Microdialysis of rat skeletal muscle and adipose tissue: dynamics of the interstitial glucose pool

Microdialysis was evaluated as a method for studying glucose metabolism in skeletal muscle. Dialysis probes (0.5 × 10 mm) were perfused at 0.5 or 1.0 μl min^-1. Based upon perfusion with glucose, the muscle interstitial glucose concentration was estimated to be 6.9 ± 0.3 mM (w = 14), which was not significantly different from the blood glucose level. With insulin infusion (1200 mU kg^-1 body wt i.v.), the insulin-induced change in the glucose concentration of the interstitial space of muscle was of equal magnitude to that of blood and adipose tissue. In spite of this, when the perfusion medium was not supplemented with glucose, the glucose concentration decreased more in skeletal muscle dialysates (to 36.7 ± 4.9% of the initial level) than in blood (to 29.7±5.0%) but less than in adipose tissue (to 17.7 ± 4.9% of the initial level) (P < 0.05). The results indicate that these differences are due to tissue-specific differences in the dynamic balance between the supply to, and removal from, the interstitial glucose pool. This balance is revealed as a result of the constant glucose drainage by the microdialysis probe. The present results show that, in skeletal muscle, increases in glucose uptake occur with a concomitant increase in tissue blood flow as revealed by the microdialysis ethanol technique, whereas in adipose tissue the glucose uptake increases in the absence of a corresponding increase in blood flow.     

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). ¹⁴C-labelled sucrose was used as intravascular reference substance and tritium-labelled water as test substance. The non-exchanging (transmitted) fraction, I-E = 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.     

Capillary permeability measured by bolus injection,residue and venous detection

Kinetic analysis of residue and outflow curves of y-emitting indicators such as chromium-51-EDTA and iodide-131-thalamate from skeletal muscle gives the possibility to determine the extraction fraction and the plasma flow, and from these two values the capillary diffusion capacity can be calculated (Sejrsen 1970, preliminary report). This is possible both for the transport from blood to tissue and from tissue to blood. This alternative method has been compared in the autoperfused cat gastrocnemius preparation with the indicator diffusion method based on venous registration of a diffusible test indicator and an intravascular reference indicator (Chinard et at. 1955, Crone 1963). The results of the five independent measurements show good agreement. Calculation of the permeability P_d based on a capillary surface area of 7 000 cm²/100 g of tissue gives values of 1.05–10^-5, 1.10–10^-5, and 1.16–10^-5 cm/s, which is in agreement with results obtained by other investigators. The permeability was equal in both directions, and thus the capillary membrane seems to function as a symmetrical membrane. Using an area of 5 000 cm²/100 g which presumably is more realistic at the plasma flow range used gives P_d values around 1.5–10^-6 cm/s. The effective pore area is calculated to constitute 1/50 000 of the capillary surface area. Calculation of volumes of distribution in the muscle tissue gave intravascular plasma volumes of 1.8 to 2.0 ml/100 g, an extravascular volume of 12.4 and 15.2 ml/100 g and a final monoexponential component constituting a compartment of 5.4 and 7.0 ml/100 g from residue and venous curves, respectively. The last mentioned compartment constitutes nearly 50 per cent of the extravascular space, and it is suggested, that it is located inside the sarcoplasmic reticulum, which anatomically constitutes about 50 per cent of the interstitial space. The total area of contact between the longitudinal and the transversal tubules in this subsystem, which is the membrane of the lateral saccus, is estimated to about 6 times the capillary surface area at a plasma flow of 15 ml/100 g min which gives a permeability about 60 times lower for this membrane compared to the capillary membrane.     

Extracellular fluid space in rabbit SA node and atria as studied with 14C-inulin and 14C-sucrose

The extracellular fluid space in rabbit SA node and atrial trabeculae was measured with 14C-inulin and 14C-sucrose. A larger 14C-inulin space in SA node than in the atrial tissue, indicated a greater extracellular fluid space in the former, and could be due to caveolae on the pacemaker cell membrane.     

