Molecular Dynamics Simulation Studies of the Limiting Conductances of CaCl2 using Extended Simple Point Charge and Revised Polarizable Models

We have carried out molecular dynamics (MD) simulations of the limiting conductances of CaCl₂ in ambient and supercritical states as a function of water density using extended simple point charge (SPC/E) and revised polarizable (RPOL) models for ions and water molecules. Both models predict the limiting conductances of CaCl₂ in supercritical water that are a linear dependence on water density. The effect of the electronic polarization on the limiting conductances is too small to cause a deduction in the lower water density of 0.6?～?0.7?g/cc in this study. The most significant effect of the electronic polarization is appeared in a decrease in the ion–water potential energy and, as a result, an increase in the limiting conductances for both ions. Different charge distributions of water molecules in the first hydration shell around the ions lead the opposite behavior of the induced dipole moment with water density for a positive and a negative ion in supercritical water; the induced dipole moment of Ca²⁺ decreases with increasing water density but for Cl^-, the opposite is observed. The same kind of opposite behavior due to the structure of water molecules around the ions is also found in hydrogen-bond correlations of water around the ions and of bulk water; hydrogen bonding around Ca²⁺ persists longer than in bulk water whereas the opposite is observed for Cl^-.     

Molecular Dynamics Simulation of Limiting Conductance for Na2+, Cl2−, Na°, and Cl° in Supercritical Water

Abstract

We report results of molecular dynamics simulations of the limiting conductance of Na²⁺, Cl^2?, Na°, and Cl° in supercritical water using the SPC/E model for water in conjuction with our previous study (Lee et al., Chem. Phys. Lett. 293, 289 (1998)). The behavior of the limiting conductances of Na²⁺ and Cl^2? in the whole range of water density shows almost the same trend as those of Na⁺ and Cl^?, but the deviation from the assumed linear dependence of limiting conductances of Na²⁺ and Cl^2? on the water density is smaller than that of Na⁺ and Cl^?. The ratio of the limiting conductance of the divalentions to that of the corresponding monovalentions over the whole range of water density is almost constant. In the cases of Na²⁺ and Cl^2?, the dominating factor of the number of hydration water molecules around ions in the higher-density region and the dominating factor of the interaction strength between the ions and the hydration water molecules in the lower-density region are also found as was the cases for Na⁺ and Cl^?. These factors, however, are not so strong as for the corresponding monovalent ions because the change in the energetics, structure, and dynamics are very small mainly due to the strong Coulomb interaction of the divalent ions with the hydration water molecules. The diffusion coefficient of Na° and Cl° monotonically increases with decreasing water density over the whole range of water density. The increase of the diffusion coefficient with decreasing water density is attributed only to the dramatic decrease of the hydration number of water in the first solvation shell around the uncharged species. Among the two important competing factors in the limiting conductance of Na⁺ and Cl^?, the effect of the number of hydration water molecules around the uncharged species is the only existing factor over the whole range of water density since the interaction strength between the uncharged species and the hydration water molecules very small through the LJ interaction. This result has confirmed the dominating factor of the number of hydration water molecules around ions in the higher-density region in the explanation of the limiting conductance of Na⁺ and Cl^? in supercritical water at 673 K.     

Molecular Dynamics Simulation of Limiting Conductance for Li + Ion in Supercritical Water using Polarizable Models

We report results of molecular dynamics simulations of the limiting conductance of Li + ion in ambient water and in supercritical water using polarizable models for water and Li + . The limiting conductances of Li + in ambient water calculated from mean square displacement (MSD) using four points transferable intermolecular potential model (TIP4P), extended simple point charge model (SPC/E), and revised polarizable model 1 (RPOL1) are larger than the experimental value. The behavior of the limiting conductance of Li + in supercritical water using the RPOL models results in good agreement with experimental results for the limiting conductance of LiCl. The agreement of the RPOL1 model with the experimental results is much better than the RPOL2 model in the higher-density regime, whereas that of the RPOL2 model is much better than the RPOL1 model in the lower-density regime. Using the RPOL models (in contrast to the SPC/E model), the number of hydration water molecules around Li + is the dominating contributor to the limiting conductance in the higher-density regime. In agreement with the SPC/E model, the interaction strength between Li + and the hydration water molecules is a non-factor in the lower-density region since the potential energy per hydration water molecule decreases with decreasing water density at the lowest water densities.     

