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Biomedical subjects

X Chen

Publications and source records attributed to X Chen.

At least 901 records · Page 50Linked to original sources
PubMed1998

The influence of alveolar structures on the torsional strain field in a gorilla corporeal cross-section.

Anthropologists have often used mandibular torsional properties to make inferences about primate dietary adaptations. Most of the methods employed are based on assumptions related to periodontal and alveolar properties. This study uses the finite element method to evaluate some of these assumptions with a cross-section through the third molar of a gorilla. Results indicate that the properties of alveolar bone play an important role in determining the strain field. In comparison, the exact stiffness values of the periodontal ligaments seem to have a much smaller impact. Replacing the dental roots and periodontal ligaments with alveolar bone, however, has a significant influence on the strain field. It underestimates the maximum shear strain by about 28% along its periosteal aspect when alveoli are modeled as cortical bone. It overestimates the strain by a smaller amount when alveoli are modeled as trabecular bone. This study supports the assumption that primate mandibles behave like a closed-section under torsion under the limiting condition that the alveolar bone stiffness is more than half of the value of cortical bone; alveolar bone can then be modeled as cortical bone with a minimal loss of accuracy. In addition, this study suggests that the minimum cortical thickness should be considered for torsional strength. Finally, modeling accuracy can be significantly increased if both dental and periodontal structures can be realistically incorporated into mandibular biomechanical models. However, this may not be always feasible in studies of fossil mandibles. This is due mainly to the difficulties involved in estimating alveolar bone densities and in distinguishing boundaries between cortical bone, alveolar bone, periodontal ligaments, and dental roots in fossil specimens.

Adaptation, Physiological↗
PubMed1998

Increased expression of Brevibacterium sterolicum cholesterol oxidase in Escherichia coli by genetic modification.

To improve expression of Brevibacterium sterolicum cholesterol oxidase in Escherichia coli, we utilized the T7lac promoter and modified the gene to encode the first 21 amino acids with high-expression E. coli codons. These changes resulted in a 60-fold improvement of expression level. N-terminal sequencing revealed that the E. coli produced cholesterol oxidase signal peptide is cleaved 6 amino acids closer to the N-terminus than in B. sterolicum. The recombinant E. coli produced protein is composed of 513 amino acids with a calculated Mr of 55,374. The kinetic rate constants of the recombinant protein and the B. sterolicum produced cholesterol oxidase are identical.

Amino Acid Sequence↗
PubMed1998

Acoustic backscatter properties of the particle/bubble ultrasound contrast agent.

Bubble-based suspensions with diameters in the 1-5 microns range have been developed for use as ultrasound contrast agents. Bubbles of these dimensions have resonance frequencies in the diagnostic ultrasonic range, thus improving their backscatter enhancement capabilities. The durability of these bubbles in the blood stream has been found to be limited, providing impetus for a number of approaches to further stabilize them. One of the approaches has been the development of micrometer-size porous particles or 'nano-sponges' with properties suitable for the entrapment and stabilization of gas bubbles. However, the complex morphology and surface chemistry involved in the production of this type of agent makes it unfeasible to directly measure the volume of the entrained gas. A model based on acoustic scattering principles is proposed which indicates that only a small volume fraction of gas should be necessary to significantly enhance the echogenicity of this type of particle-based contrast agent. In the model, the effective scattering cross-section is evaluated as a function of the volume fraction of gas contained in the overall scatterer and the overall scatterer diameter. Initially, the volume fraction of gas is considered as a discrete entity of single bubble. Using common mixture rules, it is then shown that the gas can be considered to be distributed throughout the particle and still arrive at a result that is similar to that for a single, discrete volume of gas. The main contribution to the increased scattering cross-section is due to the compressibility difference between gas and water. The backscatter coefficient is computed as the product of the resulting differential scattering cross-section and the scatterer number density. This approach facilitates comparison with known backscatter coefficients of biological targets such as liver and blood. Simple experimental results are presented for comparison with the model, and the implications relevant to clinical use are suggested.

Animals↗
PubMed1998

Electrophysiological studies of neurohypophysial neurons and peptides.

