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Biomedical subjects

M B Segal

Publications and source records attributed to M B Segal.

At least 37 records · Page 2Linked to original sources
PubMed1997

The passage of azidodeoxythymidine into and within the central nervous system: does it follow the parent compound, thymidine?

The transport of azidodeoxythymidine (AZT) into and within the central nervous system (CNS) has special clinical significance due to the ability of AZT to alleviate certain neurological symptoms associated with the acquired immunodeficiency syndrome (AIDS). AZT was thought to be similar to its parent compound, thymidine, in that it entered the CNS via the choroid plexuses (blood-CSF barrier) and could not cross the blood-brain barrier (BBB). However, a saturable transport system for thymidine at the BBB has recently been identified. The aim of this study was to test the hypothesis that AZT follows its physiological counterpart in its mode of entry into and movement within the CNS. Initial experiments using the in situ brain perfusion technique indicated that the blood-to-CNS transfer constants for [3H]AZT (blood-to-cerebrum; 0.95 +/- 0.12 microl/min/g) were significantly lower than those determined for [3H]thymidine. Also, [3H]AZT entered the CNS purely by a diffusive process. The movement of [3H]AZT within the CNS was further investigated by a ventriculocisternal perfusion technique and indicated that the majority of intraventricularly perfused [3H]AZT remained within the ventricles (79.9%), with little escaping to blood (14.1 +/- 3.1%) or brain (6.0 +/- 1.3%). Overall, these results suggest that the choroid plexus/CSF pathway was unlikely to be solely responsible for the levels of [3H]AZT observed in brain and that the BBB plays a significant role in the brain entry of this analog. However, in contrast to thymidine, AZT enters the CNS purely by a diffusional process.

Animals↗
PubMed1996

Identification of a saturable uptake system for deoxyribonucleosides at the blood-brain and blood-cerebrospinal fluid barriers.

Substances can enter the brain either directly across the blood-brain barrier or indirectly across the choroid plexuses and arachnoid membrane (blood-CSF barrier) into the CSF and then by diffusion into the brain. Earlier studies have demonstrated a saturable thymidine uptake across the blood-CSF barrier, but not across the blood-brain barrier. In this study transport of [3H]thymidine across both barriers was measured in vivo by means of a bilateral vascular brain perfusion technique in the anaesthetised guinea-pig. This method allows simultaneous and quantitative measurement of slowly penetrating solutes into both brain and CSF, under controlled conditions of arterial inflow. The results of the present study carried out over perfusion periods of up to 30 min indicated a progressive uptake of [3H]thymidine into brain and CSF, which was found to be significantly greater than the transport of D-[14C]mannitol (a plasma space marker). Furthermore, the addition of 1 mM unlabelled thymidine in the perfusate caused saturation of [3H]thymidine uptake into both brain and CSF. In conclusion, these findings suggest that thymidine can cross both the blood-brain and blood-CSF barriers in the guinea-pig by carrier-mediated transport systems.

Animals↗
PubMed1995

The entry of acidic amino acids into brain and CSF during development, using in situ perfusion in the rat.

Previous studies using the rapid single pass blood to tissue uptake of substances by the capillaries of the blood-brain barrier, have failed to show significant uptake of acidic amino acids. However, by the use of a bilateral in situ brain perfusion in neonatal and adult rats, extending the perfusion time to 30 min, the carrier-mediated uptake of aspartate and glutamate into brain and CSF has been demonstrated. The ratios of 14C-acidic amino acids in the brain and CSF to that in perfusate were measured and represented as Rbrain and RCSF respectively, after 30 min, neonatal (1-week-old) Rbrain values for both amino acids were approximately twice that of adults, while neonatal RCSF for aspartate and glutamate were 3 to 5 times that of the adult. In contrast, there was no significant entry of NMDA into either compartment for both adults and neonates. The transfer coefficient, Kin into brain and CSF was also measured in relation to stages of development. In general the Kin values for brain and CSF for aspartate and glutamate were higher in the younger age groups than the adult group (1 week > 2 week > 3 week > or = adult). In 1- and 2-week-old rats entry into CSF appears to be higher than that of brain, whereas for adults entry into the brain tissue was dominant.

Animals↗
PubMed1995

Permeability of the developing blood-brain barrier to 14C-mannitol using the rat in situ brain perfusion technique.

