The Decrease on Na+, K+-ATPase Activity in the Cortex, but not in Hippocampus, is Reverted by Antioxidants in an Animal Model of Sepsis

In the present study, we investigated whether sepsis induced by cecal ligation and puncture (CLP) modifies Na⁺, K⁺-ATPase activity, mRNA expression, and cerebral edema in hippocampus and cerebral cortex of rats and if antioxidant (ATX) treatment prevented the alterations induced by sepsis. Rats were subjected to CLP and were divided into three groups: sham; CLP??rats were subjected to CLP without any further treatment; and ATX?CCLP plus administration of N-acetylcysteine plus deferoxamine. Several times (6, 12, and 24) after CLP or sham operation, the rats were killed and hippocampus and cerebral cortex were isolated. Na⁺, K⁺-ATPase activity was inhibited in the hippocampus 24?h after sepsis, and ATX treatment was not able to prevent this inhibition. The Na⁺, K⁺-ATPase activity also was inhibited in cerebral cortex 6, 12, and 24?h after sepsis. No differences on Na⁺, K⁺-ATPase catalytic subunit mRNA levels were found in the hippocampus and cerebral cortex after sepsis. ATX treatment prevents Na⁺, K⁺-ATPase inhibition only in the cerebral cortex. Na⁺, K⁺-ATPase inhibition was not associated to increase brain water content. In conclusion, the present study demonstrated that sepsis induced by CLP inhibits Na⁺, K⁺-ATPase activity in a mechanism dependent on oxidative stress, but this is not associated to increase brain water content.     

(Na++K+)-adenosinetriphosphatase in the brain of Shiverer (Shi/Shi) mice

The myelin-deficient Shiverer (Shi/Shi) mutant mouse may be a useful model in assessing the dependence of brain (Na⁺+K⁺)-ATPase concentration and composition on myelin membrane formation. Brain microsomal membranes from age-matched control (+/+) and Shiverer (Shi/Shi) mice were fractionated by differential centrifugation and sucrose gradient sedimentation. No reduction in (Na⁺+K⁺)-ATPase specific activity was measured in whole homogenates, high-and low-speed fractions or gradient fractions from brains of Shi/Shi mice as compared to those of +/+ mice. In addition, sodium dodecylsulfatepolyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting with antisera specific for mouse brain (Na⁺+K⁺)-ATPase revealed no significant difference in catalytic subunit composition between fractions of +/+ and Shi/Shi brains. The similar results obtained for both +/+ and myelin-deficient Shi/Shi mice suggest that myelin contributes little to total brain (Na⁺+K⁺)-ATPase.     

EFFECT OF HYPOTHYROIDISM ON IONIC METABOLISM AND Na-K ACTIVATED ATP PHOSPHOHYDROLASE ACTIVITY IN THE DEVELOPING RAT BRAIN1

The effects of neonatal hypothyroidism on electrolyte contents and the Na⁺ and K⁺ activated ATPase system was studied in the cerebral cortex and cerebellum of the developing rat. Neonatal hypothyroidism increased Na⁺ and CI^? contents and decreased K⁺ and Mg²⁺ contents in both brain areas. Hypothyroidism also resulted in a decrease in the specific activity of the Na-K ATPase extracted by deoxycholate treatment from brain homogenate as well as in the specific activity of this enzyme in the heavy microsomal fraction. The decrease in Mg²⁺ content and ATPase activity is discussed in relation to the changes occurring in Na⁺ and K⁺. Both enzymic and ionic changes may underlie the biochemical and physiological abnormalities observed when the brain is deprived of thyroxine at critical stages of its development.     

