Permeability of the choroid plexus of the rabbit to several solutes

Dual function of the choroid plexus: Cerebrospinal fluid production and control of brain ion homeostasis

The choroid plexus is a small monolayered epithelium located in the brain ventricles and serves to secrete the cerebrospinal fluid (CSF) that envelops the brain and fills the central ventricles. The CSF secretion is sustained with a concerted effort of a range of membrane transporters located in a polarized fashion in this tissue. Prominent amongst these are the Na+/K+-ATPase, the Na+,K+,2Cl- cotransporter (NKCC1), and several HCO3- transporters, which together support the net transepithelial transport of the major electrolytes, Na+ and Cl-, and thus drive the CSF secretion. The choroid plexus, in addition, serves an important role in keeping the CSF K+ concentration at a level compatible with normal brain function. The choroid plexus Na+/K+-ATPase represents a key factor in the barrier-mediated control of the CSF K+ homeostasis, as it increases its K+ uptake activity when faced with elevated extracellular K+ ([K+]o). In certain developmental or pathological conditions, the NKCC1 may revert its net transport direction to contribute to CSF K+ homeostasis. The choroid plexus ion transport machinery thus serves dual, yet interconnected, functions with its contribution to electrolyte and fluid secretion in combination with its control of brain K+ levels.

Mechanisms of fluid movement into, through and out of the brain: evaluation of the evidence

Interstitial fluid (ISF) surrounds the parenchymal cells of the brain and spinal cord while cerebrospinal fluid (CSF) fills the larger spaces within and around the CNS. Regulation of the composition and volume of these fluids is important for effective functioning of brain cells and is achieved by barriers that prevent free exchange between CNS and blood and by mechanisms that secrete fluid of controlled composition into the brain and distribute and reabsorb it. Structures associated with this regular fluid turnover include the choroid plexuses, brain capillaries comprising the blood-brain barrier, arachnoid villi and perineural spaces penetrating the cribriform plate. ISF flow, estimated from rates of removal of markers from the brain, has been thought to reflect rates of fluid secretion across the blood-brain barrier, although this has been questioned because measurements were made under barbiturate anaesthesia possibly affecting secretion and flow and because CSF influx to the parenchyma via perivascular routes may deliver fluid independently of blood-brain barrier secretion. Fluid secretion at the blood-brain barrier is provided by specific transporters that generate solute fluxes so creating osmotic gradients that force water to follow. Any flow due to hydrostatic pressures driving water across the barrier soon ceases unless accompanied by solute transport because water movements modify solute concentrations. CSF is thought to be derived primarily from secretion by the choroid plexuses. Flow rates measured using phase contrast magnetic resonance imaging reveal CSF movements to be more rapid and variable than previously supposed, even implying that under some circumstances net flow through the cerebral aqueduct may be reversed with net flow into the third and lateral ventricles. Such reversed flow requires there to be alternative sites for both generation and removal of CSF. Fluorescent tracer analysis has shown that fluid flow can occur from CSF into parenchyma along periarterial spaces. Whether this represents net fluid flow and whether there is subsequent flow through the interstitium and net flow out of the cortex via perivenous routes, described as glymphatic circulation, remains to be established. Modern techniques have revealed complex fluid movements within the brain. This review provides a critical evaluation of the data.

