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S Li

Publications and source records attributed to S Li.

At least 1,441 records · Page 80Linked to original sources
PubMed1994

RF coil optimization: evaluation of B1 field homogeneity using field histograms and finite element calculations.

Two-dimensional (2D) finite element analysis has been used to solve the full set of Maxwell's equations for the 2D magnetic field of radiofrequency (RF) coils. The field histogram method has been applied to evaluate and optimize the magnetic field homogeneity of some commonly used RF coils: the saddle coil, the slotted tube resonator, the multiple elements coil and the birdcage resonator, as well as the radial plate coil. Each coil model represents a cross-section of an infinitely long cylinder. The optimum configuration of each of these five RF coils is suggested. It was found that field homogeneity is more strongly dependent on the coil's window angle than on any other parameter. Additionally, eddy currents in the coil's conductive elements distort the current and magnetic field distribution. The frequency dependence of this eddy current distortion is analyzed and discussed.

Algorithms↗
PubMed1994

Identification and characterization of kink motifs in 1-palmitoyl-2-oleoyl- phosphatidylcholines: a molecular mechanics study.

As a cis carbon-carbon double bond (delta) is introduced into the middle of an isolated all-trans hydrocarbon chain, it can be shown by molecular graphics that this delta-bond makes a bend of 130 degrees in the chain axis, thus producing a boomerang-like conformation. Such a bent structure, indeed, has been detected experimentally for oleic acid by x-ray crystallography (Abrahamson and Ryderstedt-Nahringbaur, 1962). Membrane diacyl phospholipids are largely mixed-chain lipids containing a saturated sn-1 acyl chain and an unsaturated sn-2 acyl chain. 1-Palmitoyl-2-oleoyl-phosphatidylcholine (POPC), the most abundant phospholipid in animal cell membranes, is a typical example in which the sn-2 acyl chain is the acyl chain of an oleic acid. However, this sn-2 acyl chain of POPC is unlikely to adopt a boomerang-like configuration in the gel-state lipid bilayer due to the steric hindrance imposed by neighboring chains. Instead, it has been suggested that the oleate chain in POPC is kinked in the shape of a crankshaft in the gel-state bilayer (Huang, 1977; Lagaly et al., 1977), because POPC with such a kinked sn-2 acyl chain, which is denoted here as the secondary structural element or motif, can pack efficiently against other neighboring phospholipids. In this communication, 16 different types of secondary structural elements or motifs are derived for POPC at T < Tm based on a single protocol guided by two-dimensional steric contour maps and computer-based molecular graphics. After subjecting these derived molecular species to energy minimization using the molecular mechanics method, the number of the secondary structural motifs is reduced to 13 as a result of conformational degeneracy. The structure and steric energy of each of the energy-minimized lipid rotomers are presented in this communication. Furthermore, these rotomers packed in small clusters are also simulated to mimic the lipid bilayer structure of 1-palmitoyl-2-oleoyl-phosphatidylcholines at T < Tm.

Calorimetry↗
PubMed1994

Defining the positive tilt test: a study of healthy adults with moderate acute blood loss.

STUDY OBJECTIVES: To define a set of orthostatic vital signs that minimize the frequency of false-positives among healthy individuals while maximizing sensitivity in detecting acute moderate blood loss and to determine the sensitivity and specificity of this optimized tilt test in detecting acute moderate blood loss. DESIGN AND INTERVENTION: Postural vital signs were recorded in a standardized manner before and after 450-mL phlebotomy. Paired comparisons were done for a variety of criteria for a positive tilt test using receiver-operating characteristic curves. SETTING AND TYPE OF PARTICIPANTS: Three hundred forty-five healthy euvolemic adult volunteer blood donors were tested at three community blood donation centers over a one-year period. Subjects were prospectively divided into group 1 (less than age 65; 301) and group 2 (age 65 or older; 44). MEASUREMENTS AND MAIN RESULTS: For each combination of pulse and blood pressure in group 1, a change in pulse alone had the same or higher sensitivity with at least the same specificity. Pulse alone was similarly superior in group 2 compared with previously published combinations of pulse and blood pressure. Even the optimized tilt test had limited sensitivity in detecting acute moderate blood loss with high specificity. CONCLUSION: In applying the tilt test to young adults without cardiovascular disease, pulse measurement usually is all that is necessary.

