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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Vacancy Mediated Mechanism of Nitrogen Substitution in Carbon Nanotubes

Nitrogen substitution reaction in a graphene sheet and carbon nanotubes of different diameter are investigated using the generalized tight-binding molecular dynamics method. The formation of a vacancy in curved graphene sheet or a carbon nanotube is found to cause a curvature dependent local reconstruction of the surface. Our simulations and analysis show that vacancy mediated N substitution (rather than N chemisorption) is favored on the surface of nanotubes with diameter larger than 8 nm. This predicted value of the critical minimum diameter for N incorporation is confirmed by experimental results presented.

Srivastava, Deepak↗
NASA NTRS2004

Sulfur Cycling Mediates Calcium Carbonate Geochemistry in Modern Marine Stromatolites

Modem marine stromatolites forming in Highborne Cay, Exumas (Bahamas), contain microbial mats dominated by Schizothrix. Although saturating concentrations of Ca2+ and CO32- exist, microbes mediate CaCO3 precipitation. Cyanobacterial photosynthesis in these stromatolites aids calcium carbonate precipitation by removal of HS+ through CO2 use. Photorespiration and exopolymer production predominantly by oxygenic phototrophs fuel heterotrophic activity: aerobic respiration (approximately 60 umol/sq cm.h) and sulfate reduction (SR; 1.2 umol SO42-/sq cm.h) are the dominant C- consuming processes. Aerobic microbial respiration and the combination of SR and H2S oxidation both facilitate CaCO3 dissolution through H+ production. Aerobic respiration consumes much more C on an hourly basis, but duel fluctuating O2 and H2 depth profiles indicate that overall, SR consumes only slightly less (0.2-0.5) of the primary production. Moreover, due to low O2 concentrations when SR rates are peaking, reoxidation of the H2S formed is incomplete: both thiosulfate and polythionates are formed. The process of complete H2S oxidation yields H+. However, due to a low O2 concentration late in the day and relatively high O2 concentrations early in the following morning, a two-stage oxidation takes place: first, polythionates are formed from H2S, creating alkalinity which coincides with CaCO3 precipitation; secondly, oxidation of polythionates to sulfate yields acidity, resulting in dissolution, etc. Vertical profiles confirmed that the pH peaked late in the afternoon (greater than 8.8) and had the lowest values (less than 7.4) early in the morning. Thus, the effect of this S-cycling through alkalinity production, followed by acidification during H2S oxidation, results in a six times stronger fluctuation in acidity than photosynthesis plus aerobic respiration accomplish. This implies that anaerobic processes play a pivotal role in stromatolite formation.

Visscher, P. T.↗
NASA NTRS2003

Increased Agrobacterium-mediated transformation and rooting efficiencies in canola (Brassica napus L.) from hypocotyl segment explants

An efficient protocol for the production of transgenic Brassica napus cv. Westar plants was developed by optimizing two important parameters: preconditioning time and co-cultivation time. Agrobacterium tumefaciens-mediated transformation was performed using hypocotyls as explant tissue. Two variants of a green fluorescent protein (GFP)-encoding gene--mGFP5-ER and eGFP--both under the constitutive expression of the cauliflower mosaic virus 35S promoter, were used for the experiments. Optimizing the preconditioning time to 72 h and co-cultivation time with Agrobacterium to 48 h provided the increase in the transformation efficiency from a baseline of 4% to 25%. With mGFP5-ER, the transformation rate was 17% and with eGFP it was 25%. Transgenic shoots were selected on 200 mg/l kanamycin. Rooting efficiency was 100% on half-strength Murashige and Skoog medium with 10 g/l sucrose and 0.5 mg/l indole butyric acid in the presence of kanamycin.