Comparison of β-adrenoceptors mediating vasodilatation in canine subcutaneous adipose tissue and skeletal muscle

Blood flow changes in response to various drugs in simultaneously autoperfused canine subcutaneous adipose tissue and gracilis muscle were compared to study the vascular β-adrenoceptors. Compared to isoprenaline the β₂-selective agonist salbutamol was 4–6 times more potent as a vasodilator in the muscle than in adipose tissue. Furthermore two β₁-selective agonists (Tazolol and H80/62) caused vasodilatation in adipose tissue but not in the gracilis muscle. When given by close i.a. injection after β-adrenoceptor blockade, adrenaline was a more potent vasoconstrictor than noradrenaline in both tissues. Before β-blockade, however, noradrenaline was the more potent vasoconstrictor in the gracilis muscle whereas adrenaline was more potent in adipose tissue. Intravenous infusion of adrenaline in doses causing vasodilatation in the muscle caused vasoconstriction in adipose tissue whereas intravenous infusion of noradrenaline caused vasoconstriction in both tissues. The present findings suggest that the β-adrenoceptors mediating vasodilatation in skeletal muscle are mainly of the β₂-type, whereas β₁-adrenoceptors seem to predominate in subcutaneous adipose tissue. Since adrenaline is a much more potent β₂- than β₁-agonist, these differences point to different roles of intravascular adrenaline in the two sites. In skeletal muscle circulating adrenaline is mainly a vasodilator whereas in subcutaneous adipose tissue it mainly acts as a vasoconstrictor.     

Tritiated thymidine autoradiographic study on postnatal development of epididymal adipose tissue in the normal mouse

Summary Cell proliferation was studied in the developing epididymal adipose tissue of mice by means of³H-thymidine autoradiography. 4 mice in each age group were killed 30 min after a single injection of³H-thymidine given at the 6th, 10th, 16th, 21st, 28th, 35th, 42nd, 49th or 56th postnatal day. The labeling index of the adipose tissue was high for several postnatal days but it was highest on the 6th day. The labeling index decreased thereafter, and cell proliferation appeared to almost cease after 56 days. Another 16 mice were given a single injection of³H-thymidine at 6 days old; pairs of animals were killed at 30 min, 2, 4, 6, 8, 10, 14 and 18 days later. From the 2nd to 6th days of pulse labeling, the number of labeled fat cells increased significantly, whereas the number of labeled poorly differentiated mesenchymal cells decreased reciprocally. The labeling index of fat cells remained unchanged from the 6th to the 18th day. These results suggest that there is a population of fat cell-precursor cells in the mesenchymal tissue of the new born mouse, and that it takes 2–6 days for the precursor cells to become mature fat cells in the epidiymal fatty tissue.     

Fatty acid metabolism in adipose tissue of aging mice after direct tracer injection into fat pads

We have examined previously reported age-related defects in triglyceride synthesis from [1-¹⁴C]palmitate in adipose tissue of mice. Three techniques were used: in vitro, using adipocytes isolated from epididymal fat pads of young and old mice; and in vivo, using two new methods to measure free fatty acid (FFA) esterification by adipose tissue (direct injection of labeled palmitate-albumin complexes in large or small volumes into the extracellular spaces of the epididymal or inguinal fat pads of young and old mice). When the entire fat pad was filled with tracer we no longer observed heterogeneous labeling of adipocytes in epididymal fat pads that occurred in an earlier study in which an in vivo-in vitro method has been used. Free fatty acids were converted to triacylglycerol faster by adipocytes of large cells from older animal than by those of small cells from young mice; when the cell sizes of young and old mice were approximately equal, then the rates of FFA esterification were the same in young and old adipocytes. When FFA was injected as a small bolus the fractional rates of FFA disappearance and of FFA incorporation into triacylglycerol in the different fat pads, observed during a 60-min period, were the same (about 5 min or less) regardless of the region of the fat pad studied (distal or proximal epididymal fat pad), the type of fat pad (epididymal or inguinal), or the age of the mice (12–92 weeks). Other potential applications of the direct injection technique for studying FFA metabolism and structure-function in adipose tissue in vivo are discussed. Our findings, coupled with the earlier study in which labeled FFA was added to the outside of fat pads, indicate that, in adipose tissue of old mice, there exist barriers comprising mesothelial cells, collagenous structures, and/or the outer layer of adipocytes in fat pads, that interfere in the transport of FFA to the interior adipocytes when FFA is added outside the fat pad. This age-related defect may be circumvented by injecting tracer directly into the interstitial fluid compartment.     