Cold resistance of the brain during hibernation: I. K+ transport in cerebral cortex slices

The brains of the hibernating hamsters and 13-lined ground squirrels maintain Na⁺ and K⁺ at the same concentrations as in the awake state. The ability of slices of the cerebral cortex when incubated in vitro to accumulate or retain K⁺ is similar in the awake hamster and rat at both 38 and 5 ° C. On the other hand, slices of cerebral cortex from the hibernating hamster retained slightly more K⁺ at 5 °C than did those of awake hamster or rat. It was concluded that the cerebral cortex of the awake hamster is probably not cold resistant with respect to the maintenance of cation balance. Further, the cold resistance that exists in the cerebral cortex of the hibernating hamster is largely destroyed when the brain is disrupted by slicing.     

FURTHER STUDIES ON THE K+-DEPENDENT SWELLING OF PRIMATE CEREBRAL CORTEX IN VIVO: THE ENZYMATIC BASIS OF THE K+-DEPENDENT TRANSPORT OF CHLORIDE

Abstract— The swelling of intact, exposed primate cerebral cortex perfused in vioo under, isosmotic conditions was a linear function of the concentration of K⁺ in perfusate over the range 25–117 mM. The K⁺-dependent swelling was manifested throughout the depth of the cerebral cortex studied and was associated with an increased content of chloride in the swollen tissue, despite the constancy of the concentration of external chloride. The swelling of the cerebral cortex was a linear function of the temperature of the perfusate over the range 15–38°C, despite the constancy of the concentration of external K⁺. Moreover, the content of chloride in the swollen cerebral cortex was a linear function of the temperature of the overlying perfusate, despite the constancy of the external concentration of chloride. The changes in the contents of Na⁺ and K⁺ in the swollen cerebral cortex perfused with solutions containing constant concentrations of external Na⁺ and K⁺ but differing in temperature suggested that the fluid of swelling in the tissue was rich in both K⁺ and CI^-, as had been shown previously in vitro. Perfusion of the exposed, intact cerebral cortex in uiuo with K⁺-rich fluids usually involved the reciprocal reduction of the concentrations of Na+ in the perfusate to maintain isotonicity. When comparable reductions in the concentration of external Na⁺ were achieved by replacement with choline (instead of K⁺), swelling of the perfused, exposed cortex was significantly less than that attributed to isotonic, K⁺-rich but Na⁺-poor fluids. These observations suggested that it was the elevated levels of K⁺ rather than lowered concentrations of Na+ that promoted the swelling of the perfused cerebral cortex. The apparent rate of influx of ³⁶Cl from the perfusate into the underlying exposed and intact monkey cerebral cortex in vivo was a linear function of the concentration of K⁺ in perfusate over the range 25–117 mM and conformed to Michaelis-Menten kinetics when plotted according to Lineweaver and Burk. Moreover, the apparent influx of chloride from perfusate into swollen cerebral cortex was a linear function of the percentage swelling of cerebral cortex over the range 6–30 per cent. However, the apparent rate of influx of chloride from perfusate into unswollen cortex was not consistent with the linear correlation already described for swollen cerebral cortex. One reason for this discrepancy was the reduction in the size of the true (inulin) extracellular space associated with the K⁺-dependent swelling of cerebral cortex in vivo. The anatomical locus for this K⁺-dependent swelling of cerebral cortex was an expanded glial compartment, as demonstrated by electron-microscopy. The parenteral administration (50 mg/kg) or local perfusion (5 mM) of acetazolamide inhibited the K⁺-dependent swelling of cerebral cortex in vivo. Moreover, administration of acetazolamide inhibited the K⁺-dependent increase in content of C1- and the K⁺-dependent rate of influx of ³⁶Cl into swollen cerebral cortex. We have discussed the possible enzymatic basis of these K⁺-dependent alterations in content of fluid and chloride and transport of chloride in mammalian cerebral cortex in viuo.     

