Techniques and applications of extracellular space determination in mammalian tissues

Brain physiome: A concept bridging in vitro 3D brain models and in silico models for predicting drug toxicity in the brain

In the last few decades, adverse reactions to pharmaceuticals have been evaluated using 2D in vitro models and animal models. However, with increasing computational power, and as the key drivers of cellular behavior have been identified, in silico models have emerged. These models are time-efficient and cost-effective, but the prediction of adverse reactions to unknown drugs using these models requires relevant experimental input. Accordingly, the physiome concept has emerged to bridge experimental datasets with in silico models. The brain physiome describes the systemic interactions of its components, which are organized into a multilevel hierarchy. Because of the limitations in obtaining experimental data corresponding to each physiome component from 2D in vitro models and animal models, 3D in vitro brain models, including brain organoids and brain-on-a-chip, have been developed. In this review, we present the concept of the brain physiome and its hierarchical organization, including cell- and tissue-level organizations. We also summarize recently developed 3D in vitro brain models and link them with the elements of the brain physiome as a guideline for dataset collection. The connection between in vitro 3D brain models and in silico modeling will lead to the establishment of cost-effective and time-efficient in silico models for the prediction of the safety of unknown drugs.

Diffusion in the extracellular space in brain and tumors

Diffusion of solutes and macromolecules in the extracellular space (ECS) in brain is important for non-synaptic intercellular communication, extracellular ionic buffering, and delivery of drugs and metabolites. Diffusion in tumor ECS is important for delivery of anti-tumor drugs. The ECS in brain comprises ∼20% of brain parenchymal volume and contains cell-cell gaps down to ∼50 nm. We have developed fluorescence methods to quantify solute diffusion in the ECS, allowing measurements deep in solid tissues using microfiberoptics with micron tip size. Diffusion through the tortuous ECS in brain is generally slowed by ∼3-5-fold compared with that in water, with approximately half of the slowing due to tortuous ECS geometry and half due to the mildly viscous extracellular matrix (ECM). Mathematical modeling of slowed diffusion in an ECS with reasonable anatomical accuracy is in good agreement with experiment. In tumor tissue, diffusion of small macromolecules is only mildly slowed (<3-fold slower than in water) in superficial tumor, but is greatly slowed (>10-fold) at a depth of few millimeters as the tumor tissue becomes more compact. Slowing by ECM components such as collagen contribute to the slowed diffusion. Therefore, as found within cells, cellular crowding and highly tortuous transport can produce only minor slowing of diffusion in the ECS.

Phasic oscillations of extracellular potassium (K(o)) in pregnant rat myometrium

K-sensitive microelectrodes were used to measure K(+) within the extracellular space (K(o)) of pregnant rat myometrium. Contractile activity was monitored by measuring either force or bioelectrical signals. Single and double-barreled electrodes were used. Double-barreled electrodes allowed monitoring of electrical activity 15 microns from the site of K(o) measurement. From double-barreled electrode experiments, the bioelectrical burst started first, and then K(o) began to rise 0.6 ± 0.1 seconds later. This delay indicates that K(+) leaves the cells in response to local electrical activity rather than vice versa. Four control experiments were performed to assess the influence of electrical artifacts caused by tissue motion on K(o) values. When observed, artifacts were negative and transient, and hence would result in an underestimation of K(o) rises. Artifacts were minimized when tissue motion was minimized by fixing the tissue at both ends. At 37°C, 7 single barreled experiments and 45 contractions were analyzed. Resting K(o) was within 1 mM of bath K(+) (5 mM) at the beginning and end of the experiments. K(o) rose during the contraction, fell after the completion of the contraction, and normalized before the next contraction began. Peak K(o) values observed during force production were 18.8 ± 5.9 mM, a value high enough to modulate tissue-level electrical activity. K(o) required 15.7 ± 2.8 seconds to normalize halfway (t50). Six experiments expressing 38 contractions were performed at 24°C. The contraction period was longer at 24°C. Values for peak K(o) (26.2 ± 9.9 mM) and t50 (29.8±16.2 sec) were both larger than at 37°C (p<0.0003 for both). The direct relationships between peak K(o), t50 and the contraction period, suggest elevations in K(o) may modulate contraction frequency. The myometrial interstitial space appears to be functionally important, and K(o) metabolism may participate in cell-cell interactions.

Microfiberoptic measurement of extracellular space volume in brain and tumor slices based on fluorescent dye partitioning

The fractional volume occupied by extracellular space in tissues, termed alpha, is an important parameter of tissue architecture that affects cellular functions and drug delivery. We report a technically simple fluorescent dye partitioning method to measure alpha in tissue slices based on microfiberoptic detection of dye fluorescence in tissue versus overlying solution. Microfiberoptic tip geometry and dyes were selected for alpha determination from fluorescence intensity ratios, without the need to correct for illumination profile, light scattering/absorption, or dye binding. The method was validated experimentally using cell-embedded gels of specified alpha-values and optical properties. In mouse brain slices, alpha was strongly location-dependent, ranging from 0.16 in thalamus to 0.22 in brainstem, and was sensitive to cell volume changes. Aquaporin-4 water channel gene deletion caused significant extracellular space expansion, with alpha = 0.181 +/- 0.002 in cortex in wild-type mice and 0.211 +/- 0.003 in Aquaporin-4 knockout mice. In slices of LLC1 cell tumors grown in mice to approximately 5 mm diameter, alpha decreased remarkably from approximately 0.45 in superficial tumor to <0.25 in deeper (>100 mum) tumor. Fluorescent dye partitioning with microfiberoptic detection permits rapid, accurate, and anisotropy-insensitive determination of alpha-values in tissue slices.

