Sodium ions are necessary for growth and energy transduction in the marine cyanobacterium Oscillatoria brevis

The maximal growth rate of the marine cyanobacterium Oscillatoria brevis was reached at 200–400 mM NaCl and pH 9.0–9.6. NaCl was found (i) to stimulate the rate of the light-supported generation across the cytoplasmic membrane of the cells and (ii) to decrease the sensitivity of level and motility of the O. brevis trichomes to protonophorous uncouplers. The Na⁺/H⁺ antiporter, monensin, increased both and the uncoupler sensitivity of the cells. The data obtained agree with the assumption that O. brevis possesses a primary Na⁺ pump in its cytoplasmic membrane.Abbreviations ATP adenosine-5-triphosphate - TTFB tetrachlortrifluoromethylimidazol - CCCP carbonyl cyanide m-chlorophenylhydrazone - Na⁺ transmembrane electrochemical potential differences of Na⁺ - transmembrane electric potential difference - pNa transmembrane pNa difference     

Osmoregulation and the control of phloem-sap composition in Ricinus communis L.

Phloem-sap composition was studied in plants of Ricinus communis L. grown on a waterculture medium. The sap possessed a relatively high K⁺:Na⁺ ratio and low levels of Ca²⁺ and free H⁺. Sucrose and K⁺ (together with its associated anions) accounted for 75% of the phloem-sap solute potential (_s). In plants kept in continuous darkness, a decrease in phloem-sap sucrose levels over 24h was accompanied by an increase in K⁺ levels. Measurements of phloem-sap _s and xylem water potential () indicated that this resulted in a partial maintenance of phloem turgor pressure _p. In darkness there was also a marked decrease in the malate content of the leaf tissue, and it is possible that organic carbon from this source was mobilized for export in the phloem. The results support the concept of the phloem sap as a symplastic phase. We interpret the increase in K⁺ levels in the phloem in darkness as an osmoregulatory response to conditions of restricted solute availability. This reponse can be explained on the basis of the sucrose-H⁺ co-transport mechanism of phloem loading.Abbreviations water potential - _s solute potential - _p pressure potential     

ATP is required for K+ active transport in the archaebacterium Haloferax volcanii

The Archaebacterium Haloferax volcanii concentrates K⁺ up to 3.6 M. This creates a very large K⁺ ion gradient of between 500- to 1,000-fold across the cell membrane. H. volcanii cells can be partially depleted of their internal K⁺ but the residual K⁺ concentration cannot be lowered below 1.5 M. In these conditions, the cells retain the ability to take up potassium from the medium and to restore a high internal K⁺ concentration (3 to 3.2 M) via an energy dependent, active transport mechanism with a K _m of between 1 to 2 mM. The driving force for K⁺ transport has been explored. Internal K⁺ concentration is not in equilibrium with m suggesting that K⁺ transport cannot be accounted for by a passive uniport process. A requirement for ATP has been found. Indeed, the depletion of the ATP pool by arsenate or the inhibition of ATP synthesis by N,N-dicyclohexylcarbodiimide inhibits by 100% K⁺ transport even though membrane potential m is maintained under these conditions. By contrast, the necessity of a m for K⁺ accumulation has not yet been clearly demonstrated. K⁺ transport in H. volcanii can be compared with K⁺ transport via the Trk system in Escherichia coli.Abbreviations CCCP Carbonylcyanide m-chlorophenyl-hydrazone - DCCD N,N-dicyclohexylcarbodiimide - MES 2-[N-morpholino] ethane sulfonic acid - MOPS 3-[N-morpholino] propane sulfonic acid - TRIS Tris (hydroxymethyl) aminomethane - TPP tetraphenyl phosphonium     

