PubMed Health⌕ Search

Biomedical subjects

T A Walker

Publications and source records attributed to T A Walker.

At least 19 recordsLinked to original sources
PubMed2001

Ranitidine hydrochloride: development of an isocratic stability--indicating high-performance liquid chromatographic separation.

The development of a stability-indicating assay for ranitidine hydrochloride using a mobile phase added ion-interaction reagent was achieved. The assay easily separated all known and unknown impurities/degradants. This assay may be used for the determination of purity, identity and strength for the active ingredient and finished dosage forms. Placebo samples were analyzed for all of the dosage forms and did not interfere with the separation. The system was found to be linear over a range of 0.056 to 44.4 microg/g, with a limit of detection of 0.028 microg/g and a limit of quantitation of 0.056 microg/g. The system precision was determined to be 0.7%. The development of the stability-indicating assay and the effect that each chromatographic variable had on the separation will be discussed.

Calibration↗
PubMed2001

Bupropion hydrochloride: the development of a chiral separation using an ovomucoid column.

The separation of bupropion enantiomers on an ovomucoid stationary phase was investigated. The mobile- and stationary-phase parameters that may influence the separation were identified. The parameters that were studied include: type and concentration of organic modifier, mobile phase pH, ionic strength, type of buffer, and column temperature, as well as the effect that the amount of sample injected had on the separation. The optimized chiral separation baseline-resolved the enantiomers in less than 10 min. Calibration curves for a standard were linear over a range of 0.27-53.0 microg/g (ppm) with a correlation coefficient of 0.999 for both enantiomers. A detection limit of 0.13 microg/g and a quantitation limit of 0.27 microg/g were also found. The system precision of the method was 0.2%.

Buffers↗
PubMed2001

Replacement of the mitral valve in an infant with group B streptococcal endocarditis.

Endocarditis due to group B streptococcus is very rare in infants, and may be associated with significant morbidity and mortality. Review of the literature reveals only a single reported case of an infant with this type of streptococcal endocarditis involving the mitral valve. This infant had underlying congenital heart disease, and died shortly after catheterization. We now report group B streptococcal endocarditis occurring in an infant with a structurally normal heart who was treated successfully by replacement of the mitral valve.

Endocarditis, Bacterial↗
PubMed2000

Effect of adenovector-mediated gene transfer of keratinocyte growth factor on the proliferation of alveolar type II cells in vitro and in vivo.

Alveolar type II cell proliferation occurs after lung injury and is thought to minimize the subsequent fibrotic response. Keratinocyte growth factor (KGF) has been shown to be a potent growth factor for rat alveolar type II cells. In this study, we created a replication-deficient, recombinant human type 5 adenovirus vector expressing human KGF (Ad5-KGF) to produce alveolar type II cell hyperplasia in vivo. In rat type II cells in vitro, Ad5-KGF at a multiplicity of infection (MOI) of 2, 4, and 8 plaque-forming units (PFU)/cell increased thymidine incorporation 13.3-, 16.8-, and 20. 8-fold, respectively. The KGF concentration in the medium increased up to 26.0 +/- 1.0 ng/ ml. We then instilled 10(9) PFU of Ad5-KGF, Ad5-LacZ, or phosphate-buffered saline into Fischer 344 rats and analyzed the lungs 2, 3, 7, 14, 21, and 28 d later. Ad5-KGF produced extensive alveolar type II cell hyperplasia on Days 2, 3, and 7. Surfactant protein (SP)-A and SP-D in lavage and SP-D in serum increased more in the Ad5-KGF group than in the Ad5-LacZ and PBS groups on Days 2 and 3. KGF was readily detectable for up to 7 d in lavage fluid, although only a modest number of cells expressed KGF messenger RNA as detected by in situ hybridization. These data show that Ad5-KGF stimulates extensive alveolar type II cell proliferation in vivo.

Adenoviridae↗
PubMed1999

E1A oncogene-induced cellular sensitization to immune-mediated apoptosis is independent of p53 and resistant to blockade by E1B 19 kDa protein.