The brain penetration of 14C-mannitol was investigated using a bilateral in situ brain perfusion technique followed by capillary depletion analysis. This technique measures the uptake of slowly penetrating solutes in the absence of the systemic circulation, and separates accumulation in brain endothelial cells from uptake into brain parenchyma. Penetration of 14C-mannitol was linear up to 30 min in rats aged 1, 2, 3 weeks and in adults. The brain mannitol space was higher in 1-week-old neonatal rats compared with adults (P < 0.05) and was due to a greater initial volume of distribution (Vi) for mannitol in the neonates, and not due to an elevated transfer rate (K(in)). Thirty percent of mannitol in the neonatal brain was associated with the capillary containing fraction, whereas in the adult only 13% was found in this fraction. This suggests that the permeability of the blood-brain barrier to mannitol does not change significantly with development but that more mannitol is associated with endothelial cells in the neonate. An investigation of 14C-glycine uptake was also carried out, and unlike mannitol the K(in) was greater in the neonate compared to the adult suggesting an elevated rate of transfer for this amino acid into the neonatal rat brain.

Age Factors↗
PubMed1995

Biphasic L-arginine uptake by the isolated guinea-pig heart.

L-Arginine is the physiological substrate for the formation of nitric oxide (NO) and accounts for the biological activity of endothelium-derived relaxing factor. We have studied L-arginine transport in the heart using a rapid dual-isotope dilution technique. The time course of L-[3H]arginine uptake (extraction) by the isolated perfused guinea-pig heart was found to occur in two phases. The first phase reached a plateau in 6.6 +/- 0.6 s and lasted 8.8 +/- 0.7 s, whereas the second phase developed a plateau after 16.3 +/- 0.8 s. The first phase of maximal uptake (Umax,1) accounted for 13.4 +/- 1.4% of the total uptake and the second (Umax,2) for 32.3 +/- 1.8%. The two phases of uptake were inhibited by unlabelled L-arginine in a dose-dependent manner, which suggests that both phases are carrier mediated. The degree of inhibition of Umax,1 and Umax,2 by unlabelled L-arginine was not significantly different. Studies of the kinetics of uptake of these processes revealed an apparent Km,1 of 183 +/- 10 microM with a Vmax,1 of 50 +/- 10 nmol min-1 g-1 for the first phase and Km,2 of 167 +/- 14 microM with a Vmax,2 of 93 +/- 13 nmol min-1 g-1 for the second phase of uptake. These results suggest a similar affinity for the receptors of both transport systems, but with different values for Vmax (P < 0.05). In contrast, 1 mM unlabelled D-arginine had no effect on either the first or second phase of uptake of L-[3H]arginine by the heart, which suggests that these processes are stereospecific. In the presence of the L-stereoisomer of nitro-arginine-mono-methyl ester (L-NAME), a potent inhibitor of NO synthesis, the Umax,1 was inhibited by about 60% while Umax,2 was inhibited by only 20%, which suggests that there is a difference in the effect of L-NAME on the two phases of L-arginine uptake. The first phase most probably represents uptake into the capillary wall, i.e. endothelium and smooth muscle, while the second phase represents entry into the extra-endothelial compartment, i.e. the cardiac myocytes and fibroblasts.

Animals↗
PubMed1993

Extracellular and cerebrospinal fluids.

The mechanism of formation of extracellular fluid is first described, followed by an explanation of the relation between osmotic force, reflection coefficient and molecular size. The possible mechanism of brain extracellular fluid formation is then proposed in relation to the restriction offered by the blood-brain barrier. The functions and compositions of cerebrospinal fluid (CSF) are then described followed by sections on the process of formation of CSF, the non-electrolytes and proteins in CSF, the drainage mechanisms and protein synthesis by the choroid plexus.

Animals↗
PubMed1992

The role of angiotensin II in regulation of cerebrospinal fluid formation in rabbits.