Pulmonary Transcapillary Exchange of 24Na and 51CrEDTA. An Evaluation of Factors Influencing the Extraction of these Tracers during one Passage through an Isolated Lung Preparation

Experiments with the single injection indicator diffusion method have been carried out in an isolated, ventilated and plasma-perfused rabbit lung preparation. A mixture of two diffusible tracers (²⁴Na, ⁵¹CrEDTA) and one intravascular reference tracer (¹²⁵I-albumin) were used. Relative concentrations in the first indicator-containing venous outflow fractions collected indicated that there had been some initial extraction of the diffusible indicators. A small extra-vascular space of distribution appeared to represent the most important determinator for the extent of this initial extraction during one single passage through the vascular bed. Results from several types of experiments were consistent with there being also some moderate degree of diffusion limitation for the extraction of the diffusible tracers. Extraction of the diffusible tracers appeared to be moderately increased when the intravascular concentrations of both Ca⁺⁺ and Mg⁺⁺ were reduced to such low levels that edema was developing in the preparation. It appears unlikely though that the single injection indicator diffusion method in its present form can yield more detailed information on capillary permeability to small solutes in this rabbit lung preparation.     

Effect of Sympathetic Nerve Stimulation on Net Transvascular Movement of Fluid in Canine Adipose Tissue

Net transvascular movement of fluid has been studied in the isolated, autoperfused subcutaneous adipose tissue of the dog, during and after sympathetic nerve stimulation (1–15 Hz) and during infusion of 50% glucose i.a. Net fluid movement was calculated as the difference between change in tissue volume and change in blood volume. Tissue volume was measured by plethysmography and blood volume by external monitoring of circulating ¹³¹I-albumin. No net fluid movement of statistical significance was found during or after nerve stimulation except during the first minute of stimulation at 15 Hz when a small net absorption (p<0.05) was obtained. In contrast, infusion of glucose at 25–75 mOsm/kg H₂O produced a dose-dependent net absorption lasting several minutes, amounting maximally to 0.30 ml × min^-1× 100 g^-1. The absence of prolonged net absorption in subcutaneous adipose tissue during nerve stimulation as well as the absence of net filtration after stimulation may be explained by an essentially unaltered mean hydrostatic capillary pressure. The results indicate that adipose tissue does not contribute to the fluid homeostasis of the body via sympathetic resetting of the pre-postcapillary resistance ratio. Thus, mobilisation of fluid from the nterstitial space in adipose tissue into the blood does not seem to occur by nerve activity.     

Metabolic effects of nicotine on human adipose tissue in organ culture

Summary Fragments of human adipose tissue were maintained in culture for 1 week in a medium containing 1 mU/ml insulin and 100 ng/ml dexamethasone. Under these conditions lipoprotein lipase activity was present in human adipose tissue fragments which converted [¹⁴C]glucose to ¹⁴C0₂ and [¹⁴C]triglyceride. Both metabolic parameters studied were affected by human tumor necrosis factor and brefeldin A. When fragments of human adipose tissue after 1 week in culture were incubated with nicotine tartrate for 20 h, a slight but significant increase in lipoprotein lipase activity was observed, and an increased conversion of [¹⁴C]glucose to ¹⁴CO₂ and [¹⁴C] triglyceride occurred. Nicotin was taken up by human adipose tissue, but no conversion to cotinine was observed. Our data demonstrate a direct effect of nicotine on human adipose tissue metabolism. Furthermore, it is suggested that weight loss in smokers is a multifactorial phenomenon, and one of the important factors to be considered is the direct effect of nicotine within the tissue.Abbreviations LPL lipoprotein lipase - MEM minimal essential medium - rHuTNF recombinant human tumor necrosis factor - BFA brefeldin A - TG triglyceride     

Validation of the ‘internal reference technique’ for calibrating microdialysis catheters in situ

