Involvement of prostaglandin E receptor EP3 subtype in duodenal bicarbonate secretion in rats

We investigated the involvement of prostaglandin E (PGE) receptor subtype EP3 in the regulatory mechanism of duodenal HCO₃⁻ secretion in rats. A proximal duodenal loop or a chambered stomach was perfused with saline, and HCO₃⁻ secretion was measured using a pH-stat method and by adding 2 mM HCl. Mucosal acidification was achieved through 10 min of exposure to 10 mM HCl in the duodenum or 100 mM HCl in the stomach. Various EP agonists or the EP4 antagonist were given i.v., while the EP1 or EP3 antagonist was given s.c. or i.d., respectively. Sulprostone (EP1/EP3 agonists) stimulated duodenal HCO₃⁻ secretion in a dose-dependent manner, and this response was inhibited by AE5-599 (EP3 antagonist) but not AE3-208 (EP4 antagonist). AE1-329 (EP4 agonist) also increased duodenal HCO₃⁻ secretion, and this action was inhibited by AE3-208 but not AE5-599. The response to PGE₂ or acidification in the duodenum was partially attenuated by AE5-599 or AE3-208 alone but completely abolished by the combined administration. Duodenal damage caused by mucosal perfusion with 150 mM HCl for 4 h was worsened by pretreatment with AE5-599 and AE3-208 as well as indomethacin and further aggravated by co-administration of these antagonists. Neither the EP3 nor EP4 antagonist had any effect on the gastric response induced by PGE₂ or acidification. These results clearly demonstrate the involvement of EP3 receptors, in addition to EP4 receptors, in the regulation of duodenal HCO₃⁻ secretion as well as the maintenance of the mucosal integrity of the duodenum against acid injury.     

Differential scanning calorimetric studies on bovine serum albumin: II. Effects of neutral salts and urea

Using defatted and SH-blocked bovine serum albumin (BSA), measurements of differential scanning calorimetry (d.s.c.) have been made mainly in NaSCN solution. BSA undergoes a heat-induced conformational transition in a particular range of pH and ionic strength and is separated into two thermally independent units, each of which has different thermostability in acidic and alkaline pH regions. Comparisons were made of the pH dependencies of the enthalpy of thermal denaturation (ΔH) and the temperature of thermal denaturation (T_d) in 0.01 NaSCN with those in 0.01 NaCl. It has been found that the stabilizing effect of NaSCN on BSA is larger than that of NaCl at pH 3.5–8, and that the heat-induced transition occurs by the electrostatic repulsive forces among the positively charged amino acid residues in a segment Arg 184–Arg 216 containing Trp 212 and the primary binding sites of anions. At ionic strength 0.01, the relative effectiveness of anions in suppressing the heat-induced transition and increasing the thermostability of BSA follows the order ClO₄⁻ − SCN⁻ > I⁻ > SO₄²⁻ > Br⁻ > Cl⁻. At ionic strength 0.1, the heat-induced transition is suppressed in all the salt solutions, and a T_d increase follows the order ClO₄⁻ SCN⁻ > I⁻ > Br⁻ > Cl⁻ SO₄²⁻. Thus, the highly chaotropic ions thermostabilize BSA more markedly than kosmotropic ions in the low and moderate salt concentrations. In contrast, chaotropic ions destabilize BSA and kosmotropic ions stabilize BSA at the higher concentrations. An adequate amount of NaCl or NaSCN prevents the destruction of the environment of the binding site in the segment containing Trp 212 in 4 urea solution at pH 7.0.     

The binding of bicarbonate ions to washed chloroplast grana

Radioactive labelling techniques show that isolated broken chloroplasts can take up HCO₃⁻ in the dark. There are two pools of binding sites for this ion on, or within, the thylakoid membranes. A smaller, high affinity pool exists at a concentration of one HCO₃⁻ bound per 380–400 chlorophyll molecules. Removal of HCO₃⁻ bound in this pool requires special conditions and results in greater than 90% inhibition of oxygen evolution. The inhibition is fully reversed when HCO₃⁻ is added back. HCO₃⁻ bound in the small pool does not necessarily exchange with free HCO₃⁻ in the dark or in light. Evidence presented suggests that this site is very near the site of action of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea. A second, much larger, pool of HCO₃⁻ binding sites also exists in a concentration approaching that of the bulk chlorophyll. These sites have a much lower affinity for HCO₃⁻, and their function has not yet been determined.     