The effect of central and peripheral administrations of angiotensin II (AII) on cerebrospinal fluid (CSF) formation was investigated in rabbits anesthetized with intravenous alpha-chloralose and urethane. CSF production was measured by the ventriculo-cisternal perfusion method with Blue dextran 2000 used as an indicator substance. AII infused intracerebroventricularly (i.c.v.) at rates of 5.5 and 55 pg min-1 significantly decreased CSF formation rate by 27% and 36%, respectively. This AII action could be completely blocked by simultaneously administered specific AII antagonist, [Sar1,Ala8]AII (saralasin), given i.c.v. at a rate of 5.5 ng min-1. Intracerebroventricular infusion of AII at a rate of 5.5 ng min-1 did not change CSF production. Saralasin, when given alone into the ventricular system (5.5 ng min-1), non-significantly increased CSF production by 12%. However, in 4 of the 6 animals studied, the rise in CSF production was statistically significant (by 23%). Intravenous infusion of AII at rates of 30 and 100 ng kg-1 min-1 was found not to change CSF formation rate. Also, i.c.v. administration of angiotensin I converting enzyme inhibitor, captopril (10 microliters min-1), did not influence CSF production. It is concluded that the centrally released AII can control CSF production. Our results suggest that under normal conditions, AII exerts a tonic inhibitory effect on CSF formation. In contrast, the blood-borne peptide seems not to influence this physiological process.

Angiotensin II↗
PubMed1992

Saturable uptake of [125I]L-triiodothyronine at the basolateral (blood) and apical (cerebrospinal fluid) sides of the isolated perfused sheep choroid plexus.

The uptake of 125I-labelled L-triiodothyronine (T3) was measured on the blood side of the isolated perfused choroid plexus of the sheep using steady-state and single-circulation paired tracer techniques. The steady-state uptake of T3 was 33.5% (perfusion fluid protein content was 0.05 g.dl-1) which could be reduced to 9.4% in the presence of 500 microM unlabelled T3 showing partial saturation. The CSF to blood steady-state [125I]T3 measurements gave plasma/CSF ratio, R%, of 24.6 +/- 4.8% which was reduced to 9.8 +/- 2.1% in the presence of 500 microM unlabelled T3 in the mock CSF. The transport of T3 across the blood face of the choroid plexus and the CSF to blood transport, failed to show sodium dependence. Using the single circulation paired tracer technique, the initial uptake in less than 60 s, Umax of [125I]T3 was 50.4 +/- 3.9% relative to the extracellular marker [3H]D-mannitol. However, when 250 microM unlabelled T3 was present, Umax was reduced by 66%, although further significant inhibition at higher concentrations was not observed. Uptake of T3 at the blood side of the choroid plexus was partially saturated in the presence of unlabelled reverse T3 and DT3, suggesting little uptake stereospecificity. Unlabelled thyroxine (T4) and the amino acid analogues cycloleucine (aminocyclopentane-1-carboxylic acid) and BCH (beta-2-aminobicyclo-[2,2.1]-heptane-2-carboxylic acid) each reduced [125I]T3 uptake significantly, but not to the same degree as T3 stereoisomers. The neutral amino acids alanine and phenylalanine, had no effect on uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗
PubMed1992

The uptake of anionic and cationic amino acids by the isolated perfused sheep choroid plexus.

The carrier-mediated uptake of anionic and cationic amino acids by the basolateral face (blood side) of the isolated perfused sheep choroid plexus was demonstrated using the paired-tracer dilution technique. The uptake of these two classes of amino acid was higher than at the blood-brain barrier with no cross-competition between them. In addition the uptake was markedly stereospecific with a high degree of selectivity and no interaction with the L-system amino acids.

Animals↗
PubMed1992

Effect of central administration of angiotensin II on cerebrospinal fluid formation in rabbits.

The effect of central administration of AII on CSF formation was studied in alpha-chloralose and urethane anesthetized rabbits using the ventriculocisternal perfusion method. AII infused i.c.v. at rates of 5.5 and 55 pg/min significantly decreased CSF production by 25% and 35%, respectively. In contrast, AII when given at 5.5 ng/min did not change CSF formation. It seems that drop in CSF production observed during central administration of AII at low doses is mediated by both increased vasopressin release and activation of the sympathetic nervous system. The lack of changes in CSF formation with the highest AII dose used is not clear at present and awaits further investigation. Specific AII antagonist, saralasin, was found to significantly increase CSF production in four of five animals studied. It is suggested that in normal conditions AII may exert a tonic inhibitory effect on CSF formation.

