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Biomedical subjects

X Chi

Publications and source records attributed to X Chi.

18 recordsLinked to original sources
PubMed2005

Landau quantization effects in the charge-density-wave system (Per)2M(mnt)2 (where M = Au and Pt).

A finite transfer integral t(a) orthogonal to the conducting chains of a highly one-dimensional metal gives rise to empty and filled bands that simulate an indirect-gap semiconductor upon formation of a charge-density wave (CDW). In contrast to semiconductors such as Ge and Si with band gaps approximately 1 eV, the CDW system possesses an indirect gap with a greatly reduced energy scale, enabling moderate laboratory magnetic fields to have a major effect. The consequent variation of the thermodynamic gap with magnetic field due to Zeeman splitting and Landau quantization enables the electronic band structure parameters (transfer integrals, Fermi velocity) to be determined accurately. These parameters reveal the orbital quantization limit to be reached at approximately 20 T in (Per)2M(mnt)(2) salts, making them highly unlikely candidates for a recently proposed cascade of field-induced CDW states.

Journal Article↗
PubMed2004

Amorphouslike density of gap states in single-crystal pentacene.

We show that optical and electrical measurements on pentacene single crystals can be used to extract the density of states in the highest occupied molecular orbital-lowest unoccupied molecular orbital band gap. It is found that these highly purified crystals possess band tails broader than those typically observed in inorganic amorphous solids. Results on field-effect transistors fabricated from similar crystals imply that the gap state density is much larger within 5-10 nm of the gate dielectric. Thus, organic thin-film transistors for such applications as flexible displays might be significantly improved by reducing these defects.

Journal Article↗
PubMed2004

Charge-density waves survive the Pauli paramagnetic limit.

Measurements of the resistance of single crystals of (Per)2Au(mnt)2 have been made at magnetic fields B of up to 45 T, exceeding the anticipated Pauli paramagnetic limit of Bp approximately 37 T. The continued presence of nonlinear charge-density wave electrodynamics at B> or =37 T establishes the survival of the charge-density wave state above this limit, and the probable emergence of an inhomogeneous phase analogous to that anticipated to occur in superconductors.

Journal Article↗
PubMed2004

Bias-dependent generation and quenching of defects in pentacene.

We describe a defect generation phenomenon that is new to organic semiconductors. A defect in pentacene single crystals can be created by bias-stress and persists at room temperature for an hour in the dark but only seconds with 420 nm illumination. The defect gives rise to a hole trap at Ev+0.38 eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67 eV activation energy with a small (10(8) s(-1)) prefactor, suggesting that atomic motion plays a key role in the generation and quenching process.

Journal Article↗
PubMed2002

Magneto-opto-electronic bistability in a phenalenyl-based neutral radical.

A new organic molecular conductor, based on a spiro-biphenalenyl neutral radical, simultaneously exhibits bistability in three physical channels: electrical, optical, and magnetic. In the paramagnetic state, the unpaired electrons are located in the exterior phenalenyl units of the dimer, whereas in the diamagnetic state the electrons migrate to the interior phenalenyl units and spin pair as a pi-dimer. Against all expectations, the conductivity increases by two orders of magnitude in the diamagnetic state, and the band gap decreases. This type of multifunctional material has the potential to be used as the basis for new types of electronic devices, where multiple physical channels are used for writing, reading, and transferring information.

Journal Article↗
PubMed2001

Dimeric phenalenyl-based neutral radical molecular conductors.

We report the preparation, crystallization, and solid-state characterization of ethyl (3)- and butyl (4)-substituted spiro-biphenalenyl radicals. Both of these compounds are found to be conducting face-to-face pi-dimers in the solid state but with different room-temperature magnetic ground states. At room temperature, 4 exists as a diamagnetic pi-dimer (interplanar separation of approximately 3.1 A), whereas 3 is a paramagnetic pi-dimer (interplanar separation of approximately 3.3 A), and both compounds show phase transitions between the paramagnetic and diamagnetic forms. Electrical resistivity measurements of single crystals of 3 and 4 show that the transition from the high-temperature paramagnetic pi-dimer form to the low-temperature diamagnetic pi-dimer structure is accompanied by an increase in conductivity by about 2 orders of magnitude. This behavior is unprecedented and is very difficult to reconcile with the usual understanding of a Peierls dimerization, which inevitably leads to an insulating ground state. We tentatively assign the enhancement in the conductivity to a decrease in the on-site Coulombic correlation energy (U), as the dimers form a super-molecule with twice the amount of conjugation.