Steric effects on the substitution of pentaamine-chromium(III) and -rhodium(III) complexes. Anation reaction rate constants as an indicative of the dissociative shift of the mechanism on crowding the [M(RNH2)5H2O] (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) complexes

The kinetics of the anation reactions of [M(RNH₂)₅H₂O]³⁺ (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) with several ligands (H₃PO₄/H₂PO₄⁻, H₃PO₃/H₂PO₃⁻, CF₃COO⁻, Br⁻, Cl⁻, SCN⁻) have been studied at different temperatures and acidities at I = 1.0 M (LiClO₄. Results obtained for the anation rate constants and thermal activation parameters are compared with the previously published data for R = H, in order to establish the effects of the amine substituents in the reaction mechanism proposed for the substitution reactions of these complexes. The results obtained are interpreted on the basis of a mechanism where the bond formation process is more important in the substitution on M = Cr complexes than in that of the M = Rh complexes, as already pointed out for the published ΔΛ^≠ values for the water exchange on these systems. A simple Langford-Gray classification becomes inadequate to describe these situations where the increase of the steric demand of the amine substituents shifta the I_a-I_d classification to the I_d side, although no dramatic changes in the reaction mechanism are found. It is concluded that a More O'Ferall ‘continuous’ type of approach to the mechanism classification of the substitution reactions is much more useful in this case.     

Perchlorate and nitrate reductase activity in the perchlorate-respiring bacterium perclace

The perchlorate (ClO₄⁻)-respiring organism, strain perc1ace, can grow using nitrate (NO₃⁻) as a terminal electron acceptor. In resting cell suspensions, NO₃⁻ grown cells reduced ClO₄⁻, and ClO₄⁻ grown cells reduced NO₃⁻. Activity assays showed that nitrate reductase (NR) activity was 1.31 μmol min⁻¹ (mg protein)⁻¹ in ClO₄⁻ grown cells, and perchlorate reductase (PR) activity was 4.24 μmol min⁻¹ (mg protein)⁻¹ in NO₃⁻ grown cells. PR activity was detected within the periplasmic space, with activities as high as 14 μmol min⁻¹ (mg protein)⁻¹. The NR had a pH optimum of 9.0 while the PR had an optimum of 8.0. This study suggests that separate terminal reductases are present in strain perclace to reduce NO₃⁻ and ClO₄⁻.     

Possible interaction of [H]glutamate binding sites with anion channels in rat neural tissues

The effect of various ions on [³H] -glutamic acid (Glu) binding was examined using crude synaptic membrane preparations from the rat brain. In vitro addition of sodium acetate (1–100 mM) exhibited a significant enhancement of the binding in a concentration dependent manner. Ammonium chloride (20 mM) prevented the potentiation by sodium acetate at 2°C, whereas sodium acetate exerted an inhibitory action on the ammonium chloride-induced augmentation of the binding at 30°C. Ammonium chloride (1–100 mM) itself elicited a temperature dependent stimulation of the binding, which was invariably attenuated by an antagonist for the anion channels such as picrotoxinin (10⁻³ M) as well as by inhibitors of anion transport including ethacrynic acid (10⁻³ M) and 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (10⁻⁴−10⁻³ M), respectively. The later two inhibitors also caused a significant additional raise of the sodium acetate-induced enhancement of the binding. A significant augmentation of the binding resulted from the addition (20 mM) of various anions known to penetrate the anion channels such as bromide, iodide, nitrate, bicarbonate and thiocyanate in a permeability related manner, while that of non-permeable anions including fluoride, sulfate, acetate, formate, phosphate, oxalate, lactate, succinate and tartarate had no such a profound effect on the binding. Addition of -aspartic acid resulted in the complete abolition of the Na⁺-dependent binding while sparing the Cl⁻-dependent binding. Scatchard analysis revealed that Cl⁻ ions induced a two-fold increase in the number of the binding sites without affecting their affinity, whereas Na⁺ ions reduced the affinity with a concomitant increase of the number of the binding sites. Addition of quisqualic acid (10⁻⁵−10⁻³ M) inhibited the Cl⁻-dependent binding of [³H]Glu to a significantly greater extent than the inhibition on Na⁺-dependent binding. acid and kainic acid exerted no preventive action on the basal, Cl⁻-dependent and Na⁺-dependent binding. respectively. The highest basal binding activity was found in the retina among various central structures examined. A significant basal binding activity of [³H]Glu was also detected in the pituitary and adrenal but not in the kidney. Chloride ions exhibited a significant facilitation of [³H]Glu binding to central regions without altering that to peripheral tissues such as pituitary and adrenal. In contrast, Na⁺ ions induced significant attenuation of the binding to the pituitary, adrenal and retina despite the occurrence of augmentation of the binding to other central structures.

