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At least 757 records · Page 42

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NASA NTRS2001

Plasma membrane NADH oxidase of maize roots responds to gravity and imposed centrifugal forces

NADH oxidase activities measured with excised roots of dark-grown maize (Zea mays) seedlings and with isolated plasma membrane vesicles from roots of dark-grown maize oscillated with a regular period length of 24 min and were inhibited by the synthetic auxin 2,4-dichlorophenoxyacetic [correction of dichorophenoxyacetic] acid. The activities also responded to orientation with respect to gravity and to imposed centrifugal forces. Turning the roots upside down resulted in stimulation of the activity with a lag of about 10 min. Returning the sections to the normal upright position resulted in a return to initial rates. The activity was stimulated reversibly to a maximum of about 2-fold with isolated plasma membrane vesicles, when subjected to centrifugal forces of 25 to 250 x g for 1 to 4 min duration. These findings are the first report of a gravity-responsive enzymatic activity of plant roots inhibited by auxin and potentially related to the gravity-induced growth response. c2001 Editions scientifiques et medicales Elsevier SAS.

NASA Discipline Cell Biology↗

NASA NTRS2002

Regulation of collagenase-3 and osteocalcin gene expression by collagen and osteopontin in differentiating MC3T3-E1 cells

Both collagenase-3 and osteocalcin mRNAs are expressed maximally during the later stages of osteoblast differentiation. Here, we demonstrate that collagenase-3 mRNA expression in differentiating MC3T3-E1 cells is dependent upon the presence of ascorbic acid, is inhibited in the presence of the collagen synthesis inhibitor, 3,4-dehydroproline, and is stimulated by growth on collagen in the absence of ascorbic acid. Transient transfection studies show that collagenase-3 promoter activity increases during cell differentiation and requires the presence of ascorbic acid. Additionally, we show that, in differentiating MC3T3-E1 cells, collagenase-3 gene expression increases in the presence of an anti-osteopontin monoclonal antibody that binds near the RGD motif of this protein, whereas osteocalcin expression is inhibited. Furthermore, an RGD peptidomimetic compound, designed to block interaction of ligands to the alpha(v) integrin subunit, increases osteocalcin expression and inhibits collagenase-3 expression, suggesting that the RGD peptidomimetic initiates certain alpha(v) integrin signaling in osteoblastic cells. Overall, these studies demonstrate that stimulation of collagenase-3 expression during osteoblast differentiation requires synthesis of a collagenous matrix and that osteopontin and alpha(v) integrins exert divergent regulation of collagenase-3 and osteocalcin expression during osteoblast differentiation.

NASA Discipline Musculoskeletal↗

NASA NTRS2002

Aging attenuates the vestibulosympathetic reflex in humans

BACKGROUND: The vestibular system contributes to sympathetic activation by engagement of the otolith organs. However, there is a significant loss of vestibular function with aging. Therefore, the purpose of the present study was to determine if young and older individuals differ in their cardiovascular and sympathetic responses to otolithic stimulation (ie, head-down rotation, HDR). We hypothesized that responses to otolithic stimulation would be attenuated in older adults because of morphological and physiological alterations that occur in the vestibular system with aging. METHODS AND RESULTS: Arterial blood pressure, heart rate, muscle sympathetic nerve activity (MSNA), and head rotation were measured during HDR in 11 young (26 +/- 1 years) and 11 older (64 +/- 1 years) subjects in the prone posture. Five older subjects performed head rotation (chin to chest) in the lateral decubitus position, which simulates HDR but does not alter afferent inputs from the vestibular system. MSNA responses to HDR were significantly attenuated in older as compared with young subjects (P<0.01). MSNA increased in the older subjects by only 12 +/- 5% as compared with 85 +/- 16% in the young. Furthermore, HDR elicited significant reductions in mean arterial blood pressure in older (Delta-6 +/- 1 mm Hg; P<0.01) but not young subjects (Delta1 +/- 1 mm Hg). In contrast to HDR, head rotation performed in the lateral decubitus position did not elicit hypotension. MSNA responses to baroreceptor unloading and the cold pressor test were not different between the age groups. CONCLUSIONS: These data indicate that aging attenuates the vestibulosympathetic reflex in humans and may contribute to the increased prevalence of orthostatic hypotension with age.