Molecular dynamics simulation study for diffusion of Na+ ion in water-filled carbon nanotubes at 25°C

We present results of molecular dynamics simulations for diffusion of Na⁺ ion in water-filled carbon nanotubes (CNTs) at 25°C using the extended simple point charge water potential. Simulation results indicate the general trend that the diffusion coefficients of Na⁺ ion and water molecule in CNTs decrease with an increase in water density and are larger than those in the bulk solution. The average potential energies of ion–water and water–water, the radial distribution functions, the hydration numbers and the residence times of the hydrated water molecules are discussed. The classical solventberg picture describes Na⁺ ion in water adequately for systems with the small values of diffusion coefficients.     

Hydration of counterions interacting with DNA double helix: a molecular dynamics study

In the present work, molecular dynamics simulations have been carried out to study the dependence of counterion distribution around the DNA double helix on the character of ion hydration. The simulated systems consisted of DNA fragment d(CGCGAATTCGCG) in water solution with the counterions Na⁺, K⁺, Cs⁺ or Mg²⁺. The characteristic binding sites of the counterions with DNA and the changes in their hydration shell have been determined. The results show that due to the interaction with DNA at least two hydration shells of the counterions undergo changes. The first hydration shell of Na⁺, K⁺, Cs⁺, and Mg²⁺ counterions in the bulk consists of six, seven, ten, and six water molecules, respectively, while the second one has several times higher values. The Mg²⁺ and Na⁺ counterions, constraining water molecules of the first hydration shell, mostly form with DNA water-mediated contacts. In this case the coordination numbers of the first hydration shell do not change, while the coordination numbers of the second one decrease about twofold. The Cs⁺ and K⁺ counterions that do not constrain surrounding water molecules may be easily dehydrated, and when interacting with DNA their first hydration shell may be decreased by three and five water molecules, respectively. Due to the dehydration effect, these counterions can squeeze through the hydration shell of DNA to the bottom of the double helix grooves. The character of ion hydration establishes the correlation between the coordination numbers of the first and the second hydration shells.

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Graphical Abstract Hydration of counterions interacting with DNA double helix

     

Effects of root-zone solutes on Eucalyptus camaldulensis and Eucalyptus bicostata seedlings: Responses to Na+, Mg2+ and Cl-

Specific-ion effects in salt-treated eucalypts were examined with two species known to differ in salt tolerance viz. E. camaldulensis (more tolerant) and E. bicostata (less tolerant). Sand-cultured plants were irrigated with different nutrient solutions designed to impose either osmotic stress (concentrated macronutrients with balanced cations and anions) or specific ion stress from either NaCl or MgCl₂, or from nutrient solutions rich in particular ions viz. Na⁺, Mg²⁺ and Cl^- (balancing counter ions were provided in all cases). Half-strength Hoagland nutrient solution served as control. All treatments were applied at osmotic pressures of approximately 0.52 MPa by appropriate concentrations of each solution. In general, salt-induced growth reductions were greater for E. camaldulensis than for E. bicostata, although E. camaldulensis showed strongest exclusion of Na⁺, Mg²⁺ and Cl^- from shoots. Application of NaCl and concentrated macronutrients resulted in similar growth reductions. E. bicostata seedlings exposed to high Cl^- concentrations in the presence of Mg²⁺ and concentrated cations suffered significantly more shoot and root reduction than those exposed to other salts. Treatment with solution rich in Cl^- resulted in extensive leaf damage, which suggested that Cl^- may have exerted a specific effect. No specific Na⁺ effect was observed for either species, even though shoot Na⁺ concentrations were considerably higher for E. bicostata than for E. camaldulensis. Root growth was considerably less for plants treated with Mg²⁺ salts and this effect was associated with low root Ca²⁺ concentrations.     

Interaction of Anions and Cations in Regulating Energy Distribution between the Two Photosystems

Cations such as Mg²⁺ regulate spillover of absorbed excitation energy mainly in favour of photosystem (PS) 2. Effect of low concentration (<10 mM) of the monovalent cation Na⁺ on chlorophyll (Chl) a fluorescence was completely overridden by divalent cation Mg²⁺ (5 mM). Based on Chl a fluorescence yield and 77 K emission measurements, we revealed the role and effectiveness of anions (Cl^-, SO₄ ^2-, PO₄ ^3-) in lowering the Mg²⁺-induced PS2 fluorescence. The higher the valency of the anion, the lesser was the expression of Mg²⁺ effect. Anions may thus overcome Mg²⁺ effects up to certain extent in a valency dependent manner, thereby diverting more energy to PS1 even in the presence of MgCl₂. They may do so by reversing Mg²⁺-induced changes.     

