +/- 1.4 nm (n = 9), or BSA in Ringer solution, 8.3 +/- 3.0 nm (n = 8). The separation seen with BSA in Ringer was also significantly greater than that measured with a final Ringer solution perfusion (P less than 0.01). The effects of 10 and 50 mg ml-1 BSA were not different from one another. The total glycocalyx thickness, defined as the sum of the separation layer and depth of CF layer, with plasma present, 56.2 +/- 13.7 nm, was twice the value seen with Ringer solution, 28.0 +/- 9.1 nm (P less than 0.01), while the total thickness with BSA, 30.9 +/- 5.4 nm, was not different from the Ringer solution value.(ABSTRACT TRUNCATED AT 400 WORDS)

Permeability of human venous endothelial cell monolayers perfused in microcarrier cultures: effects of flow rate, thrombin, and cytochalasin D

We have applied a multiple isotope dilution technique to examine junctional permeability of human umbilical vein endothelial cells (HUVEC) in vitro. Primary cultures were grown to confluence on porous Cytodex-3 microcarrier beads, packed into 0.3 ml columns (3 x 10(6) cells) and perfused at varying flow rates (0.3-1.2 ml/min) with HEPES-buffered Tyrodes solution containing unlabeled cyanocobalamin, insulin, and albumin. Columns were challenged periodically with mixtures of radioactive tracers of different molecular size. Permeability to 22Na+, [57Co]cyanocobalamin (1.3 kD), [125I]insulin (6 kD) or [125I]albumin (66 kD) was assessed relative to [131I]IgG (160 kD, impermeant reference tracer) by comparing column elution profiles. Although the single passage extraction of [125I]albumin by beads alone approximated 40%, the presence of confluent HUVEC rendered these beads effectively impermeable to albumin. High junctional extractions were measured for cyanocobalamin (0.79 +/- 0.02, n = 28) and insulin (0.51 +/- 0.05, n = 14) in cultures perfused at 0.3-0.4 ml/min, and tracer extraction decreased as perfusion rates increased. Permeability coefficients for cyanocobalamin (9.66 x 10(-5) cm/s) and insulin (4.18 x 10(-5) cm/s) increased significantly during perfusion with thrombin (10 U/ml) or cytochalasin D (1 microgram/ml), whereas permeability to albumin (0.39 x 10(-5) cm/s) remained unchanged. Morphological studies, using the glycocalyx stain ruthenium red, revealed that thrombin or cytochalasin D increased the penetration of the stain into junctions between endothelial cells.

31P-NMR study of cardiac preservation: St. Thomas' Hospital cardioplegic solution versus UW preservation solution

Ex vivo cardiac preservation was evaluated by measuring the catabolism of high-energy phosphate (ATP and creatine phosphate, CrP) using 31P-NMR spectroscopy. After cardioplegic arrest St. Thomas' Hospital cardioplegic solution (group A), and University of Wisconsin (UW) preservation solution (group B) were tested. The hearts were mounted in the 4.7 T horizontal bore magnet of the NMR spectrometer and were continuously perfused with the test solution under 25 cm H2O pressure for 6 h at 10 degrees C. Peak heights of the beta-phosphate of ATP and CrP were measured and expressed as percentages of the initial value. For both group A and group B. ATP declined less rapidly during preservation than CrP. In group A, ATP remained constant for 60 min while CrP decreased from the onset of preservation. After 6 h of preservation 28.3% of ATP and 24.5% of CrP remained (group A). On the other hand, in group B, levels of both ATP and CrP remained much more stable: CrP did not decrease during the first 3 h of preservation, while ATP started to decrease after 5 h. At the end of preservation 76.1% of ATP and 71.5% of CrP were still present. We conclude that UW solution is superior to St. Thomas' Hospital solution for the preservation of high-energy phosphates during 6 h cardiac preservation with continuous hypothermic low-flow perfusion.

Permeability of frog mesenteric capillaries after partial pronase digestion of the endothelial glycocalyx