NASA Discipline Neuroscience↗

NASA NTRS2002

Inhibition of caspases prevents ototoxic and ongoing hair cell death

Sensory hair cells die after acoustic trauma or ototoxic insults, but the signal transduction pathways that mediate hair cell death are not known. Here we identify several important signaling events that regulate the death of vestibular hair cells. Chick utricles were cultured in media supplemented with the ototoxic antibiotic neomycin and selected pharmacological agents that influence signaling molecules in cell death pathways. Hair cells that were treated with neomycin exhibited classically defined apoptotic morphologies such as condensed nuclei and fragmented DNA. Inhibition of protein synthesis (via treatment with cycloheximide) increased hair cell survival after treatment with neomycin, suggesting that hair cell death requires de novo protein synthesis. Finally, the inhibition of caspases promoted hair cell survival after neomycin treatment. Sensory hair cells in avian vestibular organs also undergo continual cell death and replacement throughout mature life. It is unclear whether the loss of hair cells stimulates the proliferation of supporting cells or whether the production of new cells triggers the death of hair cells. We examined the effects of caspase inhibition on spontaneous hair cell death in the chick utricle. Caspase inhibitors reduced the amount of ongoing hair cell death and ongoing supporting cell proliferation in a dose-dependent manner. In isolated sensory epithelia, however, caspase inhibitors did not affect supporting cell proliferation directly. Our data indicate that ongoing hair cell death stimulates supporting cell proliferation in the mature utricle.

Non-NASA Center↗

NASA NTRS2000

Halogenated auxins affect microtubules and root elongation in Lactuca sativa

We studied the effect of 4,4,4-trifluoro-3-(indole-3-)butyric acid (TFIBA), a recently described root growth stimulator, and 5,6-dichloro-indole-3-acetic acid (DCIAA) on growth and microtubule (MT) organization in roots of Lactuca sativa L. DCIAA and indole-3-butyric acid (IBA) inhibited root elongation and depolymerized MTs in the cortex of the elongation zone, inhibited the elongation of stele cells, and promoted xylem maturation. Both auxins caused the plane of cell division to shift from anticlinal to periclinal. In contrast, TFIBA (100 micromolar) promoted elongation of primary roots by 40% and stimulated the elongation of lateral roots, even in the presence of IBA, the microtubular inhibitors oryzalin and taxol, or the auxin transport inhibitor naphthylphthalamic acid. However, TFIBA inhibited the formation of lateral root primordia. Immunostaining showed that TFIBA stabilized MTs orientation perpendicular to the root axis, doubled the cortical cell length, but delayed xylem maturation. The data indicate that the auxin-induced inhibition of elongation and swelling of roots results from reoriented phragmoplasts, the destabilization of MTs in elongating cells, and promotion of vessel formation. In contrast, TFIBA induced promotion of root elongation by enhancing cell length, prolonging transverse MT orientation, delaying cell and xylem maturation.

Non-NASA Center↗

NASA NTRS2001

Increases in intramuscular pressure raise arterial blood pressure during dynamic exercise

This investigation was designed to determine the role of intramuscular pressure-sensitive mechanoreceptors and chemically sensitive metaboreceptors in affecting the blood pressure response to dynamic exercise in humans. Sixteen subjects performed incremental (20 W/min) cycle exercise to fatigue under four conditions: control, exercise with thigh cuff occlusion of 90 Torr (Cuff occlusion), exercise with lower body positive pressure (LBPP) of 45 Torr, and a combination of thigh cuff occlusion and LBPP (combination). Indexes of central command (heart rate, oxygen uptake, ratings of perceived exertion, and electromyographic activity), cardiac output, stroke volume, and total peripheral resistance were not significantly different between the four conditions. Mechanical stimulation during LBPP and combination conditions resulted in significant elevations in intramuscular pressure and mean arterial pressure from control at rest and throughout the incremental exercise protocol (P < 0.05). Conversely, there existed no significant changes in mean arterial pressure when the metaboreflex was stimulated by cuff occlusion. These findings suggest that under normal conditions the mechanoreflex is tonically active and is the primary mediator of exercise pressor reflex-induced alterations in arterial blood pressure during submaximal dynamic exercise in humans.

