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S I Harik

Publications and source records attributed to S I Harik.

At least 37 records · Page 2Linked to original sources
PubMed1994

Hypoxia increases glucose transport at blood-brain barrier in rats.

Prolonged hypoxia causes several adaptive changes in systemic physiology and tissue metabolism. We studied the effects of hypobaric hypoxia on glucose transport at the blood-brain barrier (BBB) in the rat. We found that hypoxia increased the density of brain microvessels seen on immunocytochemical stains using an antibody to the glucose transporting protein GLUT. In addition, we found that hypoxia increased the density of GLUT in isolated cerebral microvessels as determined by specific cytochalasin B binding. The higher GLUT density in isolated cerebral microvessels was evident after 1 wk of hypoxia and was associated with decreased activity of gamma-glutamyltranspeptidase. Consistent with these findings, we also demonstrated that 3 wk of hypobaric hypoxia caused increased unidirectional transport of glucose at the BBB in several brain regions in vivo, as determined by the doubly labeled single-pass indicator-fractionation atrial bolus injection method in anesthetized rats. We conclude that chronic hypobaric hypoxia is associated with increased glucose transport at the BBB.

Animals↗
PubMed1993

Barrier properties of testis microvessels.

The blood-testis barrier is believed to be constituted by tight junctions between Sertoli cells in seminiferous tubules and possibly by myoid cells that encircle these tubules. We now show that testis microvessels are endowed with several markers of barrier properties of brain microvessels, such as the glucose transporter, P-glycoprotein, and gamma-glutamyl transpeptidase. Quantitative EM studies show that the endothelium in testis, as in brain, is continuous and has long junctional profiles and few vesicles. However, a small proportion of testis capillaries have expansions in their junctional clefts suggestive of patent paracellular channels, which may explain their higher permeability. Because barrier features are thought to be induced and/or maintained in brain microvessels by astrocytes, we assessed whether astrocyte-like cells exist in the testis. We found that the intertubular Leydig cells, adjacent to microvessels, express the astrocyte markers: glial fibrillary acidic protein, glutamine synthetase, and S-100 protein. We suggest that the testis endothelium contributes to the blood-testis barrier and that these endothelial barrier features are influenced by Leydig cells. We believe that the endothelial and the epithelial (Sertoli) components of the blood-testis barrier are "in series" and complement each other in achieving a stable milieu for spermatogenesis.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1993

Regional blood-brain lactate influx.

Regional blood-to-brain lactate transport was studied in chloral hydrate anesthetized rats using the single pass, dual-label, indicator fractionation, right atrial injection method. Lactate influx was resolved into two components, a saturable, stereospecific (to the L-enantiomer) component and a non-saturable, non-stereospecific diffusional component. The saturable component was found to have a low efficiency and moderate capacity with transport affinity coefficients between 6 and 14 mM and transport maxima of 23-40 mumol/100 g/min in the various brain regions. Lactate transport was not inhibited by probenecid. The diffusional component was determined from D-lactate influx measurements and the regional linear diffusion coefficients ranged from 0.020 to 0.036 ml/g/min. At the usual levels of plasma lactate (1-1.5 mM) these two influx components were about equal. The relative contribution of the non-stereospecific diffusional component was increased at higher plasma lactate concentrations. Lactate clearance, estimated by the total apparent permeability x surface area products was between 6 and 8 ml/100 g/min.

Animals↗
PubMed1993

ATP-sensitive potassium channels are not expressed in brain microvessels.

We used [3H]glibenclamide binding to assess ATP-sensitive K+ channels in isolated cerebral microvessels and in the cerebral cortex of the rat. We found no measurable specific glibenclamide binding in cerebral microvessels despite its abundance in cerebral cortical membranes, implying that ATP-sensitive K+ channels are not present in cerebral microvessels.

Adenosine Triphosphate↗
PubMed1993

Effect of acetyl-L-carnitine on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is believed to induce neurotoxicity by inhibiting mitochondrial oxidative metabolism, whereas acetyl-L-carnitine (ALC) facilitates this process by transporting fatty acids into mitochondria for beta oxidation. We investigated whether large doses of ALC given by gavage for 1 week before and 1 week after MPTP could ameliorate MPTP neurotoxicity in mice. We found that ALC had no effect on MPTP-induced depletion of striatal dopamine and its metabolites.