Quantitative calculation of the role of the Na+,K+-ATPase in thermogenesis

The Na⁺,K⁺-ATPase is accepted as an important source of heat generation (thermogenesis) in animals. Based on information gained on the kinetics of the enzyme's partial reactions we consider via computer simulation whether modifications to the function of the combined Na⁺,K⁺-ATPase/plasma membrane complex system could lead to an increased body temperature, either through the course of evolution or during an individual's lifespan. The enzyme's kinetics must be considered because it is the rate of heat generation which determines body temperature, not simply the amount of heat per enzymatic cycle. The results obtained indicate that a decrease in thermodynamic efficiency of the Na⁺,K⁺-ATPase, which could come about by Na⁺ substituting for K⁺ on the enzyme's extracellular face, could not account for increased thermogenesis. The only feasible mechanisms are an increase in the enzyme's expression level or an increase in its ion pumping activity. The major source of Na⁺,K⁺-ATPase-related thermogenesis (72% of heat production) is found to derive from passive Na⁺ diffusion into the cell, which counterbalances outward Na⁺ pumping to maintain a constant Na⁺ concentration gradient across the membrane. A simultaneous increase in both Na⁺,K⁺-ATPase activity and the membrane's passive Na⁺ permeability could promote a higher body temperature.     

Identification of Electric-Field-Dependent Steps in the Na,K-Pump Cycle

The charge-transporting activity of the Na⁺,K⁺-ATPase depends on its surrounding electric field. To isolate which steps of the enzyme’s reaction cycle involve charge movement, we have investigated the response of the voltage-sensitive fluorescent probe RH421 to interaction of the protein with BTEA (benzyltriethylammonium), which binds from the extracellular medium to the Na⁺,K⁺-ATPase’s transport sites in competition with Na⁺ and K⁺, but is not occluded within the protein. We find that only the occludable ions Na⁺, K⁺, Rb⁺, and Cs⁺ cause a drop in RH421 fluorescence. We conclude that RH421 detects intramembrane electric field strength changes arising from charge transport associated with conformational changes occluding the transported ions within the protein, not the electric fields of the bound ions themselves. This appears at first to conflict with electrophysiological studies suggesting extracellular Na⁺ or K⁺ binding in a high field access channel is a major electrogenic reaction of the Na⁺,K⁺-ATPase. All results can be explained consistently if ion occlusion involves local deformations in the lipid membrane surrounding the protein occurring simultaneously with conformational changes necessary for ion occlusion. The most likely origin of the RH421 fluorescence response is a change in membrane dipole potential caused by membrane deformation.     

Effects of extracts of rat brain on the digitalis receptor

There is a possibility that an endogenous substance exists which interacts with the ouabain binding site on Na⁺, K⁺-ATPase. Recently, several reports have appeared suggesting the presence of an endogenous digitalis-like substance in acid-acetone extracts of brain. We have demonstrated that in preparing an acid-acetone extract, peroxidized lipids and lysophospholipids are produced, both of which inhibit Na⁺, K⁺-ATPase, thereby complicating interpretation. Preliminary evidence suggests, however, that when rat brains are extracted with an $\text{aqueous}$-acetone mixture under nitrogen, a principle is obtained which specifically inhibits Na⁺, K⁺-ATPase.     

Ageing in the honey-bee, Apis mellifera, as related to brain ATPases and their DDT sensitivity

The adenosine triphosphatase (ATPase) system in worker honey-bee brains showed an increased activity of 57 per cent in Na⁺K⁺ATPase and 63 per cent in Mg²⁺ATPase from adult emergence to 7 days post-emergence. Mg²⁺ATPase activity remained about the same throughout the remainder of adult life, while Na⁺K⁺ATPase remained the same until the sixth week, when a decline occurred. The percentage mortality of the bees exceeded 90 per cent at the time of decline of Na⁺K⁺ATPase. The in vitro inhibition of Mg²⁺ATPase and Na⁺K⁺ATPase by 10 μM DDT was between 40 and 50 per cent and about 20 per cent, respectively. A somewhat greater sensitivity to DDT was determined in brains of older honey-bees.     