Non-NASA Center↗

NASA NTRS2001

Baroreflex modulation of muscle sympathetic nerve activity during posthandgrip muscle ischemia in humans

To identify whether muscle metaboreceptor stimulation alters baroreflex control of muscle sympathetic nerve activity (MSNA), MSNA, beat-by-beat arterial blood pressure (Finapres), and electrocardiogram were recorded in 11 healthy subjects in the supine position. Subjects performed 2 min of isometric handgrip exercise at 40% of maximal voluntary contraction followed by 2.5 min of posthandgrip muscle ischemia. During muscle ischemia, blood pressure was lowered and then raised by intravenous bolus infusions of sodium nitroprusside and phenylephrine HCl, respectively. The slope of the relationship between MSNA and diastolic blood pressure was more negative (P < 0.001) during posthandgrip muscle ischemia (-201.9 +/- 20.4 units. beat(-1). mmHg(-1)) when compared with control conditions (-142.7 +/- 17.3 units. beat(-1). mmHg(-1)). No significant change in the slope of the relationship between heart rate and systolic blood pressure was observed. However, both curves shifted during postexercise ischemia to accommodate the elevation in blood pressure and MSNA that occurs with this condition. These data suggest that the sensitivity of baroreflex modulation of MSNA is elevated by muscle metaboreceptor stimulation, whereas the sensitivity of baroreflex of modulate heart rate is unchanged during posthandgrip muscle ischemia.

Clinical Trial↗

NASA NTRS1989

Reduced gravitropic sensitivity in roots of a starch-deficient mutant of Nicotiana sylvestris

Gravitropism was studied in seedlings of Nicotiana sylvestris Speg. et Comes wild-type (WT) and mutant NS 458 which has a defective plastid phosphoglucomutase (EC 2.7.5.1.). Starch was greatly reduced in NS 458 compared to the WT, but small amounts of starch were detected in rootcap columella cells in NS 458 by light and electron microscopy. The roots of WT are more sensitive to gravity than mutant NS 458 roots since: (1) in mutant roots, curvature was reduced and delayed in the time course of curvature; (2) curvature of mutant roots was 24-56% that of WT roots over the range of induction periods tested; (3) in intermittent-stimulation experiments, curvature of mutant roots was 37% or less than that of WT roots in all treatments tested. The perception time, determined by intermittent-stimulation experiments, was < or = 5 s for WT roots and 30-60 s for mutant roots. The growth rates for WT and NS 458 roots were essentially equal. These results and our previous results with WT and starchless mutant Arabidopsis roots (Kiss et al. 1989, Planta 177, 198-206) support the conclusions that a full complement of starch is necessary for full gravitropic sensitivity and that amyloplasts function in gravity perception. Since a presumed relatively small increase in plastid buoyant mass (N. sylvestris mutant versus Arabidopsis mutant) significantly improves the orientation of the N. sylvestris mutant roots, we suggest that plastids are the likeliest candidates to be triggering gravity perception in roots of both mutants.

Non-NASA Center↗

NASA NTRS1988

Light-regulated protein and mRNA synthesis in root caps of maize

Illumination of maize roots initiates changes in mRNA levels and in the activities of proteins within the root cap. Using Northern analysis we showed a 5-6 fold increase in the levels of three specific mRNAs and a 14-fold increase in plastid mRNA. This increase is rapid, occurring within 30 minutes of illumination. With prolonged periods of darkness following illumination, messages return to levels observed in dark, control caps. For two species of mRNA illumination results in a reduction in message levels. Light-stimulated increases in the levels of specific mRNAs are proportionally greater than are increases in the activities of corresponding proteins. We suggest that the light-stimulated increase in protein activity in root caps may be preceded by and occur as a consequence of enhanced levels of mRNA. Our work suggests that photomorphogenesis in roots could involve changes in the levels of a wide variety of mRNAs within the root cap.