Angiotensin II↗
PubMed1992

Effect of arginine vasopressin on blood vessels of the perfused choroid plexus of the sheep.

The perfused sheep choroid plexus was used to evaluate the response of the plexus blood vessels to systemically administered arginine vasopressin (AVP). AVP was found to decrease the diameter of the choroid plexus arterioles with a maximum change of 28 +/- 5% (mean +/- S.E.) at a plasma peptide concentration of 10(-7) M. This effect was blocked by the specific V1-vasopressinergic antagonist, d(CH2)5Tyr(Me)AVP. In contrast, venules were found not to show any appreciable response to AVP. Plasma AVP concentrations necessary to evoke a significant response of the choroid plexus arterioles are much higher than the highest plasma peptide levels observed in different physiological or pathophysiological situations. Some indirect evidence suggests, however, that AVP might be released within the choroid plexus from the vasopressinergic synaptic terminals, thus reaching a considerably high local concentration. It is possible then that the plexus vessels' tone could be controlled by the putative vasopressinergic neuronal fibers ending in the choroid plexus.

Animals↗
PubMed1991

The effects of (R) alpha-methyl histamine on the isolated guinea pig aorta.

The effects of the H3 agonist (R) alpha-methyl histamine ( (R) alpha-MeHA) on the isolated guinea pig aorta were investigated. At all the concentrations of (R) alpha-MeHA used, a relaxation of the aorta, which had been preconstricted with KC1 (50mM), was produced. This response was abolished by the H3 antagonist mixture of cimetidine (100 microM) and impromidine (10 microM). In contrast the same concentrations of histamine produced vasoconstriction which could be abolished by the H1 antagonist pyrilamine (3 microM). The results show that (R) alpha-MeHA produces a vasodilation response which is most probably mediated by H3 receptors and provides further evidence of a functional role for H3 receptors in the control of the cardiovascular system.

Animals↗
PubMed1991

Osmotic-hypertensive opening of the blood-brain barrier in rats does not necessarily provide access for potassium to cerebral interstitial fluid.

The blood-brain barrier was breached in urethane anaesthetized rats by infusing hypertonic mannitol or NaCl at high rate under high pressure into one internal carotid artery. Opening of the blood-brain barrier was confirmed by staining of the perfused hemisphere by intravenous Evans Blue dye. Orthodromic-evoked potentials in CA1 region of hippocampus were transiently extinguished, and the extracellular potential in hippocampus and neocortex shifted in the positive direction during hypertonic infusion. After the hypertonic infusion, the permeability of the barrier to K+ was tested by infusing into the internal carotid artery artificial cerebrospinal fluid in which K+ replaced most of the Na+, raising the concentration of K+ in the blood plasma in the superior sagittal sinus to 13-17 mM. Extracellular potential and interstitial potassium concentration ([K+]O) in hippocampus and neocortex, and evoked potentials in hippocampus, remained unchanged during prolonged infusion of high K+, unless and until spreading depression occurred. After a wave of spreading depression, [K+]O returned to baseline in spite of continued high K+ infusion. We conclude that [K+]O in brain tissue is effectively regulated even when colloidal dye can penetrate the blood-brain barrier, but excess K+ may have entered the cerebral interstitial space in scattered patches outside the region sensed by the ion-selective microelectrodes, triggering spreading depression.

Animals↗
PubMed1991

Kinetics of circulating vasopressin uptake by choroid plexus.

Uptake of circulating arginine vasopressin (AVP) by choroid plexus was studied by means of the in situ brain perfusion technique in anesthetized guinea pig and by means of single-circulation paired-tracer dilution technique in isolated perfused sheep choroid plexus. Kinetic analysis revealed saturable AVP uptake with Michaelis constant (Km) values of 32 +/- 4 and 31 +/- 5 nM and maximal saturable influx rate (Vmax) of 0.45 +/- 0.06 and 12.1 +/- 0.67 pmol.min-1.g-1 in guinea pig and sheep choroid plexus, respectively. The peptide fragments AVP-(1-8) and [pGlu4,Cyt6]AVP-(4-9), the amino acids L-phenylalanine, L-tyrosine, and 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid, and the aminopeptidase inhibitors Bestatin and bacitracin did not influence hormone kinetics. However, the V1 antagonist [(1-beta-mercapto-beta,beta-cyclo-pentamethylenepropionic acid)-O-methyl-Tyr2]AVP significantly inhibited AVP uptake with inhibitor constant (Ki) values of 0.19 +/- 0.03 (guinea pig) and 0.07 +/- 0.01 microM (sheep). The V2 agonist 1-desamino-8-D-AVP and pressinoic acid produced weak inhibitions only in guinea pig choroid plexus, and Ki/Km ratios indicated 220 and 310 times lower affinities than for AVP, respectively. It is suggested that the membrane mechanism responsible for AVP uptake in choroid plexus has a binding site with properties similar to those of V1 receptor.