Extracellular space volume measured by two-color pulsed dye infusion with microfiberoptic fluorescence photodetection

The extracellular space (ECS) is the aqueous matrix surrounding cells in solid tissues. The only method to measure ECS volume fraction (alpha) in vivo has been tetramethylammonium iontophoresis, a technically challenging method developed more than 25 years ago. We report a simple, quantitative method to measure alpha by microfiberoptic fluorescence detection of a self-quenched green dye, calcein, and a reference red dye, sulforhodamine 101, after pulsed iontophoretic infusion. The idea is that the maximum increase in calcein fluorescence after iontophoresis is proportional to the aqueous volume into which the dye is deposited. We validated the method theoretically, and experimentally, using cell-embedded gels with specified alpha and ECS viscosity. Measurements in living mice gave alpha of 0.20 +/- 0.01 in brain, 0.13 +/- 0.02 in kidney and 0.074 +/- 0.01 in skeletal muscle. The technical simplicity of the "pulsed-infusion microfiberoptic photodetection" method developed here should allow elucidation of the relatively understudied biological roles of the ECS.

Estimation of hepatic distributional volumes using non-labeled reference markers

Hepatic distributional volumes were investigated in the in situ perfused rat liver. Perfusion experiments were conducted using Krebs bicarbonate buffer delivered via the portal vein in single-pass mode at a total flow rate of 15 mL/min. A bolus dose of normal erythrocytes (RBC, vascular marker) and Evans blue (EB, extracellular marker) respectively was administered in the presence and absence of protein. At the end of the experiment, liver total water content was determined by desiccation and freeze-drying methods. Similar moment analysis results and superimposable effluent curves were obtained in the presence (RBC, mean transit time [MTT]: 7.31 +/- 0.45 s and volume of distribution [V]: 0.17 +/- 0.01 mL/g; EB, MTT: 10.9 +/- 0.62 s and V: 0.25 +/- 0.02 mL/g) and in the absence (RBC, MTT: 7.55 +/- 0.84 s and V: 0.18 +/- 0.02 mL/g; EB, MTT: 9.24 +/- 0.77 s and V: 0.20 +/- 0.02 mL/g) of protein, which indicates that the hepatic distribution of RBC and EB within the liver is not influenced by protein. Furthermore, the almost identical results obtained with the desiccation and freeze-drying methods clearly suggest that the freeze-drying method can be used as an alternative to desiccation for the estimation of liver water content.

Marked increase in the histamine content of neointima after stent implantation of pig coronary artery and growth-promoting effects of histamine in cultured smooth muscle cells

After coronary stent implantation, the unfavorable in-stent restenosis often occurs by the formation of neointima due to the proliferation of smooth muscle cells. Platelet-derived growth factor (PDGF) and other peptide growth factors contribute to this process, but little is known about the role of non-peptide factors in this process. In the present study, the role of histamine, a non-peptide factor, in the formation of neointima was investigated using a pig coronary model of in-stent restenosis and a culture system of coronary smooth muscle cells. A Palmaz-Schatz stent was implanted in the left anterior descending coronary artery of male pigs. At 1, 2 and 4 weeks after stenting, the histamine content of neointima was determined to be 326 +/- 82, 1427 +/- 280 and 440 +/- 69 pmol/mg protein, respectively, by HPLC fluorometry. In contrast, the histamine content of arterial media from the untreated control arteries was only 15.3 +/- 1.6 pmol/mg protein. These results demonstrate that the histamine content of neointima is about 20 to 90-fold that of the normal media. In vitro, histamine by itself did not stimulate the proliferation of cultured smooth muscle cells, but potentiated the PDGF-stimulated proliferation of the cultured cells via a mechanism independent of H1 and H2 histamine receptors. Thus, histamine may be an important non-peptide factor in the pathogenesis of in-stent restenosis.

