Examination of microgravity effects on spontaneous Ca2+ oscillations in AtT20 pituitary cells using heavy water

Effects of heavy water (D2O) on various organisms have been extensively studied and a majority of D2O actions were generally ascribed to the viscosity (1.23 times of H2O) and a larger inter-molecule force of D2O that may eventually alternate molecular structure of various enzymes and ion channels. It is reported that chronic application of D2O induces toxic effects and the 35% substitution of whole body water with D2O induced fatal effects in the mouse. Mitosis of a fertile egg of sea urchin was completely inhibited by 75% D2O but the paused segmentation was recovered after rinse of D2O. In addition, we also observed that neuronal development of the Lymnaea stagnalis was reversibly inhibited by D2O (M. Sakakibara, unpublished data). However, mechanism of the toxicity of D2O and the effects of D2O on cellular events have not been fully understood. The spontaneous oscillation in cytosolic free Ca2+ concentration is one of the typical physiological events in living secretory cells. We previously demonstrated that the Ca2+ oscillations are regulated by voltage-sensitive Ca2+ channels (VSCC), Ca2+ ATPases, and Ca(2+)-induced Ca2+ release from intracellular stores. To analyze the site(s) of action of D2O in the living cellular systems, the present study examined effects of D2O on the Ca2+ mobilization and resting membrane potentials in AtT20 mouse pituitary cells.     

Effect of diazoxide on AS-30D rat ascites hepatoma cells treated by Cd2+

During last years different investigators, including us, have shown that oxidative stress and mitochondrial dysfunction, mediated by disturbance of the mitochondrial respiratory chain and by induction of Ca²⁺-dependent nonselective high-conductance pore of the inner mitochondrial membrane, are involved in the mechanism(s) of cytotoxic action of heavy metals. At the same time, possible interaction of heavy metals with other channels, in particular, with selective potassium channels, such as ATP-dependent potassium channels, which are generally considered to be protective for the cells, have not been investigated. The aim of this study was to examine the influence of diazoxide, a pharmacological activator of ATP-dependent potassium channels, on mitochondrial physiology and cell viability in the presence of Cd²⁺. As a model system, we used AS-30D rat ascites hepatoma cells and isolated rat liver mitochondria. We found that diazoxide enhanced an intracellular production of reactive oxygen species and induced significant stimulation of the resting respiration rate of the cells. Apart from this, diazoxide had a protective effect on AS-30D cells increasing their viability substantially decreased in the presence of the tested concentrations of Cd²⁺ (50 and 100 μM). The protective effect of diazoxide was completely suppressed by increasing the duration of incubation of the cells with Cd²⁺, and partially by addition to the assay medium of a blocker of mitochondrial ATP-dependent potassium channels 5-hydroxydecanoic acid (100 or 300 μM). In isolated rat liver mitochondria we found that diazoxide did not prevent the toxic action of Cd²⁺, since it produced no significant effects on the mitochondrial swelling and the respiration changes evoked by the heavy metal in KCl assay media. Possible molecular mechanisms of the cytoprotective action of diazoxide are discussed.     

Effect of D2O on maize plasmalemma ATPase and electron transport coupled proton pumping.

Heavy water (D2O) has been used as a putative inhibitor of the plasma membrane H(+)-ATPase and the plasma membrane redox system. Concentrations above 50% D2O inhibited H+ secretion and the plasma membrane redox system of Zea mays L. roots. Inhibition of H+ secretion by vanadate was reduced in presence of D2O. The plasma membrane of roots was transiently depolarized after the addition of heavy water in concentrations above 5%. The repolarization of the plasma membrane that takes place while the H+ secretion is still reduced by heavy water indicates that, despite the overall inhibiting effect of D2O, the plant is still able to regulate the membrane potential.     

Pharmacological uses and perspectives of heavy water and deuterated compounds.