The blood-cerebrospinal fluid barrier: structure and functional significance

The choroid plexus (CP) of the blood-CSF barrier (BCSFB) displays fundamentally different properties than blood-brain barrier (BBB). With brisk blood flow (10 × brain) and highly permeable capillaries, the human CP provides the CNS with a high turnover rate of fluid (∼400,000 μL/day) containing micronutrients, peptides, and hormones for neuronal networks. Renal-like basement membranes in microvessel walls and underneath the epithelium filter large proteins such as ferritin and immunoglobulins. Type IV collagen (α3, α4, and α5) in the subepithelial basement membrane confers kidney-like permselectivity. As in the glomerulus, so also in CP, the basolateral membrane utrophin A and colocalized dystrophin impart structural stability, transmembrane signaling, and ion/water homeostasis. Extensive infoldings of the plasma-facing basal labyrinth together with lush microvilli at the CSF-facing membrane afford surface area, as great as that at BBB, for epithelial solute and water exchange. CSF formation occurs by basolateral carrier-mediated uptake of Na+, Cl-, and HCO3-, followed by apical release via ion channel conductance and osmotic flow of water through AQP1 channels. Transcellular epithelial active transport and secretion are energized and channeled via a highly dense organelle network of mitochondria, endoplasmic reticulum, and Golgi; bleb formation occurs at the CSF surface. Claudin-2 in tight junctions helps to modulate the lower electrical resistance and greater permeability in CP than at BBB. Still, ratio analyses of influx coefficients (Kin) for radiolabeled solutes indicate that paracellular diffusion of small nonelectrolytes (e.g., urea and mannitol) through tight junctions is restricted; molecular sieving is proportional to solute size. Protein/peptide movement across BCSFB is greatly limited, occurring by paracellular leaks through incomplete tight junctions and low-capacity transcellular pinocytosis/exocytosis. Steady-state concentration ratios, CSF/plasma, ranging from 0.003 for IgG to 0.80 for urea, provide insight on plasma solute penetrability, barrier permeability, and CSF sink action to clear substances from CNS.

[The choroid plexuses: a dynamic interface between the blood and the cerebrospinal fluid]

The choroid plexuses form one of the interfaces that control the brain microenvironment by regulating the exchanges between the blood and the central nervous system. They appear early during brain development. Originating from four different areas of the neural tube, they protrude into the ventricular system of the brain. The choroidal mechanisms involved in the control of brain homeostasis include the structural properties of the epithelial cells that restrict diffusional processes, as well as specific exchange and secretion mechanisms. In addition to the anatomical and histological organization of the choroidal tissue, this review describes the mechanism of cerebrospinal fluid secretion which is the most studied function of the choroid plexus. Experimental evidence for an implication of the choroid plexuses in neuroprotective mechanisms and in the supply of biologically active polypeptides to the brain are also reviewed.

Choroid plexus controls brain availability of anti-HIV nucleoside analogs via pharmacologically inhibitable organic anion transporters

OBJECTIVE

In AIDS, early suppression of the viral load in the central nervous system is critical for the efficacy of antiretroviral therapy, in order to prevent the emergence of a reservoir of resistant strains of virus, and brain impairment in late stages of the infection. The blood-cerebrospinal fluid (CSF) interface (i.e. the choroidal epithelium) constitutes the most direct route to reach the ventricular meningeal and perivascular infected macrophages, and may modulate the cerebral biodisposition of antiretroviral drugs through various transport systems. Our aim was to address nucleoside drug transfer specifically across the blood-CSF interface, and identify the possible mechanisms involved in their transport.

METHODS

Drug influx and efflux were measured using an in vitro cellular model that reproduces the barrier and transport properties of the blood-CSF interface in vivo. Transport mechanisms were investigated by competition studies.

RESULTS

The CSF influx rate of zidovudine was the highest, although moderate, followed by that of stavudine. The permeability coefficients of the other drugs tested were low. Zidovudine influx into the CSF is independent of thymidine transport systems, and more importantly is limited by an efflux mechanism. This efflux involves an apical (CSF-facing) carrier belonging to the solute carrier (Slc) 22 family of organic anion transporters, and can be inhibited by a therapeutic concentration of benzbromarone.

CONCLUSIONS

The demonstration and characterization of this efflux mechanism is the basis for the development of specific inhibitory agents in view to increase the delivery of antiretroviral nucleoside analogs to the brain.