Journal Article↗
PubMed2001

Down-regulation of regulatory proteins for differentiation and proliferation in murine fetal hypoplastic lungs: altered mesenchymal-epithelial interactions.

We compared proliferation (growth) and differentiation (development) related proteins in normal and hypoplastic fetal murine lungs. The hypoplastic lungs were created in CD-1 fetal mice by nitrofen exposure (25 mg per pregnant mouse given intragastrically on gestational day 8 [Gd8]), as published earlier. The lungs were harvested at Gd14, 16, 19 and from neonates. Immunoblot analyses were carried out for transcription factors (oncogenic proteins, nuclear receptor, and transmembrane receptor proteins) in severely hypoplastic murine fetal lungs with coexistent diaphragmatic hernia, and results were compared with those derived from normal lungs of equivalent age. These proteins have proposed roles in the regulation of proliferation and differentiation processes of fetal lungs. We have shown that the product of the oncogene c-myc was reduced in hypoplastic lungs at all stages of gestation, whereas c-Fos protein levels were variable. These proteins are known to regulate transcription of various developmental proteins, such as those responsible for proliferation and differentiation. Further, the nuclear transcription factors thyroid transcription factor-1 (TITF-1) and glucocorticoid receptor (GR) were reduced, and thyroid hormone receptor (TR) and retinoic acid receptors (RARs) were inhibited in severely hypoplastic lungs compared to normal lungs of equivalent gestational stage, except in neonatal lungs, where signals for RARs were seen. TITF-1 is known to localize in bronchial epithelial cells in developing lungs. It is restricted to type II pneumocytes with gestational development in the normal lungs and regulates surfactant proteins. Earlier, we have reported that surfactant proteins are reduced in hypoplastic lungs. In the current study, reduced GR and TITF-1 proteins may play a role in reducing surfactant proteins in the hypoplastic lungs. The significant inhibition in TR and RARalpha in the severely hypoplastic lungs reflects on affected epithelial cell maturation and alveolar formation, respectively. Altered RARbeta levels correlate with affected lung growth and branching morphogenesis of nitrofen-exposed lungs. A transmembrane receptor protein EGFR was reduced in hypoplastic lungs, suggesting the involvement of altered mesenchymal-epithelial signal transduction pathways. We conclude (1) Our data suggest altered levels of various nuclear transcription factors in the murine fetal hypoplastic lungs; (2) Reduced levels TITF-1 protein in hypoplastic lungs may have caused the functional immaturity of distal lung, immature airways and thus may affect overall differentiation of lungs. These results correlated with low levels of surfactant proteins in these lungs; (3) TR and RAR inhibition indicate their roles through reduced or retarded proliferation and differentiation processes in the severely hypoplastic lungs; (4) GR down-regulation in developing fetal murine hypoplastic lungs indicate delayed development, and GR up-regulation in affected neonates may be induced by stress/stretch caused at birth due to air-breathing; (5) Down- regulation of EGFR indicate altered mesenchymal-epithelial interactions and possible influence on lung proliferation and differentiation.

Animals↗
PubMed2001

Physiological levels of beta-amyloid induce cerebral vessel dysfunction and reduce endothelial nitric oxide production.

beta-amyloid (A beta), the major component of senile plaques in Alzheimer's disease (AD), normally circulates in the blood at nanomolar levels but is elevated in AD. Previous studies have found that high concentrations (10(-5)-10(-4) M) of A beta result in neuronal cell death. Here we show that physiological levels of soluble A beta can induce dysfunction in perfused rat cerebral vessels and in cultured endothelial cells. At concentrations of 10(-9)-10(-6) M, A beta induced a significant concentration-dependent reduction of NO production in endothelial cells. At 10(-8) M, A beta significantly decreased the sensitivity of cerebral vessels to acetylcholine (ACh), an endothelium dependent vasodilator. At 10(-7) M and higher concentrations, A beta significantly reduced the maximum response of vessels to ACh, and induced significant endothelial cell death. A beta (10(-9)-10(-5) M) did not cause any detectable change in nitric oxide synthase levels. The results suggest that a modest increase in the concentration of A beta above its normal physiological level in the circulation, as found in the early stages of AD, results in decreased NO production and vessel sensitivity to endothelium-dependent vasodilation that could lead to constricted blood vessels and ischemia in the surrounding tissue. Further increases in A beta concentration, which may occur in the later stages of AD, result in cell death and decreased maximum vasodilator response of cerebral vessels.

Acetylcholine↗
PubMed2000

Potassium channel openers prevent beta-amyloid toxicity in bovine vascular endothelial cells.