Effects of ammonium ions on spontaneous action potentials and on contents of sodium, potassium, ammonium and chloride ions in brain in vitro

The effects of ammonium ions on the frequency of spontaneous action potentials in guinea-pig cerebellar slices, recorded with an extracellular microelectrode, and on the contents of sodium, potassium and chloride ions in incubated guinea-pig cerebellar, and rat brain cortex, slices have been investigated. The frequencies of the spontaneous action potentials are partially suppressed by concentrations of NH₄Cl less than 2 mm and completely abolished by concentrations exceeding 2 mm . The amplitudes of the spike discharges are unaffected. A lag period of at least 15 s precedes the inhibition. The suppressing action of NH on the spike frequency is reversible, as shown by complete recovery on removal of NH after short time intervals. Deficiency of Cl^? in the superfusion medium causes conversion of inhibition by NH to excitation. Reduction of [K⁺], or of [Na⁺], causes increase of inhibition by NH in a normal [Cl¹], and reduction of excitation in a low [Cl¹], medium. The inhibitory effects of NH on spike frequency are unaffected by picrotoxin or strychnine. NH₄Cl, even at 1 or 2 mm , causes a significant increase of aerobic glycolysis. It is suggested that the lag period preceding the suppression of the frequency of spike discharges by NH is partly due to a metabolic change induced by NH, perhaps a transient lowering of pH in the responsible neurons, causing changed permeability to Cl^? and possibly to K⁺ and Na⁺, NH promotes, in guinea-pig cerebellar slices, an inward flow of Na⁺ and an outward flow of K⁺, the latter being greater than that due to exchange of K⁺ for NH. NH₄Cl at 1 or 2 mm causes an outward flow of K⁺ and an inward flow of Cl^? in rat brain cortex slices. The movement of Cl^? is biphasic, the first phase, seen with low [NH], consisting of an increase of tissue content of Cl^? with little or no fluid uptake and a second phase, seen with high (> 5 mm ) concentrations of NH, in which the uptake of Cl^? is directly proportional to the fluid uptake. It is suggested that the first phase is largely neuronal in location whilst the second is largely glial. In infant rat brain cortex slices, there seems to be predominantly an equal exchange of NH for K^+. There is little evidence of energy assisted concentrative uptake of NH by brain slices and this is thought to be due largely to the rapid diffusion of undissociated NH₃ across cell membranes. It is suggested that some NH (amounting to about 2 mequiv/1) may be bound in the brain. It is concluded that changes in ionic permeabilities, particularly that of Cl^?, partly due to a metabolic action, may be responsible for some of the acute cerebral effects of NH administration.     

Different domain organization of two main conformational states of Na<Superscript>+</Superscript>/K<Superscript>+</Superscript>-ATPase

The Na⁺/K⁺-ATPase generates an electrochemical gradient of Na⁺ and K⁺, which is necessary for the functioning of animal cells. During the catalytic act, the enzyme passes through two principal conformational states, E₁ and E₂. To assess the domain organization of the protein in these conformations, thermal denaturation of Na⁺/K⁺-ATPases from duck salt gland and from rabbit kidney has been studied in the absence and in the presence of Na⁺ or K⁺, which induce the transition to E₁ or E₂. The melting curves for the ion-free forms of the two ATPases have different shapes: the rabbit protein shows one transition at 56.1°C, whereas the duck protein shows two transitions, at 49.8 and 56.9°C. Addition of Na⁺ or K⁺ ions abolishes the difference in thermal behavior between these enzymes, but through opposite effects. The melting curves for the E₂ conformation (K⁺ bound) in both cases exhibit a single peak of heat absorption at ∼63°C. For the E₁ conformation (Na⁺ bound), each melting curve has three peaks, indicating denaturation of three domains. The difference in the domain organization of Na⁺/K⁺-ATPase in the E1 and E2 states may account for the different sensitivity to temperature, proteolysis, and oxidative stress observed for the two enzyme conformations.     

