The permeability of brain capillaries to non-electrolytes

The rapid development of functional tolerance to ethanol by mice

A method is described in which the development of tolerance to ethanol in individual mice can be measured during the inhalation of ethanol vapour. This method has been used with two behavioural end-points, loss of righting reflex and loss of rotarod performance. It demonstrates that, in the adult male, TO Swiss mouse, peak tolerance, in which approximately 2 X the original effective blood ethanol concentration is required to produce the behavioural end-point, can develop in 3--5 h. After this time the ability of the animals to perform normally in the presence of continued high concentrations of ethanol in blood begins to fall. The results are discussed in relation to current concepts of tolerance to central nervous system depressant drugs.

The blood-ocular barriers

The introduction of the concept of blood-ocular barriers in the ophthalmic literature is briefly reviewed. Two main blood-ocular barriers are proposed: the blood-aqueous barrier and the blood-retinal barrier. The blood-aqueous barrier is formed by an epithelial barrier located in the nonpigmented layer of the ciliary epithelium and in the posterior iridial epithelium, and by the endothelium of the iridial vessels. Both these layers have tight junctions of the "leaky" type. The pereability of the blood-aqueous barrier shows a significant degree of pressure-dependent diffusion associated with transport activity, resembling the standing gradient osmotic flow model. The blood-retinal barrier is located at two levels, forming an outer barrier in the retinal pigment epithelium and an inner barrier in the endothelial membrane of the retinal vessels. Both these membranes have tight junctions of the "nonleaky" type. The permeability of the blood-retinal barrier resembles cellular permeability in general, diffusion being directly related to the predominant roles of lipid solubility and transport mechanisms. Finally, the clinical significance of the blood-ocular barrier is analyzed. The metabolism of cornea and lens and the regulation of intraocular fluids are directly influenced by the blood-aqueous barrier. Similarly, an alteration of the blood-retinal barrier appears to play an important role in the development of vascular retinopathies, pigment epitheliopathies, and retinal edema.

Simultaneous measurement of regional blood flow and glucose extraction in rat brain

A method has been described which allows the measurement of cerebral blood flow (CBF) and solute transport across cerebral capillary wall in the same regional sample of rat brain. An inert diffusible indicator (iodoantipyrine) was used to measure a blood flow, in mixed gray and white matter, or approx. 1.0 ml/min/g. Using 3H2O as a reference molecule, the flux of [14C]D-glucose into brain was determined at blood glucose concentration levels between 0.1 and 60 mM. In all discrete areas of brain sampled, a consistent Vmax of 1.92 micromol/min/g and a Km of 8.35 mM was found. Glucose extraction by brain was inversely related to CBF, while a direct relationship was noted for glucose clearance.

Osmotic opening of the blood-brain barrier

The blood-brain barrier at cerebral blood vessels is due to a continuous lining of endothelial cells, which are connected by tight junctions that restrict intercellular diffusion. The endothelium excludes most water-soluble solutes and proteins but supports facilitated stereospecific transport of monosaccharides and large neutral and basic amino acids. The barrier in different species can be made permeable by infusing a hypertonic solution of urea or arabinose into the internal carotid artery. Endothelial cells presumably shrink and tight junctions between them widen to proteins and normally restricted solutes. Thus, intravascular protein tracers such as Evans' blue-albumin, 125I-labelled albumin, horseradish peroxidase (ED 1.11.1.7) and alpha-mannosidase (EC 3.2.1.24) are allowed into the brain, and uptake of [3H] norepinephrine (noradrenaline) is increased more than twofold above a normal rate of accumulation by brain. Osmotic barrier opening to amines has been used to demonstrate their effect on cerebral blood flow from within the brain parenchyma. Osmotic barrier opening is reversible, may be graded with respect to molecular size and is not followed by evidence of brain damage or of brain oedema (when measured two days after hypertonic infusion). Transient cerebral changes probably accompany osmotic opening, however, as glucose uptake and cerebral metabolism of glucose are increased after hypertonic infusion.