Acute Disease↗
PubMed1994

Unfolding patient motion with biplane radiographs.

A method of unfolding patient movement between biplane radiographs is presented. This method requires biplane projections of two points with a known separation to determine a translation motion or biplane projections of two or three point pairs to determine a general patient motion. Such a point pair can be provided by two ends of a line source or two radiopaque marks, with a fixed distance between them. The point pairs can be taped on skin, implanted in the target volume, or embedded in the bony structures. After obtaining the point locations on each film, the three-dimensional coordinates of points of interest at the instant of each view can be reconstructed, with an error of less than 0.5 mm. The motion parameters, characterized by a pseudotranslation vector and three axial rotational angles for the region of interest relative to the machine coordinate system, are also provided. In comparing with some other reconstruction algorithms, the point pair method can significantly improve source localization or point reconstruction in the presence of patient motion. Computer simulations and phantom experiments demonstrates the feasibility and accuracy of the method.

Brachytherapy↗
PubMed1994

Computer-aided geometric reconstruction of Fletcher-Suit source positions.

A computer-aided method for reconstruction of the source positions of a Fletcher-Suit applicator has been developed. The tandem source positions are determined by digitizing of the tip and two arbitrary points on the shaft from each of two orthogonal simulator films. A colpostat source position is reconstructed by digitizing of a single point on the endcap and three arbitrary points on the cylindrical sidewalls of the colpostat on the films. This computer-aided method considers the true projection geometry and applicator shape and permits localization of the source positions to within a mean error of less than 1 mm. Compared with the conventional method, the new approach (1) is more time efficient because only a few easily identified points are digitized, (2) allows localization when the tandem sources are shifted by a spacer or when colpostat sources are difficult to visualize on the lateral film, and (3) is more accurate than the conventional technique because no manual drawing of source positions on films is involved.

Algorithms↗
PubMed1994

A novel mammalian Ras GTPase-activating protein which has phospholipid-binding and Btk homology regions.

We have previously purified a novel GTPase-activating protein (GAP) for Ras which is immunologically distinct from the known Ras GAPs, p120GAP and neurofibromin (M. Maekawa, S. Nakamura, and S. Hattori, J. Biol. Chem. 268:22948-22952, 1993). On the basis of the partial amino acid sequence, we have obtained a cDNA which encodes the novel Ras GAP. The predicted protein consists of 847 amino acids whose calculated molecular mass, 96,369 Da, is close to the apparent molecular mass of the novel Ras GAP, 100 kDa. The amino acid sequence shows a high degree of similarity to the entire sequence of the Drosophila melanogaster Gap1 gene. When the catalytic domain of the novel GAP was compared with that of Drosophila Gap1, p120GAP, and neurofibromin, the highest degree of similarity was again observed with Gap1. Thus, we designated this gene Gap1m, a mammalian counterpart of the Drosophila Gap1 gene. Expression of Gap1m was relatively high in brain, placenta, and kidney tissues, and it was expressed at low levels in other tissues. A recombinant protein consisting of glutathione-S-transferase and the GAP-related domain of Gap1m stimulated GTPase of normal Ras but not that of Ras having valine at the 12th residue. Expression of the same region in Saccharomyces cerevisiae suppressed the ira2- phenotype. In addition to the GAP catalytic domain, Gap1m has two domains with sequence closely related to those of the phospholipid-binding domain of synaptotagmin and a region with similarity to the unique domain of Btk tyrosine kinase. These results clearly show that Gap1m is a novel Ras GAP molecule of mammalian cells.

Agammaglobulinaemia Tyrosine Kinase↗
PubMed1994

A controlled evaluation of psychoeducational family intervention in a rural Chinese community.