Acetylcarnitine↗
PubMed1993

Design and synthesis of 18F-labeled neurotoxic analogs of MPTP.

We report on the synthesis of two fluorine-18 labeled analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). A piperidyl triazene was fluorinated to produce [18F]-1-methyl-4-(2-fluorophenyl)-1,2,3,6-tetrahydropyridine (2'-F-MPTP, 12) in very low yield, and 1-methyl-4-[2-(fluoromethyl)phenyl]-1,2,3,6-tetrahydropyridine (2'-FCH2-MPTP, 11) was labeled with 18F by nucleophilic displacement of the corresponding chloride in 60% yield. The biodistribution in mice of the latter radiotracer and its oxidation to 1-methyl-4-[2-(fluoromethyl)phenyl]pyridinium (2'-FCH2-MPP+, 6) is also reported. The kinetics of oxidation of 2'-FCH2-MPTP and its solvolysis products (the corresponding 2'-hydroxymethyl and 2'-chloromethyl analogs) by rat liver monoamine oxidase were investigated. 2'-FCH2-MPTP accumulated to a useful degree in the brain, was oxidized by monoamine oxidase in vitro, was converted to the oxidation product in brain in vivo, and had a neurotoxic potency similar to that of MPTP. We feel it may be useful as an 18F-labeled radiopharmaceutical for positron tomographic studies of the mechanisms of MPTP toxicity.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed1993

Ontogeny of the erythroid/HepG2-type glucose transporter (GLUT-1) in the rat nervous system.

Central nervous system (CNS) microvessels of adult mammals have an unusually high density of the facilitative glucose transporter GLUT-1. Most systemic microvessels and those of the brain's circumventricular organs, which lack 'barrier' properties, do not express a high density of GLUT-1. Thus, a high GLUT-1 density is a marker of adult brain endothelium. To determine the stage at which CNS microvessels acquire GLUT-1, we studied by immunocytochemistry GLUT-1 ontogeny in the rat CNS from embryonic day (E) 11 to senescence. At E11, before blood vessels invaded the neuroectodermal tube, GLUT-1 immunoreactivity was already evident in the perineural plexus of vessels and in most of the vascular endothelium of the embryo. GLUT-1 immunoreactivity was also evident in the neuroectoderm. The neuroectoderm gradually lost GLUT-1 expression, and at about E16, GLUT-1 immunoreactivity was no longer detectable in most of the neuroectodermal epithelium, while CNS microvessels had increased their GLUT-1 immunoreactivity. By birth, GLUT-1 immunoreactivity in the CNS was restricted to the endothelium, the epithelium (but not the endothelium) of the choroid plexus, and tanycytes. This cellular distribution of GLUT-1 did not change much between birth and senescence despite considerable postnatal brain development and the increased brain capillary density. Our results suggest that while a CNS factor(s) may not have a role in the induction of the high expression of GLUT-1 in CNS endothelium, such a factor(s) is probably important in maintaining the high level of GLUT-1 in these endothelia.

Aging↗
PubMed1993

Development of fluorinated 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine analogs with potent nigrostriatal toxicity for potential use in positron emission tomography studies.

The discovery of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity stimulated intense interest in neurotoxicology and in the possible toxic etiology of Parkinson's disease. Better understanding of MPTP neurotoxicity may be achieved by studies using 18F-radiolabeled MPTP analogs and positron emission tomography in nonhuman primates. We synthesized three fluorinated analogs of MPTP: 1-methyl-4-(2-fluorophenyl)-1,2,3,6-tetrahydropyridine (2'-F-MPTP), 1-methyl-4-[2-(trifluoromethyl)phenyl]-1,2,3,6-tetrahydropyridine (2'-CF3-MPTP) and 1-methyl-4-[2-(fluoromethyl)phenyl]-1,2,3,6-tetrahydropyridine (2'-CH2F-MPTP), and developed a method for preparing the latter in 18F-labeled form. We now studied the suitability of 2'-CH2F-MPTP and its hydrolysis products as substrates for monoamine oxidase (MAO) from mouse and monkey brain preparations, and investigated the neurotoxic effect of 2'-CH2F-MPTP and 2'-F-MPTP on the nigrostriatal dopaminergic system in mice. We found that 2'-CH2F-MPTP is a better substrate for MAO and that both 2'-CH2F-MPTP and 2'-F-MPTP were more potent neurotoxins than MPTP. Like MPTP, 2'-F-MPTP was exclusively oxidized by MAO-B and its toxicity blocked by pargyline or deprenyl but not by clorgyline. In contrast, 2'-CH2F-MPTP was oxidized by both MAO-A and MAO-B, and its toxicity was not blocked by pargyline, clorgyline or deprenyl when given separately, but required clorgyline and deprenyl together.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed1992