Since the discovery of D20 (heavy water) and its use as a moderator in nuclear reactors, its biological effects have been extensively, although seldom deeply, studied. This article reviews these effects on whole animals, animal cells, and microorganisms. Both "solvent isotope effects," those due to the special properties of D20 as a solvent, and "deuterium isotope effects" (DIE), which result when D replaces H in many biological molecules, are considered. The low toxicity of D20 toward mammals is reflected in its widespread use for measuring water spaces in humans and other animals. Higher concentrations (usually >20% of body weight) can be toxic to animals and animal cells. Effects on the nervous system and the liver and on formation of different blood cells have been noted. At the cellular level, D20 may affect mitosis and membrane function. Protozoa are able to withstand up to 70% D20. Algae and bacteria can adapt to grow in 100% D2O and can serve as sources of a large number of deuterated molecules. D2O increases heat stability of macromolecules but may decrease cellular heat stability, possibly as a result of inhibition of chaperonin formation. High D2O concentrations can reduce salt- and ethanol-induced hypertension in rats and protect mice from gamma irradation. Such concentrations are also used in boron neutron capture therapy to increase neutron penetration to boron compounds bound to malignant cells. D2O is more toxic to malignant than normal animal cells, but at concentrations too high for regular therapeutic use. D2O and deuterated drugs are widely used in studies of metabolism of drugs and toxic substances in humans and other animals. The deuterated forms of drugs often have different actions than the protonated forms. Some deuterated drugs show different transport processes. Most are more resistant to metabolic changes, especially those changes mediated by cytochrome P450 systems. Deuteration may also change the pathway of drug metabolism (metabolic switching). Changed metabolism may lead to increased duration of action and lower toxicity. It may also lead to lower activity, if the drug is normally changed to the active form in vivo. Deuteration can also lower the genotoxicity of the anticancer drug tamoxifen and other compounds. Deuteration increases effectiveness of long-chain fatty acids and fluoro-D-phenylalanine by preventing their breakdown by target microorganisms. A few deuterated antibiotics have been prepared, and their antimicrobial activity was found to be little changed. Their action on resistant bacteria has not been studied, but there is no reason to believe that they would be more effective against such bacteria. Insect resistance to insecticides is very often due to insecticide destruction through the cytochrome P450 system. Deuterated insecticides might well be more effective against resistant insects, but this potentially valuable possibility has not yet been studied.     

Deuterium oxide (heavy water) arrests the cell cycle of PtK2 cells during interphase.

Deuterium oxide (D2O, heavy water) exerts an antiproliferative effect on a variety of cells in vitro and on some organisms. This effect is mainly ascribed to a tubulin-mediated antimitotic action. We evaluated the morphology, the mitotic activity, and the dynamics of the cell cycle of PtK2 cells grown in vitro in the presence of 75% D2O for up to eight weeks by microspectrophotometric DNA measurements as well as flow cytometric analysis and a determination of mitotic indices. Substitution of heavy water for water in the culture medium initially increased the mitotic index by a (pro-) metaphase block but after 2 to 3 days of incubation no mitotic figures were seen. Analysis of cells grown for 6 days in medium containing 75% D2O revealed accumulation of cells in S/G2-phase. Extended treatment stabilized the high level of cells in this specific phase, when compared to normal growing cells. Cells grown for 1 to 6 weeks in the presence of D2O remained non-proliferating, nevertheless, they were able to divide again after recovery in non-deuterated medium. The time needed for resumption of the mitotic activity was proportional to the duration of deuterium oxide exposure. Cells incubated for 8 weeks in 75% D2O did not recommence mitotic activity. Light and electron microscopic examination revealed characteristic morphological changes of size and ciliation in PtK2 cells subjected to prolonged deuteration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory activity and naphthoquinone synthesis in the fungus Fusarium decemcellulare exposed to oxidative stress