We have used hypothalamic slices of the supraoptic nucleus (SON) to investigate synaptic control of magnocellular vasopressinergic and oxytocinergic neurons. With the use of perforated patch recording techniques we identified and isolated excitatory or inhibitory postsynaptic currents elicited by electrical stimulation of afferent fibers. Both inhibitory and excitatory afferent fibers displayed presynaptic GABAB receptors; the GABAB agonist, baclofen caused a dose-dependent suppression of the evoked potentials in the absence of any effects on postsynaptic input resistance. Further evidence for a presynaptic locus included an increase in paired pulse ratio and a lack of effect on currents elicited by exogenously applied muscimol (a GABAA receptor agonist) or AMPA (a glutamate agonist). With the use of an GABAB receptor antagonist we demonstrated an action of endogenously released GABA, acting at GABAB receptors on excitatory terminals, to reduce excitatory transmission. In addition to presynaptic modulation by GABA of afferent inputs, we also observed actions of vasopressin and oxytocin, released from dendrites of magnocellular SON neurons, to gate afferent, excitatory transmission in the SON. Exogenously applied vasopressin and oxytocin, or these peptides when released by depolarizing stimuli of magnocellular neurons, reduced the size of evoked excitatory postsynaptic potentials at a presynaptic locus. We have also observed actions of arginine vasopressin to modulate the action of glutamate in slices of the ventral septal area and to attenuate a glutamate-mediated excitatory postsynaptic current in slices of the parabrachial nucleus.

Animals↗
PubMed1998

Arginine vasopressin, fever and temperature regulation.

While central administration of arginine vasopressin (VP) to the non-febrile rat at high doses can cause hypothermia, there is little evidence for a role for endogenous VP in normal thermoregulation. In contrast, VP arising from cell bodies in the bed nucleus of the stria terminalis and innervating the ventral septal areas and possibly the amygdala appears to be an endogenous antipyretic, i.e. a substance capable of reducing fever. As the synthesis of VP in bed nucleus neurons is dependent upon circulating androgens, female rats have much less VP in these cells and their projections than do male rats. In keeping with this, females may make use of VP to a lesser extent than do males to bring about antipyresis. The phenomenon whereby the VP receptor can become sensitized by previous exposure to VP may be responsible for some states of endogenous antipyresis, in which fevers are suppressed through overactivity of the vasopressinergic system. States of endogenous antipyresis can be revealed around the time of parturition in both the neonate and the mother.

Animals↗
PubMed1998

Role of neuropeptide Y in cold stress-induced hypertension.

Chronic cold stress (4 degrees C) produced a sustained increase in mean arterial pressure in both normotensive and borderline hypertensive rats (BHR). The high blood pressure in BHRs was significantly reversed by a neuropeptide Y (NPY) Y1 receptor antagonist suggesting that NPY is involved in mediating stress-induced hypertension. Corresponding increases in adrenal NPY messenger RNA and NPY immunoreactivity were found during the stress; furthermore, chronic cold stress also potentiated the pressor response of rats to a subsequent acute stress test in which NPY has been shown to play a role. These results suggest that chronic cold stress-induced hypertension is mediated by elevated NPY release and vascular tone as a result of increased NPY gene expression and storage.

Adrenal Glands↗
PubMed1998

Distinct patterns of T cell receptor distribution of peripheral blood CD8+ cells during different stages of chronic infection with hepatitis B virus.

We compared the Vbeta TCR repertoires of CD8+ peripheral blood lymphocytes between 21 patients with chronic hepatitis B (CHB) and 9 healthy individuals, using RT-PCR analysis. Several differences were seen between CHB patients and controls, including a marked increase in the expression of two to five Vbeta families in the CHB patients. There was no evidence for a superantigen effect, although an increase in Vbeta7 was seen in 64% of patients. A significant under-expression of Vbeta families were also detected, particularly in patients with active liver disease in which under-expression of Vbeta14 and Vbeta15 was associated with acute exacerbations of liver disease. We also did a longitudinal analysis of the TCR repertoire in two patients over a period of 6 months, from the initiation of a disease flare to its resolution. One patient continued to experience spontaneous flares following the completion of this study, while the other patient underwent spontaneous remission with long-term (> 12 months) loss of HBeAg following resolution of the flare. The TCR repertoires of both patients were altered during the flare, and there was a higher degree of TCR variability in the patient who went into remission.