Molecular Dynamics Simulations of Zinc Ions in Water Using CHARMM

Molecular Dynamics simulations of a zinc ion with 123 and 525 TIP3P-water molecules were carried out with CHARMM using two different Lennard-Jones parameter sets for the Zn²⁺ ion. The results were compared to published experimental and simulation data. Good agreement was found for radial distribution functions, number of hydrogen bonds, and diffusion coefficients. Experimental radial distribution functions were better reproduced by the original CHARMM22 parameter set than by the parameter set modified by Stote and Karplus. Diffusion coefficients were found to depend on the system size rather than on the parameter set used and were better reproduced by the larger systems. The divalent zinc ion exerts a strong influence on its hydration shell as indicated by the high first peak of the radial distribution function. Water molecules in the vicinity of the zinc ion show a slight deformation of the O-H bond length and of the H-O-H bond angle as compared to pure water. No water molecules from the first hydration shell were exchanged during 1 ns of MD simulation.Electronic Supplementary Material available.     

The effect of hormones on metal and metal-ATP interactions with fat cell adenylate cyclase.

1-adrenaline, ACTH and glucagon activate the adenylate cyclase of rat adipocytes by decreasing its S_0.5(Mg²⁺) (concentration yielding 0.5 V_max) from its basal value of 11.5 to 1.2, 0.3 and 1.8 mM and by increasing its K_i(ATP^4?) from 0.03 to 0.25; 0.62 and 0.16 mM respectively. The kinetic properties of the enzyme are regulated by its state of saturation with ATP^4? or Mg²⁺; its saturation with ATP^4? and citrate^3? suppressed its basal and hormone-dependent activities. The hormone-dependent decrease in K_m and increase in V_max of the enzyme occur when shifting from suboptimal low concentrations of hormone and Mg²⁺ to optimal conditions, i.e., high concentration of hormone and low concentration of Mg²⁺. The increase in the state of saturation of the enzyme with Mg²⁺ decreases the hormone-dependent effects on V_max and results in identical values of K_m (0.14 mM) for its basal and 1-adrenaline dependent activities. CaCl₂ saturation curves at 5 mM ATP with either 5, 10 or 20 mM MgCl₂ show that the substitution of 5 mM MgCl₂ by 10 mM and 20 mM MgCl₂ increased the K_i(Ca²⁺) of the enzyme from 0.19 to 0.49 and 0.94 mM but decreased its K_i(CaATP) from 0.42 to 0.19 and 0.14 mM respectively. Only when the concentration of MgCl₂ exceeded that of ATP did 1-adrenaline and ACTH activate the enzyme by increasing its K_i(Ca²⁺), although only ACTH increased its K_i(CaATP). An increase in energy charge would decrease the intracellular concentrations of Mg²⁺ and Ca²⁺ because ATP^4? has stronger binding constants for Mg²⁺ and Ca²⁺ than ADP^3? and AMP^2?. Hence, the reported properties of the enzyme suggests that changes in energy charge may allow for metabolic feedback control of the hormonal responsiveness of the Mg²⁺, Ca²⁺, ATP^4? -sensitive adenylate cyclase.     

Fast reversal of the initial reaction steps of the plasma membrane (Ca2+ + Mg2+)-ATPase