Effects of bilateral nephrectomy and angiotensin II replacement on body fluids in foetal sheep

1. To determine the importance of the kidneys in maintaining the normal volume and composition of foetal body fluids, measurements were made in 11 chronically catheterized foetuses (123-136 days) that had been bilaterally nephrectomized at least 5 days previously and compared with 10 intact foetuses (121-133 days). 2. The nephrectomized foetuses had reduced extracellular (ECV), blood, plasma and interstitial volumes per kg foetal weight (P < 0.005), reduced plasma chloride levels (P < 0.001) and were acidaemic, hypoxaemic and hypercapnaemic (P < 0.05) compared with intact foetuses. They also had reduced lung liquid production (P < 0.05) and reduced lung liquid sodium and osmolality levels (P < 0.05). Their arterial pressure was more variable between foetuses (P < 0.005) and was directly related to ECV/kg (P = 0.013). 3. To determine which of these changes were due to absence of the foetal renin-angiotensin system, seven chronically catheterized nephrectomized foetal sheep (124-132 days) were infused with replacement doses of angiotensin (Ang)II (1.5 micrograms/kg per h) for 3 days. Six nephrectomized foetuses were infused with 0.15 mol/L saline. 4. The AngII infusion was non-pressor. It prevented the fall in ECV that occurred in the control group (P < 0.05) and foetal plasma chloride concentration rose (P < 0.05). Blood gas status and lung liquid production rate did not change, but lung liquid sodium concentration fell (P < 0.05) and potassium concentration rose (P < 0.05). 5. Bolus injections of AngII (0.3-5 micrograms) were given to assess vascular sensitivity to AngII. This was not altered by either nephrectomy or AngII replacement. 6. It is concluded that the foetal kidneys are important for the maintenance of the normal volume and composition of foetal body fluids. Angiotensin II, perhaps because it promotes fluid transfer across the placenta, helps maintain foetal ECV and plasma chloride levels.

Relationship of elevated 23Na magnetic resonance image intensity to infarct size after acute reperfused myocardial infarction

BACKGROUND

Elevated 23Na MR image intensity after acute myocardial infarction has previously been shown to correspond to high tissue [Na+] and loss of myocardial viability. In this study, we explored the potential of in vivo 23Na MRI to assess infarct size and investigated possible mechanisms for elevated 23Na image intensity.

METHODS AND RESULTS

Thirteen dogs and 8 rabbits underwent in situ coronary artery occlusion and reperfusion and were imaged by 23Na MRI. For anatomically matched left ventricular short-axis cross sections (n=46), infarct size measured by in vivo 23Na MRI correlated well with triphenyltetrazolium chloride staining (r=0.87, y=0.92x+3.37, P<0.001). Elevated 23Na image intensity was observed in infarcted myocardium (206+/-37% of remote in dogs, P<0.001; 215+/-58% in rabbits, P<0.002) but was not observed after severe but reversible ischemic injury (101+/-11% of baseline, P=NS). High-resolution ex vivo imaging revealed that regions of elevated 23Na image intensity appeared to be identical to those of infarcted regions (r=0.97, y=0.92x+1.52, P<0.001). In infarcted regions, total tissue [Na+] was elevated (89+/-12 versus 37+/-9 mmol/L in control tissue, 156+/-60% increase, P<0.001) and was associated with increased intracellular sodium (254+/-68% of control, P<0.005) and an increased intracellular sodium/potassium ratio (868+/-512% of control, P<0.002). Morphometric analysis demonstrated only a minor increase in extracellular volume (17+/-8% versus 14+/-5%, P<0.05) in the infarcted territory.

CONCLUSIONS

Elevated 23Na MR image intensity in vivo measures infarct size after reperfused infarction in both a large and a small animal model. The mechanism of elevated 23Na image intensity is probably intracellular sodium accumulation secondary to loss of myocyte ionic homeostasis.

Analysis of the paired-tracer method of determining cell uptake

The paired-tracer method has been used extensively for determining cell uptake of numerous substances, although the method of calculating uptake has no published theoretical support. We have investigated the effect of capillary permeability of the tracers on v, the uptake rate calculated directly from the ratio of tracer venous concentrations. For a simple mathematical model of plasma-tissue movement of lactate and an analytic expression for v, it has been shown that values of v calculated in the first moments after tracer injection depend almost entirely on the differences in tracer permeability-surface area product (PS). The model indicates v would never give the correct value of cell uptake. It is also shown that PS differences alone can explain the published values for lactate uptake obtained from v in skeletal muscle of the rat and dog.

Effects of urea on taurine efflux and cell volume in incubated rat cerebral cortical minislices

The effects of urea on the rate of efflux of preloaded taurine and volume regulation have been examined in incubated minislices from rat superficial cerebral cortex. As external urea was increased in the range 0-100 mmol/L, there was a concentration-dependent slowing of cellular taurine efflux. Cell volumes progressively increased over the range 0-50 mmol/L urea, but decreased slightly in 100 mmol/L. Urea had no effect on cell volume in the absence of taurine. Retardation of efflux, and cell swelling in the presence of 50 mmol/L urea were entirely abolished by trimethylamine (100 mumol/L). TMA had no effect on either variable in the absence of urea. It is suggested that impaired loss of taurine and accompanying cell swelling may be factors contributing to the neurological disturbances accompanying uremia.