These results suggest the Glu binding sites may be linked to the anion channels in the rat central nervous system and that this linkage may be absent from the pituitary, adrenal and retina.     

CI-NMR measurement of chloride binding to the oxygen-evolving complex of spinach Photosystem II

The mechanism by which Cl⁻ activates the oxygen-evolving complex (OEC) of Photosystem II (PS II) in spinach was studied by ³⁵Cl-NMR spectroscopy and steady-state measurements of oxygen evolution. Measurements of the excess ³⁵Cl-NMR linewidth in dark-adapted, Cl⁻-depleted thylakoid and Photosystem II membranes show an overall hyperbolic decrease which is interrupted by sharp increases in linewidth (linewidth maxima) at approx. 0.3 mM, 0.75 mM, 3.25 mM (2.0 mM in PS II membranes), and 7.0 mM Cl⁻. The rate of the Hill reaction (H₂O → 2,6-dichlorophenolindophenol) at low light intensities (5% of saturation) as a function of [Cl⁻] in thylakoids shows three intermediary plateaus in the concentration range between 0.1 and 10 mM Cl⁻ indicating kinetic cooperativity with respect to Cl⁻. The presence of linewidth maxima in the ³⁵Cl-NMR binding curve indicates that Cl⁻ addition exposes four types of Cl⁻ binding site that were previously inaccessible to exchange with Cl⁻ in the bulk solution. These results are best explained by proposing that Cl⁻ binds to four sequestered (salt-bridged) domains within the oxygen-evolving complex. Binding of Cl⁻ is facilitated by the presence of H⁺ and vice versa. The pH dependence of the excess ³⁵Cl-NMR linewidth at 0.75 mM Cl⁻ shows that Cl⁻ binding has a maximum at pH 6.0 and two smaller maxima at pH 5.4 and 6.5 which may suggest that as many as three groups (perhaps histidine) with pK_a values in the region may control the binding.     

Simultaneous production of two different gel-forming exopolysaccharides by an Alteromonas strain originating from deep sea hydrothermal vents

Exopolysaccharide production by the marine bacterium Alteromonas sp. strain 1644 was shown to be stimulated by restricted growth conditions and was optimized in nitrogen limited fed-batch cultures. Exopolysaccharides were either partly secreted in the medium or stayed firmly cell-associated. The cell-polysaccharide associations could be destroyed by dialysis against distilled water, allowing polysaccharide purification. The chemical and rheological characterization of this last polysaccharide showed that it was different from the secreted polysaccharide that has been previously described (polysaccharide 1644). At low ionic concentration (below 0.03 M whatever the nature of the ions), solutions of this new polysaccharide had very low viscosities. However, at higher ionic concentration, it formed a gel or exhibited in solution at low polymer concentration an unusually high temperature dependent viscosity. This behaviour was also dependent on the nature of the ions and the following sequences for cations and anions were NH4 ⁺ > Mg²⁺ > Na ⁺ > Li⁺ > K⁺ > TMA⁺ and Br⁻ > NO₃⁻ > SO₄²⁻ > Cl⁻ > I⁻ respectively.     