(1) Calmodulin-depleted red cell membranes catalyse a Ca²⁺, Mg²⁺-dependent ATP-[³H]ADP exchange at 37° C. The Ca²⁺, Mg²⁺-dependent exchange, measured at 20 μM CaCl₂, 1.5 mM MgCl₂, 1.5 mM ADP and 1.5 mM ATP, is comparable to the (Ca²⁺ + Mg²⁺)-ATPase activity, between 0.3 and 0.8 mmol/litre original cells per h. (2) EDTA-washed membranes present a Ca²⁺-dependent ATP-ADP exchange whose rate is not more than 7% of that found in a Mg²⁺-containing medium, while their Ca²⁺-dependent ATPase is essentially zero. Addition of 1.5 mM MgCl₂ to the medium restores both activities to the levels found with membranes not treated with EDTA. (3) Calmodulin (16 μg/ml) produces an eight-fold stimulation of the Ca²⁺-dependent ATP-ADP exchange, slightly less than it stimulates the Ca²⁺-dependent ATP hydrolysis. The effect of 1.5 mM MgCl₂ on the exchange is greater in the presence than in the absence of calmodulin. (4) It is proposed that the reversal of the initial phosphorylation of the Ca²⁺ pump, occurring at a fast rate at 37° C, involves a conformational change in the phosphoenzyme. Thus, it would be an ADP-liganded phosphoenzyme of the form EP(ADP) that would experience the fast conformational transition at 37° C. The great difficulty in producing an overall reversal of the Ca²⁺ pump should then be due to one or more reaction steps later than and including Ca²⁺ release and dephosphorylation.     

基于典范对应分析的滨海湿地土壤季节性盐渍化特征

为了更好地开发利用黄河三角洲湿地土壤资源和生态环境建设,运用单因素方差分析(One-way ANOVA)和典范对应分析方法(CCA),对黄河口湿地0—10 cm土壤全盐量(TS)、盐分离子组成、pH、钠吸附比(SAR)、电导率(EC)的季节性变化特征及数量关系进行了分析。结果表明:该区土壤属于盐土类型,春季(5月)、夏季(8月)、秋季(10月)全盐含量均高于17.9 g/kg,且春季、秋季的含盐量高于夏季,造成春、秋季积盐,夏季脱盐。盐分阳离子以Na~+为主,阴离子以Cl~-为主,除Mg~(2+)在夏季、秋季和Cl~-在秋季表现出强烈的变异性外,其余离子在不同季节均表现出中等强度的变异性。春季,Cl~-与Na~+、Mg~(2+),SO■与K~+;夏季,Cl~-与HCO~-_3,SO■与Mg~(2+)、Ca~(2+),Ca~(2+)与Mg~(2+);秋季,Cl~-与SO■,SO■与Mg~(2+),Ca~(2+)与Mg~(2+),均有很好的关联性。pH值均介于7.7—8.1之间,各个季节受盐分离子的影响较小,分布较为均匀。SAR介于3.08—5.29之间,春季受控于HCO~-_3;夏季受控于K~+;秋季受各离子的影响均较小,分布较为均匀。EC介于7.16—13.04 mS/cm,春季受各离子的影响均较小,其空间分布较为均匀;夏季受控于SO■、Ca~(2+)、Mg~(2+);秋季受控于Na~+。TS与Cl~-、Na~+、Mg~(2+)在季节变化上的差异性一致,且各个季节均受控于Cl~-。故通过合理的措施控制或减少Cl~-来源是一条减轻黄河口湿地土壤盐渍化的合适途径。     

Absorption of magnesium and chloride by excised corn root

Absorption characteristics of Mg²⁺ and Cl⁻ were investigated with 5-day-old excised corn (Zea mays) roots. Uptake from both 0.5 and 10 milliequivalents per liter MgCl₂ solutions occurred at steady state rates for the first 6 hours. Inhibition by dinitrophenol and low temperatures established that absorption during this period was metabolically mediated in the absence and presence of Ca²⁺. Absorption isotherms indicated dual mechanisms of Mg²⁺ and Cl⁻ absorption from solutions above 1 milliequivalent per liter. The effect of H⁺ on absorption of Mg²⁺ and Cl⁻ was typical of that generally reported for other plant roots and other ions. In the physiological pH range, Ca²⁺ greatly suppressed the rate of Mg²⁺ absorption but had little effect on Cl⁻. The influence of Ca²⁺ on Mg²⁺ appeared to be noncompetitive and independent of its effect on membrane permeability.     