Regional brain blood flow in the conscious gerbil

Regional brain blood flow was determined in 23 awake, unparalyzed gerbils with a simplified indicator-fractionation technique. The use of intravenous 14C-butanol, an indicator that is freely diffusible into the brain, eliminated the need for repetitive sampling of arterial and cerebral venous blood and reduced the period of indicator circulation of 10 seconds. Gerbils spontaneously breathing room air (PaCO2 = 32 +/- 1 (SE) mm Hg) had blood flows in whole cerebrum, cerebellum, and brainstem of 102 +/- 4, 93 +/- 5, and 114 +/- 6 ml/100 gm/min respectively. Cerebral blood flow increased linearly with elevations in PaCO2 (r=0.969) and averaged 3.14 +/- 0.17 ml/100gm/min per mm Hg increase in PaCO2. Interpolated cerebral blood flow at a PaCO2 of 40 mm Hg was 127 +/- 2 ml/100 gm/min. This technique is easy and convenient to use, involves no intracranial surgery, requires steady state conditions for only 10 seconds, and minimizes blood loss in small animals. In more discrete brain regions a less volatile indicator is needed.

Chemotherapy in recurrent noncystic low-grade astrocytomas of the cerebrum in children

Six children with grade II astrocytomas were treated with combination chemotherapy consisting of 1,3 bis(2-chloroethyl) 1-nitrosourea (BCNU). Vincristine (VCR), intrathecal Methotrexate (i.t. MTX), and Dexamethasone. Of the children, 3 showed a partial remission, and 1 other showed clinical improvement. Our data suggest that chemotherapy is effective in low-grade astrocytomas, and there is now a need for larger trials in low-grade astrocytomas to document the role of chemotherapy accurately in these tumors. It is suggested that trials of chemotherapy as adjuvant to surgery and radiotherapy should be initiated.

K+-permeability of the blood-brain barrier, investigated by aid of a K+-sensitive microelectrode

The K+-permeability of the blood-brain barrier at the capillary level was estimated from determinations of brain extracellular K+-concentration in response to an isotonic bolus containing KCl injected into the carotid artery. A very low permeability appeared from the fact that the extracellular K+-concentration--measured by aid of K+-sensitive microelectrodes--remained unchanged during the passage of the bolus. An upper limit for the blood-brain barrier K+-permeability in the rat was estimated to be 2.8 X 10(-7) cm. with s-1.

Multiple pathways of capillary permeability

Morphological and physiological studies indicate multiple routes for transport across capillary endothelium. However, the identification of the morphological counterparts of specific transport processes (or the assignment of specific transport roles to morphologically identifiable pathways) has been only partly achieved: the contribution of endothelial cell membranes to transport of water and small, lipid-insoluble molecules needs to be evaluated. The identification of the "small pore" pathway for water and lipid-insoluble molecules with the intercellular junctions still remains questionable. The contributions to total macromolecular transport of junctions, single vesicles (pinocytosis, cytopempsis), chains of vesicles, and fenestrae are not yet known.

Intra-arterial basie dye studies of the blood-CSF and blood-brain barriers

1. The permeability of the blood-CSF barrier was tested by injecting highly diffusible basic dyes into the common carotid artery and aspiration of the cisterna magna during or shortly after the injection. None of the dyes appeared in the CSF. 2. The appearance of similarly administered basic dyes were observed in the exposed cerebral cortex. Some of the basic dyes appeared in the cortex immediately after a few drops of dye were injected. This contradicts the assumption that staining of the brain by basic dyes is a post-vital process. 3. The difference between the permeability of the blood-CSF barrier and the blood-brain barrier proves the fundamentally different characters of the two barriers.

Brain water alterations after unilateral nephrectomy. A study of regional circulatory factors in squirrel monkeys

White and regional gray matter distributions of water, blood flow, and the protein tracer pertechnetate were measured in five normal squirrel monkeys. A second group of five monkeys, which had undergone unilateral nephrectomy six months previously, were found at the time of study to have blood pressures similar to those of the control animals but increased brain water and altered distribution of blood flow which was increased in white matter. No alteration of capillary permeability to the protein traces attended these changes, which appeared to be influenced by blood pressure. Nephrectomy without hypertension influences brain water content, perhaps because of an effect on cerebral resistance vessels. In hypertensive encephalopathy renal lesions, as well as intraluminal pressure changes, may be related to cerebral edema.