The transport of the anti-HIV drug, 2',3'-didehydro-3'-deoxythymidine (D4T), across the blood-brain and blood-cerebrospinal fluid barriers

1. The brain is a site of infection, viral replication and sanctuary for HIV-1. The treatment of HIV-1 infection therefore requires that an effective agent be delivered to the brain. 2',3'-Didehydro-3'-deoxythymidine (D4T) is a nucleoside analogue which has been shown to have beneficial clinical effects in the treatment of HIV infection. However, although D4T has been detected in human CSF, the ability of this drug to cross both the blood-brain and blood-cerebrospinal fluid (CSF) barriers and gain entrance into the brain tissue is not known. 2. This study examined the CNS entry of D4T by means of the bilateral vascular brain perfusion technique in the anaesthetized guinea-pig. 3. The results indicated that [3H]-D4T had a limited ability to cross the blood-brain barrier (BBB), which was not significantly greater than D-[14C]-mannitol (a slowly penetrating marker molecule). Although D4T was found to cross the blood-CSF barrier, the presence of D4T in the CSF did not reflect levels of the drug in the brain tissue. 4. These results can be related to the measured low lipophilicity of D4T, the higher paracellular permeability characteristics of the choroid plexus (blood-CSF barrier) compared to the BBB, and the sink action nature of the CSF to the brain tissue. 5. In conclusion, these animal studies suggest that D4T may only penetrate the brain tissue to a limited extent and consideration should be given to these findings in the clinical situation.

Evidence for entry of plasma insulin into cerebrospinal fluid through an intermediate compartment in dogs. Quantitative aspects and implications for transport

To study the route by which plasma insulin enters cerebrospinal fluid (CSF), the kinetics of uptake from plasma into cisternal CSF of both insulin and [14C]inulin were analyzed during intravenous infusion in anesthetized dogs. Four different mathematical models were used: three based on a two-compartment system (transport directly across the blood-CSF barrier by nonsaturable, saturable, or a combination of both mechanisms) and a fourth based on three compartments (uptake via an intermediate compartment). The kinetics of CSF uptake of [14C]inulin infused according to an "impulse" protocol were accurately accounted for only by the nonsaturable two-compartment model (determination coefficient [R2] = 0.879 +/- 0.044; mean +/- SEM; n = 5), consistent with uptake via diffusion across the blood-CSF barrier. When the same infusion protocol and model were used to analyze the kinetics of insulin uptake, the data fit (R2 = 0.671 +/- 0.037; n = 10) was significantly worse than that obtained with [14C]inulin (P = 0.02). Addition of a saturable component of uptake to the two-compartment model improved this fit, but was clearly inadequate for a subset of insulin infusion studies. In contrast, the three-compartment model accurately accounted for CSF insulin uptake in each study, regardless of infusion protocol (impulse infusion R2 = 0.947 +/- 0.026; n = 10; P less than 0.0001 vs. each two-compartment model; sustained infusion R2 = 0.981 +/- 0.003; n = 5). Thus, a model in which insulin passes through an intermediate compartment en route from plasma to CSF, as a part of a specialized transport system for the delivery of insulin to the brain, best accounts for the dynamics of this uptake process. This intermediate compartment could reside within the blood-CSF barrier or it may represent brain interstitial fluid, if CNS insulin uptake occurs preferentially across the blood-brain barrier.

Blood-cerebrospinal fluid barrier alteration following intraventricularly administered cholera toxin

Cholera toxin (CT) has been reported to double cerebrospinal fluid (CSF) formation following its introduction into the ventricular system of cats and dogs. In our laboratory we noted that CT used in a similar fashion in rabbits and cats resulted in only a slight increase in CSF formation and was associated with a steadily rising protein content in the cisterna magna effluent. To further investigate this finding, rabbits and cats underwent ventriculo-cisternal perfusions, one group with CT introduced into the ventricles and the other without. In the rabbit only, radioiodinated serum albumin (125I-RISA) was given i.v. Other groups of rabbits had 125I-RISA or 125I-CT injected into the ventricles. The group of rabbits receiving intraventricular CT experienced a 4-10-fold elevation in the amount of both protein and 125I-RISA in the cisterna magna effluent compared with the control group. Electrophoretic pattern of the protein present in the effluent was similar to that of rabbit plasma. Autoradiography of the brains of those animals given intraventricular 125I-CT were found to have a very high uptake of 125I-CT in the choroid plexus and along all exposed ventricular surfaces, a finding not evident when 125I-RISA alone was given intraventricularly. It is concluded that CT altered the blood-CSF barriers allowing the reference marker to penetrate these barriers and plasma to leak into the CSF. These findings appear to account for most if not all of what was thought to be an increase in CSF formation in response to intraventricular CT.