Volume regulation and the efflux of amino acids from cells in incubated slices of rat cerebral cortex. I. Characteristics of transport mechanisms

The characteristics of amino acid efflux from pre-loaded cells in incubated slices of rat cerebral cortex have been investigated under basal conditions (isosmotic media, 315 mosmol/kg) and following mild hyposmotic shock (265 mOsmol/kg). Rates of efflux have been correlated with the extent of cell swelling in hyposmotic media. Hyposmolality accelerated the slow phase of cellular efflux of L-aspartate (+ 29%), gamma-aminoisobutyric acid (GABA) (+ 38%), L-glutamate (+ 28%) and glycine (+ 26%). The anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-sulfonic acid (DIDS, 25 or 100 microM) as well as trifluoperazine (TFP, 25 microM), an inhibitor of calmodulin activation, both retarded efflux in hyposmotic media, with associated cell swelling (increase in slice non-inulin space). The effects of DIDS and TFP were not additive. N-Ethylmaleimide (NEM, 100 microM) significantly retarded the efflux of neutral amino acids, with cell swelling: these effects were less pronounced in cells loaded with acidic amino acids. It is concluded that the hyposmotically-activated efflux of carboxylic amino acids, and associated cell swelling limitation, requires calmodulin activation and the presence of free sulfydryl groups.

Extracellular volume and blood volume in chronically catheterized fetal sheep

1. To determine the extracellular volume (ECV) in fetal sheep and its distribution between the plasma and interstitial spaces, ECV, blood volume (BV) and haematocrit were measured in ten chronically catheterized fetal sheep aged 121-133 days. Relationships with age, weight and other fetal variables, including glomerular filtration rate (GFR), were studied. 2. ECV was measured as the mean of the volumes of distribution of [3H]inulin and [14C]mannitol extrapolated to time zero. The time zero volume of distribution was 1506 +/- 79 ml (means +/- S.E.M.) for inulin and 1590 +/- 80 ml for mannitol. The ECV was 1548 +/- 79 ml (632 +/- 18 ml (kg fetal wt)-1). BV, measured using 51Cr-labelled red cells, was 351 +/- 27 ml (141 +/- 6 ml kg-1). Haematocrit, plasma volume and interstitial volume were 34 +/- 1%, 229 +/- 17 ml (92 +/- 3 ml kg-1) and 1319 +/- 63 ml (540 +/- 17 ml kg-1), respectively. 3. Interstitial volume per kilogram fell with increasing fetal weight (P = 0.026). BV per kilogram did not change with weight or age. 4. The plasma: interstitial volume ratio was 0.17 +/- 0.01. This ratio increased as fetal weight and age increased (P = 0.026 and P = 0.044), that is, the proportion of ECV that was contained outside the vascular compartment was lower in heavier or older fetuses. 5. Since GFR was 3.4 +/- 0.4 ml min-1, the entire fetal ECV was filtered by the fetal kidney only 3.1 +/- 0.3 times per day.

Taurine efflux and the regulation of cell volume in incubated slices of rat cerebral cortex

The kinetics of efflux [3H]taurine have been examined in pre-loaded slices of rat cerebral cortex incubated in media of variable osmolality. Alterations in the rate of the slowest phase of efflux, considered to represent cellular loss, have been correlated with cell volumes, provisionally identified as the slice non-inulin space. Efflux was stimulated by reduction in medium osmolality, and impaired in hyperosmotic media; these variations were accompanied by moderate (non-osmometric) cell swelling and shrinkage, respectively. The rates of taurine efflux into media in which NaCl was partly replaced by sucrose, and measurement of the corresponding cell volumes, suggest that ionic strength, rather than osmolality or cell volume per se, may be a significant controlling factor. In both isosmolal and hyposmolal media efflux was significantly impaired by the anion transport inhibitor niflumic acid, with accompanying cell swelling, or by replacement of chloride by gluconate. In hyposmotic, but not isosmotic, media efflux was impaired, and cell volumes increased, in the presence of trifluoperazine or TMB-8, a reported blocker of intracellular calcium release, and the effects of niflumic acid and trifluoperazine on both variables were strongly additive. It is suggested that in both isosmotic and hyposmotic media taurine, efflux occurs by anionic transport, mainly through exchange with external chloride, whereas in hyposmotic media a second pathway is present, probably a volume-activated calmodulin-dependent channel.

Transintestinal elimination of ciprofloxacin in humans--concomitant assessment of its metabolites in serum, ileum and colon

Ciprofloxacin (200 mg) was infused to seven patients at the beginning of elective colorectal surgery. Thirty minutes after the end of infusion (i.e. 60 min after the start of the operation) ciprofloxacin reached concentrations of 1.60 mg/l in serum and of 3.42-6.07 mg/kg fresh weight in the ileum and colon. During the next 30 min (90 min after the start of operation) the concentration of ciprofloxacin in serum decreased to 86% of its initial level, but this decrease was less rapid than that observed in the ileal (to 56.8%) or colonic (to 74.8%) mucosa. Three metabolites could be identified (desethylen-, sulpho-, oxociprofloxacin). Initially, at 60 min the amount of these metabolites was about 15% of the total drug concentration in serum, but only 2-3% of that in the gut tissues. At 90 min the relative amount of metabolites was increased in serum as well as in the gut tissues. It is concluded that transintestinal elimination of ciprofloxacin is a general feature of the whole gut. Obviously, the elimination process is not due to degradation of ciprofloxacin within the gut wall.