NASA Discipline Life Sciences Technologies↗
NASA NTRS2003

Baroreflex-mediated heart rate and vascular resistance responses 24 h after maximal exercise

INTRODUCTION: Plasma volume, heart rate (HR) variability, and stimulus-response relationships for baroreflex control of forearm vascular resistance (FVR) and HR were studied in eight healthy men after and without performing a bout of maximal exercise to test the hypotheses that acute expansion of plasma volume is associated with 1) reduction in baroreflex-mediated HR response, and 2) altered operational range for central venous pressure (CVP). METHODS: The relationship between stimulus (DeltaCVP) and vasoconstrictive reflex response (DeltaFVR) during unloading of cardiopulmonary baroreceptors was assessed with lower-body negative pressure (LBNP, 0, -5, -10, -15, -20 mm Hg). The relationship between stimulus (Deltamean arterial pressure (MAP)) and cardiac reflex response (DeltaHR) during loading of arterial baroreceptors was assessed with steady-state infusion of phenylephrine (PE) designed to increase MAP by 15 mm Hg alone and during application of LBNP (PE+LBNP) and neck pressure (PE+LBNP+NP). Measurements of vascular volume and autonomic baroreflex responses were conducted on two different test days, each separated by at least 1 wk. On one day, baroreflex response was tested 24 h after graded cycle exercise to volitional exhaustion. On another day, measurement of baroreflex response was repeated with no exercise (control). The order of exercise and control treatments was counterbalanced. RESULTS: Baseline CVP was elevated (P = 0.04) from a control value of 10.5 +/- 0.4 to 12.3 +/- 0.4 mm Hg 24 h after exercise. Average DeltaFVR/DeltaCVP during LBNP was not different (P = 0.942) between the exercise (-1.35 +/- 0.32 pru x mm Hg-1) and control (-1.32 +/- 0.36 pru x mm Hg-1) conditions. However, maximal exercise caused a shift along the reflex response relationship to a higher CVP and lower FVR. HR baroreflex response (DeltaHR/DeltaMAP) to PE+LBNP+NP was lower (P = 0.015) after maximal exercise (-0.43 +/- 0.15 beats x min-1 x mm Hg-1) compared with the control condition (-0.83 +/- 0.14 beats x min-1 x mm Hg-1). CONCLUSION: Expansion of vascular volume after acute exercise is associated with altered operational range for CVP and reduced HR response to arterial baroreceptor stimulation.

Non-NASA Center↗
NASA NTRS2003

Activation of L-type calcium channels is required for gap junction-mediated intercellular calcium signaling in osteoblastic cells

The propagation of mechanically induced intercellular calcium waves (ICW) among osteoblastic cells occurs both by activation of P2Y (purinergic) receptors by extracellular nucleotides, resulting in "fast" ICW, and by gap junctional communication in cells that express connexin43 (Cx43), resulting in "slow" ICW. Human osteoblastic cells transmit intercellular calcium signals by both of these mechanisms. In the current studies we have examined the mechanism of slow gap junction-dependent ICW in osteoblastic cells. In ROS rat osteoblastic cells, gap junction-dependent ICW were inhibited by removal of extracellular calcium, plasma membrane depolarization by high extracellular potassium, and the L-type voltage-operated calcium channel inhibitor, nifedipine. In contrast, all these treatments enhanced the spread of P2 receptor-mediated ICW in UMR rat osteoblastic cells. Using UMR cells transfected to express Cx43 (UMR/Cx43) we confirmed that nifedipine sensitivity of ICW required Cx43 expression. In human osteoblastic cells, gap junction-dependent ICW also required activation of L-type calcium channels and influx of extracellular calcium.

Non-NASA Center↗
NASA NTRS1998

Factors enhancing Agrobacterium tumefaciens-mediated gene transfer in peanut (Arachis hypogaea L.)

Parameters enhancing Agrobacterium-mediated transfer of foreign genes to peanut (Arachis hypogaea L.) cells were investigated. An intron-containing beta-glucuronidase uidA (gusA) gene under the transcriptional control of CaMV 35S promoter served as a reporter. Transformation frequency was evaluated by scoring the number of sectors expressing GUS activity on leaf and epicotyl explants. The 'Valencia Select' market type cv. New Mexico was more amenable to Agrobacterium transformation than the 'runner' market type cultivars tested (Florunner, Georgia Runner, Sunrunner, or South Runner). The disarmed Agrobacterium tumefaciens strain EHA101 was superior in facilitating the transfer of uidA gene to peanut cells compared to the disarmed strain C58. Rinsing of explants in half-strength Murashige-Skoog (MS) media prior to infection by Agrobacterium significantly increased the transformation efficiency. The use of cocultivation media containing high auxin [1.0 or 2.5 mg/l (4.53 micromolar or 11.31 micromolar) 2,4-D] and low cytokinin [0.25 or 0.5 mg/l (1.0 micromolar or 2.0 micromolar) BA] promoted higher transformation than either hormone-free or thidiazuron-containing medium. The polarity of the epicotyl during cocultivation was important; explants incubated in an inverted (vertically) manner followed by a vertically upright position resulted in improved transformation and shoot regeneration frequencies. Preculture of explants in MS basal medium or with 2.5 mg thidiazuron per l prior to infection drastically decreased the number of transformed zones. The optimized protocol was used to obtain transient transformation frequencies ranging from 12% to 36% for leaf explants, 15% to 42% for epicotyls. Initial evidence of transformation was obtained by polymerase chain reaction and subsequently confirmed by Southern analysis of regenerated plants.