In vivo calibration of microdialysis catheters with [³H]glucose as internal reference was done in rat (n= 17) and human (n= 12) subcutaneous tissue. The estimated interstititial glucose level was compared owth the glucose concentration in venous plasma which, in turn, has been shown to be identical to the interstitial glucose concentration. In subcutaneous tissue of anaesthetized male Sprageue-Dawley rats, interstitial glucose was significantly overestimated (43%, P < 0.005, n= 8, and 19%, P < 0.005, n=9, in nornoglycaemic and hyperglycaemic animals, respectively). Furthermore, fractional outflux of [³H]glucose decreased continuously during prolonged perfusion of the microdialysis catheter. Incontrast, in human subcutaneous tissue microdialysed with two catheters, correct measurements of interstitial glucose could be achieved and the precision was comparableto that obtained with equilibration calibraton in vivo. The average relative error of the mean result of two catheters was 8.9% at a perfusate flow rate of 1 μL min^-1. It may be suggested that calibration in in vivo of microdialysis catheters with internal references may be used in human subcutaneous tissue. However, it is necessary to validate the calibration technique in each different tissue under reproducible experimental conditions since accumulation of the reference substance in the tissue may create artefactual results.     

Monitoring intracellular, interstitial, and intravascular volume changes during fluid management procedures

The bioimpedance spectroscopic (BIS) analytical algorithm described in this report allows for the non-invasive measurement of intravascular, interstitial, and intracellular volume changes during various fluid management procedures. The purpose of this study was to test clinical use feasibility and to demonstrate the validity of the BIS algorithm in computing compartmental volume shifts in human subjects undergoing fluid management treatment. Validation was performed using volume changes recorded from 20 end stage renal disease patients. The validation procedure involved mathematically deriving post hoc hematocrit profiles from the BIS data-generated fluid redistribution time profiles. These derived hematocrit profiles were then compared to serial hematocrit values measured simultaneously by a CritLine^® monitor during 60 routine hemodialysis sessions. Regression and Bland–Altman analyses confirm that the BIS algorithm can be used to reliably derive the continuous and real-time rates of change of the compartmental fluid volumes. Regression results yielded a R ² > 0.99 between the two measures of hematocrit at different times during dialysis. The slopes of the regression equations at the different times were nearly identical, demonstrating an almost one-to-one correspondence between the BIS and CritLine^® hematocrits. Bland–Altman analysis show that the BIS algorithm can be used interchangeably with the CritLine^® monitor for the measurement of hematocrit. The present study demonstrates for the first time that BIS can provide real-time continuous measurements of compartmental intravascular, interstitial and intracellular fluid volume changes during fluid management procedures when used in conjunction with this new algorithm.     

Initial plasma disappearance and distribution volume of [131l]albumin and [125l]fibrinogen in man

The simultaneous plasma disappearance curves of albumin and fibrinogen were recorded in eight normal subjects from 10 to 60 min following intravenous injection. Additional samples were taken at 3, 4, 5, 6 and 7 min. The initial distribution volume (IDV) of albumin calculated by semilogarithmic extrapolation to zero time was 5.56% (range 3.73–8.53) larger than that of fibrinogen, denoting an initial high-rate function of albumin efflux extending from zero to about 10 min after tracer injection. The following slower phase of the albumin curve from 10 to 60 min was found to be similar to the so-called transcapillary escape rate (TER) of single-tracer experiments. By introducing the value C_p(o) (i.e. the estimated curve height at t= 0, from the injected amount of albumin tracer divided by the IDV_f), the entire initial part of the albumin curves was analysed. From this analysis the mean value of 0.0135+ 0.0038 min¹ was determined for initial slope, corresponding to a whole-body unidirectional albumin efflux [j(0)] of 0.0572 + 0.0160 ml 100 g^-1 min^-1. The result is about 16 times higher than normal estimates of total lymphatic albumin return, indicating a huge backflux of interstitial albumin at the whole-body capillary level. Both phases of efflux seem to reflect uptake in a variety of peripheral tissues, and the hypothesis that the second phase (TER) expresses the initial slope of albumin escape into non-liver tissues is not substantiated. Based on the difference in IDV of the tracers demonstrated, the uncritical use of albumin as a plasma volume marker is not justified.     