Rb transport in Leishmania infantum promastigotes under various in vitro culture conditions

The survival of Leishmania, which encounter drastic changes of environment during their life-cycle, requires regulation and control of ionic concentrations within the cell. We analysed the influence of growth stage, ionic composition of the medium, heat and acidic stress on ⁸⁶Rb⁺ influx in L. infantum promastigetes. Proliferating promastigotes exibited faster and higher ⁸⁶Rb⁺ uptake than stationary cells. Cl⁻ anion did not have any effect, but in the presence of physiological concentration of HCO₃⁻, ⁸⁶Rb⁺ uptake was significantly increased. This enhancing effect was only partially inhibited by N,N′-dicyclohexylcarbodiimide (DCCD), a blocker of ion-translocating ATPases. ⁸⁶Rb⁺ influx was abolished by N-ethylmaleimide (NEM), indicating a major contribution of plasma membrane transporters. Heat shock and acidic shock notably decreased ⁸⁶Rb⁺ influx. Our data provide indirect evidence that an energy-dependent system which brings K⁺ in, such as K⁺/H⁺-ATPase evidenced by Jiang et al. (1994), is active in Leishmania in different environments. Mechanism(s) other than ion-translocating ATPase occur, at least in the presence of HCO₃⁻, and their contribution to K⁺ pathways varies in different environmental conditions.     

The Cl effect on photosynthetic oxygen evolution: interaction of Cl with 18-kDa, 24-kDa and 33-kDa proteins

The interaction of Cl⁻ with the extrinsic proteins of 18 kDa, 24 kDa and 33 kDa in the photosynthetic oxygen-evolution complex was studied by comparing spinach photosystem II particles of different protein compositions. The 33-kDa protein decreased the Cl⁻ concentration optimum for oxygen evolution from 150 to 30 mM, and the 24-kDa protein decreased it from 30 to 10 mM. The 18-kDa protein did not change the optimum Cl⁻ concentration, but sustained oxygen evolution at Cl⁻ concentrations lower than 3 mM. The presence of the 24-kDa and 18-kDa proteins, but not each protein alone, markedly suppressed inactivation of oxygen evolution at a very low Cl⁻ concentration and its restoration by readdition of Cl⁻.     

Thermogravimetric study of starch derivatives with amine/ammonium ion-exchanging groups in oxidative environment

The amine/ammonium materials were prepared by cross-linking of starch (S) with epichlorohydrin (E→SE) in the presence of ammonia (A→SEA) or choline (C→SEC–HO⁻) or with 1,3-bis-(3-chloro-2-hydroxypropyl)imidazolium hydrogensulphate (BCHIHS→SHI–HO⁻) and transfered into the acid/salt forms with HCl (SEA–HCl, SEC–Cl⁻, or SHI–Cl⁻), H₂SO₄ (SEA–H₂SO₄, SEC–HSO₄⁻, or SHI–HSO₄⁻), and H₃PO₄ (SEA–H₃PO₄, SEC–H₂PO₄⁻, or SHI–H₂PO₄⁻) and analyzed with thermogravimetry (TG) under dynamic and isothermal conditions in nitrogen or oxygen environment. According to the values of thermooxidation maxima (TM) calculated from the maximal difference of measured residues on the dynamic TG curves run in nitrogen and oxygen environments the order of decreasing thermooxidation resistance is: S>SEA–H₃PO₄>SHI–H₂PO₄⁻>SHI–HSO₄⁻>SEA–H₂SO₄>SEC–HSO₄⁻>SEC–HO⁻>SEA–HCl>HCl–Cl⁻> SEC–Cl⁻>SHI–HO⁻>SEA>SEC–H₂PO₄⁻>SE. The first-order rate constants calculated by the linear regression method (regression coefficient R>0.95) represented the initial rate constants for residue formation (k_r's) and gasification (k_g's). All the derivatives had greater values of rate constants than S and the k_g's were about 1000 times greater than k_r's. The values obtained in nitrogen were smaller than those calculated from runs in oxygen environment with the exception of S. Most of the salt forms had greater values of k_g's in oxygen environment. The activation energies (E's) were usually greater in nitrogen than in oxygen as well as for residue formation than for gasification. The SHI–HO⁻ sample had high k_g's and low E_g's in oxygen environment while for SHI–H₂SO₄⁻ the opposite was true. This we consider as two extremes for labile and resistant samples for gasification.     