Cerebrovascular permeability coefficients to sodium, potassium, and chloride

CSF and regional brain concentrations of 42K, 22Na, 36Cl, and [14C]mannitol were determined 3-45 min after intravenous injection of the tracers in pentobarbital-anesthetized rats. Rapid influx of 36Cl and 22Na into ventricular CSF immediately established concentration gradients from CSF to brain extracellular fluid. The CSF contribution to brain uptake of tracers was greatest in periventricular brain regions, where brain 36Cl concentrations were up to ninefold higher than concentrations in regions distant from ventricular CSF. Acetazolamide (20 mg kg-1 i.p.), an inhibitor of CSF formation, decreased 36Cl uptake into CSF and into periventricular brain regions but not into frontal cortex. 36Cl uptake into brain was unidirectional for 10 min after intravenous injection, and, during that period, diffusion from ventricular CSF did not contribute to uptake in the frontal cortex. Therefore, cerebrovascular permeability coefficients could be calculated from tracer concentrations in frontal cortex at 10 min and equaled, in cm s-1, 13.5 X 10(-7) for 42K, 1.4 X 10(-7) for 22Na, 0.9 X 10(-7) for 36Cl, and 1.5 X 10(-7) for [14C]mannitol. The low cerebrovascular permeabilities to K, Na, and Cl, comparable to those of some cell membranes, and the permselectivity (K much greater than Na greater than Cl) suggest that a significant fraction of ion transport across cerebral capillaries is transcellular, i.e., across the endothelial cell membrane.

Permeability of epidural somatostatin and morphine into the intrathecal space of dogs

In an in vivo saline perfusate of the intrathecal space of 6 dogs, the concentration of somatostatin was determined by radioimmunoassay before and over 2 h after epidural administration of 3 mg somatostatin. The total recovered amount of somatostatin was negligible, about 0.02%. However, within 50 min after the bolus epidural injection of somatostatin, the concentration per ml perfusate increased from 0.1 +/- 0.02 ng/ml to 138 +/- 102 ng/ml (P less than 0.001) and declined to 4 +/- 1.7 ng/ml after 120 min. This increase of the somatostatin concentration by 3 orders of magnitude might explain why epidurally administered somatostatin is effective in treatment of acute and chronic pain. In a control investigation with epidural morphine in another 6 dogs to prove the feasibility of the method, the total recovered amount of morphine in the intrathecal perfusate over 2 h was about 12%.

Permeability of the blood-cerebrospinal fluid and blood-brain barriers to thyrotropin-releasing hormone

The permeability of the blood-cerebrospinal fluid (CSF) barrier to 3H-labelled thyrotropin-releasing hormone (TRH), was studied at the blood-tissue interface of the isolated perfused choroid plexus of the sheep, using a rapid (less than 30 s), single circulation paired-tracer dilution technique, in which D-[14C]mannitol serves as an extracellular marker. Arterio-venous loss of 14C radioactivity reflects the percentage of the D-mannitol dose that crosses the blood-CSF barrier using a non-specific pathway. This loss suggests that the choroidal epithelium is moderately leaky. Cellular uptake of TRH, estimated by directly comparing venous dilution profiles of [3H]TRH and D-[14C]mannitol was independent of this leakiness. The unidirectional transport of TRH could not be saturated with unlabelled TRH at a concentration as high as 10 mM, but was markedly reduced by 10 mM proline and by the inhibitor of amidase and aminopeptidase activity, bacitracin (2 mM). Permeability of the blood-brain barrier to [3H]TRH was studied in the adult rat, employing the intracarotid injection technique of Oldendorf in which [14C]butanol served as an 'internal standard'. Brain-uptake of 3H radioactivity corrected for residual vascular space indicated a low extraction from the blood of TRH during a 15 s period of exposure to the peptide. Self-inhibition of [3H]TRH uptake by unlabelled TRH (10 mM) could not be demonstrated, but L-proline (10 mM) and bacitracin (2 mM) strongly inhibited this uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