The membrane potential in whole cells of Methanobacterium thermoautotrophicum

of whole cells of Methanobacterium thermoautotrophicum was estimated under varying conditions using an electrode sensitive to the lipophilic cation tetraphenylphosphonium chloride (TPP⁺). Since was found to be extremely sensitive to air, a special reaction vessel was developed to maintain strict anaerobiosis. The cells took up TPP⁺ under energization by H₂ and CO₂ thus allowing to calculate the from the distribution of TPP⁺ inside and outside the cells. The unspecific uptake of deenergized cells was around 10% of the total uptake of energized cells. TPP⁺ itself slightly diminished the , but had no effect on the formation of methane. Typical values of were in the range of-150 to-200 mV. showed a quantitative dependence on both the electron donor H₂ and the electron acceptor CO₂. NaCl stimulated the extent of the , whereas KCl slightly diminished it. Valinomycin resulted in a linear decline of , whereas the methane production rate was only slightly affected. In contrast, monensin reduced both methanogenesis and .Abbreviations pmf proton motive force - membrane potential - TPP⁺ tetraphenylphosphonium (chloride salt) - TPMP⁺ triphenylmethylphosphonium (chloride salt, if not otherwise indicated) - d.w. dry weight - t _d doubling time - PVC polyvinylchloride     

γδ T-cell receptor repertoire in human peripheral blood and thymus

T-cell clones expressing the T-cell receptor (Tcr) were generated from peripheral blood lymphocytes (PBLs) and from a thymus sample. In the panel of ten thymus-derived clones, four Tcr phenotypes [as defined by the reaction of monoclonal antibodies (mAbs) directed against known V and V regions] were identified. All the clones lacked expression of the V3 V region, while seven clones were V1+ . V1 was found in combination with V9 or with undefined VVregions. In addition, two other Tcr phenotypes were identified on these clones: V9⁺ V1^– V3^– and V9^– V1^– V3^– One of the clones expressed CD4 and another was CD8positive. The remaining clones were CD4^– CD8^–. In the panel of 76 PBL-derived, Tcr-bearing clones, five Tcr phenotypes could be identified. In contrast to the thymus-derived clones, 30% of the clones were V3⁺ whereas V1 was expressed by a minority of the clones only. One clone was CD4-positive and approximately 30% of the clones were CD8-positive. Four of the five mAb-defined Tcr phenotypes could be identified on both thymus and PBL-derived T-cell clones. However, biochemical analysis of the Tcrs demonstrates differences in the usage of Ct- and C2-encoded y chains by T cells derived from the thymus and PBLs. The results therefore indicate that, at the clonal level, similarities and differences exist between the Tcr repertoires expressed in the thymus and by PBLs. Furthermore, they indicate that combinatorial Tcr heterogeneity is larger than has so far been described. The receptor diversity, combined with the potential of Tcr+ cells to express CD4 or CD8, indicates that these cells are a heterogeneous population that might mediate a number of immune functions.     

K+ transport in ‘Tight’ epithelial monolayers of MDCK cells

Summary Bidirectional transepithelial K⁺ flux measurements across high-resistance epithelial monolayers of MDCK cells grown upon millipore filters show no significant net K⁺ flux.Measurements of influx and efflux across the basal-lateral and apical cell membranes demonstrate that the apical membranes are effectively impermeable to K⁺.K⁺ influx across the basal-lateral cell membranes consists of an ouabain-sensitive component, an ouabain-insensitive component, an ouabain-insensitive but furosemide-sensitive component, and an ouabain-and furosemide-insensitive component.The action of furosemide upon K⁺ influx is independent of (Na⁺–K⁺)-pump inhibition. The furosemide-sensitive component is markedly dependent upon the medium K⁺, Na⁺ and Cl^– content. Acetate and nitrate are ineffective substitutes for Cl^–, whereas Br^– is partially effective. Partial Cl^– replacement by NO₃ gives a roughly linear increase in the furosemide-sensitive component. Na⁺ replacement by choline abolishes the furosemide-sensitive component, whereas Li⁺ is a partially effective replacement. Partial Na⁺ replacement with choline gives an apparent affinity of 7mm Na, whereas variation of the external K⁺ content gives an affinity of the furosemide-sensitive component of 1.0mm.Furosemide inhibition is of high affinity (K _1/2=3 m). Piretanide, ethacrynic acid, and phloretin inhibit the same component of passive K⁺ influx as furosemide; amiloride, 4,-aminopyridine, and 2,4,6-triaminopyrimidine partially so. SITS was ineffective.Externally applied furosemide and Cl^– replacement by NO ₃ ^– inhibit K⁺ efflux across the basal-lateral membranes indicating that the furosemide-sensitive component consists primarily of KK exchange.     

