The study on the mechanism of G intolerance of rabbits after simulated weightlessness

The direct consequence of cardiovascular adaptation to weightlessness (WL) is the decrease of G tolerance. In studying the mechanism of G intolerance after WL, respiration, heart rate, electrocardiogram, temporal arterial flow, loss of vision were usually used as the indices for evaluation of G tolerance. However the changes of microcirculation and blood rheological indices were seldom observed. Considering that the changes of status of blood circulation after WL may be one of the important factors causing decrease of G tolerance, the purpose of this paper is to observe the changes of microcirculation, blood rheological and the structure and circulatory status of four organs in rabbits during -4Gx after exposure to simulated weightlessness (SWL), in order to understand the cause of G intolerance after WL.     

Effect of increased acceleration on regional pleural pressure in dogs

The variation of pleural pressure was measured in anesthetized spontaneously breathing dogs subjected to increased acceleration (0-4 G) in a centrifuge. Two groups of animals were studied. In one group, the resultant acceleration was in a direction either ventral-to-dorsal (+Gx) or dorsal-to-ventral (-Gx), with a relatively small residual cranial-to-caudal acceleration. In the other group, the resultant acceleration was either cranial-to-caudal (+Gz) or caudal-to-cranial (-Gz), with a relatively small residual dorsal-to-ventral acceleration. Pleural liquid pressure (Ppl) was measured by two rib capsules that were separated by 7-9 cm and oriented either in the dorsal-to-ventral or cranial-to-caudal direction. At functional residual capacity, Ppl in the nondependent lung region became more negative when the acceleration was in the +Gx or +Gz direction. Thus the lung would be susceptible to damage that results from overexpansion in these acceleration directions. By contrast, acceleration in the -Gx or -Gz direction produced values of Ppl at functional residual capacity that were positive. Thus, in these acceleration directions, the respiratory muscles must provide greater force during inspiration to overcome lung compression before lung ventilation can occur. The Ppl gradients with respect to the acceleration directions increased approximately in proportion to acceleration in the +Gx, -Gx, and -Gz directions but remained relatively constant in the +Gz direction.     

The G-tolerance after pharmacological hypohydration

A group of 18 healthy male subjects (25-45 y.o.) participated in the studies of +8.3 Gx tolerance (profile of ballistic reentry) and +Gz tolerance (up to physiological limit) before and after pharmacological hypohydration of organism. Moderate hypohydration was carried out by one 40 mg Furosemid (Lasix) dose, or by pharmacological complex: Furosemid (40 mg) + Hypothiazide (25 mg) + Triamteren (50 mg). Subjects removed with urine about 2 l of water, that was accompanied by increase excretion of electrolytes and caused reduction of the body mass by 2.0 +/- 0.2% and diminution of the plasma volume by 13.5% +/- 2.0%. Pharmacological hypohydration does not lead to a substantial loss in +Gx tolerance. No pathological signs, limiting +Gx tolerance up to 8.3 g were found. The longitudinal +Gz tolerance was less, than +Gx one. The limit of achieved +Gz loads after diuretics reduced by 0.6 g. The scientific and applied significance of this research is that diuretics can be safety used in pre-launch period to prevent the negative effects associated with initial phase of space flight. Besides the preliminary intake of diuretics promotes to diminish of discomfort of cosmonauts, reducing urination in a period of prelaunch waiting in spacecraft.     

重复-Gx加速度暴露对家兔心脏结构的影响*

目的: 重复-Gx加速度暴露对新西兰家兔心脏结构的影响。方法: 20只新西兰家兔随机分为2组(n=10):对照组、-Gx加速度暴露组。-3.6 Gx暴露2 s,间隔5 min,每天重复20次,共30 d;对照组不受加速度作用。末次-Gx加速度暴露后,用静脉注射空气法处死动物,迅速取左心室心肌组织,常规取样并采用光学显微镜及透射电镜进行组织学观察。结果: -Gx加速度暴露组家兔心肌切片在光学显微镜下可见心肌细胞的形态及排列等与对照组无显著区别;-Gx加速度暴露组家兔心肌在透射电镜下可见,心肌纤维断裂、排列紊乱、心肌细胞水肿、核膜扩张、血管内皮基膜分离。结论: -Gx加速度暴露可造成家兔心肌细胞超微结构损伤,提示应重视长期重复-Gx加速度暴露对舰载战斗机飞行员心脏功能的影响和防护。     

LBNP and push-pull effect

The high level of the long lasting acceleration with both an extensive gradient of the acceleration onset and vector alterations belong to the flight specificity in a cockpit of agile aircraft with high manoeuvring capabilities. In these cases the so-called push-pull (PP) effect has its particular significance. This means the tolerance decrease to the plus gravitational acceleration turns up after an exposure to the microgravity on the acceleration level between zero and +0.8 Gz or after previous action of negative acceleration values. The resulting activation of the sympathicus during plus gravitational acceleration process is up to several seconds behind time and pilot's performance is reduced. At the same time both an impairment of the threshold of the vision failure and G-LOC occur. The gravitational acceleration fatigue effect escalates, too.     

