摘要:

Abstract—Tryptophan transport across the blood‐brain barrier was studied using a single injection dual isotope label technique, in the following three conditions: normal rats, rats with portacaval shunts, and rats with portacaval shunts followed 65 h later by hepatic artery ligation. In both normal rats and those with acute hepatic failure the tryptophan transport system was found to be comprised of two kinetically distinct components. One component was saturable and obeyed Michaelis‐Menten kinetics (normal:Vmax= 19.5 nmol.min−1.g−1.Km= 113 μM; hepatic failure:Vmax, = 33.8 nmol.min−1.g−1,Km= 108 μM), and the second was a high capacity system which transported tryptophan in direct proportion to concentration over the range tested (normal:K= 0.026 ml.min−1.g−1; hepatic failure:K= 0.067 ml.min−1.g−1). Since the saturable low capacity component transports several neutral amino acids, and their collective plasma concentration is high in relation to the individualKms, tryptophan transport by this component is reduced by competitive inhibition under physiological conditions. Thus it was calculated that in normal rats approx 40% of tryptophan influx occurs via the high capacity system. During acute hepatic failure transport via both components was increased substantially, approximately doubling the rate of tryptophan penetration of the blood‐brain barrier at all concentrations tested. The contribution by the high capacity component became even more significant than in normal rats, accounting for about 75% of all tryptophan passage from plasma to brain. Brain tryptophan content was 29.9 nmol/g in normal rats and rose to 45.2 nmol/g in rats with portacaval shunts and 50.5 nmol/g in those with acute hepatic failure, correlating with the increased rate of tryptophan transport. In a previous study we found that plasma competing amino acids were greatly increased during acute hepatic failure. Calculations predict that these increased concentrations would cause a reduction in tryptophan transport by the low capacity system. However, because of the increase in the rate of transport by the high capacity component, net tryptophan entry across the blood‐brain barrier was actually increased. This increased rate of transport clearly contributes to the increased content of brain tryptophan f

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05170.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Abstract—Microvessels (primarily capillaries) were isolated from the brains of rats 25‐35 days of age. This preparation was characterized by light, transmission, and scanning electron microscopy. Transmission electron microscopy revealed that the endothelial cell membranes were intact and were impermeable to horseradish peroxidase. However, scanning electron microscopy revealed that damage to the membrane occurred during isolation. The isolated microvessel preparations were metabolically competent as demonstrated by their ability to metabolize [14C]glucose.Aliquots of microvessel preparation were incubated with radioactive non‐metabolizable analogs of D‐glucose at various concentrations. The kinetics of accumulation of radioactivity in the capillaries were analyzed according to a model for carrier‐mediated diffusion and affinity constants for 3‐O‐methyl‐ D‐glucose and 2‐deoxyglucose were calculated (about 18 mM at 20°C in each case). These affinity constants are somewhat greater than that expected from whole animal experiments reported by other laboratories. This discrepancy is probably accounted for by the presence of a passive diffusion component. However, despite this complication, the primary mechanism for entry of D‐glucose analogues at physiological concentrations is compatible with carrier‐mediated transport since: the uptake of sugar analogs was shown to be saturable, to exhibit competition for uptake between structurally similar molecules, and to be non‐concentrative. In contrast, the uptake of glycerol, mannitol, and L‐glucose by isolated microvessels obeyed the kinetics of simple passive diffusion and was not saturable.Our results are compatible with the concept that the capillary is the anatomic locus of the blood‐brain barrier and that this structure contains the carrier‐mediated transport system for monos

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05171.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Abstract—The unidirectional transport of metabolic substrates from blood to brain may be defined in terms of Michaelis‐Menten saturable (Km,Vmax) and non‐saturable (Kd) components of influx. Various computation procedures have been previously reported to estimate the kinetic parameters when an intracarotid injection technique is used. Transformations of the influx data which allow linear plots to obtain estimates were compared with estimates obtained directly from a best fit on a least means squares criterion for both experimental and simulated data. Large discrepancies were apparent between the various estimates of the kinetic parameters when an equal weight was given to transformed data. For pyruvate (21‐day‐old rats),Km, values varied between 1.02 and 6.25 mM andVmaxvaried between 0.68 and 2.30 μmol g−1min−1. The estimates were almost equivalent when pyruvate data was re‐analysed using a weighting scheme based on the finding that the absolute value of the S.D. of influx increased in proportion to influx. It is recommended that estimates of kinetic parameters be obtained by an iterative, non‐linear least squares method to fit appropriately weig