E1A oncogene expression sensitizes mammalian cells to apoptosis triggered by cytolytic lymphocytes (CL) [16]. Most studies suggest that E1A-induced apoptosis involves a p53-dependent cellular pathway that is blocked by the E1B 19 kDa gene product. In this study, the roles of p53 and E1B 19 kDa were tested for E1A sensitization to CL-induced apoptosis in contrast with apoptosis triggered by TNF alpha or chemical injuries. E1A sensitization to immune-mediated (CL- or TNF-induced) apoptosis was independent of p53 expression and was resistant to blockade by E1B 19 kDa protein in mouse and hamster cells. In contrast, the p53 requirement for chemically induced apoptosis of E1A-sensitized cells varied with the agent used to treat cells. Apoptosis induced by diverse chemical agents (hygromycin, beauvericin, etoposide, H(2)O(2)) was blocked by E1B 19 kDa expression. Therefore, both the p53-dependence and the E1B 19 kDa blockade of E1A-induced cellular sensitization to apoptotic injury depend on the type of proapoptotic injury tested. These data suggest that the mechanisms by which E1A sensitizes tumor cells to immune-mediated apoptosis and to rejection by immunocompetent animals do not require cellular expression of wild-type p53 and can function independently of the Bcl-2-like, antiapoptotic mechanisms of E1B 19 kDa.

3T3 Cells↗
PubMed1999

E1A oncogene induction of cellular susceptibility to killing by cytolytic lymphocytes through target cell sensitization to apoptotic injury.

E1A oncogene expression increases mammalian cell susceptibility to lysis by cytolytic lymphocytes (CLs) at a stage in this intercellular interaction that is independent of cell surface recognition events. Since CLs can induce either apoptotic or necrotic cell death, we asked whether E1A sensitization to injury-induced apoptosis is sufficient to explain E1A-induced cytolytic susceptibility. Mouse, rat, hamster, and human cells that were rendered cytolytic susceptible by E1A were also sensitized to CL-induced and chemically induced apoptosis. In contrast, E1A-positive cells were no more susceptible to injury-induced necrosis than E1A-negative cells. Similar to induction of cytolytic susceptibility and in contrast to other E1A activities, cellular sensitization to chemically induced apoptosis depended on high-level E1A oncoprotein expression. Loss of both cytolytic susceptibility and sensitization to chemically induced apoptosis was coselected during in vivo selection of E1A-positive sarcoma cells for increased tumorigenicity. Furthermore, E1A mutant proteins that cannot bind the cellular transcriptional coactivator, p300, and that fail to induce cytolytic susceptibility also failed to sensitize cells to injury-induced apoptosis. These data indicate that E1A induces susceptibility to killer cell-induced lysis through sensitization of cells to injury-induced apoptosis.

3T3 Cells↗
PubMed1999

The acute respiratory distress syndrome in children: recent UMMC experience.

The acute respiratory distress syndrome (ARDS) is now well recognized as a cause of respiratory failure in children and is associated with a high mortality rate. We retrospectively reviewed all cases of ARDS managed in our multidisciplinary pediatric intensive care unit (PICU) from 1994 to 1998 in order to identify predisposing conditions, outcomes, complications, recent trends in therapy, and resource utilization. Twenty-seven children were identified representing approximately 1% of all intensive care admissions. Sepsis was the most common predisposing illness and air leak complicated treatment in 60%. Mortality was 30% and was most often due to non-respiratory causes. High-frequency oscillatory ventilation (HFOV) was frequently utilized and felt to be beneficial in most cases. On average, survivors required mechanical ventilation for five weeks and hospitalization for nine weeks. We conclude that despite our comparatively low mortality rate, ARDS remains a significant challenge to the pediatric intensivist. We speculate that HFOV may be an important factor in reducing mortality.

Adolescent↗
PubMed1997

Raising P50 increases tissue PO2 in canine skeletal muscle but does not affect critical O2 extraction ratio.

Affinity of hemoglobin (Hb) for O2 determines in part the rate of O2 diffusion from capillaries to myocytes by altering capillary PO2. We hypothesized that a decrease in Hb O2 affinity (increased P50) would increase capillary and tissue PO2 (PtiO2) and improve O2 consumption during ischemia. To test this hypothesis, blood flow to the pump-perfused left hindlimb of 18 anesthetized and paralyzed dogs was progressively decreased over 90 min while hindlimb O2 consumption and O2 delivery (QO2) and PtiO2 were measured at the muscle surface. Arterial PO2 was maintained at 150 +/- 10 Torr in all dogs. We increased P50 by 12.3 +/- 0.9 (SE) Torr in nine dogs with RSR-13, an allosteric modifier of Hb. This decreased arterial O2 saturation to 90-92% but increased mean PtiO2 from 35.5 +/- 11.6 to 44.1 +/- 15.2 (SD) Torr (P < 0.05) with no change in controls (n = 9). O2 extraction ratio at critical QO2 was 74 +/- 2% in controls and 79 +/- 1% in RSR-13-treated dogs (P = not significant). PtiO2 was 30-40% higher in the RSR-13-treated group at any QO2 above critical but did not differ between groups below critical QO2. Perfusion heterogeneity and convergence of the dissociation curves near critical QO2 may have mitigated any effect of increased P50 on O2 diffusion. Still, increasing P50 by 12 Torr with RSR-13 significantly increased PtiO2 at QO2 values above critical.