Compensatory and adaptive rearrangement in the human respiratory system under acute hypoxic conditions

Relationships between the parameters of external respiration (minute volume and respiration rate) and those of internal, tissue respiration (oxygen consumption, arteriovenous oxygen difference and efficiency of oxygen uptake) were studied during a period of acute hypoxia and upon its completion. The subjects were exposed to hypoxia for 25 min using oxygen-nitrogen hypoxic gas mixtures (HGMs) differing in oxygen content (8 and 12%, HGM-8 and HGM-12, respectively). From the third to the fifth minutes of exposure to HGM-8, the respiration minute volume (RMV) was found to increase by 51 ± 33% as compared to the background value; however, the body’s oxygen consumption (OC) was 35 ± 22% reduced. Afterwards, OC grew to reach, from the 20th to the 25th min of hypoxia, 108 ± 21% of the background value and 181% of the value determined from the third to the fifth minutes of hypoxia. OC growth was accompanied by an insignificant RMV increase (by 12%) as compared to the level determined from the third to the fifth minutes of hypoxia, whereas the efficiency of oxygen uptake from the arterial blood increased by 75% for the same period. RMV growth from the third to the fifth minutes of hypoxia occurred as expense result of a higher breathing depth; at the same time, the respiration rate decreased as compared to the background value. By the period from the 20th to the 25th min of exposure to HGM-8, the respiration rate increased by 21% as compared to the period from the third to the fifth minutes of hypoxia. The efficiency of oxygen uptake from the arterial blood remained higher than the background value for at least 5 min after completion of the exposure to HGM-8. During the same period, the ventilation equivalent, an indicator of the efficiency of external respiration, i.e., of oxygen supply to the body, was significantly lower than the background value. During the exposure to HGM-12, RMV increased to a lesser extent than on exposure to HGM-8, however, the efficiency of oxygen uptake was higher during exposure to HGM-12; therefore, OC was also higher in the latter case. Therefore, the assumption that, during hypoxia, intensified external respiration (ventilatory response) itself compensates oxygen deficiency in inhaled air is revised. Ventilatory response is only a portion of the entire functional system of respiration (both external and tissue respiration). The role of ventilatory response is important for conditioning the tissue respiration rearrangement to eliminate deficiency of oxygen consumption during hypoxia. The retained higher oxygen uptake from the arterial blood during the period after completion of hypoxic treatment testifies to the adaptive implication of changes in tissue respiration; the same is confirmed by a reduced ventilation equivalent after hypoxia, which is indicative of the growing efficiency of external respiration, i.e., of an improved oxygen supply to the body.     

A new evaluation method for +Gz tolerance with loratadine by using a near-infrared spectroscopy

Background

Loratadine (Claritin^®), an over the counter antihistamine in U.S. and UK, is acceptable for use without adverse side effects by aircrew with mild or moderate allergic or other situations requiring an antihistamine. Although +Gz (head to foot direction) tolerance testing for aircrew with loratadine has not been documented in the published literature, it is commonly accepted that loratadine dose not effect +Gz tolerance. The purpose of this study was to offer and validate a new evaluation method for +Gz tolerance testing with loratadine by using a near-infrared spectroscopy (NIRS).

Methods

A double-blind, placebo-controlled, randomized, crossover protocol was used to administer 10 mg of loratadine or placebo in nine healthy subjects. The subjects didn't wear anti-G suit. The +Gz exposure profiles consisted of, in series, a gradual onset ran (0.1 G·sec^-1) to the subject's visual end-point (peripheral light loss) or loss of consciousness (GLOC), and rapid onset run (1.0 G·sec^-1) to the subject's same end-point. In this study, G-level tolerance was defined as the +Gz level at visual end-point and/or at GLOC. As a subject's G-duration tolerance, we measured the total time (seconds) during rapid onset run. Otherwise, to confirm the effect of loratadine on +Gz tolerance, we measured the cerebral NIRS variables (hemoglobin concentration changes and tissue oxygenation index) as a new quantitative method for +Gz tolerance during a centrifuge experiments.

Results

No significant differences were observed in +Gz tolerance (+Gz level, duration time and NIRS variables) between subjects taking loratadine and placebo.

Conclusion

Our results demonstrate that loratadine has no detectable effect on +Gz tolerance by using a new method with cerebral NIRS variables and the traditional method with +Gz level and duration time. This study represents the first use of a quantitative parameter such as cerebral NIRS variables to assess the effects of a drug on acceleration tolerance.

     

Patterns of Root Respiration Associated with the Induction of Aluminium Tolerance in Phaseolus vulgaris L.