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05172.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Abstract—The kinetics of the uptake from blood to brain of pyruvate, lactate and glucose have been determined in rats of different ages. The carotid artery single injection technique was used in animals anaesthetized with pentobarbital. The rates of influx for each substrate were determined over a range of concentrations for the different age‐groups. Data were analysed in terms of the Michaelis‐Menten equation with a component to allow for non‐saturable diffusion. Values are given forKm,VmaxandKd. In suckling rats (15‐21 days) theVmaxvalues for both pyruvate and lactate were 2.0 μmol g−1min−1. In 28‐day‐old rats theVmaxvalues had fallen to one‐half and in adults they were less than one‐tenth.Km, values were higher in the younger animals. The rate of glucose transport in suckling rats was half that of 28‐day‐old and adults although there was no difference with age in theKmvalues.The results are discussed in relation to the net flux of these substrates in and out of brain during different stages

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05173.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Abstract—The lipid and protein compositions of PNS myelin from two species of frogs,Xenopus laevisandRana catesbiana, are compared with each other and with those of the rat. The relative proportions of PNS myelin lipids in the two frog species and rat are very similar except for a somewhat higher phosphatidyl choline content in frog myelin. The protein compositions of frog PNS myelin of the two species are also much alike, but a protein smaller than P2is found inXenopus, which is not seen inRana. The results are discussed in terms of results found by others in other species and the compositions are compared with those of the corresponding CNS myeli

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05174.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Abstract—The effects of altered osmolality on respiration and fine structure were studied in isolated cerebral mitochondria from mature rats (60‐100 days of age) and from rat pups in the first month of postnatal life (5, 10, 20 and 30 days). In the mature cerebral mitochondria, ADP‐dependent respiration was inhibited in media of decreased osmolality. There was a transient inhibition of ADP‐dependent respiration and a sustained increase in ADP‐independent respiration in media of increased osmolality. In contrast, cerebral mitochondria from 5‐day‐old rats showed both inhibition of ADP‐dependent respiration and increased ADP‐independent respiration in hypo‐osmolal media. In these mitochondria, inhibition of ADP‐dependent respiration was stable and ADP‐independent respiration was unchanged in media of increased osmolality. The transition to the mature respiratory response occurring with altered osmolality took place between 10 and 30 days of age. During this same age period, cerebral mitochondria showed an increasing resistance to matrix condensation in media of normal a

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05175.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Abstract—An enzyme with the specificity of a prolyl endopeptidase was purified about 880‐fold from rabbit brain. The enzyme hydrolyzes peptidylprolyl‐peptide and peptidylprolyl‐amino acid bonds. Several biologically active peptides such as angiotensin, bradykinin, neurotensin. substance P and thyrotropin releasing hormone are degraded by hydrolysis of the bond between the carboxyl group of proline and the adjacent amino acid or ammonia respectively. The enzyme is activated by dithiothreitol and inhibited by heavy metals and thiol blocking agents. The serine protease inhibitor phenylmethanesulfonylfluoride has no effect on activity; however, inhibition was obtained with diisopropylfluorophosphate. Prolyl endopeptidase has a molecular weight of about 66,000 and a pH optimum of about 8.3. A new chromogenic substrate,N‐benzyloxycarbonylglycyl‐L‐prolylsulfamethoxazole, was used for determination of enzyme activity. The substrate is hydrolyzed toN‐benzyloxycarbonylglycyl‐L‐proline and free sulfamethoxazole which can be conveniently determined by a col