The competition transport of sodium ions and protons in the cytoplasmic access channel of the Na+,K+-ATPase

The changes in capacitance and conductance of lipid bilayer membranes have been studied with adsorbed membrane fragments containing Na⁺,K⁺-ATPase. These changes have been initiated by fast release of protons from a bound form (“caged H⁺”) induced by an UV flash. The changes of the capacitance in the presence of Na⁺,K⁺-ATPase were affected by the frequency of the applied voltage, pH and the concentration of sodium ions. Addition of sodium ions altered the changes of capacitance caused by a pH jump in the medium due to caged H⁺ photolysis, and the magnitude and sign of this effect depended on the initial pH. These results are explained by competitive binding of sodium ions and protons to the ion-binding sites of the Na⁺,K⁺-ATPase at its cytoplasmic side. The pH at which the sign of the sodium ion effect changed allows the evaluation of the pK of the proton binding site, which is about 7.6.     

Sensitivity of cloned muscle,heart and neuronal voltage-gated sodium channels to block by polyamines

Spermidine and spermine, are endogenous polyamines (PAs) that regulate cell growth and modulate the activity of numerous ion channel proteins. In particular, intracellular PAs are potent blockers of many different cation channels and are responsible for strong suppression of outward K⁺ current, a phenomenon known as inward rectification characteristic of a major class of K_IR K⁺ channels. We previously described block of heterologously expressed voltage-gated Na⁺ channels (Na_V) of rat muscle by intracellular PAs and PAs have recently been found to modulate excitability of brain neocortical neurons by blocking neuronal Na_V channels. In this study, we compared the sensitivity of four different cloned mammalian Na_V isoforms to PAs to investigate whether PA block is a common feature of Na_V channel pharmacology. We find that outward Na⁺ current of muscle (Na_V1.4), heart (Na_V1.5), and neuronal (Na_V1.2, Na_V1.7) Na_V isoforms is blocked by PAs, suggesting that PA metabolism may be linked to modulation of action potential firing in numerous excitable tissues. Interestingly, the cardiac Na_V1.5 channel is more sensitive to PA block than other isoforms. Our results also indicate that rapid binding of PAs to blocking sites in the Na_V1.4 channel is restricted to access from the cytoplasmic side of the channel, but plasma membrane transport pathways for PA uptake may contribute to long-term Na_V channel modulation. PAs may also play a role in drug interactions since spermine attenuates the use-dependent effect of the lidocaine, a typical local anesthetic and anti-arrhythmic drug.     

The High Energy Demand of Neuronal Cells Caused by Passive Leak Currents is Not a Waste of Energy

It is estimated that maintenance of the resting potential of neurons consumes between 15 % (in gray matter) and 44 % (in fully myelinated white matter) of the brain’s total energy budget [1]. This poses the intriguing question why evolution has not strived to lower the permeability of passive ion channels to cut the high resting-state energy budget of the brain. Based on a conceptual mathematical model of neuronal ion currents and action potential (AP) firing we demonstrate that a neuron endowed with small leak currents and correspondingly low energy consumption by the Na⁺/K⁺-ATPase in the resting state may indeed recapitulate all features of normal AP firing. However, the activation and inactivation of such a “low-energy-cost neuron” turns out to be extremely sensitive to small fluctuation of Na⁺ currents associated with Na⁺-dependent secondary-active transport that is indispensable for the metabolic integrity of the cell and neurotransmitter recycling. We provide evidence that sufficiently large leak currents function as important stabilizers of the membrane potential and thus are required to allow robust AP firing. Our simulations suggest that the energy demand of the Na⁺/K⁺-ATPase needed to counterbalance passive leak currents cannot be significantly dropped below observed values.     