An Effect of Ca2+ on the Intrinsic Cl−-conductance of Rat Kidney Cortex Brush Border Membrane Vesicles

Brush-border membrane vesicles (BBMV) were prepared from superficial rat renal cortex by a divalent²⁺-precipitation technique using either CaCl₂ or MgCl₂. The dependence of the initial [¹⁴C]-d-glucose (or [³H]-l-proline) uptake rate and the extent of the overshoot of d-glucose or l-proline uphill accumulation from solutions containing 100 mm Na⁺ salt, was found to be dependent upon the precipitating divalent cation. With Mg²⁺ precipitation the initial uptake and overshoot accumulation of either d-glucose or l-proline were enhanced compared to BBMV prepared by Ca²⁺ precipitation. When the anion composition of the media was varied (uptake in Cl⁻ media in comparison to gluconate⁻-containing media) it was found that the Cl⁻-dependent component of the initial uptake was markedly depressed with Ca²⁺-prepared BBMV (104.99 ± 33.31 vs. 13.83 ± 1.44 pmoles/sec/mg protein for Mg²⁺ and Ca²⁺ prepared vesicles respectively). When Ca²⁺ was loaded into Mg²⁺ prepared BBMV using a freeze-thaw technique, it was found that the magnitude and Cl⁻ enhancement of d-glucose transport was reduced in a dose-dependent manner. Neomycin, an inhibitor of phospholipase C, had no effect on the reduction of d-glucose uptake by Ca²⁺ in Mg²⁺ prepared vesicles. In contrast, phosphatase inhibitors such as vanadate and fluoride were able to partially reverse the Ca²⁺ inhibition of d-glucose uptake and restore the enhancement due to Cl⁻ media. In addition, inhibitors of protein phosphatase 2B, deltamethrin (50 nm) and trifluoperazine (10 μm), caused partial reversal of Ca²⁺-dependent inhibition of d-glucose uptake. Direct measurement of changes in the bi-ionic (Cl⁻ vs. gluconate⁻) transmembrane electrical potential differences using the cyanine dye, 3,3′-dipropylthiodicarbocyanine iodide DiSC₃-(5) confirmed that Cl⁻ conductance was reduced in Ca²⁺-prepared vesicles. We conclude that a Cl⁻ conductance coexists with Na⁺ cotransport in rat renal BBMV and this may be subject to negative regulation by Ca²⁺ via stimulation of protein phosphatase (PP2B). Received: 14 December 1994/Revised: 27 November 1995     

Extra-matrix Mg2+ limits Ca2+ uptake and modulates Ca2+ uptake–independent respiration and redox state in cardiac isolated mitochondria

Cardiac mitochondrial matrix (m) free Ca²⁺ ([Ca²⁺]_m) increases primarily by Ca²⁺ uptake through the Ca²⁺ uniporter (CU). Ca²⁺ uptake via the CU is attenuated by extra-matrix (e) Mg²⁺ ([Mg²⁺]_e). How [Ca²⁺]_m is dynamically modulated by interacting physiological levels of [Ca²⁺]_e and [Mg²⁺]_e and how this interaction alters bioenergetics are not well understood. We postulated that as [Mg²⁺]_e modulates Ca²⁺ uptake via the CU, it also alters bioenergetics in a matrix Ca²⁺–induced and matrix Ca²⁺–independent manner. To test this, we measured changes in [Ca²⁺]_e, [Ca²⁺]_m, [Mg²⁺]_e and [Mg²⁺]_m spectrofluorometrically in guinea pig cardiac mitochondria in response to added CaCl₂ (0–0.6 mM; 1 mM EGTA buffer) with/without added MgCl₂ (0–2 mM). In parallel, we assessed effects of added CaCl₂ and MgCl₂ on NADH, membrane potential (ΔΨ_m), and respiration. We found that >0.125 mM MgCl₂ significantly attenuated CU-mediated Ca²⁺ uptake and [Ca²⁺]_m. Incremental [Mg²⁺]_e did not reduce initial Ca²⁺uptake but attenuated the subsequent slower Ca²⁺ uptake, so that [Ca²⁺]_m remained unaltered over time. Adding CaCl₂ without MgCl₂ to attain a [Ca²⁺]_m from 46 to 221 nM enhanced state 3 NADH oxidation and increased respiration by 15 %; up to 868 nM [Ca²⁺]_m did not additionally enhance NADH oxidation or respiration. Adding MgCl₂ did not increase [Mg²⁺]_m but it altered bioenergetics by its direct effect to decrease Ca²⁺ uptake. However, at a given [Ca²⁺]_m, state 3 respiration was incrementally attenuated, and state 4 respiration enhanced, by higher [Mg²⁺]_e. Thus, [Mg²⁺]_e without a change in [Mg²⁺]_m can modulate bioenergetics independently of CU-mediated Ca²⁺ transport.     