The effect of oxidants (hydrogen peroxide and juglone) on the growth, respiration, and naphthoquinone synthesis in the fungus Fusarium decemcellulare was studied. The addition of the oxidants to the exponential-phase fungus inhibited cell respiration (either partially or completely, depending on the oxidant concentration), culture growth, and naphthoquinone synthesis. The treatment of fungal cells with nonlethal concentrations of H2O2 (below 0.25 mM) and juglone (below 0.1 mM) induced the resistance of cell respiration to cyanide. The residual respiration in the presence of cyanide could be inhibited by benzohydroxamic acid, indicating the occurrence of alternative oxidase. Increased concentrations of oxidants (0.25 mM juglone and 0.5 mM H2O2) rapidly and irreversibly inhibited cell respiration. These observations suggest that the mitochondrial respiratory chain of fungal cells exposed to oxidative stress is subject to the action of active oxygen species. The treatment of fungal cells with nonlethal concentrations of H2O2 and juglone activated cellular glutathione reductase and glucose-6-phosphate dehydrogenase, which are protective enzymes against oxidative stress.     

Effect of oxygen concentration on photosynthesis and respiration in two hypersaline microbial mats

The effects of oxygen concentration on photosynthesis and respiration in two hypersaline cyanobacterial mats were investigated. Experiments were carried out on mats from Eilat, Israel, with moderate photosynthetic activity, and mats from Mallorca, Spain, with high photosynthetic activity. The oxygen concentration in the overlying water above the mats was increased stepwise from 0% to 100% O2. Subsequent changes in oxygen concentration, gross photosynthetic rates, and pH values inside the mats were measured with microelectrodes. According to published reports on the regulation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the key enzyme in the CO2-fixation pathway of phototrophs, we expected photosynthetic activity to decrease with increasing oxygen concentration. Gross photosynthetic and total respiration rates in both mats were highest when the O2 concentration was at 0% in the overlying water. Net oxygen production rates under these conditions were the same as under air saturation (21% O2), while gross photosynthetic and respiration rates were lowest at air saturation. In both mats, gross photosynthetic and respiration rates increased upon gradually increasing the oxygen concentration in the overlying water from 21% to 100%. These results contradict the expectation that photosynthesis decreases with increasing oxygen concentration. Increased photosynthetic rates at oxygen concentrations above 21% were probably caused by enhanced oxidation of organic matter and concomitant CO2 production due to the increased oxygen availability. The cause of the high respiration rates at 0% O2 in the overlying water was presumably the enhanced excretion of photosynthetic products during increased photosynthesis. We conclude that the effect of the O2/CO2 concentration ratio on the activity of Rubisco as demonstrated in vitro on enzyme extracts cannot be extrapolated to the situation in intact microbial mats, because the close coupling of the activity of primary producers and heterotrophic bacteria plays a major role in this ecosystem.     

Effect of oxygen on acetylene reduction by photosynthetic bacteria

The effect of dissolved oxygen concentration on nitrogenase activity was studied in three species of photosynthetic bacteria. The O2 concentration in the cell suspension was measured with an O2 electrode inserted into the reaction vessel. Acetylene reduction by whole cells of Rhodopseudomonas capsulata, Rhodospirillum rubrum, and Chromatium vinosum strain D was inhibited 50% by 0.73, 0.32, and 0.26 microM O2, respectively. The inhibition of the activity by O2 in R. capsulata usually was reversed completely by reestablishing anaerobic conditions. In R. rubrum and C. vinosum the inhibition was only partially reversible. The respiration rate of R. capsulata was the highest of the three, that of R. rubrum was intermediate, and that of C. vinosum was lowest. R. capsulata and R. rubrum cells were broken after their acetylene reduction activity in vivo had been completely inhibited by O2, and nitrogenase was found to be active in vitro. A concentration of cyanide that did not affect acetylene reduction activity, but which inhibited 75 to 90% of the O2 uptake by whole cells of R. capsulata, shifted the O2 concentration causing 50% inhibition of nitrogenase activity from 0.73 microM to 2.03 microM. These results are in accordance with the assumption that within a limited range of O2 concentrations, the respiratory activity of the cells is enough to scavenge the O2 and to keep the interior of the cells essentially anaerobic. It is suggested that O2 inhibits nitrogenase activity by competing for a limited supply of electrons. When cyanide is present, respiration is slower but is adequate to keep the nitrogenase environment in the cell anaerobic. The lower respiration rate may allow a greater proportion of the electrons to be used for acetylene reduction.     