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05176.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Abstract—The hypothesis that the biphasic disappearance of dopamine (DA) from the rat striatum following inhibition of synthesis with α‐methyl‐p‐tyrosine (AMPT) represents catabolism from separate‘functional’and 'storage’compartments (Javoy&Glowinski, 1971) was tested by administering3H‐tyrosine i.v. 10 min before 400 mg per kg AMPT. The levels of newly synthesized3H‐DA and total DA were then determined at 5‐min intervals. While both declined biphasically, the rate of decay of3H‐DA was significantly less than that predicted by the two‐pool hypothesis. In fact, the patterns of change of3H‐DA and total DA were identical and the specific activity of DA did not vary following AMPT. To determine if an increase in DA synthesis and release would result in the preferential catabolism of newly synthesized3H‐DA, the experiment was repeated fol!owing treatment with 0.1 mg per kg haloperidol i.v.3H‐DA and total DA still exhibited identical biphasic declines and there was no change in the specific activity of DA after AMPT even though3H‐DA levels were increased three‐fold by haloperidol. Thus (a) no evidence for the preferential catabolism of newly synthesized3H‐DA was obtained and (b) newly synthesized and total striatal DA behaved as if localized in a homogeneous kinetic compartment under all conditions employed.Rates of striatal DA metabolism were estimated in both experiments from changes in total DA after AMPT and by the formation of3H‐DA in the initial 10 min after3H‐tyrosine injection. Results of both approaches were consistent and indicated that the rates of striatal DA synthesis and catabolism may vary rapidly between approx. 20 and 90 nmol per g per h without violating single compartment kinetics. It is proposed that any labile pool contains no more than a few per cent of the DA in the striatum and the impli

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05177.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Abstract—The turnover of 5‐hydroxytryptamine in the forebrain and of dopamine in the striatum was studied in mice fasted for 20 h. Such mice showed an increased tissue concentration of 5‐hydroxyindoleacetic acid in the forebrain and an increased accumulation of this acid after probenecid. Fasted mice also showed a higher concentration of homovanillic acid in the striatum than fed mice. However, the administration of probenecid produced a smaller increase in homovanillic acid concentration in fasted than in fed mice. The decay of dopamine following α‐methyl‐p‐tyrosine was reduced in fasted mice at 2 h, but not at 1 h or 6 h after administration of the inhibitor. The possibility that fasting increases the activity of some dopaminergic neurones while decreasing the activity of others is considered. The existence of a pool of homovanillic acid at a site within the striatum where the probenecid‐sensitive transport is not effective

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05178.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY

摘要:

Abstract—The effects of LiCl on cholinergic function in rat brainin vitroandin vivohave been investigated. The high affinity transport of choline and the synthesis of acetylcholine in synaptosomes were reduced when part (25‐75%) of the NaCl in the buffer was replaced with LiCl or sucrose. This appeared to be due to lack of Na+rather than to Li+, as addition of LiCl to normal buffer had little effect.Following an injection of LiCl (10mmol/kg, i.p.) into rats the concentration of a pulsed dose of [2H4]choline (20 μmol/kg, i.v., 1 min) and its conversion to [2H4]acetylcholine, and the concentrations of [2H2]acetylcholine and [2H0]choline were measured in the striatum, cortex, hippocampus and cerebellum. The [2H4]choline and [2H4]acetylcholine were initially (15 min after LiCl) reduced (to −30% in the cortex) and later (24 h after LiCl) increased (to + 50% in the striatum). There was a corresponding initial increase (to +50% in the cerebellum) and later decrease (to −30% in the hippocampus) of the endogenous acetylcholine and choline. These results indicate an initial decrease and later increase in the utilization of acetylcholine after acute treatment with LiCl.Following 10 days of treatment with LiCl there was an increased rate of synthesis of [2H4]acetylcholine from pulsed [2H4]choline in the striatum, hippocampus and cortex (P<0.05). The high affinity transport of [2H4]choline and its conversion to [2H4]acetylcholine was activated (131% of control;P<0.01) in synaptosomes isolated from brains of 10‐day treated rats. Investigation of synaptosomes isolated from striatum, hippocampus and cortex revealed that only striatal [2H4]acetylcholine synthesis was significantly stimulated. Kinetic analysis demonstrated that the apparent KTfor choline was decreased by 30% in striatal synaptosomes isolated from rats treated for 10 days with LiCl. Striatal synaptosomes from 10‐day treated rats compared to striatal synaptosomes from untreated rats also released acetylcholine at a stimulated rate in a medium containing 35 mM‐KCl. These results indicate that LiCl treatment stimulates cholinergic activity in certain brain regions and this may play a significant role in the therapeutic effect of LiCl in neuropsychia

ISSN:0022-3042    

DOI:10.1111/j.1471-4159.1979.tb05179.x

出版商:Blackwell Publishing Ltd    

年代:1979

数据来源: WILEY