1. The proteolytic enzyme pronase, which degrades the endothelial cell glycocalyx, was perfused through single capillaries of frog mesentery. Hydraulic conductivity (Lp) of each vessel was determined before and after pronase perfusion. In three vessels in which Lp increased, the ultrastructure of interendothelial clefts was examined. In a separate group of frogs the effect of pronase on the endothelial glycocalyx was assessed by using cationized ferritin to label the capillary luminal surface. 2. Control Lp was 2.0 x 10(-7) cm s-1 cmH2O-1 (10 mg ml-1 bovine serum albumin, BSA, in frog Ringer solution). Vessels were then perfused with a solution containing 0.1 mg ml-1 pronase and 10 mg ml-1 BSA for 1 min. Lp measured in the same eleven vessels increased to 4.9 x 10(-7) cm s-1 cmH2O-1 (P less than 0.005). 3. Transverse sections of three of these vessels were examined by transmission electron microscopy at eight sites along each vessel. In these sections a total of 156 interendothelial cell clefts were found and photographed. No morphological features, such as fenestrations, transendothelial channels, or intercellular gaps associated with inflammation, were found which might account for the increases in Lp. 4. Measurement of cleft dimensions yielded a harmonic mean cleft depth (delta x) of 0.32 microns and an arithmetic mean cleft depth of 0.64 microns. Mean width (w) of the clefts outside the tight regions was 0.012 microns and the cleft length per unit area (L) was 1330 cm-1. The mean fractional pore area of vessel wall per unit cleft depth, Ap/delta x, calculated as Lw/delta x, was 48.7 cm-1. 5. There was less cationic ferritin (CF) labelling of the luminal glycocalyx in pronase-perfused than in control capillaries. On average, the proportion of the luminal surface covered by CF was 85% in controls and 42% in pronase-treated capillaries (P less than 0.01). In some vessels the CF pattern was greatly disrupted, indicating large changes in the glycocalyx structure. 6. It is concluded that the moderate increases in Lp induced by pronase perfusion are associated with partial digestion of the endothelial glycocalyx but are not accompanied by changes in the dimensions of the intercellular cleft. These observations support the fibre matrix hypothesis of capillary permeability and suggest that the endothelial glycocalyx contributes as much as 60% of the hydraulic resistance of the capillary wall.

Capillary permeability and how it may change

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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.

Glucagon effect on myocardial transport and utilization of energy-metabolites from the coronary microcirculation in the perfused rabbit heart

Effect of glucagon on energy-metabolite transport into cardiac muscle was studied during a single transit through the isolated rabbit heart using a rapid paired-tracer dilution method. Kinetic experiments revealed that 1.5 microM glucagon stimulated the influx of palmitate bound to 30 g/litre albumin, by increasing the V 2.3 times and increasing the Km for transport 2.4 times. Tracer uptake of D-glucose, as the only exogenous substrate provided, was increased by 80% by 1.5 microM glucagon. Myocardial utilization of [3H]-or [14C]-labelled short-chain monocarboxylic acids (L-lactate, pyruvate and acetate) was significantly reduced by glucagon, to the same degree as their unidirectional sarcolemmal transport. Inhibition of L-[14C]lactate uptake was dose-dependent and in positive correlation with myocardial lactate production. It is concluded that glucagon may regulate sarcolemmal permeability and myocardial utilization for energy-metabolites from the coronary circulation.

Observations on exsudation of fibronectin, fibrinogen and albumin in the brain after carotid infusion of hyperosmolar solutions. An immunohistochemical study in the rat indicating longlasting changes in the brain microenvironment and multifocal nerve cell injuries

An immunohistochemical study was carried out on rat brain to determine if a transient opening of the blood-brain barrier (BBB), leading to extravasation of serum albumin, is also associated with exudation and cellular uptake of fibronectin and fibrinogen. Both of them might exert important biological effects provided that they pass the BBB and come into contact with cells of the brain parenchyma. Hyperosmolar solutions of urea or mannitol were infused in the carotid artery for 30 s to open the BBB and the animals were killed at various time intervals thereafter. Formaldehyde-fixed, paraffin-embedded material was used for immunohistochemical demonstration of extravasated proteins by an avidin-biotin peroxidase technique. Multifocal, often confluent areas of widely different sizes with signs of albumin extravasation were observed both in the grey and the white matter of the cerebral hemispheres exposed to the hyperosmolar solutions. Much less pronounced changes were observed in rats given an intracarotid saline infusion alone. Immunoreactive material indicating extravasation of fibronectin and fibrinogen was present in the infused cerebral hemispheres but albumin immunoreactivity was much more widespread. Reaction product was observed in vascular walls, presumably in extracellular spaces and in nerve cells. Immunoreactivity in the perikaryon of neurons formed different patterns in various cells. A granular type most probably represents accumulation of the proteins in lysosomal organelles after pinocytotic uptake into the neuron. The second so-called diffuse variety is presumably the result of a severe nerve cell injury with an uncontrolled leakage of proteins into the cytoplasm. Our results indicate that vascular walls, extracellular spaces, glial cells and neurons will be exposed to extravasated fibronectin and fibrinogen as well as to albumin and that antigenic sites in such compounds remain for a long period after the BBB opening. In addition, there are indications that carotid infusions of hyperosmolar solutions may cause nerve cell injuries in regions with BBB opening. These findings have obvious clinical and experimental significance.