Inhibition of human surfactant protein A function by oxidation intermediates of nitrite

Nitration of protein tyrosine residues by peroxynitrite (ONOO⁻) has been implicated in a variety of inflammatory diseases such as acute respiratory distress syndrome (ARDS). Pulmonary surfactant protein A (SP-A) has multiple functions including host defense. We report here that a mixture of hypochlorous acid (HOCl) and nitrite (NO₂⁻) induces nitration, oxidation, and chlorination of tyrosine residues in human SP-A and inhibits SP-A’s ability to aggregate lipids and bind mannose. Nitration and oxidation of SP-A was not altered by the presence of lipids, suggesting that proteins are preferred targets in lipid-rich mixtures such as pulmonary surfactant. Moreover, both horseradish peroxidase and myeloperoxidase (MPO) can utilize NO₂⁻ and hydrogen peroxide (H₂O₂) as substrates to catalyze tyrosine nitration in SP-A and inhibit its lipid aggregation function. SP-A nitration and oxidation by MPO is markedly enhanced in the presence of physiological concentrations of Cl⁻ and the lipid aggregation function of SP-A is completely abolished. Collectively, our results suggest that MPO released by activated neutrophils during inflammation utilizes physiological or pathological levels of NO₂⁻ to nitrate proteins, and may provide an additional mechanism in addition to ONOO⁻ formation, for tissue injury in ARDS and other inflammatory diseases associated with upregulated ^•NO and oxidant production.     

Nitrite reduces the effects of HOCl on unsaturated phosphatidylcholines--a MALDI-TOF MS study

The concentration of nitrite (NO₂⁻) increases under inflammatory conditions. However, the physiological role of nitrite is so far controversial discussed: it was reported that effects of HOCl (an important inflammation mediator) on phospholipids (PL) may be enhanced but also reduced in the presence of nitrite.

In this paper a simple model system was used: unsaturated phosphatidylcholine (PC) vesicles were treated with HOCl in the presence of varying NaNO₂ concentrations and the yield of reaction products was determined by MALDI-TOF MS: the extent of chlorohydrin generation was significantly reduced in the presence of NaNO₂ because HOCl is consumed by the oxidation of NO₂⁻ to NO₃⁻.

Similar results were obtained when HOCl was generated by the myeloperoxidase (MPO)/H₂O₂/Cl⁻ system or the experiments were carried out in the presence of a simple peptide. It is concluded that the transient products of the reaction between HOCl and NO₂⁻ do not have a sufficient reactivity to modify PL.     

The action of amytal on frog gastric mucosa

The frog gastric mucosa has been shown to be sensitive to amytal. At 2 mM acid secretion was completely inhibited with a rise of resistance, fall in short-circuit current and no significant change in potential difference. Simultaneously there was 75% inhibition of O₂ consumption and 50% depression of ATP levels. Dual-beam spectrophotometric studies of intact mucosa with amytal showed a crossover point between NAD⁺ and FAD. The microsomal NADH oxidase ferricyanide reductase has also been shown to be amytal sensitive. Cl⁻ transport was relatively insensitive to amytal, suggesting a qualitative distinction between the mechanisms underlying the transport of H⁺ and Cl⁻ in the mucosa. This was further brought out by the effects of anoxia in which H⁺ transport was inhibited at 5 min but Cl⁻ transport at minimally 20 min following the onset of anoxia.     

Resonance Raman studies of amaylose — iodine complexes

Resonance Raman measurements have been performed with solutions of iodine-complexed synthetic amyloses (DP 25–200), malto-oligomers (DP 3–18, and -cylodextrin. Interest was focused on the minimum chain length for helical complex formation and a possible preferred length for the polyiodine chain. Four fundamental vibrations are observed at 164, 112, 52 and 24 cm⁻¹. The 112 cm⁻¹ Raman line was shown to arise both from free I₃⁻ (enhanced at 363.8 nm excitation) and from bound iodine (relatively most intense at 457.9 nm excitation). The main signal of complexed iodine at 164 cm⁻¹is enhanced at an excitation wavelength close to the long wavelength absorption maximum. This signal is observed firt with malto-octaose and -cyclodextrin. The less intense signals at 52 and 24⁻¹ are only detected at DP 15 and higher. Raman spectra give no evidence for a preferred length of the polyiodine chain. Significantly identical Raman spectra are obtained when using different molar ratios of I₂/KI solution or I₂ solution initially free of I⁻ ions. The results are discussed in view of previous assignments of the Raman lines to I₂⁻, I₃⁻/I₂, and I₅⁻ subunits. Our findings are incompatible with I₃⁻ units as the only bound species. They are compatible with both I₃⁻/I₂ and I₃⁻ subunits under certain conditions. In the case of I₂ solution used for complexation we favour the polyiodine chain model proposed previously by Cramer^35,36. The I₃⁻ ions formed could function mainly as chain initiators, as has been suggested by Cesàro and Brant³⁰.     