BACKGROUND: The shortage of mental health care in rural China might be tackled by community care. This study tests the hypothesis that family intervention increases patients' treatment compliance, which is crucial in community care. METHOD: A cohort of psychiatric patients were randomly selected to a controlled trial for four months. Compliance rates, clinical and social outcomes were measured double-blindly. RESULTS: The compliance rate was increased significantly in the trial group, and they had better clinical and social outcomes. CONCLUSIONS: Family intervention is an effective method of improving treatment compliance and should be an important part of community mental health care.

Adolescent↗
PubMed1994

Effect of estrone on the growth of 7,12-dimethylbenz(a)anthracene-induced mammary carcinoma in the rat: a model of postmenopausal breast cancer.

Estradiol (E2) is well known as stimulator of the growth of 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary tumors. The effect of estrone (E1), however, has not been described in this model of human breast cancer. As E1 is the predominant estrogen precursor in postmenopausal women, we have investigated the effect of this steroid and, simultaneously, the potential role of the enzyme required for interconversion of the weak estrogen E1 into the potent estrogen E2, namely 17 beta-hydroxysteroid dehydrogenase, in the growth of DMBA-induced mammary carcinoma in the rat. Treatment for 20 days of ovariectomized animals bearing DMBA-induced mammary tumors with twice daily doses of 0.375, 0.75, 1.5, and 3.0 micrograms E1 increased total tumor area by 48%, 101%, 116%, and 129%, respectively. Treatment with the highest dose of E1 increased progesterone receptor levels by 20.4- and 2.3-fold in the DMBA-induced tumors and uterus, respectively. After treatment with E1, the concentration of this steroid was similar in the serum and tumor tissue, whereas concentrations of E2 were approximately 3-fold higher in the tumor tissue compared to serum. Treatment with a 1.0-microgram dose of E1 caused a 60% increase in tumoral 17 beta-hydroxysteroid dehydrogenase activity in ovariectomized animals, thus favoring E2 formation from E1 in tumors. In addition, treatment with the 1.0-microgram dose of E1 or 0.1 microgram E2 gave similar stimulatory effects on tumor growth and uterine weight in ovariectomized animals; the values were comparable to those found in intact animals. The present data indicate that ovariectomized rats bearing DMBA-induced mammary tumors and treated with E1 can be a useful model of postmenopausal breast cancer.

17-Hydroxysteroid Dehydrogenases↗
PubMed1994

Glucocorticoid regulation of rat whey acidic protein gene expression involves hormone-induced alterations of chromatin structure in the distal promoter region.

Milk protein gene expression is regulated by the synergistic interactions of several lactogenic hormones, including insulin, PRL, and glucocorticoids. Whey acidic protein (WAP) gene expression is highly dependent on glucocorticoids, and to a lesser extent than casein gene expression, on the presence of PRL. Previous studies have demonstrated that a distal DNase I hypersensitive site in the rat WAP gene 5'-flanking region containing several binding sites for nuclear factor I is required for high level WAP gene expression in transgenic mice. In this study several specific glucocorticoid receptor (GR) binding sites were identified flanking these nuclear factor I sites using an in vitro DNase I footprinting assay with baculovirus-expressed GR. These sites were able to confer dexamethasone inducibility to a heterologous thymidine kinase-chloramphenicol acetyltransferase reporter gene construct in transient cotransfection experiments with GR in CV1 cells. Administration of dexamethasone to adrenal-ectomized mice carrying the +2020 rat WAP transgene during lactation demonstrated that glucocorticoids are required to maintain transgene expression in the mammary gland. Furthermore, glucocorticoid-induced changes in transgene expression were correlated with the appearance of DNase I hypersensitive sites. These results indicate that at least part of glucocorticoid regulation of WAP gene expression is mediated through the direct interaction of GR with glucocorticoid response elements in the distal promoter region resulting in steroid hormone-dependent alterations in chromatin structure.

