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Biomedical subjects

H Xu

Publications and source records attributed to H Xu.

At least 235 records · Page 13Linked to original sources
PubMed2000

Modulation of kir4.1 and kir5.1 by hypercapnia and intracellular acidosis.

CO2 chemoreception may be mediated by the modulation of certain ion channels in neurons. Kir4.1 and Kir5.1, two members of the inward rectifier K+ channel family, are expressed in several brain regions including the brainstem. To test the hypothesis that Kir4.1 and Kir5. 1 are modulated by CO2 and pH, we carried out experiments by expressing Kir4.1 and coexpressing Kir4.1 with Kir5.1 (Kir4.1-Kir5. 1) in Xenopus oocytes. K+ currents were then studied using two-electrode voltage clamp and excised patches. Exposure of the oocytes to CO2 (5, 10 and 15 %) produced a concentration-dependent inhibition of the whole-cell K+ currents. This inhibition was fast and reversible. Exposure to 15 % CO2 suppressed Kir4.1 currents by approximately 20 % and Kir4.1-Kir5.1 currents by approximately 60 %. The effect of CO2 was likely to be mediated by intracellular acidification, because selective intracellular, but not extracellular, acidification to the measured hypercapnic pH levels lowered the currents as effectively as hypercapnia. In excised inside-out patches, exposure of the cytosolic side of membranes to solutions with various pH levels brought about a dose-dependent inhibition of the macroscopic K+ currents. The pK value (-log of dissociation constant) for the inhibition was 6.03 in the Kir4.1 channels, while it was 7.45 in Kir4.1-Kir5.1 channels, an increase in pH sensitivity of 1.4 pH units. Hypercapnia without changing pH did not inhibit the Kir4.1 and Kir4.1-Kir5.1 currents, suggesting that these channels are inhibited by protons rather than molecular CO2. A lysine residue in the N terminus of Kir4.1 is critical. Mutation of this lysine at position 67 to methionine (K67M) completely eliminated the CO2 sensitivity of both the homomeric Kir4. 1 and heteromeric Kir4.1-Kir5.1. These results therefore indicate that the Kir4.1 channel is inhibited during hypercapnia by a decrease in intracellular pH, and the coexpression of Kir4.1 with Kir5.1 greatly enhances channel sensitivity to CO2/pH and may enable cells to detect both increases and decreases in PCO2 and intracellular pH at physiological levels.

Acidosis↗
PubMed2000

Purification of the newly found selenium-containing proteins in the arterial wall and brain of the rat.

Previously several selenium-containing proteins with different subunit molecular masses (M(r)) were detected in the arterial wall and brain of rats. In continuation of this work, after labeling of rats in vivo with [(75)Se]selenite, the new selenium-containing proteins of interest were purified on a Sephadex G-200 column followed by preparative isoelectric focusing. Nuclear analytical methods (gamma-counter and gamma-detector) were applied in the detection and identification of the (75)Se-labeled proteins. The two (75)Se-containing proteins from the arterial wall migrated as 15.0- and 67.0-kDa species on SDS-PAGE gels with pI values of 4.5 and 5.1, respectively. The three (75)Se-containing proteins from brain purified to homogeneity had M(r) values of 18.0, 30.0, and 42.9 kDa and pI values of 6.3, 6.5, and 6.0, respectively. Of these proteins, the 67.0-, 42.9-, and 30.0-kDa species may be yet not characterized selenoproteins with important biological functions.

Animals↗
PubMed2000

Enhancement of chiral recognition by formation of a sandwiched complex in capillary electrophoresis.

For chiral primary amino compounds not separable by cyclodextrins alone, chiral recognition was successfully achieved by the formation of a sandwiched complex of the non-chiral 18-crown-6, the chiral amine and cyclodextrin (CD) [18-crown-6+amino compound+CD]. The separation of 1-methyl-3-phenylpropylamine and 1,2,3,4-tetrahydro-1-naphthylamine racemates showed the special function of the non-chiral 18-crown-6 on chiral recognition. By formation of the sandwiched complex, the chiral center of 1-methyl-3-phenylpropylamine was successfully recognized, and resolution of 1,2,3,4-tetrahydro-1-naphthylamine dramatically increased. In these studies, the mobility differences of the enantiomers were evaluated as a function of the concentration of cyclodextrins with and without the 18-crown-6, and as a function of the concentration of the 18-crown-6. In addition, the separations by this method were compared to those by the chiral 18-crown-6 reagent.