Evidence for frequency-dependent extracellular impedance from the transfer function between extracellular and intracellular potentials

Dopaminergic (DA) neurons of the mammalian midbrain exhibit unusually low firing frequencies in vitro. Furthermore, injection of depolarizing current induces depolarization block before high frequencies are achieved. The maximum steady and transient rates are about 10 and 20 Hz, respectively, despite the ability of these neurons to generate bursts at higher frequencies in vivo. We use a three-compartment model calibrated to reproduce DA neuron responses to several pharmacological manipulations to uncover mechanisms of frequency limitation. The model exhibits a slow oscillatory potential (SOP) dependent on the interplay between the L-type Ca²⁺ current and the small conductance K⁺ (SK) current that is unmasked by fast Na⁺ current block. Contrary to previous theoretical work, the SOP does not pace the steady spiking frequency in our model. The main currents that determine the spontaneous firing frequency are the subthreshold L-type Ca²⁺ and the A-type K⁺ currents. The model identifies the channel densities for the fast Na⁺ and the delayed rectifier K⁺ currents as critical parameters limiting the maximal steady frequency evoked by a depolarizing pulse. We hypothesize that the low maximal steady frequencies result from a low safety factor for action potential generation. In the model, the rate of Ca²⁺ accumulation in the distal dendrites controls the transient initial frequency in response to a depolarizing pulse. Similar results are obtained when the same model parameters are used in a multi-compartmental model with a realistic reconstructed morphology, indicating that the salient contributions of the dendritic architecture have been captured by the simpler model.     

Physicochemical approaches to the alcohol-membrane interaction in brain

The effects of ethanol on physicochemical and enzymatic perturbations of neuronal membranes were examined. Using synaptic plasma membrane (SPM) isolated from cerebral cortex of Sprague-Dawley rats, a biphasic mode of action for ethanol was observed with (Na⁺+K⁺)-ATPase, but not with Ca²⁺-ATPase or acetylcholinesterase. (Na⁺+K⁺)-ATPase was found to be more sensitive to low concentration of sodium deoxycholate treatment than Ca²⁺-ATPase. A sharp transition break of (Na⁺+K⁺)-ATPase activity in response to temperature changes was found with SPM preparation. Arrhenius plots of the response also indicated that (Na⁺+K⁺)-ATPase is more sensitive to temperature changes than Ca²⁺-ATPase. The fluorescence polarization of TNS-membrane complex decreases as ethanol concentration increases, indicating an increase in membrane fluidity. However, ethanol, at low concentration (<0.3%) appears to elevate TNS fluorescence, but a hhigher concentration (3%) ethanol tends to lower the intensity of maximal emission. The results of this study indicate that ethanol may interact with the synaptic plasma membranes and elicit specific biochemical responses depending on the concentration of the alcohol used.     

Effect of tissue specificity of brain soluble fractions on Na+, K+-ATPase activity

Previous evidence from this laboratory indicated that catecholamines and brain endogenous factors modulate Na⁺, K⁺-ATPase activity of the synaptosomal membranes. The filtration of a brain total soluble fraction through Sephadex G-50 permitted the separation of two fractions-peaks I and II-which stimulated and inhibited Na⁺, K⁺-ATPase, respectively (Rodríguez de Lores Arnaiz and Antonelli de Gomez de Lima, Neurochem. Res.11, 1986, 933). In order to study tissue specificity a rat kidney total soluble was fractionated in Sephadex G-50 and kidney peak I and II fractions were separated; as control, a total soluble fraction prepared from rat cerebral cortex was also processed. The UV absorbance profile of the kidney total soluble showed two zones and was similar to the profile of the brain total soluble. Synaptosomal membranes Na⁺, K⁺- and Mg²⁺-ATPases were stimulated 60–100% in the presence of kidney and cerebral cortex peak I; Na⁺, K⁺-ATPase was inhibited 35–65% by kidney peak II and 60–80% by brain peak II. Mg²⁺-ATPase activity was not modified by peak II fractions. ATPases activity of a kidney crude microsomal fraction was not modified by kidney peak I or brain peak II, and was slightly increased by kidney peak II or brain peak I. Kidney purified Na⁺, K⁺-ATPase was increased 16–20% by brain peak I and II fractions. These findings indicate that modulatory factors of ATPase activity are not exclusive to the brain. On the contrary, there might be tissue specificity with respect to the enzyme source.     