NASA Program Space Biology↗

NASA NTRS1991

Cardiovascular function in space flight

Changes in orthostatic heart rate have been noted universally in Soviet and U.S. crewmembers post space flight. The magnitude of these changes appears to be influenced by mission duration, with increasing orthostatic intolerance for the first 7-10 days of flight and then a partial recovery in the orthostatic heart rate response. Fluid loading has been used as a countermeasure to this postflight orthostatic intolerance. Previous reports have documented the effectiveness of this technique, but it has also been noted that the effectiveness of volume expansion diminishes as flight duration exceeds one week. The response of carotid baroreceptor function was investigated utilizing a commercially available neck collar which could apply positive and negative pressure to effect receptor stimulation. Bedrest studies had validated the usefulness and validity of the device. In these studies it was shown that carotid baroreceptor function curves demonstrated less responsiveness to orthostatic stimulation than control individuals. Twelve Space Shuttle crewmembers were examined pre- and postflight from flights lasting from 4-5 days. Plots of baroreceptor function were constructed and plotted as change in R-R interval vs. carotid distending pressure (an orthostatic stimulus). Typical sigmoidal curves were obtained. Postflight the resting heart rate was higher (smaller R-R interval) and the range of R-R value and the slope of the carotid sigmoidal response were both depressed. These changes were not significant immediately postflight (L + O), but did become significant by the second day postflight (L + 2), and remained suppressed for several days thereafter. It is hypothesized that the early adaptation to space flight involves a central fluid shift during the initial days of flight, but subsequent alterations in neural controlling mechanisms (such as carotid baroreceptor function) contribute to orthostatic intolerance.

Review↗

NASA NTRS1995

Gravitropic responses of the Avena coleoptile in space and on clinostats. I. Gravitropic response thresholds

We conducted a series of gravitropic experiments on Avena coleoptiles in the weightlessness environment of Spacelab. The purpose was to test the threshold stimulus, reciprocity rule and autotropic reactions to a range of g-force stimulations of different intensities and durations The tests avoided the potentially complicating effects of earth's gravity and the interference from clinostat ambiguities. Using slow-speed centrifuges, coleoptiles received transversal accelerations in the hypogravity range between 0.l and 1.0 g over periods that ranged from 2 to 130 min. All responses that occurred in weightlessness were compared to clinostat experiments on earth using the same apparatus. Characteristic gravitropistic response patterns of Atuena were not substantially different from those observed in ground-based experiments. Gravitropic presentation times were extrapolated. The threshold at 1.0 g was less than 1 min (shortest stimulation time 2 min), in agreement with values obtained on the ground. The least stimulus tested, 0.1 g for 130 min, produced a significant response. Therefore the absolute threshold for a gravitropic response is less than 0.1 g.

Flight Experiment↗

NASA NTRS1987

Water content and the conversion of phytochrome regulation of lettuce dormancy

In an effort to determine which biological reactions can occur in relation to the water content of seeds, the regulation of lettuce seed dormancy by red and far red light was determined at various hydration levels. Far red light had an inhibiting effect on germination for seeds at all moisture contents from 4 to 32% water. Germination was progressively stimulated by red light as seed hydration increased from 8 to 15%, and reached a maximum at moisture contents above 18%. Red light was ineffective at moisture contents below 8%. Seeds that had been stimulated by red light and subsequently dried lost the enhanced germinability if stored at moisture contents above 8%. The contrast between the presumed photoconversion of phytochrome far red-absorbing (Pfr) to (Pr) occurring at any moisture content and the reverse reaction occurring only if the seed moisture content is greater than 8% may be explained on the basis of the existence of unstable intermediates in the Pr to Pfr conversion. Our results suggest that the initial photoreaction involved in phytochrome conversion is relatively independent of water content, while the subsequent partial reactions become increasingly facilitated as water content increases from 8 to 18%.