Urea and hearing in patients with menière's disease

Transient hearing improvement was recorded in around 60% of patients with Menière's disease 2 hours after the ingestion of 20 g of urea perorally. Urea slightly increased the serum osmolality, which is thought to cause a transient reduction of the endolymphatic hydrops. Urea has no inconvenient side effects and is recommended primarily as a test substance for the verification of endolymphatic hydrops, in conformity with glycerol.

Transport of leucine and sodium in central nervous tissue: studies on retina in vitro

Unidirectional leucine fluxes were measured in isolated rabbit retina maintained under steady state conditions in medium resembling CSF but with leucine varied from 2 to 20,000 microM. At physiological leucine concentration (11 microM), 1/2 time for outward transport was 88 s and intracellular fluid was cleared of isotopically labelled leucine at 2.3 ml/g dry wt./min; 1/2 time for inward transport was 16 s and interstitial fluid was cleared at 7.5 ml/g dry wt./min. The rate of leucine influx corresponded quite well with its rate of disappearance from the intracellular fluid, over a wide range of concentrations. Exchange diffusion was demonstrated for transport in both directions. There was competition by other amino acids, but no interaction between Na+ and leucine transport could be demonstrated. Kinetic analysis indicated the presence of more than one transport system for leucine. There was an unexpected fall in the efflux coefficient, with reduction in leucine concentration at the lower end of the concentration range, for which an explanation is proposed. Under control conditions, 1/2 time for efflux of intracellular 24Na+ was about 0.9 min. With intracellular Na+ increased 4 fold, 1/2 time for efflux was slightly reduced. Problems encountered in measuring fluxes in organized tissue are discussed.

The blood-retinal barriers

The Blood-Retinal Barrier (BRB) is a situation of restricted permeability which is present between the blood and the retina. This barrier has a well defined anatomic substrate, particular permeability characteristics and appears to play a role of major importance in the pathophysiology and therapeutics of retinal disease. The BRB phenomenon operates fundamentally at two levels, retinal vessels and chorioepithelial interface, forming which may be better called an inner BRB and an outer BRB. The main structures involved are, for the inner BRB, the endothelial membrane of the retinal vessels, and for the outer BRB, the retinal pigment epithelium. 'Zonulae occludentes' are present in these membranes forming complete belts around the cells, sealing off the spaces between them. Other structures appear to play an accessory role. Both barriers show an apparent predominance of processes of active transport over mechanisms of passive transfer, these being extremely restricted. Much information on the pathophysiology of the BRB mechanism has been obtained from studies of its experimental breakdown. In this way, a breakdown of the inner BRB may be induced by acute distension of the vessel walls, ischaemia, chemical influences, defects in the endothelial cells and failure of the active transport system, whereas experimental ischaemia, mechanical distension of the pigment epithelial membrane, defects in the pigment epithelium and failure of the active transport systems can cause a breakdown of the outer BRB. The increased permeability of the inner BRB, and of the outer BRB, appears to be related to changes in the vascular endothelial membrane and retinal pigment epithelium, respectively. In clinical ophthalmology there are two methods for the diagnosis of breakdown of the BRB, fundus fluorescein angiography and vitreous fluorophotometry. Vitreous fluorophotometry being capable of detecting functional alterations of the barrier before any pathological changes are apparent. There is evidence of an intimate relationship between breakdown of the BRB and almost every retinal disease, particularly the vascular retinopathies and the pigment epitheliopathies. Diabetic retinopathy, hypertensive retinopathy, retinal vein obstruction, blood diseases, trauma or surgery to the eye, temporary arterial obstruction, perivasculitis, Behçet's and Coats' diseases, retinoblastoma, hemangioblastoma and retinal neovascularization are examples of situations where a breakdown of the inner BRB has been demonstrated. On the other hand, examples of breakdown of the outer BRB include situations of choroidal ischaemia, detachment of the pigment epithelium, choroidal neovascularization, photocoagulation, retinal detachment, Koyanagi's disease, central serous choroidopathy, multifocal inner choroiditis and acute placoid pigment epitheliopathy.