Clarification of the Relationship Between Free Radical Spin Trapping and Carbon Tetrachloride Metabolism in Microsomal Systems

It has been proposed that the C-phenyl-N-tert-butylnitrone/trichloromethyl radical adduct (PBN/^•CCl₃) is metabolized to either the C-phenyl-N-tert-butylnitrone/carbon dioxide anion radical adduct (PBN/^•CO₂⁻) or the glutathione (GSH) and CCl₄-dependent PBN radical adduct (PBN/[GSH-^•CCl₃]). Inclusion of PBN/^•CCl₃ in microsomal incubations containing GSH, nicotinamide adenine dinucleotide phosphate (NADPH), or GSH plus NADPH produced no electron spin resonance (ESR) spectral data indicative of the formation of either the PBN/[GSH-^•CCl₃] or PBN/^•CO₂⁻ radical adducts. Microsomes alone or with GSH had no effect on the PBN/^•CCl₃ radical adduct. Addition of NADPH to a microsomal system containing PBN/^•CCl₃ presumably reduced the radical adduct to its ESR-silent hydroxylamine because no ESR signal was observed. The Folch extract of this system produced an ESR spectrum that was a composite of two radicals, one of which had hyperfine coupling constants identical to those of PBN/^•CCl₃. We conclude that PBN/^•CCl₃ is not metabolized into either PBN/[GSH-^•CCl₃] or PBN/^•CO₂⁻ in microsomal systems.     

Evidence that de novo protein synthesis participates in a time-dependent augmentation of the chemotactic peptide-induced respiratory burst in neutrophils: Effects of recombinant human colony stimulating factors and dihydrocytochalasin B

We examined the effects of the recombinant human colony stimulating factors GM-CSF and G-CSF, cycloheximide (a protein synthesis inhibitor) and dihydrocytochalasin B (a microfilament disrupting agent) upon FMLP (N-formyl-methionyl-leucylphenylalanine)-stimulated O₂ ⁻ production by neutrophils. We confirmed a time dependent augmentation of O₂ ⁻ production following preincubation of neutrophils either alone or with colony stimulating factors. Furthermore, we found that GM-CSF, but not G-CSF, increased O₂ ⁻ production at some concentrations of the stimulus. Preincubation of neutrophils with cycloheximide in the absence of CSF caused a marked fall in O₂⁻-production that was first evident at 2 hours. The fall in O₂⁻-forming capacity caused by cycloheximide was much less pronounced if dihydrocytochalasin B was also included in the preincubation buffer. These findings suggest a previously unrecognized role for de novo protein synthesis in maintaining the ability of neutrophils to manufacture O₂⁻, and support earlier studies indicating that the cycling of FMLP receptors between the cell membrane and an intracellular compartment is important in determining the magnitude of the respiratory burst in FMLP-stimulated neutrophils.     

Physiological responses to salt stress in young umbu plants

Soil salinity affects plant growth and development due to harmful ion effects and water stress caused by reduced osmotic potential in the soil solution. In order to evaluate the effects of salt stress in young umbu plants, research was performed in green house conditions at the Laboratory of Plant Physiology at Federal Rural University of Pernambuco, Brazil. Growth, stomatal behaviour, water relations, and both inorganic and organic solutes were studied aiming for a better understanding of the responses of umbu plants to increasing salinity. Plants were grown in washed sand with Hoagland and Arnon nutrient solution with 0, 25, 50, 75, and 100 mM NaCl. Growth, leaf water potential, transpiration, and diffusive resistance were evaluated. Na⁺, K⁺, Cl⁻, soluble carbohydrates, and free amino acid contents were measured in several plant organs. Most variables were affected with salinity above 50 mM NaCl showing decreases in: number of leaves, plant height, stems diameter, and dry masses, and increases in root-to-shoot ratio. Reductions in ψ_pd were observed in plants grown under 75 and 100 mM NaCl. All salt levels above zero increased Na⁺ and Cl⁻ contents in leaves. However, K⁺ content was not affected. Na⁺ and Cl⁻ in stems and roots reached saturation in treatments above 50 mM NaCl. Organic solute accumulation in response to salt stress was not observed in umbu plants. These results suggest that umbu plants tolerate salt levels up to 50 mM NaCl without showing significant physio-morphological alterations.     