Mitochondrial respiratory chain inhibitors modulate the metal-induced inner mitochondrial membrane permeabilization

To elucidate the molecular mechanisms of the protective action of stigmatellin (an inhibitor of complex III of mitochondrial electron transport chain, mtETC) against the heavy metal-induced cytotoxicity, we tested its effectiveness against mitochondrial membrane permeabilization produced by heavy metal ions Cd2(+), Hg2(+), Cu2(+) and Zn2(+), as well as by Ca2(+) (in the presence of P(i)) or Se (in form of Na?SeO?) using isolated rat liver mitochondria. It was shown that stigmatellin modulated mitochondrial swelling produced by these metals/metalloids in the isotonic sucrose medium in the presence of ascorbate plus tetramethyl-p-phenylenediamine (complex IV substrates added for energization of the mitochondria). It was found that stigmatellin and other mtETC inhibitors enhanced the mitochondrial swelling induced by selenite. However, in the same medium, all the mtETC inhibitors tested as well as cyclosporin A and bongkrekic acid did not significantly affect Cu2(+)-induced swelling. In contrast, the high-amplitude swelling produced by Cd2(+), Hg2(+), Zn2(+), or Ca2(+) plus P(i) was significantly depressed by these inhibitors. Significant differences in the action of these metals/metalloids on the redox status of pyridine nucleotides, transmembrane potential and mitochondrial respiration were also observed. In the light of these results as well as the data from the recent literature, our hypothesis on a possible involvement of the respiratory supercomplex, formed by complex I (P-site) and complex III (S-site) in the mitochondrial permeabilization mediated by the mitochondrial transition pore, is updated.     

Electrophysiological evidence for the herbicidal mode of action of phosphonic acid esters

The electrical membrane potential of leaf cells of the higher aquatic plant Egeria densa Planchon, measured with microelectrodes, was immediately depolarized after treatment with 0.29 m M of the dialkyl phosphonic ester, O, O-di- n -butyl-(1- n -butylamino-cyclohexyl)-phosphonate (PABT). This depolarization was followed by a strong electrolyte efflux after ca 90 min. Active photosynthesis or respiration as well as an intact plasma membrane was essential for this effect. An increased concentration of thiobarbituric acid (TBA)-reactive agents observed within this period suggests that membrane destruction by lipid peroxidation was responsible for the electrolyte efflux. Antioxidants such as α-tocopherol (0.25 m M ) and ascorbic acid (1 m M ) stopped electrolyte efflux, but did not affect the depolarization.
Fusicoccin (1 μ M ) prevented PABT-induced membrane depolarization and the subsequent electrolyte efflux. Also, the ATPase inhibitor, DES (50 ü M ), as well as substances which stimulate the proton pump such as sucrose (30 m M ), AIB (10 μ M ), and acetate (1 m M ), prevented PABT-mediated electrolyte efflux. The depolarizing effect of PABT was also obviated above pH 7.5. Thus, if the PABT-induced depolarization was inhibited no membrane destruction occurred whereas depolarization alone was not a sufficient condition for the development of the PABT action. The initial depolarizing effect of PABT cannot be explained by a physical interaction with the lipid part of the plasma membrane. Thus, a metabolism-driven mode of action connected to plasma membrane energization has to be assumed.     