Electrophoresis, Capillary↗
PubMed2000

Identification of a urokinase receptor-integrin interaction site. Promiscuous regulator of integrin function.

Adhesion and signaling by integrins require their dynamic association with nonintegrin membrane proteins. One such protein, the glycolipid-anchored urokinase receptor (uPAR), associates with and modifies the function of the beta(2)-integrin Mac-1 (CD11b/CD18). In this study, a critical non-I-domain binding site for uPAR on CD11b (M25; residues 424-440) is identified by homology with a phage display peptide known to bind uPAR. Recombinant soluble uPAR and cells expressing uPAR bound to immobilized M25, binding being promoted by urokinase and blocked by soluble M25, but not a scrambled control or homologous peptides from other beta(2)-associated alpha-chains. Mac-1, but not a mutated Mac-1 in which M25 was replaced with the homologous sequence of CD11c, co-precipitated with uPAR. In the beta-propeller model of alpha-chain folding, M25 spans an exposed loop on the ligand-binding, upper surface of alphaM, identifying uPAR as an atypical alphaM ligand. Although not blocking ligand binding to Mac-1, M25 (25-100 microM) inhibited leukocyte adhesion to fibrinogen, vitronectin, and cytokine-stimulated endothelial cells. M25 also blocked the association of uPAR with beta(1)-integrins and impaired beta(1)-integrin-dependent spreading and migration of human vascular smooth muscle cells on fibronectin and collagen. These observations indicate that uPAR associates with integrins directly and that disruption of this association broadly impairs integrin function, suggesting a novel strategy for regulation of integrins in the settings of inflammation and tumor progression.

Amino Acid Sequence↗
PubMed2000

Heroin-induced neuronal activation in rat brain assessed by functional MRI.

The present study demonstrates the application of fMRI technology to neuropharmacology and the interaction of drug/receptor in the rat brain. Specifically, we have observed two different types of fMRI signal changes induced by acute i.v. heroin administration in rat brains under conditions of spontaneous and artificial respiration. Under spontaneous respiration, a global decrease in fMRI signal was observed; under artificial respiration, a region-specific increase in fMRI signal was identified and the activation sites are consistent with the distribution of opiate mu-receptors in rat brain as previously reported by autoradiography. Both heroin-induced fMRI signal changes were suppressed by pretreatment of naloxone, an opiate mu-receptor antagonist, and reversed by injection of naloxone following heroin infusion. These results suggest that fMRI has specific advantages in spatial and temporal resolution for studies of neuropharmacology and drugs of abuse.

Analgesics, Opioid↗
PubMed2000

Expression of rat, renal NHE2 and NHE3 during postnatal development.

The current studies were designed to characterize the expression of sodium-hydrogen exchangers NHE2 and NHE3 during rat, renal ontogeny. NHE2 mRNA and immunoreactive protein were more highly expressed at 2 and 3 weeks of age, with declining levels into adulthood. In situ hybridization of NHE2 mRNA localized the message to the renal inner cortex and outer medullary regions and suggested higher mRNA levels in suckling animals as compared to adults. Immunohistochemical analysis of rat kidney with the NHE2 antiserum showed specific staining of the distal convoluted tubules. In contrast, NHE3 mRNA expression was lowest in 2-week animals and higher in older rats, while NHE3 immunoreactive protein showed constant expression levels during development. Additionally uptake experiments utilizing HOE694 showed no change in NHE2 or NHE3 functional protein expression in 2-week-old rats versus adults. We conclude that the developmental increase in NHE2 mRNA and immunoreactive protein expression cannot be detected by functional assays, which suggests that NHE2 does not play a role in sodium absorption by the renal tubules (as has been previously suggested). Additionally, molecular changes seen in NHE3 mRNA expression do not affect functional protein activity, suggesting increased mRNA translational efficiency or protein stability in suckling rats.

Age Factors↗
PubMed2000

The action of soybean lipoxygenase-1 on 12-iodo-cis-9-octadecenoic acid: the importance of C11-H bond breaking.