The transport of sugars across the perfused choroid plexus of the sheep

The blood-perfused choroid plexuses from the lateral ventricles of the sheep were used to determine the nature of sugar exchanges between blood and cerebrospinal fluid (c.s.f.). There was a net entry of sugars from blood to c.s.f. at all concentrations of sugars which were used and this net entry was seen when the sugars were measured either directly by enzymic analysis or by the use of isotopically labelled sugars. From competition experiments the order of affinity of the transporting system from both blood to c.s.f. and c.s.f. to blood was the same, i.e. 2-deoxy-D-glucose much greater than D-glucose greater than 3-O-methyl-D-glucose much greater than D-galactose. The transport of sugars from c.s.f. to blood and blood to c.s.f. consists in both cases of a non-saturable and a saturable component. However, the affinity of the two systems is markedly different, the blood to c.s.f. being a system of low affinity and high capacity while that of the c.s.f. to blood has a high affinity and a low capacity. The concentration of glucose in the newly formed c.s.f. was estimated from the rate of c.s.f. secretion and the net flux of glucose across the choroid plexus. The concentration of glucose in this fluid was some 45-60% of that in plasma and so the low glucose concentration observed in bulk c.s.f. would appear to be a result of the entry process and not that of cerebral metabolism.

Cerebrospinal fluid and extracellular fluid: their relationship to pressure and duration of canine hydrocephalus

Fifteen greyhound dogs were made hydrocephalic by the transsphenoidal injection of silicone into the basal cisterns at the level of the tentorial incisura. Six of these animals had ventriculocisternal perfusions 4 weeks later and six at 8 weeks, half at 150 and half at 100 mm H2O. Three 12-week dogs were perfused at 150 mm H2O. Serial sections of brain from the ependyma of the left frontal horn to the overlying pia were counted for 14C inulin and 3H methotrexate uptake. Tissue concentrations of both markers varied indirectly with distance from ependyma and from pia, and varied directly with perfusion pressure. The data indicate that the diffusional pathway between cerebrospinal fluid (CSF) and extracellular fluid (ECF) can be modified by CSF pressure changes, i.e., CSF flows from the ventricles and subarachnoid space into the extracellular space when CSF pressures are raised. Brain uptake of inulin and methotrexate was significantly increased in the dogs made hydrocephalic 4 weeks prior to perfusion, but was less so in the 8-week hydrocephalics. Uptake of the tracers in three 12-week animals was similar to that found previously in normal dogs at elevated pressures. These findings correspond in location and time to the periventricular lucencies that are seen by computed tomography in human subacute hydrocephalus. They are apparently due to pressure-related changes in the volume of the ECF.

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

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

Recent research into the nature of cerebrospinal fluid formation and absorption

Recent information regarding the nature of bulk cerebrospinal fluid formation and absorption is reviewed, integrated with previous knowledge, and applied to the clinical setting.

Minireview. Peptides and the blood-brain barrier

Most neuropeptides are known to occur both in the central nervous system and in blood. This, as well as the occurrence of central nervous peptide effects after peripheral administration, show the importance of studying the relationships between the peptides in the two compartments. For many peptides, such as the enkephalins, TRH, somatostatin and MIF-1, poor penetration of the blood-brain barrier was shown. In other cases, including beta-endorphin and angiotensin, peptides are rapidly degraded during or just after their entry into brain or cerebrospinal fluid. Some peptides, such as insulin, delta-sleep-inducing peptide, and the lipotropin-derived peptides, enter the cerebrospinal fluid to a slight or moderate extent in the intact form. Many peptide hormones, such as insulin, calcitonin and angiotensin, act directly on receptors in the circumventricular organs, where the blood-brain barrier is absent. Oxytocin, vasopressin, MSH, and an MSH-analog alter the properties of the blood-brain barrier, which may result in altered nutritient supply to the brain. In conclusion, the diffusion of most peptides across the brain vascular endothelium seems to be severely restricted. There are, however, several alternative routes for peripheral peptides to act on the central nervous system. The blood-brain barrier is a major obstacle for the development of pharmaceutically useful peptides, as in the case of synthetic enkephalin-analogs.