An analysis of the permeability of a fenestra

In this analysis of published data we evaluate the permeabilities of unit area of fenestral pathway to water and small solutes. These properties are then used to assess the functional significance of the fenestral diaphragm which, along with the glycocalyx and basement membrane, makes up the fenestral pathway. Endothelial hydraulic conductance and permeability to small lipophobic solutes increase with fenestral density in a variety of mammalian tissue. The increase in conductance per unit increment in fenestral area (Kfen) averages 0.38 micron X s-1 X mm Hg-1 (regression analysis). This value means that a single fenestra (diam 0.06 micron, area 0.003 micron2) has a greater conductance than 1 micron2 of continuous (skeletal muscle) endothelium. Similarly the diffusional permeability of unit fenestral area (Pfen) to B12 and inulin is high. But neither Kfen nor Pfen is as great as might be expected from the extreme thinness of the diaphragm (less than or equal to 5 nm). Pfen depends on diffusivity (D) and pathlength (delta chi): Pfen = D'/delta chi. D' depends on the fraction of the fenestral surface available for exchange and on restriction to diffusion. These relations were applied to test the view that the diaphragm, rather than the glycocalyx or basement membrane, is the major barrier to fluid and small solutes in the fenestral pathway. If this were turn, over 98 1/2% of the diaphragm's area would have to be totally impermeable in order to yield the observed values of Pfen--a result consequent upon the extreme thinness of the diaphragm. In this event there would be less than one equivalent pore (radius 5-11 nm) per diaphragm on average-which is incompatible with ultrastructural evidence. The resistance of a 5-nm-thick diaphragm containing one or more pores is not high enough to account for fenestral resistance. It is concluded that much thicker structures, such as the glycocalyx and/or basement membrane, and not the diaphragm, account for fenestral resistance to small-solute and water transport.

Evidence for a lactate transport system in the sarcolemmal membrane of the perfused rabbit heart: kinetics of unidirectional influx, carrier specificity and effects of glucagon

The kinetics and specificity of L-lactate transport into cardiac muscle were studied during a single transit through the isolated perfused rabbit heart using a rapid (15 s) paired-tracer dilution technique. Kinetic experiments revealed that lactate influx was highly stereospecific and saturable with an apparent Kt = 19 +/- 6 mM and a Vmax = 8.4 +/- 1.5 mumol/min per g (mean +/- S.E., n = 14 hearts). At high perfusate concentrations (10 mM), the inhibitors alpha-cyano-4-hydroxycinnamate (Ki = 7.3 mM), pyruvate (Ki = 6.5 mM), acetate (Ki = 19.4 mM) and chloroacetate (Ki = 28 mM) reduced L-lactate influx, and Ki values were estimated assuming a purely competitive interaction of the inhibitors with the monocarboxylate carrier. The monocarboxylic acids [14C]pyruvate and [3H]acetate were themselves transported, and sarcolemmal uptakes of respectively 38 +/- 1% and 70 +/- 8% were measured relative to D-mannitol. Perfusion of hearts for 10-30 min with 0.15 or 1.5 microM glucagon increased myocardial lactate production and simultaneously inhibited tracer uptake of lactate, pyruvate and acetate. It is concluded that a stereospecific lactate transporter exhibiting an affinity for other substituted monocarboxylic acids is operative in the sarcolemmal plasma membrane of the rabbit myocardium.

Serum factors other than albumin are needed for the maintenance of normal capillary permselectivity in rat hindlimb muscle

To investigate the effects of different perfusates on capillary permeability, we determined the capillary filtration coefficient (CFC), the capillary diffusion capacity (PS) for Cr-EDTA and clearance of albumin during isogravimetric conditions and maximal vasodilatation in the isolated, perfused rat hindquarter preparation. Experiments were conducted in 30 rats with different perfusates. We were able to confirm the classical 'protein effect'. Absence of proteins, using pure dextran solution as perfusate, induced a 45% increase in CFC and a three-fold increase in albumin clearance. However, we also found evidence for a 'serum effect'. Hence, the clearance of albumin was normal when the serum content exceeded 5% (v/v) in perfusates otherwise composed of albumin in Tyrode, but increased three-fold from 0.0305 to 0.0912 ml (min X 100 g)-1 when the rats were perfused with albumin in Tyrode with no serum present, without any change in CFC, PS for Cr-EDTA or vascular resistance to flow. Thus, certain non-dialysable serum factors, other than albumin, seem to be needed for the maintenance of normal capillary permselectivity in rat hindquarters. These factors are probably needed for the capillary membrane to maintain its character of a negatively charged barrier.