Animals↗
PubMed1990

The steady-state amino acid fluxes across the perfused choroid plexus of the sheep.

The steady-state flux of labelled amino acids was investigated across the isolated perfused choroid plexus of the sheep. The extraction of anionic, cationic, small and large neutral amino acids by the blood side of the choroid plexus was demonstrated. However, there was no uptake of the analogue MeAIB, confirming the absence of the 'A' carrier system on this side of the blood--CSF barrier. The direction of the net flux of amino acids across the tissue varied depending on the amino acid and its concentration. At a concentration of 0.01 mM the net movement for phenylalanine, serine, aspartate and glycine was from blood to CSF. When the concentration of amino acid was increased to 0.1 mM, the net flux of phenylalanine and serine remained from blood to CSF whereas the net flux of the transmitters, aspartate and glycine, was in the opposite direction, from CSF to blood. When the level was raised further, to 1 mM, all four amino acids showed a net CSF to blood flux. The concentration of amino acids is newly formed CSF was calculated from the blood to CSF fluxes and was found to be between 2 and 200 microM, similar to that found in mammalian bulk phase CSF.

Amino Acids↗
PubMed1990

A saturable mechanism for transport of immunoglobulin G across the blood-brain barrier of the guinea pig.

The existence of an immunological blood-brain barrier to homologous blood-borne immunoglobulin G (IgG) was investigated in the guinea pig using a vascular brain perfusion technique in situ. Cerebrovascular unidirectional transfer constants (Kin) for 125I-labeled IgG (2.5 micrograms/ml) estimated from the multiple-time brain uptake data, ranged from 0.53 to 0.58 ml min-1 g-1 X 10(3) in the parietal cortex, hippocampus, and caudate nucleus, the transfer rate being some 10 times higher than that for [3H]dextran (MW 70,000). In the presence of 4 mg/ml unlabeled IgG, unidirectional blood to brain transfer of 125I-IgG was markedly inhibited. Immunohistochemical analysis of the brain tissue after vascular perfusion with unlabeled IgG revealed a distribution of the blood-borne immunoglobulin in the endothelial cells of microvessels and in the surrounding perivascular tissue. It is concluded that there is a specific transfer mechanism for IgG at the blood-brain barrier in the guinea pig, which is saturated at physiological plasma levels of IgG.

Animals↗
PubMed1990

Thyroxine transport from blood to brain via transthyretin synthesis in choroid plexus.

The transport of thyroxine from the bloodstream to the brain and the synthesis and secretion of transthyretin (formerly called prealbumin) were studied in rats and in sheep choroid plexus perfused in vitro. Rat choroid plexus contained 4.4 micrograms and rat liver 0.39 micrograms transthyretin mRNA per gram wet tissue. The specific radioactivity of transthyretin isolated from cerebrospinal fluid of rats 60 min after intravenous injection of [14C]leucine was greater than 50 times that of transthyretin from serum. After adding [14C]leucine to the perfusion medium of an in vitro perfused sheep choroid plexus, highly radioactive transthyretin was isolated from freshly secreted cerebrospinal fluid collected from the exposed choroid plexus surface. Secretion of newly synthesized transthyretin into the perfusion medium could not be demonstrated. After intravenous injection of [125I]-thyroxine into rats, a maximum in the curve of radioactivity in tissue plotted against time after injection was observed first for choroid plexus, thereafter for cerebrospinal fluid, and still later for cortex and striatum. Based on the obtained data, a hypothesis is derived for the mechanism of the transport of thyroid hormones from the bloodstream to the brain involving transthyretin synthesized in choroid plexus and secreted into the cerebrospinal fluid.

Animals↗