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L Stephens

Publications and source records attributed to L Stephens.

At least 37 records · Page 2Linked to original sources
PubMed1994

Evidence for the temporal processing of odor mixtures in humans.

The neural mechanisms underlying the perception of complex odor mixtures are largely unknown. The present study investigated the hypothesis that odorants in a mixture are processed and perceived in series. The results indicate that (i) odorants in mixtures are temporally processed with up to several hundred milliseconds separating individual components; (ii) 'fast' odorants are more likely to be a suppressor of 'slow' odorants than the reverse; and (iii) separation times can be altered by changing the concentrations of mixture components. These findings provide a new mechanism to account for the limited ability of humans to identify odorants in mixtures and for odor suppression.

Brain↗
PubMed1994

Activation of phosphoinositide 3-kinase is required for PDGF-stimulated membrane ruffling.

BACKGROUND: There is substantial evidence that phosphoinositide 3-kinase (PI 3-kinase) is a critical component of signalling pathways used by the cell-surface receptors for a variety of mammalian growth factors and other hormones. The physiological product of this enzyme is a highly polar membrane lipid called phosphatidylinositol (3,4,5)-trisphosphate This lipid has been postulated to act as a second-messenger in cells but its putative targets are still unknown. RESULTS: A particular rearrangement of actin filaments, which results in membrane ruffling, is elicited by the activation of PDGF beta-receptors expressed in cultured porcine aortic endothelial cells. We have found that this consequence of PDGF beta-receptor activation is inhibited by three independent manipulations of PI 3-kinase activity: firstly, by the deletion of tyrosine residues in the PDGF beta-receptor to which PI 3-kinase binds; secondly, by the overexpression of a mutant 85 kD PI 3-kinase regulatory subunit to which the catalytic kinase subunit cannot bind; and thirdly, by the addition of the fungal metabolite wortmannin, which is a potent inhibitor of the catalytic activity of PI 3-kinase. CONCLUSIONS: These results argue strongly that phosphatidylinositol (3,4,5)-trisphosphate synthesis is required for growth-factor-stimulated membrane ruffling in porcine aortic endothelial cells, and suggest that synthesis of this lipid may be part of a signalling pathway leading to direct or indirect activation of the small GTP-binding protein Rac.

Androstadienes↗
PubMed1994

A novel phosphoinositide 3 kinase activity in myeloid-derived cells is activated by G protein beta gamma subunits.

Phosphoinositide 3 kinase (PI3K) is a key signaling enzyme implicated in receptor-stimulated mitogenesis, oxidative bursting in neutrophils, membrane ruffling, and glucose uptake. A PI3K has already been purified, cloned, and shown to be regulated by receptors that act via tyrosine kinase-dependent regulatory mechanisms. We report that an immunologically, pharmacologically, and chromatographically distinct form of PI3K activity present in neutrophils and U937 cells is specifically activated by G protein beta gamma subunits. This data suggests PI3Ks conform to the paradigm set by receptor regulation of phosphoinositidase Cs: different receptor transduction systems specifically regulate dedicated isoforms of effector protein.

Adenosine Triphosphate↗
PubMed1994

Characterization of a phosphatidylinositol-specific phosphoinositide 3-kinase from mammalian cells.