NASA Discipline Life Support Systems↗
NASA NTRS1991

Phytochrome-mediated germination and early development in spores of Dryopteris filix-mas L.: phase-specific and non phase-specific inhibition by staurosporine

The alkaloid staurosporine, currently known as the most potent inhibitor of protein kinase C, PKC, was tested for its ability to inhibit phytochrome-mediated spore germination in Dryopteris filix-mas L., evaluated by the induction of chlorophyll synthesis. Approximately half-maximal inhibition was obtained at a concentration of 10(-5) M. This effect of staurosporine was phase-specific and was found during the same period in which the presence of extracellular calcium is necessary for realization of the light signal. Furthermore, the ability of staurosporine to prevent progression of a germinated spore into early gametophyte development, evaluated by the accumulation of chlorophyll, was examined. Again, staurosporine (10(-5) M) significantly diminished chlorophyll accumulation, determined quantitatively in vivo by single-cell measurements, in a non-phase specific way. The fact that the phase-specific inhibitory effect of staurosporine in preventing germination was coincident with the phase-specific requirement of Ca2+ suggests that both Ca2+ and staurosporine affect the same step in the signal-transduction chain. A phosphorylation event catalysed by PKC or any Ca2+ -dependent protein kinase is proposed as the target of staurosporine and Ca2+.

Non-NASA Center↗
NASA NTRS1986

Evaluation of ethylene as a mediator of gravitropism by tomato hypocotyls

Assessments of the participation of ethylene in gravitropism by hypocotyls of tomato (Lycopersicon esculentum Mill.) indicate that gravitropism can occur without substantial change in ethylene production. Moreover, lowering or evaluating ethylene over a considerable range, as well as inhibiting ethylene action, fails to influence gravitropic bending. This vitiates the possibility that ethylene is a mediator of the primary, negative gravitropic response of tomato shoots.

NASA Program Space Biology↗
NASA NTRS2000

Prostaglandin E2-induced up-regulation of c-fos messenger ribonucleic acid is primarily mediated by 3',5'-cyclic adenosine monophosphate in MC3T3-E1 osteoblasts

The mechanism by which the proto-oncogene, c-fos, is up-regulated in response to PGE2 in the mouse osteoblastic (MC3T3-E1) cell line was investigated using RT-PCR. c-fos messenger RNA up-regulation by dmPGE2 is rapid, starting 10 min post stimulation, and transient. The specific protein kinase A (PKA) inhibitor, H89, inhibited c-fos induction. Moreover, down-regulation of protein kinase C (PKC) activity by chronic TPA treatment had no effect on the induction of c-fos by dmPGE2. We conclude that up-regulation of c-fos by dmPGE2 is primarily dependent on PKA in MC3T3-E1 osteoblasts. In S49 lymphoma wild-type but not S49 cyc- cells, which are deficient in cAMP signaling, dmPGE2 up-regulates c-fos and increases cell growth compared with unstimulated cells. Thus in S49 lymphoma cells, c-fos induction by PGE2 is also dependent on cAMP signaling. The minimal c-fos promoter region required for dmPGE2-induced expression was identified by transfecting c-fos promoter deletion constructs coupled to the chloramphenicol acetyltransferase (CAT) reporter gene into Vero cells. Transfection of a plasmid containing 99 bp c-fos proximal promoter was sufficient to direct c-fos/CAT expression following stimulation with dmPGE2. Because induction of c-fos is mediated by cAMP, these data are consistent with activation of c-fos via the CRE/ATF cis element.