Pollen tube extract supports the movement of actin filaments in vitro

Summary Muscle actin filaments labeled with rhodamine-phalloidin were observed to move on the surface coated with a crude extract of pollen tubes ofLilium longiflorum with an average velocity of 1.99±0.55 m/sec. The movement required both Mg²⁺ and ATP. These results indicate that the extract of pollen tubes contains a myosin-like translocatorAbbreviations ATP adenosine-5-triphosphate - DTT dithiothreitol - EGTA ethyleneglycol-bis-(-aminoethylether)N,N,N,N-tetraacetic acid - PIPES piperazine-N,N-bis-(2-ethanesulfonic acid) - PMSF phenylmethylsulfonyl fluoride     

Conformation of the circular dumbbell d〈pCGC-TT-GCG-TT〉: Structure determination and molecular dynamics

Summary The circular DNA decamer 5-dpCGC-TT-GCG-TT-3 was studied in solution by means of NMR spectroscopy and molecular dynamics in H₂O. At a temperature of 269 K, a 50/50 mixture of two dumbbell structures (denoted L2L2 and L2L4) is present. The L2L2 form contains three Watson-Crick C-G base pairs and two two-residue loops in opposite parts of the molecule. On raising the temperature from 269 K to 314 K, the L2L4 conformer becomes increasingly dominant (95% at 314 K). This conformer has a partially disrupted G(anti)-C(syn) closing base pair in the 5-GTTC-3 loop with only one remaining (solvent-accessible) hydrogen bond between NH of the cytosine dC(1) and O6 of the guanine dG(8). The opposite 5-CTTG-3 loop remains stable. The two conformers occur in slow equilibrium (rate constant 2–20 s^–1). Structure determination of the L2L2 and L2L4 forms was performed with the aid of a full relaxation matrix approach (IRMA) in combination with restrained MD. Torsional information was obtained from coupling constants. Coupling constant analysis (³J_HH, ³J_HP, ³J_CP) gave detailed information about the local geometry around backbone torsion angles , , and , revealing a relatively high flexibility of the 5-GTTC-3 loop. The values of the coupling constants are virtually temperature-independent. Weakly constrained molecular dynamics in solvent was used to sample the conformational space of the dumbbell. The relaxation matrices from the MD simulation were averaged over r^–3 to predict dynamic NOE volumes. In order to account for the 1:1 conformational mixture of L2L2 and L2L4 present at 271 K, we also included S² factors and r^–6 averaging of the r^–3-averaged relaxation matrices. On matrix averaging, the agreement of NOE volumes with experiment improved significantly for protons located in the thermodynamically less stable 5-GTTC-3 loop. The difference in stability of the 5-CTTG-3 and 5-GTTC-3 loops is mainly caused by differences in the number of potential hydrogen bonds in the minor groove and differences in stacking overlap of the base pairs closing the minihairpin loops. The syn conformation for dC(1), favored at high temperature, is stabilized by solvation in the major groove. However, the conformational properties of the dC(1) base, as deduced from R-factor analysis and MD simulations, include a large flexibility about torsion angle .     

Study of the involvement of osmotic adjustment and H<Superscript>+</Superscript>-ATPase activity in the resistance of <Emphasis Type="Italic">Catharanthus roseus</Emphasis> suspension cells to salt stress

The salt-induced H⁺-ATPase activity and osmotic adjustment responses of Catharanthus roseus (L.) G. Don suspension cultures were studied. Cells were treated with 0, 50 or 100mM NaCl for 7days or were maintained for 8 months with 50 mM NaCl (50T cells). Growth, osmotic potential (), ions content, soluble sugars, proline and total amino acids were determined in the sap of control and salt-treated cells. Salinity reduced cell growth and . The higher decrease in the in salt-treated cells was due to higher accumulation of Na⁺ and Cl^–. The levels of organic solutes, such as soluble sugars, free proline and total amino acids, increased with salt treatment. These results suggest that salt-tolerant cells are able to osmotically adjust. Salinity treatments stimulated H⁺-ATPase activity. Immunodetection of the enzyme showed that the increased activity was due to an increased amount of protein in the plasmalemma. The induction by NaCl, especially at 100 mM NaCl and for 50T cells, could account for the K⁺ and Cl^– uptake but not for higher or lower tolerance.     