Hypoxia affects the physiological behavior of rat cortical synaptosomes

Nerve cells, especially synaptosomes, are very susceptible to hypoxia and the subsequent oxidative stress. In this paper, we examined the effects of hypoxia (93% N(2):2% O(2):5% CO(2), v/v/v) on rat cortical synaptosomes by evaluating modifications of synaptosomal mitochondrial respiration rate and ATP production, membrane potential, intrasynaptosomal mitochondrial Ca(2+) concentration ([Ca(2+)](i)), and desferoxamine-chelatable free iron and esterified F2-isoprostane levels after different periods of hypoxia and after 30 min of reoxygenation. Oxygen consumption decreased significantly during 120 min of hypoxia and was restored after reoxygenation. At the same time, ATP production decreased and remained significantly lower even after reoxygenation. This involved a depolarization of the synaptosomal mitochondrial membrane, although the [Ca(2+)](i) remained practically unchanged. Indeed, iron and F2-isoprostane levels, representing useful prediction markers for neurodevelopmental outcome, increased significantly after hypoxia, and there was a strong correlation between the two variables. On the whole our results indicate that synaptosomal mitochondria undergo mitoptosis after 2 h of hypoxia.     

Deuterium oxide effects upon three parameters characterizing the activity of glycerol-extracted muscle: phosphate release, ATP-induced shortening, and 22Na+ distribution

ATP splitting activity is progressively reduced with increasing heavy water (D2)) concentration. In contrast, sarcomere shorteining inhibition produced by D2O does not significantly depend on its concentration. Even at low concentration, the presence of D2O does reduce the excessive accumulation of radioactive sodium within glycerinated frog muscle. These heavy water effects on muscular contraction and soidum distribution can be interpreted to indicate adsorbed water within the cells. Evaluation of these experimental results in terms of Gibbs free enthalpy of binding at the adsorption sites of D20 or H20 is in good agreement with the data in the literature.     

Respiratory resistance of methylotrophic bacteria to formate and cyanide

Whole cells and cell-free preparations of the methylotrophic bacteria, Pseudomonas sp. AM 1 and Achromobacter parvulus, can oxidize formate at tis concentration in the reaction medium up to 1 M. The respiration of whole cells is registered at a concentration of formate greater than 10(-2) M, while that of cell-free extracts at a formate concentration greater than 5 X 10(-5) M. This seems to be due to the presence of a permeability barrier in cells for formate. The oxidation of reduced TMPD and exogenous cytochrome c by the membrane preparations of the two bacteria is inhibited by formate and cyanide; Ki50% = 2.5 X 10(-2) and 10(-6) M, respectively. The oxidation of NADH by the membrane preparations of the bacteria is not inhibited by 1 M formate and 5 X 10(-4) M cyanide but is inhibited by formaldehyde with Ki50% = 3 X 10(-2) M. Formaldehyde has no effect on the oxidation of reduced TMPD and cytochrome c at concentrations greater than 2 X 10(-1) M. These data indicate that respiration of the studied methylotrophic bacteria in the presence of high formate concentrations should be attributed in the presence of a branched electron transport chain in them; one branch of the chain is resistant to formate and cyanide, but is sensitive to formaldehyde.     

Respiration-Linked Proton Transport, Changes in External pH, and Membrane Energization in Cells of Escherichia coli

The kinetics of respiration-dependent proton efflux and membrane energization have been studied in intact cells of logarithmic-phase Escherichia coli. Parallel measurements of the rate and extent of proton efflux into the external medium (half-time, about 10 s; ratio of H(+) to O, about 0.5) and the oxidation of E. coli cytochrome b (half-time, 相似文献   

Aerobic microbial growth at low oxygen concentrations

Sterilizable membrane probes were used to study the relation between oxygen concentration and respiration rate in Candida utilis growing on acetate. When the organism was grown in a continuous fermentor at various dissolved oxygen concentrations (0.23 x 10(-6) to 32 x 10(-6)m), with time allowed for full adaptation to each oxygen concentration, the relationship between oxygen concentration and growth rate simulated Michaelis-Menten behavior, giving an apparent K(m) for oxygen of 1.3 x 10(-6)m. When respiration rate was measured at various oxygen concentrations without allowing time for adaptation, it was found that the respiration rate was directly proportional to O(2) concentration at low O(2) concentrations, and independent of O(2) concentration at high O(2) concentrations. Transition from one type of behavior to the other was fairly abrupt. The respiration rate in the presence of excess oxygen depended on the O(2) concentration at which the cells were grown, but the rate at low O(2) concentrations did not. There was evidence that, at low oxygen concentrations, oxygen diffusion through the cell substance limits respiration rate, at least in part.     