Previous work has demonstrated that the ferric form of soybean lipoxygenase-1 will catalyze an elimination reaction on 12-iodo-cis-9-octadecenoic acid (12-IODE) to produce 9, 11-octadecadienoic acid and iodide ion. Elimination is accompanied by irreversible inactivation of the enzyme on 1 out of 10 turnovers. In the present work, 11,11-dideuterio-12-IODE (D(2)-12-IODE) was synthesized and used to demonstrate that both the elimination reaction and inactivation of the enzyme exhibit very large kinetic isotope effects. The rates with the deuterated compound are so low that the isotope effects are difficult to quantify, but they appear to be comparable to the isotope effects previously observed for the normal reaction catalyzed by lipoxygenase and much larger than can be explained by zero-point energy considerations. ESR spectroscopy was used to demonstrate that 12-IODE can reduce ferric lipoxygenase to the ferrous form, and a large isotope effect on this process was observed with D(2)-12-IODE. It is proposed that the pathway leading to reduction and inactivation by 12-IODE is initiated by homolytic cleavage of the C(11)-H bond. Elimination could be initiated either by homolytic or by heterolytic cleavage of this bond. The results suggest that very large isotope effects may be a general feature of C-H bond cleavages catalyzed by this enzyme.

Deuterium↗
PubMed2000

Characterisation of receptor-specific TNFalpha functions in adipocyte cell lines lacking type 1 and 2 TNF receptors.

Tumour necrosis factor-alpha (TNFalpha) is a multifunctional cytokine that exerts a myriad of biological actions in numerous different tissues including adipocytes through its two distinct cell surface receptors. To address the role of each TNF receptor in the biological actions of TNFalpha in adipocytes, we have developed four new preadipocyte cell lines. These were established from wild type controls (TNFR1(+/+)R2(+/+)) and from mice lacking TNFR1 (TNFR1(-/-)), TNFR2 (TNFR2(-/-)) or both (TNFR1(-/-)R2(-/-)). All four new cell lines can fully differentiate to form mature adipocytes, under appropriate culture conditions, as judged by cell morphology, expression of multiple adipogenic markers and the ability to mediate agonist-stimulated lipolysis and insulin-stimulated glucose transport. In wild type (TNFR1(+/+)R2(+/+)) and TNFR2(-/-) adipocytes, TNFalpha stimulated lipolysis and inhibited insulin-stimulated glucose transport as well as insulin receptor autophosphorylation. In contrast, these activities were completely lost in the TNFR1(-/-)R2(-/-) and TNFR1(-/-) cells. Taken together, these studies demonstrate that TNFalpha-induced lipolysis, as well as inhibition of insulin-stimulated glucose transport are predominantly mediated by TNFR1 and that the presence of TNFR2 is not necessary for these functions. This new experimental system promises to be useful in dissecting the molecular pathways activated by each TNF receptor in mediating the biological functions of TNFalpha in differentiated adipocytes.

Adipocytes↗
PubMed2000

Three-layered atmospheric structure in accretion disks around stellar-mass black holes

Modeling of the x-ray spectra of the Galactic superluminal jet sources GRS 1915+105 and GRO J1655-40 reveals a three-layered atmospheric structure in the inner region of their accretion disks. Above the cold and optically thick disk with a temperature of 0.2 to 0.5 kiloelectron volts, there is a warm layer with a temperature of 1.0 to 1.5 kiloelectron volts and an optical depth around 10. Sometimes there is also a much hotter, optically thin corona above the warm layer, with a temperature of 100 kiloelectron volts or higher and an optical depth around unity. The structural similarity between the accretion disks and the solar atmosphere suggests that similar physical processes may be operating in these different systems.

Journal Article↗
PubMed2000

Effect of temperature on the separation of DNA fragments by high-performance liquid chromatography and capillary electrophoresis: a comparative study.

This study investigates the effect of experimental temperature on the separation of DNA fragments, 21-587 bp, by both high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE). The results show that the temperature plays an important role in the HPLC separation of DNA fragments. The optimum temperature was found to be between 40 and 50 degrees C for HPLC, while 25 degrees C was the optimum temperature for the CE separation. Also, although CE migration times became shorter, efficiency and resolution decreased with an increase in temperature from 25 to 50 degrees C, but the separation was not significantly affected. Also, the optimum HPLC temperature might be different depending on the fragment sizes to be resolved.