Metabolic control of respiratory neuronal activity and the accompanying changes in breathing movements of the rabbit. 1. Mainpulation of inspiratory and expiratory-inspiratory neurons

The property of the neuronal membrane to be permeable to metabolic modifiers of two regulatory enzymes has been utilized to manipulate the spike activity of inspiratory (I) and expiratory-inspiratory (EI) neurons of the bulbar respiratory centre. The neurons have been classified according to their response to lung distention or collapse (alpha- or beta-type) and to hyperventilation (tonic firing denoted by "+", cessation of activity by "-"). Using extracellular microelectrodes for single unit recording, the medulla oblongata was superfused with a metabolite-containing CSF. The various neuronal sub-types exhibited a differential activating or inhibitory response to one or several metabolic effectors. For example Ialpha+ units were activated by 5 mM glucose-6-phosphatase (G-6-P) and 3.5 mM 3-phosphoglycerate (3-PGA), which both inhibited Ibeta+ neurons, while 5 mM AMP inhibited Ialpha+ much more strongly than Ibeta+ cells. The spike density of Ialpha- and Ibeta- neurons was increased in the presence of 2.5 mM fructose-6-phosphate and 3.5--5 mM AMP, but became reduced by G-6-P. In contrast, 3 mM fructose-1,6-diphosphate and 5 mM 3-PGA activated the Ialpha- but inhibited the Ibeta- neurons. The EIbeta units were characteristically activated by 10 mM citrate, which inhibited all I-type neurons. Activations of the Ialpha and Ibeta neurons led to an accelerated respiratory rate and a higher tidal volume, while the opposite was true for EIbeta neurons. Intravenous injection of metabolites could not duplicate the striking effects under local applications.

Blood-brain barrier studies in the rat: an indicator dilution technique with tracer sodium as an internal standard for estimation of extracerebral contamination

(1)The single injection-indicator dilution method of Crone was adapted for use in the rat. A bolus containing a non-diffusible reference tracer and the test tracer was injected into the internal carotid artery. Simultaneously venous blood was sampled from the confluence of sinuses. It proved necessary to take special steps to avoid extracerebral contamination of the samples. Therefore the extracerebral branches of the carotid tree on both sides were ligated, and the venous sampling was done without suction. Results obtained in animals prepared surgically in different ways were compared. (2) 24Na+ was used to assess the degree of extracerebral contamination, the transcapillary loss of 24Na+ being a sensitive measure since the permeability of cerebral capillaries for sodium is about 4-5% that of muscle. (3) The transcapillary fractional loss, extraction E, was determined in this preparation with minimal extracerebral contamination. ENA+ was 2.9%, Eglucose was 12.5% at normal blood glucose levels whilst the net glucose extraction (calculated from arterio-venous difference) was about 10%. (4) It is concluded that extracerebral contamination is an important pitfall in indicator dilution studies on the brain. Use of 24Na+ as an indicator of extracerebral contamination is therefore recommendable.

The characteristics of glucose transport across the blood brain barrier and its relation to cerebral glucose metabolism

The evidence suggests that glucose transport across the blood brain barrier (BBB) in the dog is normally not a rate-limiting step in cerebral metabolism; however, transport may become rate-limiting under conditions of extreme hypoglycemia or anoxia. Studies on the mechanism of glucose transport from blood to brain do not at this time permit us to distinguish between active transport and facilitated diffusion; however, a decrease in the rate of unidirectional transport during anoxia suggests that an energy-dependent process may be involved. In spite of this evidence, glucose transport across the BBB is similar to the facilitated diffusion of glucose into the red cell in terms of the structural requirements of the glucose molecule, the pattern of inhibition by phlorizin, phloretin and cytochalasin B, and the lack of sensitivity to Na+ or insulin.