Energy-Dependent Solute Transport from the Apoplast into the Symplast of Leaves during Transpiration

Inorganic and organic salts, amino acids, sugars, and phosphate esters (concentrations usually 25 mM) were fed via the transpiration stream through the petiole into detached leaves of Lepidium sativum and Solanum tuberosum. While water was lost by transpiration, solutes did not accumulate in the apoplast. Uptake into leaf cells was indicated by stimulation of respiration and by changes of membrane potential and apoplastic pH. Apoplastic alkalinization (followed by transient acidification) and membrane depolarization (followed by repolarization) indicated energization of transport at the expense of the proton motive force (PMF) across the plasma membrane in all examined cases. Loss of ATP in the symplast during proton extrusion into the apoplast by the plasmalemma ATPase is thought to be responsible for stimulation of respiration. Even unphysiological solutes such as -morpholinoethane sulfonate (Mes), or potentially toxic salts such as CdCl₂ or AlCl₃, and metabolites involved in energy conservation such as AMP and NAD, were readily transported into leaf cells at the expense of metabolic energy. At the maximum stimulation of CO₂ release by D-serine (which is unlikely to be metabolized) respiration exceeded basal respiration by an average of 33%. Occasionally, and with other solutes, basal respiration was almost doubled. The ratio of transported solute to released extra CO₂ was 6.9 ± 1.1 (n = 11) in the case of D-serine. From this, maximum energized transport of D-serine was calculated to be close to 500 nmol/(m² leaf area s). Solute/CO₂ ratios similar to those observed with D-serine were also obtained for sucrose. Lower ones were observed with organic solutes such as L-glycine, pyruvate, malate or citrate where secondary metabolic conversions may contribute to CO₂ release.     

Pyridine nucleotide oxidation by a plasma membrane fraction from red beet (Beta vulgaris L.) storage tissue

The potential role of pyridine nucleotide oxidation in the energization and/or regulation of membrane transport was examined using sealed plasma membrane vesicles isolated from red beet (Beta vulgaris L.) storage tissue. In this system, pyridine nucleotide oxidation, which was enhanced in the presence of ferricyanide, occurred. In the presence or absence of ferricyanide, the oxidation of NADH was several-fold greater than the oxidation of NADPH, indicating that it was the preferred substrate for oxidation in this system. Ferricyanide reduction coupled to NADH oxidation did not require the transmembrane movement of reducing equivalents since ferricyanide incorporated inside the vesicles could not be reduced by NADH added externally to the vesicles, unless the vesicles were made leaky by the addition of 0.05% (v/v) Triton X-100. Using fluorescent probes for the measurement of transmembrane pH gradients and membrane potentials, it was determined that NADH oxidation did not result in the production of a proton electrochemical gradient or have any effect upon the proton electrochemical gradient produced by the plasma membrane H+-ATPase. The oxidation of NADH in the presence of ferricyanide did result in the acidification of the reaction medium. This acidification was unaffected by the addition of Gramicidin D and stimulated by the addition of 0.05% (v/v) Triton X-100, suggesting a scalar (nonvectorial) production of protons in the oxidation/reduction reaction. The results of this study suggest that the oxidation of pyridine nucleotides by plasma membrane vesicles is not related to energization of transport at the plasma membrane or modulation of the activity of the plasma membrane H+-ATPase.     