The effects of bovine serum albumin and a form of cationised ferritin upon the molecular selectivity of the walls of single frog capillaries

Single frog mesenteric capillaries have been perfused with Ringer solutions containing the neutral macromolecule Ficoll 70 at a concentration of 40 mg ml-1, while the tissues have been washed with Ringer solution containing no macromolecules but otherwise of similar composition. The effective osmotic pressures (sigma delta II) set up across the capillary wall by the Ficoll 70 were used to assess the wall's molecular selectivity. The hydraulic conductivities, Lp, of the capillary walls were also measured. In seven capillaries the addition of bovine serum albumin (BSA) to the perfusate at concentrations of 1 and 10 mg ml-1 increased sigma delta II from a mean value of 7.14 +/- 1.81 cm H2O to one of 18.71 +/- 2.33 cm H2O and at the same time halved Lp. In another eight capillaries, the addition of a form of cationised ferritin (CF) to the perfusate at a concentration of 1 mg ml-1 increased sigma delta II from a mean value of 5.69 +/- 0.87 cm H2O to one of 16.69 +/- 0.262 cm H2O and reduced Lp to between a third and a half of its original value. In a further seven capillaries, the addition of native ferritin at a concentration of 1 mg ml-1 to the perfusate had no effect on either sigma delta II or Lp. It is suggested that both CF and BSA increase the reflection coefficients of capillary walls to Ficoll 70 by binding to the surface coat of the endothelium. The results are discussed in terms of a development of the fiber matrix theory of Curry and Michel (1980).

The failure of serum albumin to affect capillary permeability in the isolated rabbit heart

The method of local tissue clearance was used to measure capillary permeability-surface area products (PS) for [3H]inulin and [14C]sucrose in the left ventricular wall of the isolated rabbit heart. As soon as a heart was excised, its coronary arteries were perfused with Ringer solution at 37 degrees for at least 30 min before clearance trials were begun. In paired trials, Ringer perfusion fluid containing 1% bovine serum albumin (Sigma) was compared with protein-free Ringer solution in terms of sucrose PS (PSs), inulin PS (PSi), and the PS ratio (Pi/Ps). With or without protein, the mean Pi/Ps was significantly less than the ratio of the free diffusion coefficients. With the untreated albumin, flow resistance rose markedly, and the PSs of both solutes fell but not Pi/Ps. To remove the unidentified vasoactive contaminant (which apparently resisted dialysis), the albumin was "defatted" by the procedure of R. F. Chen (1967, J. Biol. Chem. 242, 173-181). Defatted albumin (1% in the perfusion fluid) did not affect the volume of distribution (lambda) of sucrose or inulin in the myocardium, the heart rate, coronary flow, flow resistance, PSs, PSi, or Pi/Ps. Apparently bovine serum albumin does not influence capillary permeability in the rabbit heart. A protein effect on permeability, however, could have been missed if it has a long latent period (more than 15 min) or a long persistence (more than 30 min).

Rapid transcapillary exchange and unidirectional neuronal uptake of noradrenaline in the perfused rabbit heart

Capillary permeability and cellular uptake of noradrenaline by the isolated artificially perfused rabbit heart was measured using rapid (less than 30 s) single-circulation tracer-dilution techniques. In a single coronary circulation capillary extractions of L-[14C]noradrenaline and D-[3H]mannitol (extracellular reference) relative to an intravascular marker, 125I-labelled albumin, were similar and above 60%. The 'apparent' volume of distribution for tracer noradrenaline was 2.5-fold larger than that measured for D-mannitol (0.32 ml g-1) suggesting cellular uptake of the amine. Unidirectional noradrenaline uptake was estimated by directly comparing coronary sinus dilution profiles of L-[3H]noradrenaline and D-[14C]mannitol. Michaelis-Menten saturation kinetics based on a single-entry system were determined (Km = 2.8 +/- 1.5 microM, Vmax = 2.1 +/- 0.5 nmol min-1 g-1, n = 4) by perfusing hearts with varying concentrations of L-noradrenaline (1-10 microM). Various known inhibitors of noradrenaline uptake were investigated to determine whether uptake was mediated by neuronal (uptake1) and/or extraneuronal (uptake2) mechanisms. Desipramine (5 microM), imipramine (5 microM) and metaraminol (2 microM) resulted in a 66-94% inhibition of noradrenaline influx. In comparison, the steroids, 17 beta-oestradiol (1 microM) and corticosterone (10 microM), and the noradrenaline metabolite normetanephrine (5 microM) caused virtually no inhibitory effects. The beta-adrenergic antagonist propranolol (5 microM) was also relatively ineffective. These results together with the kinetic constants estimated suggest that the rapid noradrenaline uptake reflects transport into adrenergic neurones lying in the coronary interstitium. The high resolution of this paired-tracer dilution technique has permitted a 'non-invasive' study of neuronal uptake mechanisms and its application may be of clinical value.