Non-NASA Center↗

NASA NTRS1988

Effect of salicylhydroxamic acid on endosperm strength and embryo growth of Lactuca sativa L. cv Waldmann's Green seeds

Salicylhydroxamic acid (SHAM) stimulated germination of photosensitive lettuce (Lactuca sativa L. cv Waldmann's Green) seeds in darkness. To determine whether SHAM acts on the embryo or the endosperm, we investigated separately effects of SHAM on growth potential of isolated embryos as well as on endosperm strength. Embryo growth potential was quantified by incubating decoated embryos in various concentrations of osmoticum and measuring subsequent radicle elongation. Growth potential of embryos isolated from seeds pretreated with 4 millimolar SHAM was equal to that of untreated controls. Rupture strength of endosperm tissue excised from seeds pretreated with SHAM was 33% less than that of controls in the micropylar region. To determine if the embryo must be in contact with the endosperm of SHAM to weaken the endosperm, some endosperms were incubated with SHAM only after dissection from seeds. Rupture strength of SHAM-treated, isolated endosperms in the micropylar region was 25% less than that of untreated controls. There was no difference in rupture strength in the cotyledonary region of endosperm isolated from seeds treated with SHAM in buffer or buffer alone. SHAM therefore stimulates germination not by enhancing embryo growth potential, but by weakening the micropylar region of the endosperm enclosing the embryo.

NASA Discipline Plant Biology↗

NASA NTRS1991

Partial purification and characterization of a Ca(2+)-dependent protein kinase from pea nuclei

Almost all the Ca(2+)-dependent protein kinase activity in nuclei purified from etiolated pea (Pisum sativum, L.) plumules is present in a single enzyme that can be extracted from chromatin by 0.3 molar NaCl. This protein kinase can be further purified 80,000-fold by salt fractionation and high performance liquid chromatography, after which it has a high specific activity of about 100 picomoles per minute per microgram in the presence of Ca2+ and reaches half-maximal activation at about 3 x 10(-7) molar free Ca2+, without calmodulin. It is a monomer with a molecular weight near 90,000. It can efficiently use histone III-S, ribosomal S6 protein, and casein as artificial substrates, but it phosphorylates phosvitin only weakly. Its Ca(2+)-dependent kinase activity is half-maximally inhibited by 0.1 millimolar chlorpromazine, by 35 nanomolar K-252a and by 7 nanomolar staurosporine. It is insensitive to sphingosine, an inhibitor of protein kinase C, and to basic polypeptides that block other Ca(2+)-dependent protein kinases. It is not stimulated by exogenous phospholipids or fatty acids. In intact isolated pea nuclei it preferentially phosphorylates several chromatin-associated proteins, with the most phosphorylated protein band being near the same molecular weight (43,000) as a nuclear protein substrate whose phosphorylation has been reported to be stimulated by phytochrome in a calcium-dependent fashion.

NASA Discipline Number 40-50↗

NASA NTRS1994

Influence of low temperature preincubation on somatic embryogenesis and ethylene emanation from orchardgrass leaves

The objectives of this study were to determine the effects of low temperature (4 degrees C) preincubation on somatic embryogenesis from orchardgrass (Dactylis glomerata L.) leaf cultures and to relate these effects to ethylene emanation during the preincubation and incubation periods. Experiments were also conducted with an ethylene biosynthesis inhibitor aminooxyacetic acid (AOA). Segments from the innermost two leaves were cultured on SH medium with 30 micromoles dicamba at 4 degrees C for 1 to 7 d before transfer to 21 degrees C. Results from a paired design showed that the embryogenic response of leaf segments preincubated at 4 degrees C was equal or superior to nonpreincubated leaves at all time periods. Ethylene emanation was decreased during the low temperature incubation. Transfer of leaf segments from 4 degrees C to 21 degrees C was accompanied by a burst of ethylene which rose to control levels within 30 min. AOA at 20 and 40 micromoles decreased ethylene emanation but did not stimulate the embryogenic response. We conclude that the stimulation of somatic embryogenesis by low temperature is probably due to factors other than suppression of ethylene biosynthesis.