Chromatography, High Pressure Liquid↗
PubMed2000

Testosterone reduces neuronal secretion of Alzheimer's beta-amyloid peptides.

Alzheimer's disease (AD) is characterized by the age-related deposition of beta-amyloid (Abeta) 40/42 peptide aggregates in vulnerable brain regions. Multiple levels of evidence implicate a central role for Abeta in the pathophysiology of AD. Abeta peptides are generated by the regulated cleavage of an approximately 700-aa Abeta precursor protein (betaAPP). Full-length betaAPP can undergo proteolytic cleavage either within the Abeta domain to generate secreted sbetaAPPalpha or at the N- and C-terminal domain(s) of Abeta to generate amyloidogenic Abeta peptides. Several epidemiological studies have reported that estrogen replacement therapy protects against the development of AD in postmenopausal women. We previously reported that treating cultured neurons with 17beta-estradiol reduced the secretion of Abeta40/42 peptides, suggesting that estrogen replacement therapy may protect women against the development of AD by regulating betaAPP metabolism. Increasing evidence indicates that testosterone, especially bioavailable testosterone, decreases with age in older men and in postmenopausal women. We report here that treatment with testosterone increases the secretion of the nonamyloidogenic APP fragment, sbetaAPPalpha, and decreases the secretion of Abeta peptides from N2a cells and rat primary cerebrocortical neurons. These results raise the possibility that testosterone supplementation in elderly men may be protective in the treatment of AD.

Alzheimer Disease↗
PubMed2000

Dose-dependent effects of propofol on renal sympathetic nerve activity, blood pressure and heart rate in urethane-anesthetized rabbits.

To evaluate the role of the autonomic nervous system in hemodynamic changes after propofol bolus injection, we used direct recordings of renal sympathetic nerve activity to examine the dose-dependent effects of propofol (2.5, 5, 10, and 20 mg/kg) on heart rate, mean blood pressure and renal sympathetic nerve activity in urethane-anesthetized rabbits. The animals were divided into four groups: animals with an intact neuraxis (intact group), cervical vagal nerve-sectioned animals (vagotomy group), carotid sinus and aortic-nerve sectioned animals (SAD group), and animals with SAD plus vagotomy (SADV group). Heart rate did not change significantly even after administration of 2.5 and 5 mg/kg but decreased markedly on 20 mg/kg injection in all groups. The intact and vagotomy groups had augmented renal sympathetic nerve activity with insignificant changes in mean blood pressure after 5 mg/kg injection of the agent. Insignificant changes of renal sympathetic nerve activity but a remarkable decrease of mean blood pressure appeared after 10 mg/kg propofol. Sustained hypotension in parallel with a profound depression of renal sympathetic nerve activity developed at the dose of 20 mg/kg. In SAD and SADV groups, however, dose-dependent depressions of renal sympathetic nerve activity were accompanied by decreases of mean blood pressure. These results suggest the following: (1) propofol-induced hypotensive effects are probably produced by the central-mediated sympathetic depression. (2) The baroreceptor reflex may be preserved at the lower dose of the agent. (3) Heart rate does not change significantly unless a large dose of propofol is used. The difference in effects on heart rate and on mean blood pressure may denote a greater inhibition of sympathetic vascular outflow than of the cardiac sympathetic outflow regulating cardiac rate and contractility. This hypothesis needs further clarification.

Anesthesia↗
PubMed2000

I-CeuI fragment analysis of the Shigella species: evidence for large-scale chromosome rearrangement in S. dysenteriae and S. flexneri.

I-CeuI fragments of four Shigella species were analyzed to investigate their taxonomic distance from Escherichia coli and to collect substantiated evidence of their genetic relatedness because their ribosomal RNA sequences and similarity values of their chromosomal DNA/DNA hybridization had proved their taxonomic identity. I-CeuI digestion of genomic DNAs yielded seven fragments in every species, indicating that all the Shigella species contained seven sets of ribosome RNA operons. To determine the fragment identities, seven genes were selected from each I-CeuI fragment of E. coli strain K-12 and used as hybridization probes. Among the four Shigella species, S. boydii and S. sonnei showed hybridization patterns similar to those observed for E. coli strains; each gene probe hybridized to the I-CeuI fragments with sizes similar to that of the corresponding E. coli fragment. In contrast, S. dysenteriae and S. flexneri showed distinct patterns; rcsF and rbsR genes that located on different I-CeuI fragments in E. coli, fragments D and E, were found to co-locate on a fragment. Further analysis using an additional three genes that located on fragment D in K-12 revealed that some chromosome rearrangements involving the fragments corresponding to fragments D and E of K-12 took place in S. dysenteriae and S. flexneri.