Contrast media: quantitative criteria for designing compounds with low toxicity

Toxicity of contrast media that are ionized iodobenzoic acids or their derivatives is highly correlated with lipid solubility, as measured by the octanol/water partition coefficient. New contrast media have been designed with lower lipid solubility than media in current use, taking into account the additive-constitutive nature of the partition coefficient of an organic compound. If these contrast media are chemically stable, they should also be less toxic. It remains to be tested whether the relation between clinical toxicity and lipid solubility applies to non-ionized contrast media as well.

Inhibition of glucose transport into brain by phlorizin, phloretin and glucose analogues

An indicator dilution technique with 22Na+ as the intravascular marker was used to measure unidirectional transport of D-[6-3H]glucose from blood into the isolated, perfused dog brain. 18 compounds which are structurally related to glucose were tested for their ability to inhibit glucose transport. The data suggest that no single hydroxyl group is absolutely required for glucose transport, but rather that glucose binding to the carrier probably occurs through hydrogen bonding at several sites (hydroxyls on carbons 1, 3, 4 and 6). In addition, alpha-D-glucose has higher affinity for the carrier than does beta-D-glucose. A separate series of experiments demonstrated that phlorizin and phloretin are competitive inhibitors of glucose transport into brain; however, phloretin is partially competitive and inhibits at lower concentrations than does phlorizin. Inhibition by phlorizin and phloretin is mutually competitive, indicating that these compounds compete for binding to the glucose carrier. Comparison with the results reported in the literature for similar studies using the human erythrocyte demonstrates a fundamental similarity between glucose transport systems in the blood-brain barrier and erythrocyte.

Complex tight junctions of epithelial and of endothelial cells in early foetal brain

?The morphology of epithelial and of endothelial intercellular junctions in human foetal (9-15 weeks gestation) and sheep foetal (50, 60 and 125 days gestation, term 147 days) brain has been studied using the freeze-fracture technique and thin section transmission electronmicroscopy. Freeze-fracture replicas of the choroid plexus of both early human and sheep foetuses showed that the choroidal ependymal cells are linked at the ventricular surface by tight junctions. Freeze-fracture replicas of foetal cortical endothelial cell junctions showed that they are still more complex than those of choroidal epithelial cells, in all specimens so far examined. In some 60 day sheep foetuses the dye Alcian blue, which binds to plasma albumin and which iselectrondense when treated with osmium tetroxide, was injected intravenously a few minutes prior to fixation. The dye penetrated from blood into brain extracellular space and c.s.f. but apparently not by an intercellular route. The dye was found in a tubular system (endoplasmic reticulum) in both choroidal epithelial and cortical endothelial cells. The possibility that protein penetrates into the foetal brain and c.s.f. by a transcellular route is discussed. The possible significance of these findings in relation to previous ideas and studies of the development of blood-brain barrier mechanisms is also considered.

Equipotency of hypertonic solutions of mannitol and sodium chloride in eliciting thirst in the dog

The dogs were infused intravenously with either 3.6% NaCl or 20% mannitol solutions. Both infusions stimulated the animals to drink water when similar osmotic loads were introduced and exactly the same degree of cellular dehydration achieved. The amounts of water drunk at thirst threshold did not differ significantly. The results are discussed with relation to the hypothesis of osmometric control of water intake.

The third circulation revisited

The author reviews modern information concerning the formation, flow and functions of the cerebrospinal fluid. Particular attention is given to the lymphatic-like features of the third circulation and to its importance as an internal milieu for nervous tissue.

Blood-brain transfer of d-glucose, l-leucine, and l-tryptophan in the rat

Recently Oldendorf developed a method for measurement of the early uptake of substances from the capillary bed into brain tissue after a single capillary passage of a bolus injected rapidly into the common carotid artery of the rat. The uptake of a test substance is expressed relative to the uptake of a highly diffusible reference substance, tritiated water. In this study experimental data are presented allowing to correct the uptake values for the unknown loss of tritiated water, so that the fractional unidirectional uptake (Extraction, E) can be calculated. This method is used to investigate the uptake kinetics for D-glucose, L-leucine, and L-tryptophan. For the three substances investigated the uptake kinetics involved both saturation and linear kinetics. Km values of 11 mM for D-glucose, 0.16 mM for D-glucose, 0.16 mM for L-leucine, and 0.19 mM for L-tryptophan were found. The uptake capacity, Vm, was calculated using a blood flow value of 0.85 ml/(g x min); Vm was for D-glucose 1.6, for L-leucine 0.027, and for L-trytophan 0.024 mumol/(g x min). The D-glucose Vm in the present study is comparable with Vm values in the literature, and indicates that the method may be employed for quantitative analyses of the blood-brain transfer of solutes.