Transferrin receptors of rat and human brain and cerebral microvessels and their status in Alzheimer's disease.

We studied the regional distribution of specific [125I]transferrin binding to transferrin receptors in the brains and cerebral microvessels of humans and rats. We also assessed transferrin receptors in subjects with Alzheimer's disease. Human diferric [125I]transferrin bound to regional brain and cerebral microvessels with high affinity (dissociation constants of 1-10 nM), and the maximal binding densities ranged from 30 to 90 pmol/mg protein in the brain and were several-fold higher in cerebral microvessels. In Alzheimer's disease, transferrin receptor densities were significantly reduced in the hippocampus and the temporal and occipital cortex but were unchanged in the frontal and parietal cortex and the cerebellum. Although [125I]transferrin binding was higher in cerebral microvessels from subjects with Alzheimer's disease than in those of age-matched controls, this difference did not attain statistical significance. These results suggest that transferrin receptor density was decreased in some cortical areas including the hippocampus in Alzheimer's disease but relatively unchanged in cerebral microvessels.

Alzheimer Disease↗
PubMed1992

Carnitine acetyltransferase activity in the human brain and its microvessels is decreased in Alzheimer's disease.

L-Carnitine and acetyl-L-carnitine facilitate mitochondrial beta-oxidation of fatty acids. In the brain, they may also have a role in acetylcholine synthesis. Carnitine acetyltransferase catalyzes the interchange between L-carnitine and acetyl-L-carnitine. Recently, acetyl-L-carnitine was reported to have a beneficial effect in patients with Alzheimer's disease. We therefore assessed carnitine acetyl-transferase activity in selected brain regions and in isolated cerebral microvessels obtained at autopsy from patients with Alzheimer's disease and from age-matched control subjects. We found a 25 to 40% decrease in carnitine acetyltransferase activity in patients with Alzheimer's disease, which attained statistical significance in most brain regions and in cerebral microvessels. These findings document another neurochemical abnormality in patients with Alzheimer's disease and provide a rationale for the use of acetyl-L-carnitine in the treatment of patients with Alzheimer's disease.

Aged↗
PubMed1992

Monoamine oxidases of the brains and livers of macaque and cercopithecus monkeys.

We assessed the two forms of monoamine oxidase (MAO), MAO-A and MAO-B, in discrete regions of the brain and in cerebral micro- and macrovessels, choroid plexus, and liver of three species of monkeys: African Green, rhesus, and cynomolgus. MAO was determined by specific [3H]pargyline binding which is stoichiometric and irreversible and by measuring the rate of oxidation of several substrates. Cerebral micro- and macrovessels had low MAO content. Regional brain MAO did not vary by more than one-fold in the brains of each of the three species of monkeys and was higher in the basal ganglia than in the cerebral cortex or cerebellum. MAO in the choroid plexus was low, while the liver had higher MAO activity than any of the brain samples. The vast majority of MAO in all the tissues that we examined was of the MAO-B type, and specific [3H]pargyline binding correlated well with the oxidation rate of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. These results show marked similarities in brain MAO distribution between monkey and man. Or the three monkey species, the African Green monkey had the lowest MAO activity in its cerebral microvessels, which constitute the blood-brain barrier, although the small number of observations in each group did not allow statistical analyses of the differences.

Animals↗
PubMed1992

Nicotine enhances 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine neurotoxicity.