BACKGROUND: As phosphoinositides can serve as signalling molecules within cells, the enzymes responsible for their synthesis and cleavage are likely to be involved in the transduction of signals from the cell surface through the cytoplasm. The precise role of the phosphoinositide 3-kinase that has been cloned from mammalian cells is not known, but it has been implicated in receptor-stimulated mitogenesis, glucose uptake and membrane ruffling. The enzyme can use phosphatidylinositol (PtdIns), PtdIns 4-phosphate and PtdIns (4,5)-bisphosphate as substrates in vitro, but it seems to phosphorylate PtdIns (4,5)-bisphosphate preferentially in vivo. The VPS34 gene product of yeast, by contrast, is a phosphoinositide 3-kinase homologue implicated in vacuolar protein sorting that apparently utilizes only PtdIns as a substrate. The significance of this difference in lipid-substrate preference and its relationship to the functions of the two phosphoinositide kinases is unknown. RESULTS: We have characterized a distinct PtdIns-specific phosphoinositide 3-kinase activity in mammalian cells. Unlike the previously identified, broad-specificity mammalian phosphoinositide kinase, this enzyme is resistant to the drug wortmannin and uses only PtdIns as a substrate in vitro; it therefore has the capacity to generate PtdIns 3-phosphate specifically. The newly characterized enzyme, which was purified by chromatography from cytosol, has biochemical and pharmacological characteristics distinct from those of the broad-specificity enzyme. CONCLUSIONS: The enzyme we have characterized may serve to generate PtdIns 3-phosphate for fundamentally different roles in the cell from those of PtdIns (3,4)-bisphosphate and/or PtdIns (3,4,5)-trisphosphate. Furthermore, the functions of the VSP34 gene product, which may not be relevant to the broad-specificity mammalian phosphoinositide 3-kinase, may be related to those of the enzyme we describe.

Amino Acid Sequence↗
PubMed1994

Ethical dilemmas and decisions concerning the do-not-resuscitate patient undergoing anesthesia.

The growing geriatric population in this country makes it increasingly difficult to deal with the number of do-not-resuscitate (DNR) orders. In part, this is due to an increase in the number of elderly undergoing anesthesia and surgery. It can also be attributed to a rise in complex legal, ethical, and moral issues these orders pose for the healthcare professional caring for the DNR patients, including anesthetists. The term "DNR" is confusing to many, including healthcare professionals. As patients progress through the perioperative period, this confusion is compounded by the fact that administration of anesthesia encompasses interventions that include intubation, ventilation, and fluid replacement. These interventions may be regarded as resuscitative efforts outside the operating room. The anesthetist must identify and sort through a maze of conflicting courses of action, which must match the patient's desires and personal rights. The topic of DNR orders is addressed as well as some of the moral and ethical dilemmas they pose for the Certified Registered Nurse Anesthetist (CRNA). Some solutions are offered to help the nurse anesthetist make those decisions that are most "right" for the patient.

Decision Making↗
PubMed1993

Activation of phosphatidylinositol 4,5-bisphosphate supply by agonists and non-hydrolysable GTP analogues.

PtdIns(4,5)P2 serves as a precursor of a diverse family of signalling molecules, including diacylglycerol (and hence phosphatidic acid), Ins(1,4,5)P3 [and hence Ins(1,3,4,5)P4] and PtdIns(3,4,5)P3. The production of these messengers can be activated by agonists, and therefore the rate of utilization of PtdIns(4,5)P2 can vary dramatically. Although cells can only meet these large changes in demand for PtdIns(4,5)P2 by increasing its synthesis and/or by continuously cycling it at a rate that exceeds its potential consumption (avoiding the need for a co-ordinated activation mechanism), no satisfactory explanation for how this is achieved in agonist-stimulated cells has yet been provided. We show here that, in streptolysin-O-permeabilized neutrophils, N-formylmethionyl-leucyl-phenylalanine (FMLP), platelet-activating factor (PAF) and non-hydrolysable GTP analogues can cause large activations of PtdIns4P 5-kinase, suggesting that cells can accommodate agonist-activated rates of consumption of PtdIns(4,5)P2 without having to sustain continuous, comparably rapid and energetically expensive 'futile cycling' reactions.

Cell Membrane Permeability↗
PubMed1993

Synthesis of phosphatidylinositol 3,4,5-trisphosphate in permeabilized neutrophils regulated by receptors and G-proteins.