Aniline Compounds↗
PubMed1995

Chiral separation of MDL 73,005EF enantiomers using an alpha 1-acid glycoprotein column.

The enantiomeric separation of MDL 73,005EF (racemic mixture of two enantiomers) has been accomplished using a Chiral AGP (alpha 1-acid glycoprotein) column. The enantiomers are baseline resolved with a runtime of less than 10 min. This separation is used to quantitate the enantiomers present in bulk drug samples, tabletted formulations, and drug used in pharmacological and toxicological studies. Variables found to have an effect on the enantiomeric separation were studied and include: mobile phase ionic strength, type and concentration of organic modifier added to the mobile phase, mobile phase, mobile phase pH, column temperature and the amount of analyte injected. The enantiomeric separation was optimized on the Chiral AGP column based on the effects that each variable had on the separation. Calibration curves for a standard were linear over a range of 0.24 to 61.2 micrograms/g with a correlation coefficient of better than 0.999. A detection limit of 0.012 micrograms/g and a quantitation limit of 0.24 micrograms/g were also found.

Chromatography, High Pressure Liquid↗
PubMed1995

Effects of nitric oxide synthase inhibition on regional hemodynamics and oxygen transport in endotoxic dogs.

Nitric oxide synthase (NOS) inhibition has been used to increase blood pressure in humans with septic shock despite a lack of data regarding its effects on O2 delivery (QO2). We studied the effects of NG-nitro-L-arginine methyl ester (L-NAME) on systemic, gut, and hindlimb circulations of endotoxic dogs. Twelve dogs were infused with 2 mg/kg of LPS over 1 h followed by 60 mL/kg of 6% dextran over 2 h. Six dogs also received 20 mg/kg of L-NAME, LPS caused mean arterial pressure (MAP), flow and QO2 to whole body, hindlimb and gut to decrease, but O2 uptake (VO2) did not change. Dextran resuscitation alone produced a hyperdynamic state with increased blood flow to or above baseline. With L-NAME, systemic and regional resistances increased twofold and MAP returned to near baseline. Late in the study, these dogs had significantly lower blood flow and QO2 to the gut but maintained VO2 by increasing oxygen extraction to near critical levels. These data suggest that in acute endotoxicosis, L-NAME may significantly improve blood pressure but may markedly encroach on O2 transport reserves to the gut.

Animals↗
PubMed1994

A liquid chromatographic assay for urea in over-the-counter carbamide peroxide products.

A liquid chromatographic assay has been developed and validated for determining the amount of urea present in carbamide peroxide formulations. The method employs an interaction CHO-620 carbohydrate column loaded in the Ca+2 form, a flow rate of 0.6 ml min-1, a column temperature of 85 degrees C, and UV detection at 200 nm. The mobile and stationary phase variables that may have an effect on the urea separation were studied and are discussed. The method was found to be linear over a range of 20 micrograms/g to 340 micrograms/g of urea, with a limit of detection of 1 microgram/g and a quantitation limit of 15 micrograms/g. System precision was found to have a %RSD of less than 1.0% while the method precision was typically less than 5.0%. Recoveries of 97.4% (1.0% RSD) and 98.3% (1.1% RSD) of the theoretical amount of urea (6.6% on a weight to weight basis) contained in two different formulations were obtained.

Carbamide Peroxide↗
PubMed1994

Viral respiratory infections.

Viral infections constitute more than 60% of acute lower respiratory illnesses. Respiratory syncytial virus (RSV) and parainfluenza viruses are the most frequent etiologic agents. After transmission by large droplet aerosol or direct contact, the viruses gain entry into host cells through specific viral surface proteins; subsequently, pathogenetic mechanisms cause tissue injury and result in clinical disease. In the intensive care unit the mainstay of treatment is primarily supportive. Nonspecific treatment may include nebulized beta-agonists, aminophylline, and steroids. Ribavarin is the only specific antiviral agent approved for respiratory syncytial virus infection but its efficacy remains controversial. New therapies and vaccines offer hope for improved outcome from viral respiratory infections such as RSV.

Adolescent↗
PubMed1993

E1A oncogene expression level in sarcoma cells: an independent determinant of cytolytic susceptibility and tumor rejection.