Vesicular transport of cationized ferritin by the epithelium of the rat choroid plexus

We have studied the transport of ferritin that was internalized by coated micropinocytic vesicles at the apical surface of the choroid plexus epithelium in situ. After ventriculocisternal perfusion of native ferritin (NF) or cationized ferritin (CF), three routes followed by the tracers are revealed: (a) to lysosomes, (b) to cisternal compartments, and (c) to the basolateral cell surface. (a) NF is micropinocytosed to a very limited degree and appears in a few lysosomal elements whereas CF is taken up in large amounts and can be followed, via endocytic vacuoles and light multivesicular bodies, to dark multivesicular bodies and dense bodies. (b) Occasionally, CF particles are found in cisterns that may represent GERL or trans-Golgi elements, whereas stacked Golgi cisterns never contain CF. (c) Transepithelial vesicular transport of CF is distinctly revealed. The intercellular spaces of the epithelium, below the apical tight junctions, contain numerous clusters of CF particles, often associated with surface-connected, coated vesicles. Vesicles in the process of exocytosis of CF are also present at the basal epithelial surface, whereas connective tissue elements below the epithelium are unlabeled. Our conclusion is that fluid and solutes removed from the cerebrospinal fluid by endocytosis either become sequestered in the lysosomal apparatus of the choroidal epithelium or are transported to the basolateral surface. However, our results do not indicate any significant recycling via Golgi complexes of internalized apical cell membrane.

Uptake of [14C]urea by the in vivo choroid plexus--cerebrospinal fluid--brain system: identification of sites of molecular sieving

1. The time course of the uptake of [(14)C]urea by the lateral ventricular choroid plexus of the adult rat in vivo was analysed to delineate further the permeability characteristics of the epithelial membrane of this secretory tissue.2. Eight hours after I.P. injection, [(14)C]urea attained a steady-state distribution in 78% of the tissue water of lateral ventricular choroid plexus; similarly, approximately 8 hr was required for radiourea to reach a steady-state concentration in both the cerebral cortex and cerebrospinal fluid (c.s.f.).3. Results obtained for compartment analysis were used to calculate the concentration of [(14)C]urea in the epithelium of the lateral ventricular plexus during the approach to and at steady-state distribution. Even after 1 hr of distribution, the [(14)C]urea concentration in choroid cell water was less than 15% of that in plasma water.4. Although the concentration of radiourea in choroid cell water continually increased after 3 hr, it remained in equilibrium with the concentration of [(14)C]urea in c.s.f. water. At the steady state (i.e., 8 hr), the distribution of [(14)C]urea between the water of plasma and that of the choroidal epithelium was considerably away from equilibrium (i.e., by 25-30%).5. An analysis of the concentration gradients for [(14)C]urea across both the apical (c.s.f.-facing) and basolateral (plasma-facing) membranes of the epithelium of the lateral ventricular plexus suggests that the movement of urea is hindered to a greater extent by the basolateral membrane than by the apical membrane.6. Only a single half-time component (1.3 hr) can be resolved from analysis of the curve describing the time course of uptake of radiourea by the choroid epithelial cell compartment.7. The concentration gradient data suggest that urea penetrates from blood to c.s.f. via the choroid plexus by a transcellular pathway; however, it is not possible to rule out a paracellular pathway for urea movement.8. At the steady state, radiourea distributes into 88% of the water of cerebral cortex. This observation, together with the finding of a steady-state concentration gradient for [(14)C]urea from cortical tissue to c.s.f., constitutes evidence that urea movement is hindered at the blood-brain barrier as well as at the blood-c.s.f. barrier.

Fine structure of tight junctions between rat choroidal cells after osmotic opening induced by urea and sucrose

After ventriculo-cisternal perfusion of hypertonic urea or sucrose, both the choroid plexus permeability to horseradish peroxidase and the structure of tight junctions between choroidal cells are modified. Intercellular spaces are swollen, continuous ridges are fragmented and intrajunctional spaces are invested by many membranous particles. These morphological alterations appear to be reversible. These ultrastructural data are related to an osmotic maladjustment induced by the introduction of hypertonic solutions into the cerebro-spinal fluid.