Electrochemical potential of protons in vesicles reconstituted from purified,proton-translocating adenosine triphosphatase

Summary Measurements were made of the difference in the electrochemical potential of protons ( ) across the membrane of vesicles reconstituted from the ATPase complex (TF ₀ ·F ₁) purified from a thermophilic bacterium and P-lipids. Two fluorescent dyes, anilinonaphthalene sulfonate (ANS) and 9-aminoacridine (9AA) were used as probes for measuring the membrane potential () and pH difference across the membrane ( pH), respectively.In the presence of Tris buffer the maximal and no pH were produced, while in the presence of the permeant anion NO ₃ ^– the maximal pH and a low were produced by the addition of ATP. When the ATP concentration was 0.24mm, the was 140–150 mV (positive inside) in Tris buffer, and the pH was 2.9–3.5 units (acidic inside) in the presence of NO ₃ ^– . Addition of a saturating amount of ATP produced somewhat larger and pH values, and the attained was about 310 mV.By trapping pH indicators in the vesicles during their reconstitution it was found that the pH inside the vesicles was pH 4–5 during ATP hydrolysis.The effects of energy transfer inhibitors, uncouplers, ionophores, and permeant anions on these vesicles were studied.     

Surcose transport in isolated plasma-membrane vesicles from sugar beet (Beta vulgaris L.) Evidence for an electrogenic sucrose-proton symport

An analysis of the molecular mechanism of sucrose transport across the plasmalemma was conducted with isolated plasma-membrane (PM) vesicles. Plasma membrane was isolated by aqueous two-phase partitioning from fully expanded sugar beet (Beta vulgaris L.) leaves. The isolated fraction was predominantly PM vesicles as determined by marker-enzyme analysis, and the vesicles were oriented right-side-out as determined by structurally linked latency of the PM enzyme, vanadate-sensitive Mg²⁺-ATPase. Sucrose uptake was investigated by equilibrating PM vesicles in pH 7.6 buffer and diluting them 20-fold into pH 6.0 buffer. Using this pH-jump technique, vesicles accumulated acetate in a pH-dependent, protonophore-sensitive manner, which demonstrated the presence of a pH gradient (pH) across the vesicle membrane. Addition of sucrose to pH-jumped PM vesicles resulted in a pH-dependent, protonophoresensitive uptake of sucrose into the vesicles. Uptake was sucrose-specific in that a 10-fold excess of mannose, glucose, fructose, mannitol, melibiose, lactose or maltose did not inhibit sucrose accumulation. The rate of pH-dependent uptake was saturable with respect of sucrose concentration and had an apparent K _m, of 0.45 mM. Sucrose uptake was stimulated approximately twofold by the addition of valinomycin and K⁺, which indicated an electrogenic sucrose-H⁺ symport. Membrane potentials () were imposed across the vesicle membrane using valinomycin and K⁺. A membrane potential, negative inside, stimulated pH-dependent sucrose uptake while a , positive inside, inhibited uptake. Conditions that produce a negative in the absence of a pH gradient supported, although weakly, sucrose uptake. These data support an electrogenic sucrose-H⁺ symport as the mechanism of sucrose transport across the PM in Beta leaves.Abbreviations and symbols CCCP carbonyl cyanide m-chlorophenylhydrazone - cyt cytochrome - PM plasma-membrane(s) - electrical potential difference     

Daily fluctuations in water potential and associated ionic changes in Atriplex canescens

Summary Water potential () measurements of Atriplex canescens at the base of the Red Desert near Tipton in Wyoming, revealed a range between-15.5 to-45.1 bars. Minimum values coincided with the lowest air and soil temperatures, maximum with the greatest atmospheric evaporative demand. Change in exceeded 12 bars h^-1 during periods of rapidly moving storm systems. Changes in appeared to be independent of plant size, age, sex, and the spatial location of plants. Chemical analyses revealed that xylem sap was up to three times more concentrated at high than at low . It was observed that the flow rate of sap was greater at lower than at higher and that the increase in water movement accounted for the dilution of the baseline concentration of sap solutes. Together, Ca, Mg, K and Na contributed 58% of the mean osmolality of the xylem sap; the dominant ions, however, were K and Cl. We suggest that the ability of the species to respond rapidly to changing atmospheric conditions affords it a distinct advantage in a harsh environment.     

Winter water relations of a deciduous timberline conifer,Larix lyallii Parl.