NASA Discipline Cell Biology↗
NASA NTRS2000

Nitric oxide inhibits calpain-mediated proteolysis of talin in skeletal muscle cells

We tested the hypothesis that nitric oxide can inhibit cytoskeletal breakdown in skeletal muscle cells by inhibiting calpain cleavage of talin. The nitric oxide donor sodium nitroprusside prevented many of the effects of calcium ionophore on C(2)C(12) muscle cells, including preventing talin proteolysis and release into the cytosol and reducing loss of vinculin, cell detachment, and loss of cellular protein. These results indicate that nitric oxide inhibition of calpain protected the cells from ionophore-induced proteolysis. Calpain inhibitor I and a cell-permeable calpastatin peptide also protected the cells from proteolysis, confirming that ionophore-induced proteolysis was primarily calpain mediated. The activity of m-calpain in a casein zymogram was inhibited by sodium nitroprusside, and this inhibition was reversed by dithiothreitol. Previous incubation with the active site-targeted calpain inhibitor I prevented most of the sodium nitroprusside-induced inhibition of m-calpain activity. These data suggest that nitric oxide inhibited m-calpain activity via S-nitrosylation of the active site cysteine. The results of this study indicate that nitric oxide produced endogenously by skeletal muscle and other cell types has the potential to inhibit m-calpain activity and cytoskeletal proteolysis.

NASA Discipline Musculoskeletal↗
NASA NTRS2000

The auxin-resistant diageotropica mutant of tomato responds to gravity via an auxin-mediated pathway

Hypocotyls of the diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.) do not elongate in response to exogenous auxin, but can respond to gravity. This appears paradoxical in light of the Cholodny-Went hypothesis, which states that shoot gravicurvature results from asymmetric stimulation of elongation by auxin. While light-grown dgt seedlings can achieve correct gravitropic reorientation, the response is slow compared to wild-type seedlings. The sensitivity of dgt seedlings to inhibition of gravicurvature by immersion in auxin or auxin-transport inhibitors is similar to that of wild-type plants, indicating that both an auxin gradient and auxin transport are required for the gravitropic response and that auxin uptake, efflux, and at least one auxin receptor are functional in dgt. Furthermore, dgt gravicurvature is the result of asymmetrically increased elongation as would be expected for an auxin-mediated response. Our results suggest differences between elongation in response to exogenous auxin (absent in dgt) and elongation in response to gravistimulation (present but attenuated in dgt) and confirm the presence of two phases during the gravitropic response, both of which are dependent on functional auxin transport.

Non-NASA Center↗
NASA NTRS2000

Extracellular calcium-sensing-receptor (CaR)-mediated opening of an outward K(+) channel in murine MC3T3-E1 osteoblastic cells: evidence for expression of a functional CaR

The existence in osteoblasts of the G-protein-coupled extracellular calcium (Ca(o)(2+))-sensing receptor (CaR) that was originally cloned from parathyroid and kidney remains controversial. In our recent studies, we utilized multiple detection methods to demonstrate the expression of CaR transcripts and protein in several osteoblastic cell lines, including murine MC3T3-E1 cells. Although we and others have shown that high Ca(o)(2+) and other polycationic CaR agonists modulate the function of MC3T3-E1 cells, none of these actions has been unequivocally shown to be mediated by the CaR. Previous investigations using neurons and lens epithelial cells have shown that activation of the CaR stimulates Ca(2+)-activated K(+) channels. Because osteoblastic cells express a similar type of channel, we have examined the effects of specific "calcimimetic" CaR activators on the activity of a Ca(2+)-activated K(+) channel in MC3T3-E1 cells as a way of showing that the CaR is not only expressed in those cells but is functionally active. Patch-clamp analysis in the cell-attached mode showed that raising Ca(o)(2+) from 0.75 to 2.75 mmol/L elicited about a fourfold increase in the open state probability (P(o)) of an outward K(+) channel with a conductance of approximately 92 pS. The selective calcimimetic CaR activator, NPS R-467 (0.5 micromol/L), evoked a similar activation of the channel, while its less active stereoisomer, NPSS-467 (0.5 micromol/L), did not. Thus, the CaR is not only expressed in MC3T3-E1 cells, but is also functionally coupled to the activity of a Ca(2+)-activated K(+) channel. This receptor, therefore, could transduce local or systemic changes in Ca(o)(2+) into changes in the activity of this ion channel and related physiological processes in these and perhaps other osteoblastic cells.