Capillary permeability in the isolated rabbit heart as measured by local tissue clearance

The relatively simple method of local tissue clearance was used to measure capillary permeability-surface area products (PS) in the isolated, Ringer-perfused rabbit heart. Ten microliters of a mixture of [3H]inulin and [14C]sucrose was injected at a depth of 2 mm into the left ventricular myocardium and clearance rate constants (k in min-1) were determined by analyzing the draining perfusion fluid. PS (ml/min per 100 ml) for each solute was calculated by the following equation: PS = -F ln(1 - lambda k/F), where F is perfusate flow (ml/min per 100 ml) and lambda is the equilibrium tissue/Ringer partition coefficient. At a perfusion pressure of 40 mm Hg, F = 133 +/- 9.7 (mean +/- SEM), PSsucrose = 77 +/- 8.0, and PSinulin = 13.9 +/- 0.7. These PS products are within the range of values previously reported by others using several different techniques. The mean inulin/sucrose permeability ratio was 0.189 +/- 0.018 which is significantly less than the separately measured free diffusion coefficient ratio (= 0.41 +/- 0.005), thus indicating that sucrose and inulin crossed myocardial capillary walls by restricted diffusion. The reasons why some investigators did not find similar evidence of restricted diffusion are discussed.

The role of the coronary microcirculation in myocardial recovery from ischemia

The aim of thrombolysis, angioplasty, and coronary artery bypass surgery is to "reperfuse" ischemic myocardium; however, reperfusion can cause further cardiac damage and compromise the coronary microcirculation. Because nutrient supply and exchange and delivery of pharmacologic agents require a patent microvasculature, the coronary microcirculation plays a major role in myocardial recovery from ischemia. It is known that ischemia-reperfusion can cause an increase in coronary permeability and microvascular plugging (No-reflow). The permeability to macromolecules is increased more than the permeability to smaller molecules. The permeability increase leads to extravasation of plasma proteins and a permeability edema. Furthermore, proteins that normally remain extravascular are now free to wash out the heart. Both microvascular effects, increased coronary permeability and No-reflow, compromise cardiac function. The degree of damage depends on the nature (No-flow versus low-flow) and length of ischemia. Unfortunately, both the increase in coronary permeability and the reduction in perfused capillarity advance with time during early reperfusion. Although the increase in permeability does not require the presence of platelets or leukocytes, it is apparent that the No-reflow response does. Mechanisms that may explain the microvascular responses to ischemia include cell swelling, damage caused by oxygen free radicals, and inflammatory responses that may or may not involve granulocytes. The permeability response may involve a calcium-mediated endothelial contraction because the macromolecular leakage that follows ischemia can be prevented by pretreating hearts with the calcium blocker nisoldipine. Protection of the coronary microcirculation should be included in any attempt to improve treatment of occlusive coronary artery disease.

Techniques and applications of extracellular space determination in mammalian tissues

This review summarizes the ways in which the extracellular space (ECS) may be estimated in mammalian tissues, and briefly describes some of the uses to which the EC confinement of certain molecules (markers or tracers) may be put in the elucidation of physiological functions. The introductory section is followed by a description of the more commonly used marker molecules and their functional characteristics, and of factors likely to lead to the spurious over- or under-estimation of the ECS. Certain alternative methods are also described, in particular those based on morphological and electrical criteria which seek to demonstrate small, functionally important, changes in the size of specialized regions of the ECS (e.g. lateral cellular interspaces) without necessarily being required to provide a quantitatively precise estimate of their size. Section III describes the results of measurements of ECS in various mammalian tissues (muscle, gastro-intestinal tract, nervous tissue, crystalline lens, placenta, lung and kidney) and some applications of EC markers to investigation of cellular function (e.g. uptake of metabolic substrates and epithelial transport) and, in outline, characterization of capillary permeability. The available literature in this field is very extensive, and in the interests of brevity the reader is, where appropriate, referred to previous reviews covering specialized aspects of ECS determination and related topics.