Adrenalectomy↗
PubMed1994

Pituitary ontogeny of the Snell dwarf mouse reveals Pit-1-independent and Pit-1-dependent origins of the thyrotrope.

The anterior pituitary provides a model to study the molecular mechanisms responsible for emergence of distinct cell types within an organ. Dwarf mice (Snell) that express a mutant form of the tissue-specific POU-domain transcription factor Pit-1 fail to generate three cell types, including the thyrotrope (S. Li, E. B. Crenshaw, E. J. Rawson, D. S. Simmons, L. Swanson and M. G. Rosenfeld (1990), Nature 347, 528-533). Analyses of wild-type and Pit-1-defective mice, presented here, have revealed that thyrotropes unexpectedly arise from two independent cell populations. The first population is Pit-1-independent and appears on e12 in the rostral tip of the developing gland, but phenotypically disappears by the day of birth. The second is Pit-1-dependent and arises subsequently in the caudomedial portion of the developing gland (e15.5), following the initial expression of Pit-1 in this region. The failure of caudomedial thyrotrope cells to appear in the Snell dwarf, and the observation that Pit-1 can bind to and transactivate the TSH beta promoter, apparently enhanced by its phosphorylation, suggests that Pit-1 is directly required for the appearance of this distinct population that serves as the precursors of the mature thyrotrope cell type. These data suggest that different molecular mechanisms, based on the actions of distinct transcription factors, can serve to independently generate a specific cell phenotype during mammalian organogenesis.

Animals↗
PubMed1994

Expression of transforming growth factor-alpha in the human ovary during follicular growth, regression and atresia.

The cytologic localization and cellular levels of TGF alpha in the human ovary during follicular growth and regression were examined by avidin/biotin immunoperoxidase techniques with a monoclonal antibody to TGF alpha. In primordial follicles, only the oocyte showed intense immunostaining for TGF alpha, whereas the flattened pregranulosa cells were negative for the immunostaining. The earliest stage of follicular growth at which immunostaining for TGF alpha in granulosa cells and theca interna cells became apparent was the preantral stage. With the increase in the size of follicles, the intensity of TGF alpha immunostaining in the oocyte decreased, whereas the staining intensity of the granulosa and theca cells increased. The immunostaining for TGF alpha in granulosa and theca interna cells persisted in the corpus luteum, and further intensified during the midluteal phase. In the regressing corpus luteum, the immunostaining was present only in the peripheral theca lutein cells adjacent to the central scar tissue. The corpus albicans was negative for the immunostaining. Stromal cells exhibited weak staining for TGF alpha. In atretic follicles, the theca interna cells exhibited intense staining for TGF alpha without appreciable staining in the scattered granulosa cells. This is the first report to demonstrate that TGF alpha expression in the oocyte is maximal in primordial follicles, whereas TGF alpha expression in granulosa and theca cells increases with the progress of follicles and reaches its maximum in the lutein cells during the mid luteal phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Stage-limited expression of myc oncoprotein in the human ovary during follicular growth, regression and atresia.

The cytologic localization and cellular levels of myc oncoprotein in the human ovary during follicular growth, regression and atresia were examined by the avidin/biotin immunoperoxidase method with a specific antibody to myc oncoprotein. In primordial follicles, only the oocyte showed intense immunostaining for myc protein, whereas the granulosa cells were negative for the staining. In preantral follicles, both the oocyte and granulosa cells were moderately immunostained for myc protein. In antral and preovulatory follicles, there was no appreciable staining for myc protein in the granulosa or theca cells, while myc protein staining in the oocyte persisted with less intensity. It is of interest that myc protein expression in granulosa cells was apparent only during the preantral follicle stage. Corpora lutea during the early and mid luteal phase were negative for myc protein staining, whereas in regressing corpora lutea during the late luteal phase, peripheral theca lutein cells adjacent to the central core of scar tissue were immunostained for myc protein. Corpora albicans showed no staining for myc protein. In atretic follicles, granulosa cells and theca interna cells demonstrated positive staining for myc protein. Ovarian stromal cells were negative for the immunostaining throughout the menstrual cycle. This demonstrates that myc protein is expressed in a stage-limited manner in the human ovary during follicular growth and regression. The abundant expression of myc protein in the oocyte at the primordial and preantral follicle stages and in the granulosa cells at the preantral follicle stage suggests a role for myc expression in the initial growth of the oocyte as well as in the autonomous growth of granulosa cells during the preantral stage seemingly independent of gonadotropic stimulation. Furthermore, notable expression of myc protein in the granulosa cells and theca interna cells of atretic follicles and in the peripheral theca lutein cells of regressing corpora lutea implies the possible participation of myc expression in remodelling the ovarian local tissue following atresia and luteolysis in the human ovary.