Formylated Met-Leu-Phe (fMLP), platelet-activating factor (PAF), ATP, and various nonhydrolyzable guanine nucleotides stimulated accumulation of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) in intact human neutrophils. A protocol was devised to selectively inhibit the capacity of the nucleotide-sensitive receptors to elicit accumulation of PtdIns (3,4,5)P3. This enabled study of the regulation of phosphoinositide 3OH-kinase (PI3K) activities in permeabilized neutrophils free from interference due to activation of cell-surface nucleotide receptors. FMLP, PAF, and nonhydrolyzable GTP analogues stimulated an increase in the concentration, and rate of synthesis, of PtdIns(3,4,5)P3 in permeabilized neutrophils by increasing the rate of a PtdIns(4,5)P2-directed PI3K-catalyzed reaction. A number of characteristics of these responses, including their relative sensitivities to inhibition by pertussis toxin and guanosine 5'-beta-(thio)diphosphate, suggested that fMLP and PAF increased this PI3K activity via the actions of heterotrimeric G-proteins. Basal and guanosine 5'-gamma-(thio)triphosphate/fMLP-stimulated increases in PI3K activity were resistant to changes in free calcium concentrations, staurosporine, acute treatment with phorbol esters, and evidently to permeabilization. This, in conjunction with other work, indicates that the PAF and fMLP-induced increase in PtdIns(4,5)P2-directed PI3K activity is not being produced via activation of a currently defined G-protein regulated effector enzyme, or a protein tyrosine kinase coordinated mechanism of a type already known to regulate PI3K activities.

Adenosine Triphosphate↗
PubMed1993

The detection, purification, structural characterization, and metabolism of diphosphoinositol pentakisphosphate(s) and bisdiphosphoinositol tetrakisphosphate(s).

A new class of inositol phosphates containing energy-rich pyrophosphoryl residues has been characterized. D/L-1-Diphosphoinositol pentakisphosphate(s) and D/L-bis-(1,4)-diphosphoinositol tetrakisphosphate(s) are present as soluble ionic species in the cytosol of amoebae (Dictyostelium discoideum) at concentrations in the range of 0.05-0.25 mM. These compounds are rapidly metabolized in intact cells and can be synthesized in cell lysates from myo-inositol hexakisphosphate in the presence of ATP. Their phosphomonoester groups have predicted C-O-P bond energies of between 3.3 and 4 kcal mol-1; however, the bond energies of the P-O-P links in their diphosphate moieties are 6.6 kcal mol-1 and hence similar to the equivalent bonds in ADP, indicating a potential role for these compounds as phosphate donors in phosphotransferase reactions. Compounds with similar chromatographic properties are found in a variety of mammalian cell types.

Animals↗
PubMed1993

Receptor stimulated accumulation of phosphatidylinositol (3,4,5)-trisphosphate by G-protein mediated pathways in human myeloid derived cells.

Phosphoinositide 30H-kinase (PI3K) activities are thought to be critical regulatory enzymes in a new intracellular signalling pathway, the activation of which results in the rapid accumulation of a putative signalling molecule, phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5) P3]. To date, activation of PI3K has always correlated with its recruitment into complexes containing protein tyrosine kinases (PTK). Here we report that agonists which utilize G-protein mediated transduction pathways can stimulate very rapid and large accumulations of PtdIns(3,4,5)P3 via a novel mechanism, possibly involving direct coupling between the G-protein and a PI3K activity. In addition, some of these agonists also stimulate small increases in PI3K activity in anti-phosphotyrosine and anti-src-type PTK antibody directed immunoprecipitates, indicating activation of PI3K via a 'conventional' PTK mediated mechanism; these pathways however, play only a minor role in the initial, agonist sensitive production of PtdIns(3,4,5)P3 in myeloid derived cells.

Adenosine Triphosphate↗
PubMed1992

Physiological evidence for involvement of a kinesin-related protein during anaphase spindle elongation in diatom central spindles.