Sensitivity of cloned muscle,heart and neuronal voltage-gated sodium channels to block by polyamines: A possible basis for modulation of excitability in vivo

Spermidine and spermine, are endogenous polyamines (PAs) that regulate cell growth and modulate the activity of numerous ion channel proteins. In particular, intracellular PAs are potent blockers of many different cation channels and are responsible for strong suppression of outward K⁺ current, a phenomenon known as inward rectification characteristic of a major class of K_IR K⁺ channels. We previously described block of heterologously expressed voltage-gated Na⁺ channels (Na_V) of rat muscle by intracellular PAs and PAs have recently been found to modulate excitability of brain neocortical neurons by blocking neuronal Na_V channels. In this study, we compared the sensitivity of four different cloned mammalian Na_V isoforms to PAs to investigate whether PA block is a common feature of Na_V channel pharmacology. We find that outward Na⁺ current of muscle (Na_V1.4), heart (Na_V1.5), and neuronal (Na_V1.2, Na_V1.7) Na_V isoforms is blocked by PAs, suggesting that PA metabolism may be linked to modulation of action potential firing in numerous excitable tissues. Interestingly, the cardiac Na_V1.5 channel is more sensitive to PA block than other isoforms. Our results also indicate that rapid binding of PAs to blocking sites in the Na_V1.4 channel is restricted to access from the cytoplasmic side of the channel, but plasma membrane transport pathways for PA uptake may contribute to long-term Na_V channel modulation. PAs may also play a role in drug interactions since spermine attenuates the use-dependent effect of the lidocaine, a typical local anesthetic and anti-arrhythmic drug.     

Growth,ion composition,and stomatal conductance of peas exposed to salinity

Availability of irrigation water of appropriate quality is becoming critical in many regions. Excess salt in irrigation water represents a risk for crop yield, crop quality, and soil properties. During the short vegetation period, field peas require high amounts of water, and irrigation is often indispensable for successful production. Steady presence of NaCl (0.1, 0.2, 0.6 or 1.2 g NaCl L⁻¹ in 1/2 strength Hoagland nutrient solution) under semi-controlled conditions reduced growth and resulted in shorter vegetation. Disturbances in the peas’ water regime were provoked by NaCl, as water content in pea tissues was reduced and stomatal density and stomatal diffusive resistance increased in the presence of higher NaCl concentrations. Concentration of Na⁺ increased in all pea tissues with increased NaCl concentration in the nutrient medium. In the presence of NaCl, concentrations of K⁺, Ca²⁺ and P_i increased in roots, stems and leaves, and decreased and in pods and grains. Concentration ratios Na⁺/K⁺, Na⁺/Ca²⁺, K⁺/Ca²⁺ and (Na⁺+K⁺)/Ca²⁺ in various plant parts were affected as well, but magnitudes of changes were variable. Continuous presence of NaCl in concentrations frequently met in irrigation waters significantly reduced pea growth, impaired the water regime, and altered plant chemical composition.     

The Mechanism of Na+/K+ Selectivity in Mammalian Voltage-Gated Sodium Channels Based on Molecular Dynamics Simulation

Voltage-gated sodium (Na_v) channels and their Na⁺/K⁺ selectivity are of great importance in the mammalian neuronal signaling. According to mutational analysis, the Na⁺/K⁺ selectivity in mammalian Na_v channels is mainly determined by the Lys and Asp/Glu residues located at the constriction site within the selectivity filter. Despite successful molecular dynamics simulations conducted on the prokaryotic Na_v channels, the lack of Lys at the constriction site of prokaryotic Na_v channels limits how much can be learned about the Na⁺/K⁺ selectivity in mammalian Na_v channels. In this work, we modeled the mammalian Na_v channel by mutating the key residues at the constriction site in a prokaryotic Na_v channel (Na_vRh) to its mammalian counterpart. By simulating the mutant structure, we found that the Na⁺ preference in mammalian Na_v channels is collaboratively achieved by the deselection from Lys and the selection from Asp/Glu within the constriction site.