Adult↗
PubMed1994

Characterization of freshly dispersed porcine myometrial cells: evidence for voltage-dependent Ca2+ channels and regulatory receptors.

A patch-clamp study of Ca2+ currents and spectrofluorometric detection of the intracellular Ca2+ concentration [Ca2+]i was performed on porcine myometrial cells that had been isolated by collagenase. Isolated myometrial cells were the typical long cylinder shape. The main length and diameter of myometrial cells were 505 +/- 20 and 11 +/- 0.5 microns (n = 40), respectively, in the prepartum period and 265 +/- 22 and 7 +/- 0.4 microns (n = 40), respectively, in the luteal phase. Immunocytochemistry with an antibody against desmin stained 90% of the cells positively, and about 95% of the cells excluded Trypan blue dye. The basal [Ca2+]i of myometrial cells in the luteal phase and the prepartum period was 119 +/- 12 (n = 30) and 154 +/- 31 nmol l-1 (n = 48), respectively. In prepartum myometrial cells, oxytocin (10(-7) mol l-1) and carbachol (10(-4) mol l-1) increased [Ca2+]i in a biphasic pattern, with a sharp peak followed by a plateau. In cells in the luteal phase, adrenaline (10(-7) mol l-1) plus propranolol (10(-6) mol l-1) produced a biphasic increase in [Ca2+]i. However, in the absence of propranolol, the increase in [Ca2+]i by adrenaline was small. Prostaglandin F2 alpha (10(-6) mol l-1) induced a monophasic increase in the [Ca2+]i in cells in the luteal phase. By depolarizing the cells from -30 to +50 mV from a holding potential of -50 mV, Ca2+ currents were evoked with a threshold at -20 mV, reaching a maximum between 10 and 30 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

The effect of dietary fat content on amino acid digestibility in young pigs.

Studies were carried out with 12 pigs (Yorkshire x Landrace) to determine the effect of dietary fat content on amino acid digestibility. The pigs were weaned at 21 d of age and fitted with a simple T-cannula at the distal ileum at 27 or 28 d of age. After a 7-d recuperation period, the pigs were fed one of four isonitrogenous cornstarch-based soybean meal diets (22.5% CP) containing 3.2, 6.2, 9.2, or 12.2% canola oil according to a balanced two-period change-over design. The pigs were fed four times daily, equal amounts, at 6-h intervals. The diets were supplied at a rate of 5% of the average body weight that was determined at the initiation of the first (11.0 kg) and second (12.5 kg) experimental period. Each experimental period consisted of 10 d. Feces were collected for 48 h on d 6 and 7 and ileal digesta for 24 h during d 8, 9, and 10. Chromic oxide was used as digestibility marker. The apparent ileal digestibilities of most of the amino acids increased linearly (P < .05) with increasing dietary fat levels. There were differences (P < .05) in the ileal digestibilities of most of the amino acids between the diets containing 3.2 and 12.2% canola oil. Conversely, the dietary level of inclusion of canola oil did not affect (P > .05) the fecal amino acid digestibilities. The protein-sparing effect of additional canola oil inclusion results, in part, from an increase in ileal amino acid digestibility.

Amino Acids↗