Summary The degree of winter desiccation resistance exhibited by Larix lyallii Parl. was assessed by determination of water potential components and content of buds, xylem pressure potential ( _xylem) of twigs and amount of damage through winter at timberline in the Rocky Mountains of Canada. Comparative measurements were made on sympatric evergreen tree species to evaluate differences in winter desiccation avoidance and tolerance between evergreen and deciduous trees. Total () and osmotic plus matric potentials ( ₊ ) of L. lyallii buds were lowest in December (-5.0 to-5.3 MPa and-6.6 to-7.0 MPa, respectively) when temperatures were lowest. Bud and water content increased in late winter while _xylem of twigs continued to decline until March. The buds were isolated from the xylem from October through February, as indicated by large differences in water potential between the two organs during this time. Buds thus avoided desiccation as water was lost from the twigs. At the same time the buds were tolerant of very low and ₊ , a characteristic which is an important component of freezing damage resistance. Desiccation damage to buds of L. lyallii was much less than that to buds of similar-sized nearby trees of Abies lasiocarpa, although _xylem of both species was similar. The deciduous habit apparently confers a significant advantate to L. lyallii, which dominated the upper timberline sites, in reduced susceptibility to winter desiccation damage. Other deciduous timberline species might also benefit from this advantage where winter conditions are desiccating.Seedlings of L. lyallii were also studded for their winter desiccation resistance because they have a large component of non-deciduous (wintergreen) needles that are photosynthetically active through two growing seasons and must overwinter as mature tissue. Experimental exposure of these needles, which are normally protected by the snowpack, caused nearly complete mortality of the wintergreen needles when twig _xylem was only-3.9 MPa. The buds on these twigs were undamaged.     

Transmembrane signal defect and absence of cancer extract induced leukocyte adherence inhibition (LAI) for leukocytes from patients with advanced cancer

Summary Leukocytes from patients with early cancer exhibit leukocyte adherence inhibition (LAI) when incubated with extracts of cancer of the same organ and histogenesis, whereas leukocytes from patients with advanced cancer seldom do. To understand the reason for this refractory state, tumor antigen-induced LAI and transmembrane signalling were measured in the same leukocytes. Transmembrane signalling was measured by changes in membrane potential () by the [³H]tetraphenylphosphonium equilibration technique. When leukocytes from patients with early breast cancer were incubated with extracts of breast cancer and malignant melanoma they showed changes consisting of depolarization and hyperpolarization beginning within 0.5 min after addition of the breast cancer extract and finishing 15 min later. Moreover, they showed no changes when incubated with extracts of normal breast tissue. Leukocytes from subjects without cancer seldom showed changes. In criss-cross experiments, leukocytes from patients with melanoma only exhibited changes when incubated with the melanoma extract. There was a strong correlation between cancer extract-induced change and LAI. The change was triggered by leukotriene-like mediators from antibody-dependent monocytes. Authentic leukotrienes triggered changes in all subpopulation of leukocytes. Leukocytes from patients with advanced breast cancer when incubated with breast cancer extract did not transmit a signal or show LAI. Brief elevation of intracellular cyclic AMP restored both change and LAI induced by breast cancer extracts, indicating that reactive leukocytes are present but in a refractory state. We conclude that leukocytes from patients with advanced cancer do not react in LAI because tumor antigen does not trigger a transmembrane signal to initiate the cascade of biochemical reactions and physiological changes for LAI.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - cyclic AMP cyclic adenosine monophosphate - ETYA eicosatetraynoic acid - HEPES 4-(2-hydroxyethyl)-1-piperazine ethansulfonate - LAI leukocyte adherence inhibition - NAI nonadherence index - OSN organ-specific cancer neoantigen - PBL peripheral blood leukocytes - PGE₂ prostaglandin E₂ - [³H]TPP⁺ [phenyl³H]tetraphenylphosphonium bromide - transmembrane potential     

The effect of humidity and light on cellular water relations and diffusion conductance of leaves ofTradescantia virginiana L.