There is increasing evidence that the cerebrovasculture may be involved in the pathology of Alzheimer's disease. Here, we report that potassium channel openers (KCOs) inhibit dose and time dependent necrosis induced by beta-amyloid (Abeta) in cultured vascular endothelial cells. Cell proliferation rate was assayed by a colorimetric method. Abeta cytotoxicity and inhibition by the K(ATP) channel opener diazoxide and the K(Ca) channel opener NS1619 was correlated with changes in nitric oxide (NO) production. The protective effects were partly blocked by potassium channel blockers. Toxicity of Abeta and KCO protection was verified by histological examination of endothelial cells with scanning electron microscopy. eNOS levels in endothelial cells were not changed by any of the treatments. The results suggest that disruption of K(+) channels function may be a critical step in Abeta-induced cytotoxicity in endothelial cells by alteration of NO release.

Amyloid beta-Peptides↗
PubMed2000

[A theoretical study on vibrational spectra of S7 rings].

The geometries of S7 rings with the chair-type and boat-type conformations have been optimized using SCF-HF and hybrid density functional methods. (B3LYP) with the split valence 3-21 + G(d) and 6-31 + G(d) basis sets. Vibrational spectra of S7 rings have been predicted by using the same two methods: B3LYP/6-31 + G(d) and HF/3-21 + G(d) over the geometries optimized above, respectively. The symmetries of normal vibrational modes have been assigned. We also make comparisons between the present predictions and the previous experimental data.

Models, Chemical↗
PubMed1999

The protective effect of K+ channel openers on beta-amyloid induced cerebrovascular endothelial dysfunction.

Amyloid angiopathy is characterized by amyloid beta-peptide (A beta) deposition and may contribute to the cerebrovascular abnormalities that precede the onset of Alzheimer's Disease (AD). That aberrant potassium (K+) channel function occurs in AD patients is supported by deleterious effects of A beta on normal fibroblast K+ channels and prevention of A beta-induced toxicity by potassium channel openers (KCOs) in neuronal cell culture. We report here that KCOs protect cerebral and peripheral vessels against the endothelial damage induced by A beta. Pressurized posterior cerebral artery and aortic ring segments from the rat were constricted and then relaxed with the endothelium-dependent vasodilator acetylcholine before and after incubation with A beta (10(-6) M), or pre-treatment with KCOs before the addition of beta-amyloid. Vessels treated with A beta exhibited features of endothelial dysfunction: enhanced vasoconstriction and diminished endothelium-dependent vasodilation. Pre-treatment with KCOs significantly antagonized the A beta effect in both cerebral and aortic vessel segments. This protection was provided by both KCa and KATP channel openers. Endothelial damage by A beta and protection by KCOs was verified by electron microscopy. The K+ channel blocker, TEA, reversed the protective effect of KCO. The results suggest that potassium channel openers protect against A beta induced endothelial dysfunction and that KCOs may have a role in the treatment of degenerative cerebrovascular disease as seen in stroke, AD and aging.

Acetylcholine↗
PubMed1999

A hypergraphic model of medical informatics: curriculum development guide.

Medical informatics, as a descriptive, scientific study, must be mathematically or theoretically described. Is it important to define a model for medical informatics? The answer is worth pursuing. The medical informatics profession stands to benefit three-fold: first, by clarifying the vagueness of the definition of medical informatics, secondly, by identifying the scope and content for educational programs, and, thirdly, by defining career opportunities for its graduates. Existing medical informatics curricula are not comparable. Consequently, the knowledge and skills of graduates from these programs are difficult to assess. The challenge is to promote academics that develops graduates for prospective employers to fulfill the criteria of the health care industry and, simultaneously, compete with computer science programs that produce information technology graduates. In order to meet this challenge, medical informatics programs must have unique curricula that distinguishes its graduates. The solution is to educate students in a comparable manner across the domain of medical informatics. This paper discusses a theoretical model for medical informatics.

Computers↗
PubMed1998

Validation of computer-mediated open-ended standardized patient assessments.