Hexahydrated Mg2+ Binding and Outer-Shell Dehydration on RNA Surface

The interaction between metal ions, especially Mg²⁺ ions, and RNA plays a critical role in RNA folding. Upon binding to RNA, a metal ion that is fully hydrated in bulk solvent can become dehydrated. Here we use molecular dynamics simulation to investigate the dehydration of bound hexahydrated Mg²⁺ ions. We find that a hydrated Mg²⁺ ion in the RNA groove region can involve significant dehydration in the outer hydration shell. The first or innermost hydration shell of the Mg²⁺ ion, however, is retained during the simulation because of the strong ion-water electrostatic attraction. As a result, water-mediated hydrogen bonding remains an important form for Mg²⁺-RNA interaction. Analysis for ions at different binding sites shows that the most pronounced water deficiency relative to the fully hydrated state occurs at a radial distance of around 11 Å from the center of the ion. Based on the independent 200 ns molecular dynamics simulations for three different RNA structures (Protein Data Bank: 1TRA, 2TPK, and 437D), we find that Mg²⁺ ions overwhelmingly dominate over monovalent ions such as Na⁺ and K⁺ in ion-RNA binding. Furthermore, application of the free energy perturbation method leads to a quantitative relationship between the Mg²⁺ dehydration free energy and the local structural environment. We find that $\text{ΔΔ}{G}_{\text{hyd}},$ the change of the Mg²⁺ hydration free energy upon binding to RNA, varies linearly with the inverse distance between the Mg²⁺ ion and the nearby nonbridging oxygen atoms of the phosphate groups, and $\text{ΔΔ}{G}_{\text{hyd}}$ can reach ?2.0 kcal/mol and ?3.0 kcal/mol for an Mg²⁺ ion bound to the surface and to the groove interior, respectively. In addition, the computation results in an analytical formula for the hydration ratio as a function of the average inverse Mg²⁺-O distance. The results here might be useful for further quantitative investigations of ion-RNA interactions in RNA folding.     

Enhancement of yeast ethanol tolerance by calcium and magnesium

Ethanol-induced changes of CO₂ production were compared in three strains ofSaccharomyces cerevisiœ. CaCl₂ and MgCl₂ exerted protective effects against the action of ethanol. Optimal concentrations ensuring maximum of CO₂ production at 10% (V/V) of ethanol under non-growing conditions were 3 mmol/L Ca²⁺ and 2 mmol/L Mg²⁺. Yeast growth with and without ethanol addition was stimulated by Mg²⁺ more than by Ca²⁺ during fermentation, whereas ethanol production was more efificient when both Ca²⁺ and Mg²⁺ were added.     

Water and ion balance in hydrating oocytes of the grey mullet, Mugil cephalus (L.), during hormone-induced final maturation

Sequential changes in water content and inorganic ion concentrations occurring in intra-ovarian oocytes of the grey mullet, Mugil cephalus L., during hormone-induced final maturation are described. Post-vitellogenic oocytes have a water content of 59.4%. During final maturation, oocyte water increases rapidly to a maximum of 84.8% at oviposition. Hydration is accompanied by a relatively small increase in dry matter due to the concurrent uptake of inorganic ions and an apparent uptake of organic matter. Net uptake of Ca²⁺ during hydration is negligible. Net uptake of Mg^{2 +} and Na⁺ occurs in less than equimolar proportions. Net uptake of K⁺ and Cl^? occurs in greater than equimolar proportions prior to ovulation, returning to equimolarity at oviposition. Hence, hydration results in a dilution of intracellular Ca²⁺, Mg²⁺ and Na⁺ to minimum concentrations at oviposition. In contrast, K⁺ and Cl^? concentrations are slightly elevated prior to ovulation, returning to initial levels at oviposition. A 1:1 stoichiometric relationship in net uptake of K ⁺ andCl^? is evident. Intracellular osmotic pressure in post-vitellogenic oocytes is 203.4 ± 18.0 mOsmolkg^?1. Osmotic pressure rises to a peak of 421.3+ 17.3 mOsmol kg^?1 prior to ovulation, then declines to 313.3 ± 15.0 mOsmol kg^?1 at oviposition. This rise in osmotic pressure prior to ovulation cannot be accounted for solely on the basis of inorganic ion uptake and must be attributed to changes in intracellular concentrations of organic solutes as well. An apparent increase in non-ash dry matter during hydration suggests that a net uptake of organic solute is involved. A relatively small organic molecule with high osmotic activity (e.g., amino acids) is implicated.     