Turgor (_p) and osmotic potential (_s) in epidermal and mesophyll cells, in-situ xylem water potential (-xyl) and gas exchange were measured during changes of air humidity and light in leaves ofTradescantia virginiana L., Turgor of single cells was determined using the pressure probe. Sap of individual cells was collected with the probe for measuring the freezing-point depression in a nanoliter osmometer. Turgor pressure was by 0.2 to 0.4 MPa larger in mesophyll cells than in epidermal cells. A water-potential gradient, which was dependent on the rate of transpiration, was found between epidermis and mesophyll and between tip and base of the test leaf. Step changes of humidity or light resulted in changes of epidermal and mesophyll turgor (_p-epi, _p-mes) and could be correlated with the transpiration rate. Osmotic potential was not affected by a step change of humidity or light. For the humidity-step experiments, stomatal conductance (g) increased with increasing epidermal turgor.g/_p-epi appeared to be constant over a wide range of epidermal turgor pressures. In light-step experiments this type of response was not found and stomatal conductance could increase while epidermal turgor decreased.Symbols E transpiration - g leaf conductance - w leaf/air vapour concentration difference - -epi water potential of epidermal cells - -mes water potential of mesophyll cells - -xyl water potential of xylem - _p-epi turgor pressure of epidermal cells - _p-mes turgor pressure of mesophyll cells - _s-epi osmotic potential of epidermal cells - _s-mes osmotic potential of mesophyll cells     

Significance of the β-Subunit in the Biogenesis of Na+/K+-ATPase

This review summarizes our experiments on the significance of the -subunit in the functional expression of Na⁺/K⁺-ATPase. The -subunit acts like a receptor for the -subunit in the biogenesis of Na⁺/K⁺-ATPase and facilitates the correct folding of the -subunit in the membrane. The -subunit synthesized in the absence of the -subunit is subjected to rapid degradation in the endoplasmic reticulum. Several assembly sites are assigned in the sequence of the -subunit from the cytoplasmic NH₂-terminal domain to the extracellular COOH-terminus: the NH₂-terminal region of the extracellular domain, the conservative proline in the third disulfide loop, the hydrophobic amino acid residues near the COOH-terminus and the cysteine residues forming the second and the third disulfide bridges. Upon assembly, the -subunit confers a resistance to trypsin on the -subunit. The conformations induced in the -subunit of Na⁺/K⁺-ATPase by Na⁺/K⁺- and H⁺/K⁺-ATPase -subunits are somehow different from each other and are named the NK-type and KH-type, respectively. The extracellular domain of the -subunit is involved in the folding of the -subunit leading to trypsin-resistant conformations. The sequences from Cys150 to the COOH-terminus of the Na⁺/K⁺-ATPase -subunit and from Ile89 to the COOH–terminus of the H⁺/K⁺-ATPase -subunit are necessary to form trypsin-resistant conformations of the NK- and HK-type. respectively. The first disulfide loop of the extracellular domain of the -subunits is critical in the expression of functional Na⁺/K⁺-ATPase.     

Modulation of a plant mitochondrial K+ATP channel and its involvement in cytochrome c release

Pea stem mitochondria, resuspended in a KCl medium (de-energized mitochondria), underwent a swelling, as a consequence of K⁺ entry, that was inhibited by ATP. This inhibition was partially restored by GTP and diazoxide (K⁺ _ATP channel openers). In addition, glyburide and 5-hydroxydecanate (K⁺ _ATP channel blockers) induced an inhibition of the GTP-stimulated swelling. Mitochondrial swelling was inhibited by H₂O₂, but stimulated by NO. The same type of responses was also obtained in succinate-energized mitochondria. When the succinate-dependent transmembrane electrical potential () had reached a steady state, the addition of KCl induced a dissipation that was inhibited by H₂O₂ and stimulated by NO. The latter stimulation was prevented by carboxy-PTIO, a NO scavenger. Phenylarsine oxide (a thiol oxidant) and NEM (a thiol blocker) stimulated the KCl-induced dissipation of , while DTE prevented this effect in both cases. In addition, DTE transiently inhibited the NO-induced dissipation of , but then it caused a more rapid collapse. These results, therefore, show that the plant mitochondrial K⁺ _ATP channel resembles that present in mammalian mitochondria and that it appears to be modulated by dithiol–disulfide interconversion, NO and H₂O₂. The aperture of this channel was linked to the partial rupture of the outer membrane. The latter effect led to a release of cytochrome c, thus suggesting that this release may be involved in the manifestation of programmed cell death.     