Pharmacokinetic analysis of drug absorption from muscle based on a physiological diffusion model: effect of molecular size on absorption

In a rabbit hind leg perfusion experiment, the absorption of radiolabeled water and carbohydrates of various molecular sizes from muscle was analyzed using a physiological diffusion model and, also, by statistical moment analysis. The model takes into account diffusion in the interstitial space, transcapillary movement, and removal by the blood circulation and pharmacokinetic parameters representing these processes were computed by curve-fitting. The apparent diffusion coefficients of water and small sugars in the interstitial space (Dm) were proportional to their free diffusion coefficients in water (Df), whereas the diffusion of 14C-inulin was hampered by interstitial structures. The first moments of each absorption process were also determined to assess the quantitative contribution of each absorption process to overall absorption. For carbohydrate molecules, residence time in the depot (td) accounted for most of the absorption time after injection, whereas for 3H-water, residence times in muscle (tm) and in the depot (td) were similar.

A new method for assessment of drug absorption from muscle: application of a local perfusion system

Drug absorption from muscle has been examined using the rabbit hind leg, isolated by cannulating the femoral artery and vein, and perfused with or without bovine serum albumin (BSA) using a single-pass technique. A model compound, [14C]sucrose, was injected into the musculus gastrocnemius, and its appearance in the venous outflow was monitored and kinetically analysed by statistical moment theory. The fraction absorbed and mean arrival time in the venous outflow were calculated to be 103% and 50.5 min, respectively, in the BSA-containing system. The results of perfusion experiments without BSA were not significantly different, suggesting possible use of the simpler BSA-free perfusion system to obtain information on the muscular absorption of drugs.

Effects of prostaglandin F2 alpha and thromboxane A2 analogue on bovine cerebral arterial tone and calcium fluxes

We determined sources of activator calcium for prostanoid-induced cerebrovascular constriction by measuring isometric tension and calcium-45 (45Ca) fluxes in bovine middle cerebral arteries. Constriction induced by prostaglandin F2 alpha or the stable thromboxane A2 analogue SQ-26,655 was near-maximally inhibited in calcium-deficient solutions but only partially inhibited by calcium antagonists (10(-5) M verapamil or 3.3 x 10(-7) M nifedipine). Studies of 45Ca binding at different external Ca2+ concentrations showed that cerebral arteries possess two calcium binding sites, a high-affinity site and a low-affinity site. Each prostanoid significantly increased low-affinity 45Ca uptake (external Ca2+ concentration = 1.2 mmol/l) during 5 minutes of 45Ca loading; for prostaglandin F2 alpha 45Ca uptake increased from 69 to 108 nmol/g and for SQ-26,655, from 78 to 141 nmol/g. The prostanoid-induced increases in low-affinity 45Ca uptake were completely abolished by pretreatment with verapamil or nifedipine. Prostaglandin F2 alpha, SQ-26,655, verapamil, and nifedipine had no effect on high-affinity 45Ca uptake (external Ca2+ concentration = 45 mumol/l) or 45Ca efflux (after 60 minutes' preincubation in calcium-deficient media). Prostaglandin F2 alpha and SQ-26,655 each appear to constrict cerebral arteries by two mechanisms: first, by promoting calcium uptake from low-affinity binding sites through receptor-operated channels sensitive to the calcium antagonists, and second, by releasing calcium from depletable internal stores.

Plasma reciprocal pool specific activity predicts that of intracellular free leucine for protein synthesis

We previously proposed that, during the infusion of either labeled leucine or its alpha-ketoacid, alpha-ketoisocaproate (KIC), the plasma specific activity (SA) of the transaminated product of the infused tracer ("reciprocal pool SA") may better reflect the intracellular leucine SA than the plasma SA of either infused tracer ("primary pool SA"). To test this hypothesis, 14 dogs were simultaneously infused intravenously with [3H]leucine and [14C]KIC, and blood and tissue compartments were sampled. The ratios of [3H]-leucine to [14C]leucine [( 3H]/[14C]leucine) in mixed tissue proteins and in the intracellular space of striated muscle were the same as the ratio of the isotope infusion rates and similar, although slightly lower (P less than 0.01), than [3H]KIC/[14C]leucine SA (ratio of reciprocal pool SA) in plasma. Plasma [3H]KIC/[14C]leucine SA were essentially identical to the [3H]/[14C] of leucine in 1) mixed liver proteins, 2) intrahepatic free leucine, and 3) fibrin. The [3H]/[14C]leucine in mixed renal proteins and in the intracellular space of kidney and erythrocytes were similar to those of the venous plasma [3H]/[14C]leucine SA. The plasma [3H]KIC and [14C]leucine SA (the reciprocal pool SA) were similar to the SA of [3H]- and [14C]leucine in the intracellular space of all organs investigated with the exception of kidney. Therefore, in postabsorptive dogs, the plasma SA of the transaminated product of the infused labeled KIC or leucine is an excellent predictor of the intracellular leucine SA in all tissues investigated with the exception of kidney.