We have developed a new model system for studying spindle elongation in vitro using the pennate, marine diatom Cylindrotheca fusiformis. C. fusiformis can be grown in bulk to high densities while in log phase growth and synchronized by a simple light/dark regime. Isolated spindles can be attained in quantities sufficient for biochemical analysis and spindle tubulin is approximately 5% of the total protein present. The spindle isolation procedure results in a 10-fold enrichment of diatom tubulin and a calculated 40-fold increase in spindle protein. Isolated spindles or spindles in permeabilized cells can elongate in vitro by the same mechanism and with the same pharmacological sensitivities as described for other anaphase B models (Cande and McDonald, 1986; Masuda et al., 1990). Using this model, in vitro spindle elongation rate profiles were developed for a battery of nucleotide triphosphates and ATP analogs. The relative rates of spindle elongation produced by various nucleotide triphosphates parallel relative rates seen for kinesin-based motility in microtubule gliding assays. Likewise ATP analogs that allow discrimination between myosin-, dynein-, and kinesin-mediated motility produce relative spindle elongation rates characteristic of kinesin motility. Also, isolated spindle fractions are enriched for a kinesin related protein as identified by a peptide antibody against a conserved region of the kinesin superfamily. These data suggest that kinesin-like motility contributes to spindle elongation during anaphase B of mitosis.

Adenosine Triphosphate↗
PubMed1992

"We overcame all".

Explore the source record for details and available documents.

Breast Feeding↗
PubMed1990

Cheilitis granulomatosa: report of six cases and review of the literature.

Six cases of cheilitis granulomatosa, a rare inflammatory disorder of unknown origin, are reported. The condition produces nontender, persistent swelling of one or both lips and affects primarily young adults. Histologically, nonnecrotizing granulomatous inflammation is seen. The clinical findings and results of therapy in these six cases are presented. One patient was treated with hydroxychloroquine sulfate (Plaquenil) that stabilized the process. One of our patients had vesicular-appearing lesions. Microscopic examination showed the lesions to be dilated superficial lymphatic channels, a finding that to our knowledge has not been previously described.

Adult↗
PubMed1990

Cold subcutaneous abscesses.

Cold abscesses are defined as having no associated erythema, heat, or tenderness. They may be present in immunodeficiency disorders, deep mycoses, and other infectious diseases. As there is a dearth information on this subject in the dermatology, surgery, and infectious disease literature, we present a case of cold abscesses secondary to coccidioidomycosis and discuss the possible role of humoral immunity, cell-mediated immunity, prostaglandins, T cells, and other mediators in cold abscess pathogenesis. In addition, therapeutic guidelines for abscesses are reviewed.

Abscess↗
PubMed1989

Metabolic and structural evidence for the existence of a third species of polyphosphoinositide in cells: D-phosphatidyl-myo-inositol 3-phosphate.

When human 1321 N1 astrocytoma cells were labelled to steady state with [3H]inositol and briefly with [32P]orthophosphate, a compound which contained both radiotracers and which co-migrated with phosphatidylinositol-myo-inositol 4-phosphate during t.l.c. could be extracted in acidic chloroform/methanol. Treatment with methylamine under conditions which lead to deacylation of conventional glycerophospholipids yielded a water-soluble moiety which was labelled with both radioisotopes and was eluted from an anion-exchange h.p.l.c. column with a retention time similar to, but distinct from, that of glycerophosphoinositol 4-phosphate. Experiments using sodium periodate and selective phosphatase enzymes to degrade this compound systematically generated a series of products which suggested the structure of the parent phospholipid was phosphatidyl-myo-inositol 3-phosphate (PtdIns3P). PtdIns3P is metabolically closely related to the pool(s) of inositol phospholipid(s) that serves as substrate(s) for an agonist-sensitive phosphoinositidase C, as the levels of PtdIns3P fell significantly when 1321 N1 cells were stimulated with carbachol. The relative rate of turnover of the inositol moiety of PtdIns3P is similar to that of both of the major polyphosphoinositides and significantly higher than that of total cellular phosphatidyl-myo-inositol. This suggests that all three polyphosphoinositides are synthesized from a common, rapidly metabolized, pool of phosphatidyl-myo-inositol.

Astrocytoma↗