The diffusion permeability to water of the rat blood-brain barrier

The diffusion permeability to water of the rat blood-brain-barrier (BBB) was studied. Preliminary data obtained with the Oldendorf tissue uptake method (Oldendorf 1970) in seizure experiments suggested that the transfer from blood to brain of labelled water is diffusion-limited. More definite evidence of such a limitation was obtained using the single injection technique of Crone (1963). 14-C-labelled sucrose was used as intravascular reference substance and tritium-labelled water as test substance. The non-exchanging (transmitted) fraction, I-E equals T, of labelled water during a single passage increased from 0.26 to 0.67 when the arterial carbon dioxide tension was changed from 15 to 85 mm Hg, a change increasing the cerebral blood flow about sixfold. This finding suggests that water does not pass the blood-brain barrier as freely as lipophilic gases.

Brain uptake of lactate, antipyrine, water and ethanol

Brain uptake of antipyrine, water and ethanol was studied in rats under normo-, hypo- and hypercapnic conditions. Brain uptake of D- and L-lactate was studied in normal rats. The uptake was determined with the Oldendorf method, using single common carotid arterial injections of a mixture of -14C-labeled test substance and tritiated reference substance. The results demonstrate that L-lactate is taken up by the brain in significant amounts. The results also demonstrate marked differences in the uptakes of antipyrine, water and ethanol. The brain uptake of antipyrine is lower, the brain uptake of ethanol higher, than of water. The brain uptake of all 3 substances was shown not to be affected by changes in cerebral blood flow, although a decrease of brain uptake of antipyrine relative to ethanol was observed during hypo- and hypercapnia.

Glycerol effects on the perilymphatic and cerebro-spinal fluid pressure

Glycerol has been injected intravenously in guinea pig and its effects on the pressure in the cochlear fluids have been studied. Simultaneously, arterial and cerebro-spinal pressures have been recorded. Glycerol lowered the intracochlear as well as cerebro-spinal and blood pressures, the latter only temporarily. Different possible mechanisms for the glycerol effect on the intracochlear pressure and its transient effect on hearing in Menière's disease are discussed.

Permeability changes in the blood-brain barrier: causes and consequences

1. Generalized changes in blood-brain barrier (BBB) permeability are accompanied by extravasation of plasma proteins; thus, they are readily studied with protein markers or protein-dye complexes. Selective changes in permeability involve alterations in BBB transport systems; they are best studied with techniques which detect the qualitative hallmarks of carrier-mediated transport, namely saturation, competition, and stereospecificity. 2. Quantitative assessments of the selective permeability of the BBB can be made from the saturation data expressed in terms of Michaelis-Menten kinetics. The advantages of the latter are twofold: (a) alterations elicited by modified barrier affinity (Km) can be distinguished from alterations in carrier capacity (Vmax); (b) the relative rates of flux of a metabolite across the BBB can be placed in the perspective of cerebral metabolism. Kinetic data on transport processes in the BBB are obtained by either constant infusion or single injection techniques. Results obtained with both methodologies have been comparable. 3. Independent transport systems for glucose, neutral amino acids, basic amino acids, and monocarboxylic acids have been identified in the BBB. The description of these transport systems in kinetic terms provides a background of information on intact mechanisms to which altered transport can be compared. 4. Experimental evidence indicates that the availability of key metabolic substrates, such as glucose or essential amino acids, may be rate-limiting in cerebral metabolism. A working hypothesis was developed that the consequences of a selective change in barrier permeability to one or more of these essential substrates are directly related to altered rates of reaction in substrate-limited pathways, e.g., cerebral protein or neuro-transmitter biosynthesis. 5. Toxicological causes of generalized changes in BBB permeability include hypertonic solutions, organic solvents, surface-active agents, enzymes, and heavy metals. Some agents, e.g., mercury or hypertonic urea, induce selective changes in BBB transport at doses much lower than those required for nonspecific barrier break-down. Subtle changes in transport of metabolic substrates may remain unrecognized unless specifically investigated, yet may have profound consequences on brain metabolism. 6. Pathological processes can also induce selective changes in BBB permeability. Such changes often temporally precede the more generalized alterations in permeability that can occur during pathogenesis. For example, in brain edema due to an ischemic infarct, glucose transport increases during the early cytotoxic phase, whereas generalized changes are not detected until the later vasogenic phase.