Ad2/5 E1A oncogene expression induces cytolytic susceptibility of rodent cells to natural killer lymphocytes. To determine whether the requisite thresholds of E1A oncoprotein expression differ for induction of cytolytic susceptibility as compared with other E1A-related activities, sarcoma cells expressing low or normal levels of E1A oncoproteins were compared for differences in morphological transformation, transactivation of viral genes, cytolytic susceptibility and tumorigenicity. Low-level E1A expression transformed sarcoma cells and transactivated the Ad5 E2A gene but did not induce the increased cytolytic susceptibility observed with normal levels of E1A expression. Furthermore, low-level E1A expressers retained the tumorigenicity of parental cells, whereas normal-level expressers were non-tumorigenic in hosts with intact natural killer (NK)-cell responses. In contrast to E1A, E1B oncogene expression caused no changes in morphological, cytolytic or tumorigenic phenotypes in these sarcoma cells. These data define an expression threshold for E1A-induced cytolytic susceptibility and associated NK-cell-dependent tumor rejection. The results suggest that the cellular mechanisms involved in E1A induction of cytolytic susceptibility differ from those involved in E1A-mediated cellular transformation and viral gene transactivation.

Adenovirus E1A Proteins↗
PubMed1991

E1A oncogene induction of cytolytic susceptibility eliminates sarcoma cell tumorigenicity.

The manner in which oncogenes influence tumorigenicity beyond their ability to immortalize cells is uncertain. We tested the hypothesis that, in addition to subverting cellular growth controls, oncogenes can actively determine tumor-inducing capacity by affecting neoplastic cell susceptibility to destruction by the host cellular immune response. The adenovirus type 5 E1A oncogene, which induces susceptibility to lysis by natural killer cells and encodes epitopes recognized by cytotoxic T lymphocytes, was transfected into highly tumorigenic sarcoma cells. E1A expression in these sarcoma cells eliminated their tumorigenicity in recipients with natural killer cell activity that was competent to lyse these E1A-positive targets. Thymus-dependent responses were not required for tumor rejection. These results indicate that oncogene-regulated cellular pathways that affect neoplastic cell susceptibility to natural killer cell lytic mechanisms may influence tumor development in the immunocompetent host.

Adenovirus Early Proteins↗
PubMed1989

Role of tumor necrosis factor-alpha in E1A oncogene-induced susceptibility of neoplastic cells to lysis by natural killer cells and activated macrophages.

NIH-3T3 cells transfected with adenovirus E1A oncogene cDNA were found to exhibit cytolytic susceptibility to murine NK cells and activated macrophages associated with a threshold level of oncogene product expression exceeding that required for morphological transformation. A similar correlation was observed between threshold levels of E1A gene product expression and target cell susceptibility to direct cytotoxicity by rTNF. Inhibition of splenic NK cell and peritoneal macrophage cytolysis by antisera specific for murine rTNF confirmed the importance of E1A-induced TNF susceptibility as one determinant of target cell cytolytic susceptibility. Anti-TNF antibody was, however, unable to block killing of E1A-expressing targets by the NK cell line, NKB61A2. These results suggest a direct link between the functions of E1A oncogene products and cellular mechanisms of action of TNF elaborated by host effector cells and indicate that E1A expression also affects target cell susceptibility to TNF-independent cytolytic mechanisms.

Adenoviridae↗
PubMed1989

Differential induction of cytolytic susceptibility by E1A, myc, and ras oncogenes in immortalized cells.

The E1A oncogene of adenovirus serotypes 2 and 5 induces susceptibility to the cytolytic effects of natural killer lymphocytes and activated macrophages when expressed in infected and transformed mammalian cells (cytolysis-susceptible phenotype). E1A and the oncogenes v-myc, long-terminal-repeat-promoted c-myc, and activated c-ras share the ability to immortalize transfected low-passage rodent cells. The cytolytic phenotypes of well-characterized rodent cell lines immortalized by these three oncogenes were defined. In contrast to target cells expressing the intact E1A gene, myc- and ras-expressing, immortalized primary transfectants were resistant to lysis by both types of killer cell populations. The same patterns of susceptibility (E1A) and resistance (myc and ras) to cytolysis were observed in oncogene-transfected continuous rat (REF52) and mouse (NIH 3T3) cell lines, indicating that differences in the cytolytic phenotypes associated with expression of these oncogenes are not due to cell selection during immortalization. The results suggest that the E1A oncogene may possess a functional domain that is different from those of other oncogenes, such as myc and ras, and that the activity linked to this postulated domain is dissociable from the process of immortalization.

Adenovirus Early Proteins↗