Kinetics and distribution volumes for tracers of different sizes in the brain plasma space

Regional brain and plasma concentrations were determined for a series of radiotracers that differ in molecular weight and size in pentobarbital-anesthetized rats at 1, 5 and 30 min after i.v. injection. The tracers, [3H]inulin (mol. wt. 5000 Da, radius 1.5 nm), 5 [3H]dextrans (10,000-200,000 Da, 2.3-9.5 nm) and [51Cr]transferrin (79,000 Da, 3.8 nm), are not taken up into erythrocytes and do not measurably cross the blood-brain barrier in 30 min. Results were expressed as a brain distribution volume, defined as (dpm/g brain)/(dpm/ml plasma). Within 1 min after injection, all tracers attained an initial distribution volume which varied regionally from 0.4 to 1.6 X 10(-2) ml/g. The volumes remained constant between 1 and 30 min for tracers with radii greater than or equal to 3.8 nm, whereas the volumes increased up to 90% for tracers with radii less than or equal to 3.1 nm. Rates of equilibration for tracers with radii less than or equal to 3.1 nm were size dependent with smaller tracers equilibrating before larger tracers. These results indicate that the brain distribution volume for plasma tracers consists of two compartments: one which is quickly filled (less than or equal to 1 min) by all tracers and comprises approximately 60% of the total volume, and one which allows only tracers with radii less than or equal to 3.1 nm and comprises 40% of the total volume. The inverse relation between the rate of equilibration in the second compartment and molecular size may indicate a diffusion limitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular volume in the bovine corneal epithelium

Three different substances, [14C]inulin, [14C]polyethylenglycol (PEG, molecular weight 4000) and [3H]sucrose have been examined for suitability as markers for determination of the extracellular volume (ECV) in the bovine corneal epithelium and stroma. In both tissues, sucrose was the best marker reaching equilibrium of distribution within 90 min. Inulin and 4000 PEG have been found to be unacceptable because of a slow rate of penetration of the ECV. In addition, both inulin and 4000 PEG gave considerably lower values for the ECV than sucrose. Using the value for ECV determined with sucrose as a marker in the bovine corneal epithelium, i.e. 21.6 +/- 1.4% of tissue water, and the results obtained previously by the chemical analysis of this tissue (Midelfart 1987) the intracellular concentrations of 34.4 +/- 8.6 mmol Na/kg H2O and 154.3 +/- 11.1 mmol K/kg H2O were determined. These results were compared with the results obtained by other methods in corneae from other species.

The volume and ionic composition of cells in incubated slices of rat renal cortex, medulla and papilla

The apparent extracellular space in incubated slices of rat renal cortex, medulla and papilla has been measured using three differently sized marker molecules, mannitol, sucrose and inulin. Cellular volumes have been estimated by following the efflux of 3-O-methyl-D-glucose from equilibrated slices. Sucrose appears to be the most accurate extracellular marker in each of the regions examined, in that the sum of its volume of distribution plus cellular volume approximates most closely to the total slice fluid volume. Inulin has the same volume of distribution as sucrose in cortical slices, but under-penetrates medullary and papillary tissue. Mannitol overestimates the extracellular space in all three regions, although its larger volume of distribution, relative to that of sucrose, was not statistically significant in papillary slices. When cell volume and composition are estimated (a) using sucrose as extracellular marker and (b) making appropriate allowance for the presence of bound tissue electrolytes, it is found that cells in each region have low Na+ and high K+ concentrations and contents. When papillary slices are incubated in medium of very high osmolality (NaCl plus urea, 2000 mosmol/kg H2O) there is a moderate (approx. 23%) decrease in cell volume and an increase in cell fluid Na+ and Cl- concentrations equal to approx. 50% of the increase in the extracellular concentrations. Cell K+ concentrations remain unchanged. The results show that cells in renal slices are able to maintain high K+-to-Na+ ratios when incubated in isosmotic (cortex) or moderately hyperosmotic media (medulla and papilla), and suggest that regulation of papillary cell volume following hyperosmotic shock can only partly be ascribed to uptake of extracellular electrolytes.

Effects of calcium antagonists on isolated bovine cerebral arteries: inhibition of constriction and calcium-45 uptake induced by potassium or serotonin

The purpose of this study was to determine the mechanisms by which organic calcium channel blockers inhibit cerebral vasoconstriction. Isolated bovine middle cerebral arteries were cut into rings to measure contractility or into strips to measure radioactive calcium (45Ca) influx and efflux. Calcium channel blockers (10(-5) M verapamil or 3.3 X 10(-7) M nifedipine) and calcium-deficient solutions all produced near-maximal inhibition of both potassium- and serotonin-induced constriction. In calcium-deficient solutions containing potassium or serotonin, verapamil and nifedipine each blocked subsequent calcium-induced constriction in a competitive manner. Potassium and serotonin significantly increased 45Ca uptake into cerebral artery strips during 5 minutes of 45Ca loading; for potassium 45Ca uptake increased from 62 to 188 nmol/g, and for serotonin from 65 to 102 nmol/g. Verapamil or nifedipine had no effect on basal 45Ca uptake but significantly blocked the increase in 45Ca uptake induced by potassium or serotonin. Potassium, and to a lesser extent serotonin, each induced a brief increase in the rate of 45Ca efflux into calcium-deficient solutions. Verapamil or nifedipine had no effect on basal or potassium-stimulated 45Ca efflux. The results demonstrate that verapamil and nifedipine block 45Ca uptake through both potential-operated (potassium) and receptor-operated (serotonin) channels in bovine middle cerebral arteries.