Quantitative information regarding the development of students' clinical reasoning skills is valuable in assessing third- and fourth-year medical students' clinical knowledge. Standardized patient cases are often used to obtain this quantified information. These cases typically involve a post-patient examination consisting of a series of closed-ended multiple-choice questions. Many medical educators question whether the results from the closed-ended multiple-choice questions truly reflect students' clinical knowledge and reasoning proficiency. Since 1995, the Kirksville College of Osteopathic Medicine (KCOM) has used a computer program, SOAP Note Plus, for standardized patient assessment of students to automate the post-encounter documentation and evaluation. This paper describes the development of the SOAP Note Plus program and a validation study which was conducted to determine the empirical association between the computer-mediated closed-ended and open-ended post-encounter standardized patient assessments. Correlation and GENOVA statistics were used in the analysis of the performance of third-year medical students on the closed-ended and the open-ended assessments following standardized patient encounters and the relationship to their undergraduate GPA, first 2-year medical school GPA and the actual clinical rotation evaluations. The initial results show the positive relationship between the open-ended assessment and the actual clinical rotation evaluations. undergraduate GPA and the first two-years of medical school GPA.

Clinical Competence↗
PubMed1998

Identification of a novel alternatively spliced mRNA of murine pulmonary surfactant protein B.

An alternatively spliced mRNA of pulmonary surfactant protein B (SP-B) was identified in murine lung. Sequencing analysis revealed a 69 base-pair (bp) deletion at the beginning of exon 7 of SP-B, presumably the result of an alternative splicing event. Reverse transcription-polymerase chain reaction (RT-PCR) of mouse, rat, and rabbit lung RNA revealed the existence of full-length and the 69-bp deleted short form. Ribonuclease protection assay of the SP-B messenger RNA (mRNA) demonstrated expression of both isoforms in five strains of adult and fetal mice with different genetic backgrounds, as well as in rabbit, but not in human. Splice junction sequences in exon 6 and at the exon 7 splice boundary for the two isoforms are similar, including AG doublet identity, but sequence differences do not account for species variation in isoform abundance. The abundance of the short SP-B mRNA isoform was approximately 30% of total SP-B mRNA in mouse and rabbit. Analysis of precursor SP-B protein in mouse lung suggested that the two mRNA species are expressed as stable protein isoforms.

Alternative Splicing↗
PubMed1997

[Physiological characteristics of single K+ channel currents (Ik) in cerebellar cortical neurons of neonatal rat].

The characteristics of single K+ channels in cultured cerebellar granule neurons of neonatal rat were studied by the cell- attached and inside-out configurations of patch clamp technique. The results are as follows: the predominant conductance of single channel was 25PS, the currents amplitude, the mean open time and the open probability were dependent of membrane potential, the open time distribution histograms were fitted by two exponential components, the channel activity was not inactivated during 20 s period, independent of calcium and blocked by tetraethylammonium. These results indicated that the single channel was a delayed rectifier (Ik). It is suggested that the non-inactivating channel with conductance of 25pS which exists in the cerebellar granule cell might be a new subtype of Ik channel.

Animals↗
PubMed1997

[A deep regional impedance method measuring the blood distribution in the lung].

A new deep regional impedance method to measure the blood distribution in the lung has been developed. Using a multi-electrode system, the method simultaneously detects impedance information of chest, and back including lung and the thorax, then extracts the impedance information directly from lung with a special data processing approach in order to eliminate thorax effect on the measurement. The salt pool experiment and the preliminary application of measuring the blood distribution in the lung have been made. The results show that the method raises the relative sensitivity and the distinguishability of the blood measurement in the lung, and as a new deep regional impedance measurement method, it has a good prospects of clinical application in the future.

Adult↗
PubMed1995

Long-term effects of instilled mineral dusts on pulmonary surfactant isolated from monkeys.

Experiments were carried out to determine the long-term effect of instillation of 500 mg of generic bituminous, anthracite, quartz, or titanium dioxide (TiO2) dust on the composition of pulmonary surfactant. Dust was instilled in the caudal lobe of the right lungs of female pigtailed macaque monkeys (Macaca nemestrina). The composition of surfactant isolated from cell-free bronchoalveolar lavage (CF-BAL) samples obtained from right lungs (dust exposed) at various times over the following year was compared with that of surfactant isolated from CF-BAL from left lungs (dust free). Little change was seen in the amount of surfactant-associated lipid phosphorus as a result of exposure to dust. Exposure to quartz, anthracite, or TiO2 dust induced a significant increase in the total amount of protein in the surfactant-enriched fraction. The relative amount of specific proteins was also altered: surfactant-associated protein A decreased, and the amount of the heavy and light chains of immunoglobulin molecules (identified by NH2-terminal amino acid sequence analysis) increased. These changes were visible more than a year after instillation of quartz and at least 3 months after instillation of anthracite dust. Despite variation in the responses of the individual animals, the changes observed might serve as an indicator of the severity of the effect of exposure of the lung to mineral dust and/or to pathogens.

Administration, Inhalation↗