Regional Cerebral Blood Flow in the Rat Measured by the Tissue Sampling Technique; a Critical Evaluation Using Four Indicators C14-Antipyrine,C14-Ethanol H3-Water and Xenon133

The validity of the assumptions inherent in the tissue sampling technique for measuring cerebral blood flow was tested in the rat, using the 4 indicators C¹⁴-antipyrine, C¹⁴-ethanol, H³-water and Xenon¹³³. Each one of the indicators was infused i.v. during either 30, 60 or 120 s and the blood flow was calculated from the integrated arterial curve and from the tracer concentration in the tissue. The calculated flows varied with the time of infusion, and at high perfusion rates all tracers gave flow values that were lower than those obtained with the Kety and Schmidt technique. For C¹⁴-antipyrine, the method gives an error already at normal perfusion rates. It is concluded that the cerebral uptake of the tracers is diffusion-limited, and that the tissue sampling technique cannot be used for quantitative measurement of cerebral blood flow in high flow situations. However, the error of the method is considerably reduced if C¹⁴-ethanol is used and if the time of infusion is limited to 30 s.     

Absence of Restricted Diffusion in Adipose Tissue Capillaries

Capillary permeability in adipose tissue for ⁵⁷Co-cyanocobalamin (⁵⁷Co-B12) was determined by the single injection, external registration method. The capillary diffusion capacity, CDC, (the permeability-surface area product, PS) was 1.1 ml/100 g·min at a capillary extraction of 0.21 and a plasma flow of 6.7 ml/100 g·min. Results were compared to ⁵¹Cr-EDTA data from a previous study with similar method and preparation. As CDC(⁵¹Cr-EDTA)/CDC (⁵⁷Co-B 12) was 1.81 and as D(⁵¹Cr-EDTA)/D(⁵⁷Co-B 12), the ratio between the free diffusion coefficients in water at 37°C, was 1.79, it is concluded that restricted diffusion does not occur in cutaneous tissue for ⁵⁷Co-B 12 as compared to ⁵¹Cr-EDTA, i.e., ⁵¹Cr-EDTA and ⁵⁷Co-B 12 diffuse across the capillary membrane of adipose tissue at rates proportional to their respective free diffusion coefficients in water. The Pappenheimer equivalent pore radius estimate of 30 Å and the Karnovsky interendothelial 40 Å slit width are both defective in explaining the experimental data. The transendothelial channel system of fused vesicles (Simionescu, Simionescu and Palade 1975) is a possible structural equivalent for the present findings. The results support the hypothesis that capillaries of continuous type exhibit similar permeation characteristics regardless of the tissue in which they are located.     

The extracellular space of rat intestine “in vivo”

Summary The extracellular space (ECS) of rat jejunum, ileum and colon were determined in in vivo condition both by means of a continuous perfusion technique and after ligature of the renal pedicles. As a marker, intravenously injected¹⁴C-polyethylenglycol (¹⁴C-PEG) was used.The data indicate that the ECS of rats whose renal pedicles were ligated, increases throughout the experiment, due most probably to a hypertonic expansion of the extracellular space. On the contrary, in the experiments with a continuous perfusion the ECS remains constant throughout the experiment.Total tissue water does not change in the two types of experiments whereas the intracellular water decreases only if the renal pedicles are ligated.     

Acta Physiologica Scandinavica - 100 years

High levels (110–120 cmH₂O) of lower body negative pressure (LBNP) were used in male volunteers (n = 7) to produce pronounced hypovolaemic circulatory stress in an attempt to reveal the potential in man for compensatory absorption of extra vascular fluid from skeletal muscle and skin as studied in the upper arm by plethysmographic technique. LBNP evoked clear-cut hypovolaemic symptoms or even accidental syncope as well as a marked tachycardiac response and a significant fall in systolic blood pressure. In the studied arm there was concomitantly a very rapid net transcapillary absorption of extravascular fluid into the circulation at an average rate of 0.35 ml min^-1 100 mr¹ soft tissue during 5 min of LBNP exposure. These data demonstrate an amazingly great potential to increase plasma volume by ‘autotransfusion’ of fluid from tissue to blood, as may be visualized by extrapolation of the data from the arm to apply to the whole mass of skeletal muscle and skin in the body. The observed absorption rate would then correspond to a total fluid gain of no less than 700 ml within a period no longer than 5 min. At present, however, there is no evidence to indicate that such impressive fluid volumes can be rapidly transferred from the extra- to the intravascular space after actual blood loss.     