NASA Discipline Musculoskeletal↗
NASA NTRS2000

Epstein-Barr virus reactivation associated with diminished cell-mediated immunity in antarctic expeditioners

Epstein-Barr virus (EBV) reactivation and cell-mediated immune (CMI) responses were followed in 16 Antarctic expeditioners during winter-over isolation at 2 Australian National Antarctic Research Expedition stations. Delayed-type hypersensitivity (DTH) skin testing was used as an indicator of the CMI response, that was evaluated 2 times before winter isolation and 3 times during isolation. At all 5 evaluation times, 8 or more of the 16 subjects had a diminished CMI response. Diminished DTH was observed on every test occasion in 4/16 subjects; only 2/16 subjects exhibited normal DTH responses for all 5 tests. A polymerase chain reaction (PCR) assay was used to detect EBV DNA in saliva specimens collected before, during, and after the winter isolation. EBV DNA was present in 17% (111/642) of the saliva specimens; all 16 subjects shed EBV in their saliva on at least 1 occasion. The probability of EBV shedding increased (P = 0.013) from 6% before or after winter isolation to 13% during the winter period. EBV appeared in saliva during the winter isolation more frequently (P < 0.0005) when DTH response was diminished than when DTH was normal. The findings indicate that the psychosocial, physical, and other stresses associated with working and living in physical isolation during the Antarctic winter result in diminished CMI and an accompanying increased reactivation and shedding of latent viruses.

NASA Program Biomedical Research and Countermeasur↗
NASA NTRS1999

Effects of nitric oxide synthase inhibition on sympathetically-mediated tachycardia

The aim of the present study was to determine whether inhibition of nitric oxide (NO) synthesis directly alters the tachycardia produced by sympathetically-derived norepinephrine. The NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME; 50 micromol/kg, i.v.), produced a marked rise in mean arterial blood pressure. This pressor response was associated with a fall in heart rate which involved the withdrawal of cardiac sympathetic nerve activity. The NO-donor, sodium nitroprusside (5 microg/kg, i.v.), produced a pronounced fall in mean arterial blood pressure but only a minor increase in heart rate. The beta-adrenoceptor agonist, isoproterenol (0.5 micromol/kg, i.v.), and the membrane-permeable cAMP analogue, 8-(4-chlorophenylthiol)-cAMP (10 micromol/kg, i.v.), produced falls in mean arterial blood pressure and pronounced increases in heart rate. The indirectly acting sympathomimetic agent, tyramine (0.5 mg/kg, i.v.), produced a pressor response and a tachycardia. The effects of sodium nitroprusside, tyramine, isoproterenol and 8-(4-chlorophenylthiol)-cAMP on mean arterial blood pressure were not markedly affected by L-NAME. However, the tachycardia produced by these agents was considerably exaggerated in the presence of this NO synthesis inhibitor. These findings suggest that L-NAME potentiates the tachycardia produced by sympathetically-derived norepinephrine. The increased responsiveness to norepinephrine may involve (i) a rapid up-regulation of cardiac beta1-adrenoceptors and cAMP signaling in cardiac pacemaker cells due to the loss of the inhibitory influence of cardiac NO, and (ii) the up-regulation of beta1-adrenoceptor-mediated signal transduction processes in response to the L-NAME-induced withdrawal of cardiac sympathetic nerve activity.

NASA Discipline Regulatory Physiology↗
NASA NTRS1994

Signal transduction pathways mediating parathyroid hormone regulation of osteoblastic gene expression