Protein synthesis and degradation in biopsies of rat skeletal muscle

The use of 20- to 40-mg biopsies of rat skeletal muscle to measure protein synthesis and degradation rates in vitro was investigated and compared to that of the intact extensor digitorum longus (EDL) and soleus muscles. During incubations in oxygenated Krebs-Ringer bicarbonate buffer with glucose, insulin, 23 amino acids at 10 times rat plasma levels, and [14C]tyrosine, the specific activity of intracellular tyrosine approximated that of the incubation medium and was constant in the biopsy, the EDL, and the soleus. The rate of incorporation of tyrosine into the protein of the biopsy was constant for 3 hr and was 39 and 32% of the rates of the EDL and soleus, respectively. The rate of release of tyrosine from protein in the biopsy during incubations in buffer with glucose and cycloheximide was constant for 3 hr and was intermediate between the rates of the EDL and soleus. The effects of starvation on the in vitro protein metabolism of the biopsy were the same as on the intact muscles. The 42% decrease in synthesis and the 53% increase in degradation in the biopsy were intermediate between the changes measured in the EDL and soleus muscles. The ability of this technique to identify proportional changes in the in vitro protein synthesis and degradation rates makes this a valid technique suitable for the measurement of changes of in vitro protein metabolism using serial biopsies from larger animals, including man.

Uptake, transfer and metabolism of prostaglandin E2 in the isolated perfused human placental cotyledon

(3H) PGE2 uptake and transfer in the isolated perfused human placental cotyledon was assessed by a single pass paired isotope dilution technique utilising (14C) sucrose as an extracellular marker. Metabolism of (3H) PGE2 was measured by analysing maternal and fetal effluents from perfused human placental cotyledons after bolus injection of (3H) PGE2 into either the maternal or fetal sides. Maximal uptake of (3H) PGE2 was greater on the maternal (81 +/- 8%) than the fetal sides (42 +/- 12%) and showed saturation with increasing concentrations of PGE2 only on the fetal side with an apparent Km of 12 +/- 4.9 nmol/l and vmax of 1.5 +/- 0.2 pmol/min/g. Total recoveries of (3H) PGE2 were 84.6 +/- 11.8% and 32.6 +/- 6.3% of the injected dose after injection on the fetal and maternal sides respectively. Transfer of (3H) PGE2 was the same in both directions being 6.4 +/- 1.2% of the injected dose in the fetal-maternal direction and 5.8 +/- 2.7% of the injected dose in the maternal-fetal direction. Metabolism was greater on the maternal side (35% of injected (3H) PGE2) than the fetal side (18% of injected (3H) PGE2) and was principally to the 13,14-dihydro-15-keto-PGE2 metabolite. Metabolism of (3H) PGE2 after passage across the placenta was the same in both directions and was of the order of approximately 60%.

A new method for assessment of drug disposition in muscle: application of statistical moment theory to local perfusion systems

A new experimental system is used to determine exact information concerning local drug disposition. Rabbit hind leg is perfused in situ using a single-pass technique, and outflow curves of drugs are analyzed using statistical moment theory. By the introduction of chromatographic concepts and the application of the well-stirred model to the local perfusion system, physiologically and/or physicochemically meaningful parameters are derived from the first three moments. Moreover, in the assessment, drug disposition is divided into elimination and distribution. The elimination process is also evaluated with respect to rate and extent. This system is used to elucidate the disposition characteristics of mitomycin C and its lipophilic derivative nonyloxycarbonyl mitomycin C.

Intracellular pH in hibernation and respiratory acidosis in the European hamster

Intracellular pH was determined (DMO method) in European hamsters, in the spontaneously-occurring respiratory acidosis of hibernation, in hypercapnia due to breathing 12% CO2 in air in euthermy in spring, and in euthermicnormocapnic controls. From euthermy to hibernation, the temperature coefficient of pH was lowest in blood plasma and brain, intermediate in striated muscles (thigh muscles and diaphragm), and highest in heart and liver (Fig. 1). Correspondingly, the estimated dissociation ratio of the protein imidazole buffer groups, alpha Im, decreased markedly in plasma and brain, denoting an acid titration, but varied little in liver and heart. Striated muscles were intermediate (Fig. 2). Like in other mammals, intracellular responses to short-term euthermic respiratory acidosis were characterized by a partial metabolic compensation in the brain and a small metabolic acidification in striated muscles. In hibernation, a powerful metabolic compensation took place in liver and heart, nearly restoring alpha Im, but none occurred in brain (Figs. 3 to 5). The existence of an intracellular acidosis in brain and striated muscles during hibernation is in keeping with an inhibitory role of acidosis, whereas the homeostasis of intracellular alpha Im in liver and heart would subserve the eurythermal functioning of metabolic regulations in these organs, like in most organs of ectotherms.