Ion-pair mechanism in square planar substitution. Reactivity of cationic platinum (II) complexes with negatively charged nucleophiles in solvents of high, medium and low polarity

The rates of displacement of dimethyl sulfoxide from the cation [Pt(phen) (CH₃) (Me₂SO)]⁺ by a series of uncharged and negatively charged nucleophiles have been measured in a methanol/water (19:1 vol./vol.) mixture. The starting complex and the reaction products were characterized either as solids or in solution by their IR and ¹H NMR spectra. The substitution reactions take place by way of a direct bimolecular attack of the ligand on the substrate. The sequence of reactivity observed is as expected on the basis of a nucleophilicity scale relevant for + 1 charged substrates ([Pt(en) (NH₃)Cl]⁺ used as standard). The difference of reactivity between the first (t-BuNH₂) and the last (SeCN⁻) members of the series spans five orders of magnitude. The value measured for the nucleophilic discrimination (1.55) is the highest found so far for cationic substrates. This is a result of the easy transfer of some of the electron density brought in by the incoming ligand into the ancillary ligands. When the reaction is carried out in a series of protic and dipolar aprotic solvents, using chloride ion as nucleophile, the rate of formation of [Pt (phen) (CH₃)Cl] is dominated by the extent of solvation of Cl⁻, as measured by its values of the Gibbs molar energy of transfer Δ_tG⁰. Conductivity measurements at 25°C in dichloromethane were fitted to the Fuoss equation and the values of the dissociation constants K_d for the ion pairs were calculated as follows: 2.27 × 10⁻⁵ M for Bu₄NCl, 2.75 × 10⁻⁵ M for Bu₄NSCN and 17.05 × 10⁻⁵ M for [Pt(phen) (CH₃) (Me₂SO)]PF₆. The pseudo-first-order rate constants k_obs for the reactions with Bu₄NCl, Bu₄NBr, Bu₄NSCN and Bu₄NI showed a curvilinear dependence on the concentration of the salt which levels off very soon (at concentrations higher than 0.005 M the kinetics are zero order in [Bu₄NX]). On addition of the inert electrolyte Bu₄NPF₆ the rates slow down and the kinetics follow the rate law k_obs = kK_ip[Bu₄NX]/[Bu₄NPF₆] + K_ip[Bu₄NX]). These findings fit well with a reaction scheme which involves a pre-equilibrium K_ip between ion pairs, followed by unimolecular substitution within the contact ion pair [Pt(phen) (CH₃) (Me₂SO)X]_ip. Values of the equilibrium constants K_ip for ion-pair exchange and of the internal substitution rates k were derived. The latter showed that the discrimination in reactivity between Cl⁻, Br⁻, SCN⁻ and I⁻ is greatly reduced with respect to aqueous solutions. The reason behind this may be desolvation of the ions coupled to the fact that a contact ion pair is already at a certain distance along the reaction coordinate in the direction of the transition state. Applications of the special salt effect and of ion pairing to synthesis are discussed.     

Stimulatory effect of vasoactive intestinal peptide (VIP) on the secretory activity of dispersed rat adrenocortical cells. Evidence for the interaction of VIP with ACTH receptors

VIP dose-dependently increased basal, but not submaximally ACTH (10⁻¹⁰ M)-stimulated, aldosterone (ALDO) and corticosterone (B) secretion of dispersed rat capsular and inner adrenocortical cells, respectively. The maximal stimulatory effect (60–70% rise) was obtained with a VIP concentration of 10⁻⁸ M. [4-Cl-D-Phe⁶,Leu¹⁷]-VIP, a VIP-receptor antagonist (VIP-A), and corticotropin inhibiting peptide (CIP), an ACTH receptor antagonist (both 10⁻⁶ M), completely annulled VIP (10⁻⁸M)-evoked rises in basal ALDO and corticosterone secretions. The ACTH (10⁻¹⁰ M)-enhanced (about 5-fold) production of both hormones was completely reversed by CIP (10⁻⁶ M) and only partially reduced (about −30%) by VIP-A (10⁻⁶ M). The hypothesis is advanced that the weak secretagogue effect of VIP on dispersed rat capsular and inner adrenocortical cells may be due to its positive interaction with ACTH receptors.