Parathyroid hormone (PTH) plays a central role in regulation of calcium metabolism. For example, excessive or inappropriate production of PTH or the related hormone, parathyroid hormone related protein (PTHrP), accounts for the majority of the causes of hypercalcemia. Both hormones act through the same receptor on the osteoblast to elicit enhanced bone resorption by the osteoclast. Thus, the osteoblast mediates the effect of PTH in the resorption process. In this process, PTH causes a change in the function and phenotype of the osteoblast from a cell involved in bone formation to one directing the process of bone resorption. In response to PTH, the osteoblast decreases collagen, alkaline phosphatase, and osteopontin expression and increases production of osteocalcin, cytokines, and neutral proteases. Many of these changes have been shown to be due to effects on mRNA abundance through either transcriptional or post-transcriptional mechanisms. However, the signal transduction pathway for the hormone to cause these changes is not completely elucidated in any case. Binding of PTH and PTHrP to their common receptor has been shown to result in activation of protein kinases A and C and increases in intracellular calcium. The latter has not been implicated in any changes in mRNA of osteoblastic genes. On the other hand activation of PKA can mimic all the effects of PTH; protein kinase C may be involved in some responses. We will discuss possible mechanisms linking PKA and PKC activation to changes in gene expression, particularly at the nuclear level.

Review↗
NASA NTRS1992

Direct muscarinic and nicotinic receptor-mediated excitation of rat medial vestibular nucleus neurons in vitro

We have utilized intracellular recording techniques to investigate the cholinoceptivity of rat medial vestibular nucleus (MVN) neurons in a submerged brain slice preparation. Exogenous application of the mixed cholinergic agonists, acetylcholine (ACh) or carbachol (CCh), produced predominantly membrane depolarization, induction of action potential firing, and decreased input resistance. Application of the selective muscarinic receptor agonist muscarine (MUSC), or the selective nicotinic receptor agonists nicotine (NIC) or 1,1-dimethyl-4-phenylpiperazinium (DMPP) also produced membrane depolarizations. The MUSC-induced depolarization was accompanied by decreased conductance, while an increase in conductance appeared to underlie the NIC- and DMPP-induced depolarizations. The muscarinic and nicotinic receptor mediated depolarizations persisted in tetrodotoxin and/or low Ca2+/high Mg2+ containing media, suggesting direct postsynaptic receptor activation. The MUSC-induced depolarization could be reversibly blocked by the selective muscarinic-receptor antagonist, atropine, while the DMPP-induced depolarization could be reversibly suppressed by the selective ganglionic nicotinic-receptor antagonist, mecamylamine. Some neurons exhibited a transient membrane hyperpolarization during the depolarizing response to CCh or MUSC application. This transient inhibition could be reversibly blocked by the gamma-aminobutyric acid (GABA) antagonist, bicuculline, suggesting that the underlying hyperpolarization results indirectly from the endogenous release of GABA acting at GABA receptors. This study confirms the cholinoceptivity of MVN neurons and establishes that individual MVN cells possess muscarinic as well as nicotinic receptors. The data provide support for a prominent role of cholinergic mechanisms in the direct and indirect regulation of the excitability of MVN neurons.

Non-NASA Center↗
NASA NTRS2006

The Role of Non-Targeted Effects as Mediators in the Biological Effects of Proton Irradiation

In recent years, the hypothesis that non-DNA targets are primary initiators and mediators of the biological effects of ionizing radiation, such as proton beams and heavy ions, has gained much interest. These phenomena have been denoted as non-targeted or bystander effects to distinguish them from the more traditionally studied model that focuses on direct damage to DNA causing chromosomal rearrangements and mutations as causative of most biological endpoints such as cell killing, tissue damage, and cancer. We review cellular and extra-cellular structures and signal transduction pathways that have been implemented in these recent studies. Non-targeted effects of interest include oxidative damage to the cytoplasm and mitochondria, disruption of the extra-cellular matrix, and modification of cytokine signaling including TGF-beta, and gap junction communication. We present an introduction to these targets and pathways, and contrast there role with DNA damage pathways.

Cucinotta, Francis A.↗
NASA NTRS2016

Electride Mediated Surface Enhanced Raman Scattering (SERS)

An electride may provide surface enhanced Raman scattering (SERS). The electride, a compound where the electrons serve as anions, may be a ceramic electride, such as a conductive ceramic derived from mayenite, or an organic electride, for example. The textured electride surface or electride particles may strongly enhance the Raman scattering of organic or other Raman active analytes. This may also provide a sensitive method for monitoring the chemistry and electronic environment at the electride surface. The results are evidence of a new class of polariton (i.e., a surface electride-polariton resonance mechanism) that is analogous to the surface plasmon-polariton resonance that mediates conventional SERS.

Anderson, Mark S.↗