Taste responses to divalent cations in the frog glossopharyngeal nerve: competitive inhibition of the Mg2+ response by Ca2+

In the frog glossopharyngeal nerve, single water fibers respondto low CaCl₂ (1–2 mM) and relatively high MgCl₂ (100 mM).In the present study, it was found that stimulation by a mixtureof low CaCl₂ and relatively high MgCl₂ led to a small response.This suggests that the Ca⁺ response is inhibited by the presenceof Mg²⁺ and the Mg²⁺ response is inhibited by the presence ofCa²⁺. Hence, it is suggested that there are different receptorsites for divalent cations in single water fibers of the frogglossopharyngeal nerve, a calcium receptor site (X_Ca) responsiblefor the Ca²⁺ response and a magnesium receptor site (X_Mg) responsiblefor the Mg²⁺ response. It has been reported that Mg²⁺ inhibitsthe Ca²⁺ response by competing with Ca²⁺ for X_Ca (Kitada andShimada, 1980). In the present study, the inhibition of theMg²⁺ response by Ca²⁺ was examined quantitatively under theassumption that the magnitude of the neural response is proportionalto the amount of MgX_Mg complex minus a constant (the thresholdconcentration of the MgX_Mg complex). The results obtained indicatethat Ca²⁺ competes with Mg²⁺ for X^Mg. The apparent dissociationconstants for MgX_Mg complex and CaX_Mg complex, which were obtainedfrom the present study, were 8.0 x 10^–2 M and 7.2 x 10^–4M, respectively. Thus, competition between Ca⁺ and Mg²⁺ forthe distinct receptor sites involved in taste reception wasdemonstrated by the results described in this paper. Since thedivalent cations do not always bring about activation of tastereceptors, the responses to salts in the frog glossopharyngealnerve cannot be explained in terms of changes in the surfacepotential outside the taste cells. The present results suggestthat there exist multiple specific receptor sites for cationsinvolved in salt taste responses, and only the binding of eachseparate cation to its appropriate receptor sites leads to activationof the receptor and the initiation of impulses in sensory nerveendings.     

Enhancing effects of transition metals on the salt taste responses of single fibers of the frog glossopharyngeal nerve: specificity of and similarities among Ca2+, Mg2+ and Na+ taste responses

Fibers of the frog glossopharyngeal nerve (water fibers) thatare sensitive to water also respond to CaCl₂, MgCl₂ and NaCl.In the present study, interaction among cations (Ca²⁺, Mg²⁺and Na⁺) on taste cell membrane in frogs was studied using transitionmetals (NiCl₂, CoCl₂ and MnCl₂), which themselves are barelyeffective in producing neural response at concentrations below5 mM. Unitary discharges from single water fibers were recordedfrom fungiform papillae with suction electrode. Transition metalions (0.05–5.0 mM) had exclusively enhancing effects onthe responses to 50 mM Ca²⁺, 100 mM Mg²⁺ and 500 mM Na⁺. Theeffects of transition metal ions were always reversible. Therank order of effectiveness of transition metals at 1 mM inthe enhancement of the responses to 50 mM CaCl₂, 100 mM MgCl₂and 500 mM NaCl was NiCl₂ > CoCl² > MnCl₂. The concentrationof transition metal ions effective to enhance salt responsewas almost the same among Ca²⁺, Mg²⁺ and Na⁺ responses. Theresults suggest that a common mechanism is involved in the enhancementof Ca²⁺, Mg²⁺ and Na⁺ taste responses. The enhanced Mg²⁺ responseand the enhanced Na⁺ response were greatly inhibited by theaddition of Ca²⁺ ions, and the enhanced Ca²⁺ response was inhibitedby the addition of Mg²⁺ or Na⁺ ions, suggesting that competitiveantagonism occurs between Ca²⁺ and Mg²⁺ ions and between Ca²⁺and Na⁺ ions in the presence of Ni²⁺ ions. Ni²⁺ ions had a dualeffect on the Ca²⁺ response induced by low concentration (0.1mM) of CaCl₂: enhancement at lower concentrations (0.02–0.1mM) of NiCl₂ and inhibition at higher concentrations (0.5–5mM)of NiCl₂. The present results suggest that transition metalions do not affect the receptor-antagonist complex, but affectonly the receptor-agonist complex.