NASA Discipline Number 40-50↗

NASA NTRS1988

The influence of calcium and pH on growth in primary roots of Zea mays

We investigated the interaction of Ca2+ and pH on root elongation in Zea mays L. cv. B73 x Missouri 17 and cv. Merit. Seedlings were raised to contain high levels of Ca2+ (HC, imbibed and raised in 10 mM CaCl2) or low levels of Ca2+ (LC, imbibed and raised in distilled water). In HC roots, lowering the pH (5 mM MES/Tris) from 6.5 to 4.5 resulted in strong, long-lasting growth promotion. Surprisingly, increasing the pH from 6.5 to 8.5 also resulted in strong growth promotion. In LC roots acidification of the medium (pH 6.5 to 4.5) resulted in transient growth stimulation followed by a gradual decline in the growth rate toward zero. Exposure of LC roots to high pH (pH shift from 6.5 to 8.5) also promoted growth. Addition of EGTA resulted in strong growth promotion in both LC and HC roots. The ability of EGTA to stimulate growth appeared not to be related to H+ release from EGTA upon Ca2+ chelation since, 1) LC roots showed a strong and prolonged response to EGTA, but only a transient response to acid pH, and 2) promotion of growth by EGTA was observed in strongly buffered solutions. We also examined the pH dependence of the release of 45Ca2+ from roots of 3-day-old seedlings grown from grains imbibed in 45Ca2+. Release of 45Ca2+ from the root into agar blocks placed on the root surface was greater the more acidic the pH of the blocks. The results indicate that Ca2+ may be necessary for the acid growth response in roots.

Non-NASA Center↗

NASA NTRS1993

Influence of electrolytes on growth, phototropism, nutation and surface potential in etiolated cucumber seedlings

A variety of electrolytes (10-30 mol m-3) increased the relative growth rate of etiolated cucumber (Cucumis sativus L. cv. Burpee's Pickler) hypocotyls by 20-50% relative to water-only controls. The nonelectrolyte mannitol inhibited growth by 10%. All salts tested were effective, regardless of chemical composition or valence. Measurements of cell-sap osmolality ruled out an osmotic mechanism for the growth stimulation by electrolytes. This, and the nonspecificity of the response, indicate that an electrical property of the solutions was responsible for their growth-stimulating activity. Measurements of surface electrical potential supported this reasoning. Treatment with electrolytes also enhanced nutation and altered the pattern of phototropic curvature development. A novel analytical method for quantitating these effects on growth was developed. The evidence indicates that electrolytes influence an electrophysiological parameter that is involved in the control of cell expansion and the coordination of growth underlying tropisms and nutations.

Non-NASA Center↗

NASA NTRS1990

Gravitropism of cucumber hypocotyls: biophysical mechanism of altered growth

The biophysical basis for the changes in cell elongation rate during gravitropism was examined in aetiolated cucumber (Cucumis sativus L.) hypocotyls. Bulk osmotic pressures on the two sides of the stem and in the epidermal cells were not altered during the early time course of gravitropism. By the pressure-probe technique, a small increase in turgor (0.3 bar, 30 kPa) was detected on the upper (inhibited) side, whereas there was a negligible decrease in turgor on the lower (stimulated) side. These small changes in turgor and water potential appeared to be indirect, passive consequences of the altered growth and the small resistance for water movement from the xylem, and indicated that the change in growth was principally due to changes in wall properties. The results indicate that the hydraulic conductance of the water-transport pathway was large (.25 h-1 bar-1) and the water potential difference supporting cell expansion was no greater than 0.3 bar (30 kPa). From pressure-block experiments, it appeared that upon gravitropic stimulation (1) the yield threshold of the lower half of the stem did not decrease and (2) the wall on the upper side of the stem was not made more rigid by a cross-linking process. Mechanical measurements of the stress/strain properties of the walls showed that the initial development of gravitropism did not involve an alteration of the mechanical behaviour of the isolated walls. Thus, gravitropism in cucumber hypocotyls occurs principally by an alteration of the wall relaxation process, without a necessary change in wall mechanical properties.

Non-NASA Center↗