Blotting, Southern↗
PubMed2000

Cellular and molecular basis of beta-amyloid precursor protein metabolism.

In molecular neurobiology, perhaps no molecule has been as thoroughly examined as Alzheimer's beta-amyloid precursor protein (beta-APP). In the years since the cDNA encoding beta-APP was cloned, the protein has been the subject of unparalleled scrutiny on all levels. From molecular genetics and cellular biology to neuroanatomy and epidemiology, no scientific discipline has been left unexplored - and with good reason. beta-amyloid (Abeta) is the main constituent of the amyloidogenic plaques which are a primary pathological hallmark of Alzheimer's disease, and bta-APP is the protein from which Abeta is cleaved and released. Unraveling the molecular events underlying Abeta generation has been, and remains, of paramount importance to scientists in our field. In this review we will trace the progress that has been made in understanding the molecular and cellular basis of beta-APP trafficking and processing, or alternatively stated, the molecular basis for Abeta generation. Imperative to a complete understanding of Abeta generation is the delineation of its subcellular localization and the identification of proteins that play either direct or accessory roles in Abeta generation. We will focus on the regulation of beta-APP cleavage through diverse signal transduction mechanisms and discuss possible points of therapeutic intercession in what has been postulated to be a seminal molecular step in the cascade of events terminating in the onset of dementia, loss of neurons, and eventual death from Alzheimer's disease.

Alzheimer Disease↗
PubMed2000

Functional magnetic stimulation facilitates gastrointestinal transit of liquids in rats.

The purpose of this study was to investigate the effect of a relatively novel technology, functional magnetic stimulation (FMS), on gastrointestinal transit of liquids in rats. Orogastric gavage with technetium-99 solution was used to assess gastric emptying and gastrointestinal transit time in 92 rats. FMS was performed over the anterior cervical and/or dorsal thoracolumbar regions using a figure-8 coil. Stimulation protocols were 1, 2, or 4 h in length. FMS accelerated gastric emptying and decreased gastrointestinal transit time. The acceleration was dependent on the stimulation parameters used as well as on the duration of the protocol; high levels of FMS produced a quicker effect, whereas lower levels were effective at later times. This study provides evidence that FMS could be an alternative or adjunct therapy to treat disorders in gastrointestinal motility.

Analysis of Variance↗
PubMed2000

CO(2) inhibits specific inward rectifier K(+) channels by decreases in intra- and extracellular pH.

Hypercapnia has been shown to affect cellular excitability by modulating K(+) channels. To understand the mechanisms for this modulation, four cloned K(+) channels were studied by expressing them in Xenopus oocytes. Exposures of the oocytes to CO(2) for 4-6 min produced reversible and concentration-dependent inhibitions of Kir1.1 and Kir2.3 currents, but had no effect on Kir2.1 and Kir6.1 currents. Intra- and extracellular pH (pH(i), pH(o)) dropped during CO(2) exposures. The inhibition of Kir2.3 currents was mediated by reductions in both intra- and extracellular pH, whereas the suppression of Kir1.1 resulted from intracellular acidification. In cell-free excised inside-out patches with cytosolic-soluble factors washed out, a decrease in pH(i) produced a fast and reversible inhibition of macroscopic Kir2.3 currents. The degree of this inhibition was similar to that produced by hypercapnia when compared at the same pH(i) level. Exposure of cytosolic surface of patch membranes to a perfusate bubbled with 15% CO(2) without changing pH failed to inhibit the Kir2.3 currents. These results therefore indicate that (1) hypercapnia inhibits specific K(+) channels, (2) these inhibitions are caused by intra- and extracellular protons rather than molecular CO(2), and (3) these effects are independent of cytosol-soluble factors.

Animals↗