An investigation of the role of solutes in the xylem sap and in the xylem parenchyma as the source of root pressure

Summary Solute osmotic potentials (_x) in the vessels of hydroponically grown maize roots were measured to assess the osmotic-xylem-sap mechanism for generating root pressure (indicated by guttation). Solutes in vessels were measured in situ by X-ray microanalysis of plants frozen intact while guttating. Osmotic potentials outside the roots (_o) were changed by adding polyethylene glycol to the nutrient solution. Guttation rate fell when _o was decreased, but recovered towards the control value during 3–5 days when _o was greater than or equal to –0.3 MPa, but not when _o was equal to –0.4 MPa. In roots stressed to _o = –0.3 MPa, _x, was always more positive than _o, and _x changed only slightly (ca. 0.05 MPa). Thus the adjustment in the roots which increased root pressure cannot be ascribed to _x, contradicting the osmotic-xylem-sap mechanism. An alternative driving force was sought in the osmotic potentials of the vacuoles of the living cells (_v), which were analysed by microanalysis and estimated by plasmolysis. _v showed larger responses to osmotic stress (0.1 MPa). Some plants were pretreated with abundant KNO₃ in the nutrient solution. These plants showed very large adjustments in _v (0.4 MPa) but little change in _x (0.08 MPa). They guttated by 4 h after _o was lowered to –0.4 MPa. It is argued that turgor pressure of the living cells is a likely alternative source of root pressure. Published evidence for high solute concentrations in the xylem sap is critically assessed.Abbreviations _o external water potential - _x osmotic potential of xylem sap - _v osmotic potential of vacuolar sap - EDX energy dispersive X-ray microanalysis - CSEM cryo-scanning electron microscope - LN₂ liquid nitrogen - PEG polyethylene glycol     

Current-voltage relations of sodium-coupled sugar transport across the apical membrane ofNecturus small intestine

Summary The current-voltage (I–V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane ofNecturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, ^mc , and that across the entire epithelium, ^ms , when the latter was varied over the range ±200 mV, (i) under steady conditions in the presence of galactose and (ii) after the current across the apical membrane carried by the cotransporter,I _SNa ^m , is blocked by the addition of phloridzin to the mucosal solution.I _SNa ^m was found to be strongly dependent upon ^mc over the range –50 mV < ^mc <E _SNa ^m whereE _SNa ^m is the zero current or reversal potential. Over the range of values of ^mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so thatI _SNa ^m =g _SNa ^m (E _SNa ^m – ^mc ) whereg _SNa ^m is the contribution of this mechanism to the conductance of the apical membrane and is near constant. In several instancesI _SNa ^m saturated at large hyperpolarizing or depolarizing values of ^mc .The values ofE _SNa ^m determined in the presence of 1, 5, and 15mm galactose strongly suggest that if the Na-galactose cotransporters are kinetically homogeneous, the stoichiometry of this coupled process is unity.Finally, the shapes of the observedI–V relations are consistent with the predictions of a simple kinetic model which conforms with current notions regarding the mechanico-kinetic properties of this cotransport process.     

No effects of intermittent 50 Hz EMF on cytoplasmic free calcium and on the mitochondrial membrane potential in human diploid fibroblasts

The recently described increase in DNA strand breaks of cultured human diploid fibroblasts after intermittent exposure to extremely-low-frequency electromagnetic fields (ELF-EMF) of more than about 70 µT ELF-EMF is difficult to explain by a direct induction of covalent bond disruption. Therefore the hypothesis has been tested that ELF-EMF-induced DNA strand breaks might be mediated by cellular processes that cause alteration of the intracellular concentration of free calcium ([Ca²⁺]_i) and/or the membrane potential (_m). [Ca²⁺]_i was determined by the ratiometric fura-2 technique. Changes in _m were assessed by using the potential-dependent lipophilic cationic probe JC-1. Human fibroblasts were exposed to intermittent ELF-EMF (50 Hz, 1000 µT). Although exposure of fiboblasts to ELF-EMF resulted in a highly significant increase in DNA strand breaks as determined by the comet assay, no effect on JC-1 fluorescence emission or on [Ca²⁺]_i has been observed when comparing exposed with sham-exposed cells. Therefore, it is suggested that ELF-EMF-induced DNA strand breaks are unlikely to be caused by intracellular changes that affect [Ca²⁺]_i and/or _m.