Cholesterol metabolism in squirrel monkeys: Analysis of long-term kinetic studies in plasma and body tissues

Thirty adult male Brazilian-type squirrel monkeys (Saimiri sciureus) were fed a semipurified diet containing 45% of the calories as fat. One milligram of cholesterol per calorie was added to the diet of one-half of the animals. After 4 months on their respective diets, a single intravenous dose of [³H] cholesterol was administered to each animal. At 2, 4, and 6 months following intravenous labeling, five animals from the cholesterol-fed group and five animals from the non-cholesterol-fed group were killed for analysis of tissue cholesterol specific activity. The analysis of ratios between cholesterol specific activity of various tissues to the specific activity of terminal blood samples of the cholesterol-fed group indicated to us that cholesterol in liver, spleen, adrenal gland, ileum, stomach, kidney, testes, lung, heart, skin, tendon, fascia, and skeletal muscle had reached equilibrium with plasma cholesterol radio-activity by 2 months. The tissue/plasma ratios of adipose tissue, aorta, and gallstones demonstrated a slower rate of exchange. In the non-cholesterol-fed group the tissue/plasma ratios of all the tissues mentioned above remained close to unity in all three subgroups, indicating equilibration with plasma by 2 months. As before, the tissue/plasma ratios in adipose tissue and aorta had a slower rate of exchange. In both groups, cerebrum and spinal cord had a very slow rate of exchange with plasma cholesterol. Computer analysis of the plasma cholesterol specific activity time curve suggested to us a significantly better three-exponential fit rather than a two-exponential fit in $\text{5}\text{14}$ of the cholesterol-fed group and in none of the non-cholesterol-fed group. The minimal mass of the miscible body pool of cholesterol derived from kinetic analysis agreed reasonably well with, but consistently overestimated, the whole-body miscible pool determined by carcass analysis in both groups.     

Uptake of inorganic phosphate by the maternal border of the guinea pig placenta

Mechanism of uptake of inorganic phosphate (P_i) by the maternal border of the dually perfused guinea pig placenta was studied using the paired-tracer dilution technique with³²P-phosphate and¹⁴C-sucrose being the tracers. Placental uptake of radioactive phosphate increased when the concentration of P_i in the perfusion fluid was reduced, and it decreased during anoxia, in presence of CN or during perfusion with low-Na or Na-free fluids. Iodoacetate was without effect. These observations are consistent with placental uptake of P_i being effected by a carrier mediated process dependent on external Na and, partly, on placental metabolism. Unidirectional flux of P_i from the maternal vascular space into the cell compartment of the placenta, estimated from the values of instantaneous extraction of³²P, correlated significantly with foetal weight. The flux per unit weight of the foetus was 17.0±1.0 nmol · min^–1 g^–1.     

Estimation of capillary permeability of inulin,sucrose and mannitol in rat brain cortex

The present study analyzed the brain uptake of the differently sized hydrophilic molecules ¹⁴C-inulin, ¹⁴C-sucrose and ¹⁴C-mannitol. Constant tracer concentrations were maintained in blood plasma after renal ligation. Accumulation of the indicators was measured in brain cortex during the approach to steady-state. At the conclusion of infusion periods of 5, 10, 15, 30 and 60 min, samples of cerebral cortex were analyzed for radioactivity. Fractions attributable to blood in the tissue were subtracted and time-dependent apparent distribution volumes of the indicators in the tissue were estimated. The brain level did not rise to more than 1–2% of that in the plasma for inulin and sucrose and to 6–7% for mannitol. The explanation for this could be a combination of restricted penetration from blood into brain and a sink effect of cerebrospinal fluid (csf). To determine the removal of the indicators into csf, ventriculo-cisternal perfusion was performed during the period of tracer uptake in the tissue; the rate of passage of the indicators into perfusion fluid was found to be negligible in comparison to the rates of uptake in the tissue. As diffusion in the extracellular space could limit uptake rates from blood to brain, estimates of diffusion limited half-times were also made. Calculations showed that the major hindrance to indicator uptake in the tissue is located in the wall of the brain capillaries; thus the uptake data permit calculation of brain capillary permeability. The half-times for the distribution of the indicators in their equilibrated tissue distribution spaces were used to estimate brain capillary permeability. By comparison with the aqueous diffusion coefficients of the indicators it is concluded that the substances in the molecular weight range of 182 to 5 500 daltons are subjected to restricted diffusion during the passage of the blood-brain barrier and that in addition non-discriminative transendothelial pathways are available.