Adult↗
PubMed1998

Characterization of acidic Ca(2+)-independent phospholipase A2 of bovine lung.

An acidic Ca(2+)-independent phospholipase A2 (aiPLA2) has been isolated previously from rat lung and a human cDNA has been described. This study applied the method to larger scale isolation of the native protein from the bovine lung. A polyclonal antibody was generated to a 15 amino acid synthetic peptide based on a conserved rat/human sequence. This antibody recognized a single protein band with an estimated molecular mass of approximately 29 kDa in a soluble fraction obtained from bovine lung homogenate. A 29 kDa protein that reacted with the aiPLA2 antipeptide antibody was detected in fractions containing aiPLA2 activity on sequential column chromatographies. The partially purified enzyme showed 176-fold increase over the homogenate in Ca(2+)-independent PLA2 activity at pH 4. Activity was maximal with phosphatidylcholine substrate and was significantly less with phosphatidylethanolamine and anionic phospholipids. The enzyme had no acyl group preference in phosphatidylcholine and showed no preference for oxidized substrate, but activity was less with 1-O-alkyl phosphatidylcholine. aiPLA2 activity was inhibited by a transition state phospholipid analog (MJ33, 1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol), serine protease inhibitors, and the anti-peptide antibody but was insensitive to arachidonoyl trifluoromethyl ketone, bromoenol lactone, p-bromophenacyl bromide, and ATP. Analysis of N-terminal amino acid sequence for the 29 kDa protein demonstrated its high homology to human 26 kDa aiPLA2. These was no significant change in molecular mass of the protein following treatment with endoglycosidase F. Western blot of subcellular fractions from rat lung indicated aiPLA2 immunoreactivity with lamellar body, lysosomal, and cytosolic fractions. These results indicate isolation from bovine lung of a 29 kDa acidic Ca(2+)-independent phospholipase A2 homologue of the rat and human enzyme and provide evidence for specificity in the metabolism of lung surfactant phosphatidylcholine.

Amino Acid Sequence↗
PubMed1998

The effect of early burn wound excision on regional gastric blood flow in rats.

This study examined the effects of early burn wound excision on gastric blood flow and on morphologic changes in mucosal vessels. Wistar rats were given a 30% total body surface area burn and divided into four groups, consisting of control animals (group 1), animals with burn injury without and with fluid resuscitation (groups 2 and 3, respectively), and animals with both fluid resuscitation and early wound excision (group 4). Gastric mucosal blood flow (GMBF) was measured by the hydrogen gas clearance method up to 24 h post-burn. Morphologic changes in mucosal vessels were examined by scanning electron microscopy (SEM) at 3 and 24 h post-burn. The GMBF sharply decreased in the acute period after the burn. In group 4, however, it recovered to the initial value by 6 h post-burn and there was no significant change throughout the experiment. Morphologically, although the mucosal capillaries revealed some changes such as irregularity in diameter in groups 2-4 at 3 h, most of mucosal capillaries retained their original appearance in group 4 at 24 h post-burn. These result suggest that early excision does not aggravate the state of gastric ischemia.

Animals↗
PubMed1998

Methods for quantifying ultrasound backscatter and two-dimensional video intensity: implications for contrast-enhanced sonography.