Solute-excluded volumes near the Novikoff cell surface

A differential centrifugation technique, in which all extracellular water except that intimately associated with the cell (pericellular domain) is removed, has been applied to isolated Novikoff hepatoma cells. The pericellular volumes accessible to albumin, inulin, raffinose, and sucrose were inversely related to the molecular weights of the test solutes. This phenomenon was not detectable in erythrocytes or in fat cells. Selective removal of cell surface components by enzymatic treatment produced proportional changes in the relative volumes of distribution accessible to the solutes. This discrimination in the volume accessible to each of the solutes is analogous to that obtained in gel chromatographic separation and represents, in effect, excluded volumes which are inversely related to solute size. This exclusion is associated with components of the Novikoff cell surface, including the surface coat and the microvilli that cover the Novikoff cell. These structures provide an additional level of discrimination for the Novikoff cell not seen in certain other cell types.

Asymmetric calcium influx and efflux at maternal and fetal sides of the guinea-pig placenta: kinetics and specificity

Unidirectional influx of calcium across maternal and fetal sides of the syncytiotrophoblast was investigated in the guinea-pig placenta by using a rapid (less than 30 s) paired-tracer dilution technique. Experiments were performed in an in situ placenta artificially perfused through the umbilical vessels or in an isolated placenta in which both the maternal and fetal circulations were perfused. At equimolar Ca2+ concentrations, unidirectional calcium influx was always significantly lower on the maternal side than on the fetal side. Saturation kinetics were observed: on the fetal side the estimated Km was 1.8 +/- 0.7 mM and Vmax was 1.66 +/- 0.28 mumol/min X g (mean +/- S.E. of mean) and on the maternal side Km ranged from 0.18 to 1.15 mM and Vmax ranged from 0.12 to 0.59 mumol/min X g. When the inhibition of calcium influx was investigated on the fetal side of the trophoblast by using competing cations, the following sequence was observed: Ba2+ greater than Ca2+ congruent to Ni2+ greater than Sr2+ greater than Mg2+ congruent to Li+. Efflux of 45Ca2+ from the trophoblast into the ipsilateral circulation (backflux) was rapid (20-100% in 6 min) and asymmetric since the fetal:maternal ratio was 1.35 +/- 0.11 (mean +/- S.E. of mean) in the presence of 0.1 mM-Ca2+. In the dually perfused placenta, transplacental transfer (6 min) of 45Ca2+ varied over a wide range (0-80%); however, it was similar to that of the extracellular reference tracer, 22Na+, in either maternal-to-fetal or fetal-to-maternal directions. It is suggested that this is a consequence of the 'leakiness' of the dually perfused placenta since the transplacental transfer of 22Na+ and D-[3H]mannitol (or L-[14C]glucose) measured simultaneously were also variable but similar. Transplacental transfer of 45Ca2+ could not be used to characterize specific calcium-transport mechanisms, whereas highly sensitive trophoblast uptake measurements were provided by the single-circulation, paired-tracer technique. Our findings suggest the presence of a specific carrier-mediated transport system for calcium on both maternal and fetal surfaces of the trophoblast. The asymmetries in unidirectional influx into the trophoblast and rapid backflux indicate a mechanism by which the net transfer of calcium from the maternal to the fetal circulation is maintained in favour of the fetus.

Some effects of monovalent anion replacement on the volume and composition of cells in incubated slices of rat renal cortex

Steady-state cellular water content and cation content and concentration have been studied in slices of rat renal cortex incubated in media in which Cl- was replaced by various monovalent anions at pH 7.35. Anions derived from low molecular weight aliphatic acids caused net uptake of cell water and K+. In the presence of larger anions cell water content fell in a manner related to anionic equivalent weight, but water content in media containing anions from weak aliphatic or alicyclic acids was slightly but consistently higher than that in media containing anions from strong acids. Cell K+ content did not decrease in these media. Aromatic anions caused enhanced cell water loss. When external pH adjusted to 6.8 or 7.8 it was found that in media containing anions from weak acids, but not from strong acids, increasing pH was associated with significantly decreased cell water content.

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.

Application of an automatic electronic image analyzer to the measurement of myocardial extracellular space

The volume of the rat myocardial extracellular compartment was determined by electronic image analysis (Zeiss Micro-Videomat) and compared with values obtained by point-counting on the same histological micrographs. The data suggest that the results obtained by the two methods may be expected to be approximately within 10% of each other. The subjective component of the morphometry was not entirely eliminated due to some heterogeneity in the staining of the histological sections. Until the deficiencies in staining homogeneity are eliminated, the main advantage of the image analysis compared to point-counting will be limited to a greater labor efficiency.