Epidemiologic evidence of an inverse relationship between cigarette smoking and Parkinson's disease suggests that a component of cigarette smoke protects against nigrostriatal degeneration. Nicotine, a major component of cigarette smoke, is similar in chemical structure to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and is metabolized in part by the same enzymes that detoxify MPTP. We investigated the effect of chronic nicotine on MPTP neurotoxicity in two strains of mice and found that nicotine increases rather than decreases MPTP toxicity. These results are not compatible with the hypothesis that nicotine is that component of cigarette smoke that protects against nigrostriatal degeneration, at least in the MPTP experimental model of Parkinson's disease.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed1992

Changes in the glucose transporter of brain capillaries.

Brain capillary endothelium has a high density of the GLUT-1 facilitative glucose transporter protein. This is reasonable in view of the brain's high metabolic rate for glucose and its isolation behind unique capillaries with blood-brain barrier properties. Thus, the brain endothelium, which constitutes less than 0.1% of the brain weight, has to transport glucose for the much larger mass of surrounding neurons and glia. I describe here the changes that occur in the density of glucose transporters in brain capillaries of subjects with Alzheimer disease, where there is a decreased cerebral metabolic rate for glucose, and in a novel clinical entity characterized by defective glucose transport at the blood-brain barrier. In subjects with Alzheimer disease, cerebral microvessels showed a marked decrease in the density of the glucose transporter when compared with age-matched controls, but there was no change in the density of glucose transporters in erythrocyte membranes. Thus, I believe that the decreased density of glucose transporters in the brains of subjects with Alzheimer disease is the result rather than the cause of the disease. In contradistinction, the primary defect in glucose transport at the blood-brain barrier in subjects with the recently described entity is associated with decreased density of GLUT-1 in erythrocyte membranes.

Animals↗
PubMed1991

Correlation of MPTP neurotoxicity in vivo with oxidation of MPTP by the brain and blood-brain barrier in vitro in five rat strains.

We studied 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity in 5 strains of rats by assessing mortality and brain monoamine changes after MPTP injections into the internal carotid artery. We then attempted to correlate the differences among rat strains in their susceptibility to MPTP neurotoxicity in vivo with MPTP oxidation by monoamine oxidase (MAO) of the cerebral cortex, striatum, and brain microvessels in vitro. Despite the fact that the carotid route delivers much higher amounts of MPTP to the ipsilateral cerebrum than can be achieved by systemic injections, no significant dopamine depletion occurred in ipsilateral striata of Sprague-Dawley rats (the most resistant strain), but significant reductions of about 40% in striatal dopamine were evident in the more sensitive strains. Decreased striatal dopamine levels in these latter rat strains were associated with increased dopamine turnover. Higher doses of MPTP resulted in acute death. MPTP-induced mortality was not affected, but striatal dopamine depletion was prevented, by MAO inhibition. Differences among rat strains in their susceptibility to MPTP neurotoxicity correlated best with MAO activity in their isolated brain microvessels, but not with MAO activity in their striata or cerebral cortices. These results are consistent with the hypothesis that the rats' resistance to MPTP neurotoxicity is to some extent a property of their unique brain endothelium which has high MAO activity.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed1991

The erythrocyte-type glucose transporter in blood vessels of primary and metastatic brain tumors.

We previously demonstrated that brain microvessels have a high density of the erythrocyte-type glucose transporter and suggested this could be used as a marker of cells with "occluding" junctions. Now, we have studied by immunocytochemistry the tissue distribution of the glucose transporter in a variety of primary and metastatic human brain tumors with a monoclonal antibody to the glucose transporter of human erythrocytes. Microvessels in normal brain tissue distant from the tumor, and in brain surrounding the tumor, immunostained strongly for the glucose transporter. Microvessels in primary and metastatic tumors, however, lacked glucose transporter immunoreactivity. Only in the relatively benign juvenile cerebellar astrocytomas was glucose transporter immunoreactivity retained in some of the tumor microvessels. Because a high glucose transporter density is a marker of vessels with barrier properties, our results indicate that the vast majority of brain tumor microvessels do nc, have an intact blood-brain barrier.

Astrocytoma↗