Quantification of acoustic backscatter energy is believed to be useful for assessing "tissue character" and for quantifying the regional concentration of echo contrast. Measurement of ultrasonic video intensity has been the traditional means of quantifying backscatter energy, with "integrated backscatter" considered the gold standard. The purpose of this work is to review the commonly used methods for quantifying ultrasonic backscatter and to describe the difference between detected backscatter energy and the intrinsic tissue backscatter coefficient. Many of the quantification pitfalls that can lead to erroneous conclusions will also be discussed. A set of eight rubber phantoms with backscatter coefficient from -6 dB to +15 dB relative to liver were imaged at 2.5, 3.5, and 5.0 MHz. Methods for calculating the acoustic backscatter intensity from calibrated video intensity measurements and for calculating the tissue backscatter coefficient are described and tested using equipment from two different manufacturers. A commercially available automatic "acoustic densitometry" system with on-board quantitative integrated backscatter is also evaluated. Ultrasound attenuation and ultrasound system factors were found to strongly influence the detected backscatter intensity using either calibrated video intensity or on-board integrated backscatter. Special system transfer functions and attenuation correction were found to be useful in converting video intensity and integrated backscatter to a measure of the intrinsic tissue backscatter coefficient. With these correction factors, the correlation between the measured tissue backscatter coefficient and the phantom backscatter coefficient was excellent (r = 0.99, intercept 0.0, regression slope essentially 1.0) at all three imaging frequencies with traditional video intensity or on-board integrated backscatter. Uncalibrated video intensity and on-board integrated backscatter have limitations when used in isolation for tissue characterization. Rigorous attention to the imaging parameters and the use of calibration functions are necessary before video intensity measurement or integrated backscatter can be used reliably to measure the tissue backscatter coefficient.

Acoustics↗
PubMed1998

A mouse homologue of FAST-1 transduces TGF beta superfamily signals and is expressed during early embryogenesis.

The transcription factor FAST-1 has recently been shown to play a key role in the specification of mesoderm by TGF beta superfamily signals in the early Xenopus embryo. We have cloned Fast1, a mouse homologue of Xenopus FAST-1, and characterized its expression during embryogenesis and function in activin/TGF beta signal transduction. In vitro, Fast1 associates with Smads in response to an activin/TGF beta signal to form a complex that recognizes the Xenopus activin responsive element (ARE) targeted by Xenopus FAST-1. In intact cells, introduction of Fast1 confers activin/TGF beta regulation of an ARE-luciferase reporter. In embryos, Fast1 is expressed predominantly throughout the epiblast before gastrulation and declines as development progresses. We propose that mouse Fast1, like Xenopus FAST-1, mediates TGF beta superfamily signals specifying developmental fate during early embryogenesis.

Animals↗
PubMed1998

A simple reproducible model of free radical-injured isolated heart induced by 1,1-diphenyl-2-picryl-hydrazyl (DPPH).

1,1-Diphenyl-2-Picryl-Hydrazyl (DPPH), a stable free radical, has been used for detecting antioxidant activity in chemical analysis. However, it is still unknown if DPPH triggers free radical injury in cardiac tissue. In order to establish a simple free radical-injured isolated heart model, we investigated the action of DPPH on isolated guinea pig heart by Langendorff perfusion and compared it with cardiac effect of superoxide anion (O2.-), generated by the hypoxanthine (HX)-xanthine oxidase (XO) system. Free radical scavengers, dimethyl sulphoxide (DMSO), superoxide dismutase (SOD), and L-cysteine, were also used to analyze the characteristic of the DPPH free radical-derived cardiac dysfunction. In isolated guinea pig hearts, DPPH 100 nM and 250 nM in Krebs-Henseleit solution significantly decreased the left ventricular developed pressure (LVDP) and maximum velocity changes of left ventricular pressure (+/-LVdP/dtmax), elevated the left ventricular end-diastolic pressure (LVEDP), and increased lactate dehydrogenase (LDH) release and thiobarbituric acid-reactive substances (TBARS) formation in cardiac tissue. The cardiac dysfunction induced by DPPH 250 nM was more intense than 100 nM. L-Cysteine improved the DPPH-impaired cardiac function, while DMSO and SOD had no beneficial effect on this injury. The cardiac membrane fluidity was decreased by DPPH. Free radical signals, detected by electron spin resonance (ESR) in the DPPH-injured heart, were reduced by L-cysteine-treatment. These results suggest that DPPH free radical-induced cardiac dysfunction is attributed to neither the superoxide anion nor the hydroxyl radical. In conclusion, our data indicate that DPPH-induced isolated heart dysfunction serves as a simple and reproducible free radical-injured heart model.

Animals↗
PubMed1998

GAA instability in Friedreich's Ataxia shares a common, DNA-directed and intraallelic mechanism with other trinucleotide diseases.