Microbial synthesis of 2H-labelled L-phenylalanine with different levels of isotopic enrichment by a facultative methylotrophic bacterium Brevibacterium methylicum with RuMP assimilation of carbon

Using the L-phenylalanine secreting strain of Gram-negative aerobic facultative methylotrophic bacteria Brevibacterium methylicum, assimilating methanol via the ribulose-5-monophosphate (RuMP) cycle of carbon assimilation, as an example, we have continued studies on the use of methylotrophic bacteria for the preparative microbial synthesis of amino acids labeled with stable isotopes, including deuterium (²H), suitable for biomedical applications and clinical diagnostics. Here we demonstrate the data on adaptation of the methylotrophic bacterium B. methylicum to the maximal concentration of deuterium in the growth medium with 98% (v/v) ²H₂O and 2% (v/v) [²H]MeOH, and biosynthesis of deuterium labeled L-phenylalanine with different levels of isotopic enrichment. The strain was adapted to ²H₂O by means of plating of initial cells on solid (2% agarose) minimal growth media M9 with an increasing gradient of ²H₂O concentration from 0, 24.5, 49.0, 73.5 up to 98% (v/v) ²H₂O and subsequent selection of individual colonies stable to the action of ²H₂O, which were capable to produce L-phenylalanine. L-phenylalanine was extracted from the growth medium with isopropanol followed by subsequent crystallization in ethanol (output 0.65 g/L). Using the developed method of microbial synthesis it is possible to obtain deuterated L-phenylalanine with different levels of isotopic enrichment, depending on concentration of ²H₂O in growth media, from 17% (the growth medium with 24.5% (v/v) ²H₂O) right up to 75% (the growth medium with 98% (v/v) ²H₂O) of deuterium as evidenced by results of the electron impact (EI) mass-spectrometry analysis of methyl ethers of N-dimethylamino(naphthalene)-5-sulfonyl chloride (dansyl) phenylalanine isolated from growth media under different experimental conditions.     

干旱胁迫对降香黄檀幼苗光合生理特性的影响

采用温室盆栽方法,设置对照(CK)、轻度(LS)、中度(MS)和重度(HS)干旱胁迫4个水分条件,研究不同水分条件对降香黄檀幼苗光合和生理特性的影响。结果表明:(1)随着干旱胁迫程度增加,降香黄檀幼苗叶片叶绿素总含量总体呈现出下降趋势。(2)降香黄檀幼苗叶片净光合速率、气孔导度、胞间CO2浓度和蒸腾速率随着干旱胁迫强度增加均呈现出先增加后降低趋势,且MS和HS处理下的气孔导度和胞间CO2浓度同时降低,此时幼苗光合能力的下降主要受气孔因素限制。(3)随着干旱胁迫强度的增加,降香黄檀幼苗叶片细胞膜相对透性、丙二醛含量、游离脯氨酸含量和POD活性均呈现出增加趋势,而同期SOD和CAT活性呈现出先升高后降低趋势。可见,降香黄檀幼苗在轻度干旱胁迫下可通过增加叶片保护酶活性来清除活性氧对其组织造成的伤害,但胁迫超过一定程度后保护酶活性下降,表明降香黄檀幼苗的耐旱能力有限。     

Heavy Water and Hydrostatic Pressure Effects on Tetrahymena pyriformis1

SYNOPSIS. Heat-synchronized cultures of Tetrahymena pyriformis strain GL subjected to pulses of high hydrostatic pressure (10,000 psi for 2 min) had increasing division delays during the 1st 40 min after the last heat shock (40 min after heat treatment). Pressure treatment during the subsequent 10-min interval disrupted cell synchrony. Comparable pressures applied to the cells at later stages, before the 1st synchronous division, caused negligible division delay. Continuous exposure to 10% (v/v) heavy water hardly affected division; higher concentrations delayed or blocked division. Ten-min pulses with heavy water (40%, 50%, 70%) resulted in increasing division delays depending on the stage of the cell cycle during which the heavy water was applied. Amelioration of the division-delaying effects of pressure was observed in cells treated simultaneously with pressure (3,000 psi for 30 min), and 30% D₂O. The results are consistent with the hypothesis that some of the pressure and D₂O effects could be attributed to changes in the sol-gel state of the cytoplasm.