A concurrent flow model for extraction during transcapillary passage

A model for capillary-tissue exchange in a uniformly perfused organ with uniform capillary transit times and no diffusional capillary interactions was designed to permit the exploration of the influences of various parameters on the interpretation of indicator-dilution curves obtained at the venous outflow following the simultaneous injection of tracers into the arterial inflow. These parameters include tissue geometric factors, longitudinal diffusion and volumes of distribution of tracers in blood and tissue, hematocrit, volumes of nonexchanging vessels and the sampling system, capillary permeability, P. capillary surface area, S , and flow of blood- or solute-containing fluid, Fs ′. An assumption of instantaneous radial diffusion in the extravascular region is appropriate when intercapillary distances are small, as they are in the heart, or permeabilities are low, as they are for lipophobic solutes. Numerical solutions were obtained for dispersed input functions similar to normal intravascular dye-dilution curves. Axial extravascular diffusion showed a negligible influence at low permeabilities. The “instantaneous extraction” of a permeating solute can provide an estimate of PS /Fs ′, the ratio of the capillary permeability–surface area product to the flow, when PS /Fs ′ lies between approximately 0.05 and 3.0; the limits of the range depend on the extravascular volume of distribution and the influences of intravascular dispersion. The most accurate estimates were obtained when experiments were designed so that PS /Fs ′ was between 0.2 and 1.0 or peak extractions were between 0.1 and 0.6.

On the uptake of materials by the intact liver. The transport and net removal of galactose

D-galactose, a monosaccharide rapidly phosphorylated within liver cells, is irreversibly removed from the portal circulation. We have studied the kinetic relations between the hepatic cell entry process and the metabolic sequestration process, by means of the multiple indicator dilution technique. Labeled red blood cells (a vascular indicator), labeled sucrose (an extracellular reference), and labeled galactose were rapidly injected into the portal vein, and from rapidly sampled hepatic venous blood, normalized outflow-time patterns were secured. The labeled red cell curve rises to the highest and earliest peak, and decays rapidly; and that for labeled sucrose rises to a later and lower peak. Its extrapolated recovery is equivalent to that of the labeled red cells. At low blood galactose concentrations, the labeled galactose appears at the outflow with labeled sucrose, but is much reduced in magnitude, and exhibits a long tailing. Its outflow recovery is much reduced. At high blood galactose concentrations, the initial part of the profile increases towards that for labeled sucrose, the tailing becomes much larger in magnitude, and the outflow recovery becomes virtually complete. We have modeled the uptake of labeled galactose, and find two parts to the predicted outflow pattern, corresponding to our experimental observations; throughput material, which sweeps past the cell surface in the extracellular space; and returning material, which has entered the cells but escaped the sequestration process. Analysis of the data by use of this model provides estimates of both transmembrane fluxes and rates of sequestration. The capacity of the process subserving cell entry is found to be 40 times that for phosphorylation; and, whereas the K(m) value for sequestration is less than 15 mg/100 ml, that for entry is approximately 500 mg/100 ml. Both processes are relatively stereospecific; the entry of the L-stereoisomer is very slow and it undergoes no significant amount of metabolic sequestration. The sequestration process produces a lobular intracellular concentration gradient; and this gradient, in turn, produces some uncertainty in the estimate of the true K(m) value for the sequestration process.