We show that GAA instability in Friedreich's Ataxia is a DNA-directed mutation caused by improper DNA structure at the repeat region. Unlike CAG or CGG repeats, which form hairpins, GAA repeats form a YRY triple helix containing non-Watson-Crick pairs. As with hairpins, triplex mediates intergenerational instability in 96% of transmissions. In families with Friedreich's Ataxia, the only recessive trinucleotide disease, GAA instability is not a function of the number of long alleles, ruling out homologous recombination or gene conversion as a major mechanism. The similarity of mutation pattern among triple repeat-related diseases indicates that all trinucleotide instability occurs by a common, intraallelic mechanism that depends on DNA structure. Secondary structure mediates instability by creating strong polymerase pause sites at or within the repeats, facilitating slippage or sister chromatid exchange.

Alleles↗
PubMed1998

Stimulation of MAP kinase and S6 kinase by vanadium and selenium in rat adipocytes.

To explore the mechanism underlying the insulin-mimetic actions of vanadium and selenium we examined their effects on the mitogen activated protein/myelin basic protein kinases (MAPK) and ribosomal S6 protein kinases, which are among the best characterized of the kinases that comprise the phosphorylation cascade in insulin signal transduction. We observed a transient activation of MAPK and S6 kinases by insulin in rat adipocytes, while both sodium selenate and vanadyl sulphate produced prolonged activation of the kinases. Vanadyl sulphate stimulated the activity of MAPK and S6 kinase by as much as 6 fold and 15 fold, respectively. Pretreatment of the cells with genistein did not affect the activation of MAPK by insulin, but partially blocked the effects of sodium selenate and vanadyl sulphate. Genistein did not change the activation of S6 kinase by insulin, but blocked the activation in vanadyl sulphate- and sodium selenate-treated-cells, suggesting that a genistein sensitive tyrosine kinase may be involved in the activation by these two compounds. Rapamycin, a specific inhibitor of the p70s6k isoform of S6 kinase, partially reduced the activation of S6 kinase activity by sodium selenate, indicating a role for this kinase in the overall activity of the S6 kinase in sodium selenate-treated cells. A similar trend was noted in vanadyl sulphate-treated cells. Thus, this study supports the involvement of MAPK and S6 kinases in the insulin-mimetic actions of vanadium and selenium.

Adipose Tissue↗
PubMed1998

The gene 4 of rice yellow stunt rhabdovirus encodes the matrix protein.

The complete nucleotide sequence of the gene 4 of rice yellow stunt rhabdovirus (RYSV) was determined from cDNAs corresponding to the viral genomic RNA. Gene 4 is 913 nucleotides (nt) long, comprising a 17-nt untranslated 5' region, a 786-nt open reading frame encoding a polypeptide with a molecular mass of 29,125 Da, and a 110-nt untranslated 3' region. Western blot analysis of the RYSV proteins using the antiserum raised against the protein expressed from the cloned gene in Escherichia coli indicates that gene 4 encodes the M protein of RYSV. Comparisons of the deduced amino acid sequence of the M protein of RYSV with those of other rhabdoviruses revealed no significant homologies. However, it shared a similar basic property and a similar distribution of charges with the other rhabdovirus matrix proteins and showed a relatively closer relationship to the sonchus yellow net virus (SYNV) M1 protein.

Amino Acid Sequence↗
PubMed1998

Child-rearing attitudes and behavioral inhibition in Chinese and Canadian toddlers: a cross-cultural study.

Behavioral inhibition data were collected from samples of 2-year-olds from the People's Republic of China and Canada. Information on child-rearing attitudes and beliefs was obtained from mothers of the children. Chinese toddlers were significantly more inhibited than their Canadian counterparts. Inhibition was associated positively with mothers' punishment orientation and negatively with mothers' acceptance and encouragement of achievement in the Canadian sample. However, the directions of the relations were opposite in the Chinese sample; child inhibition was associated positively with mothers' warm and accepting attitudes and negatively with rejection and punishment orientation. The results indicated different adaptational meanings of behavioral inhibition across cultures.

Canada↗