The nuclear permeability, intracellular distribution, and diffusion of inulin in the amphibian oocyte

[(3)H]Inulin (mol wt approximately 5,500) solutions are microinjected into the cytoplasm of mature oocytes of Rana pipiens and the subsequent movement of the solute recorded by quantitative ultralow temperature autoradiography. The autoradiographs show transient cellular diffusion gradients, the influence of the nucleus on these gradients, and the nuclear:cytoplasmic distribution of inulin. Analysis leads to the following conclusions: (a) Inulin diffuses in cytoplasm at about 3 x 10(-6) cm(2)/s, or one-fifth as rapidly as in water. Most of this decrease is attributable to the increased tortuosity of the diffusional path due to the presence of inclusions and macromolecules. (b) The nuclear envelope is very permeable to inulin; its resistance to inulin's passage is similar to that of cytoplasm. The envelope appears to play a negligible role in regulating the nucleocytoplasmic movement of solutes smaller than macromolecules, (c) Inulin concentrates in the nucleus to four times its cytoplasmic level; this is attributed to solute exclusion from cytoplasmic water. Evidence is presented that among hydrophilic solutes the degree of exclusion increases with molecular size. The potential significance of cytoplasmic exclusion processes to understanding secretion and the intracellular movement of macromolecules is briefly discussed.

Stereospecific permeability of rat blood-brain barrier to lactic acid

Blood and whole brain 14 C and 32 P activities were determined in hepatectomized rats one, two, five and ten minutes after intravenous (I.V.) injection of 14 C-labeled L-lactate or D-lactate and 32 P-labeled rat red blood cells. Whole brain homogenate 14 C was corrected for blood 14 C and chemically partitioned into 14 C-lactate, 14 CO 2 and other 14 C compounds. In controls, lactate was replaced with 14 C-D-glucose and 125 l-antipyrine. At one minute postinjection, whole brain 14 C expressed as percent of total injected 14 C activity and as percent of the antipyrine value were: antipyrine 1.78% (100%); D-glucose 1.45% (81%); L-lactate 0.36% (20%); and D-lactate 0.13% (7%). One minute after L-lactate injection, brain 14 C was 74% lactate, 5% CO 2 and 21% other compounds. Preloading rats with cold racemic Na-lactate reduced L-lactate uptake to 0.14% of the injectate (8% of antipyrine), and reduced D-Iactate uptake to 0.09% (= 5% of antipyrine). At two, five and ten minutes, brain contained more 14 C with larger fractions metabolized to CO 2 and other compounds from both L-lactate and D-lactate. The blood-brain barrier appears to contain a saturable lactate carrier exhibiting threefold L-stereospecificity to D-stereospecificity, but resulting in far less net transport than the comparable glucose carrier. Lactate transport may be limited by the scarcity of neutral lactic acid at normal blood pH.

Ouabain-sensitive carrier-mediated transport of glucose from the cerebral ventricles to surrounding tissues in the cat

1. Artificial cerebrospinal fluid containing isotopically labelled sugars was perfused from the lateral cerebral ventricles to an effluent catheter inserted into the cerebral aqueduct of anaesthetized cats. This system was used for a quantitative study of the absorption of the sugars during steady state.2. A saturable mechanism was involved in the absorption of [U-(14)C]D-glucose and [(14)C]D-galactose. Absorption of [U-(14)C]D-glucose in the dead animal was similar to that of [(3)H]D-mannitol.3. 5 x 10(-5)M ouabain in the inflow reduced cerebrospinal fluid formation and the unidirectional fluxes of glucose from the ventricles into brain tissue and plasma. Ouabain did not alter the absorption of [(3)H]D-mannitol.4. Three types of unidirectional fluxes of glucose from the cerebral ventricles were separated. One was ouabain-sensitive and followed Michaelis-Menten kinetics. The second was insensitive to ouabain and the third occurred by simple diffusion.5. At normal ventricular glucose concentrations (3.5 mM) the three fluxes comprised (roughly): 25% (ouabain-sensitive), 35% (ouabain-insensitive) and 40% (simple diffusion) of total, unidirectional transport.