Aluminium (Al) tolerance in an Al-tolerant cultivar of Phaseolusvulgaris L. (‘Dade’) was found to be an inducibletrait. Upon exposure to 10 µM Al, the rate of root elongationwas inhibited in comparison to controls. During the following72 h, the rate of elongation returned to levels comparable tocontrols. In contrast, root elongation of an Al-sensitive cultivar(‘Romano’) did not recover after exposure to Al.In Dade, the resumption of root elongation following exposureto Al was accompanied by increased rates of root respiration,whereas respiration rates slowly declined over the 72 h treatmentperiod in Romano. When partitioned into growth and maintenanceexpenditures, a larger proportion of root respiration of Dadeexposed to Al was allocated to maintenance processes, potentiallyreflecting diversion of energy to metabolic pathways that offsetthe adverse effects of Al toxicity. Romano did not show sucha pattern and respiration associated with both growth and maintenancewas reduced after exposure to Al. Root and shoot growth of bothcultivars were also measured to determine the effects of long-term(21 d) exposure to 10 µM Al. Dade plants exposed to Alexhibited enhanced growth in comparison to controls, whereasRomano plants were characterized by reduced shoot and root growth.Modelling the time-course of root respiration and measuringthe long-term growth responses to Al is a valuable method ofelucidating respiratory costs of stress tolerance. Key words: Aluminium, differential tolerance, maintenance respiration, Phaseolus vulgaris, root respiration     

Brief exposure to -2 Gz reduces cerebral blood flow velocity during subsequent +2 Gz acceleration

In order to determine the implication of the cerebral vasoconstriction occurring under -Gz acceleration in the mechanism of the push-pull effect, four healthy male non-pilots were submitted to a control centrifugation at +2 Gz, and then to an experimental run with identical +2 Gz plateau, but preceded by -2 Gz exposure. Cerebral blood flow velocity (CBFV), pulsatility index, and resistance index (RI) were continually measured with a transcranial Doppler instrument. The decrease in blood pressure and in CBFV was more important during the experimental run, when the change in RI was not different. We concluded that the cerebral vasoconstriction occurring under -2 Gz exposure seems not to be a major contributor in the mechanism of the push-pull effect appearing during subsequent +2 Gz acceleration.     

Continuous culture algal bioassays for organic pollutants in aquatic ecosystems

Short-term responses of phytoplankton to organic pollutants are highly transitory. Time-course studies of non-steady state cells in continuous culture showed varying growth or photosynthetic responses such as enchancement, inhibition, adaptation (or development of resistance) or rebound, depending on the direction of changes in the intracellular toxicant concentration and the duration of exposure. However, steady-state cells in a two-stage chemostat system exhibited an increased tolerance to toxicants and subtle physiological effects such as photosynthetic enhancement which was accompanied by a considerable leakage of photosynthesates. It is important to understand such steady-state responses for the prediction and assessment of ecological impact by organic pollution on phytoplankton, since the time scale of changes in the toxicant/biomass ratio in most natural waters is long enough to approximate an equilibrium state.     

The Role of External Potassium Ion in Activation of Sea Urchin Spermatozoa

When sperm of the sea urchin, Hemicentrotus pulcherrimus, are diluted into K⁺-free seawater, the pH of the suspension gradually decreases, whereas a rapid decline in pH is observed following dilution into regular seawater. Sperm motility and respiration are also activated after dilution into K⁺-free seawater, but levels of activity are less than those observed following dilution into regular seawater. Upon addition of 10 mM K⁺ to K⁺-free seawater, rapid acid release occurs and motility and respiratory rate in sperm are reactivated. The effect of K⁺ on respiration was competitive with respect to the external Na⁺ concentrations. Harmaline, a potent inhibitor of Na⁺/K⁺-ATPase, causes a decrease in movement and respiration of the sperm. Harmaline does not inhibit the rapid decline in pH, although it depresses the release of acid from mitochondria. These results suggest that external K⁺ plays an important role in intracellular alkalinization of sea urchin sperm.     

Cold tolerance and cold hardening strategy of the Japanese pine sawyer Monochamus alternatus (Coleoptera: Cerambycidae)

The Japanese pine sawyer, Monochamus alternatus , is an important pine forest pest and vector transmitting the pine wilt nematode that causes pine wilt disease. Low temperatures in autumn, winter and spring often differentially affect mortality of M. alternatus larvae. In this paper, we mainly compared the differences of mortality and cold hardening of larvae from different seasons, based on supercooling point (SCP) and cumulative probability of individuals freezing (CPIF). The cold hardening of the larvae from autumn, winter and spring seasons were largely different. Correlations between mortality and CPIF of autumn and spring larvae were highest on day 1/4, and gradually decreased with prolonged exposure duration. This beetle's death mainly resulted from freezing in short exposure duration. However, the correlation between mortality and CPIF of winter larvae increased gradually with the prolonged exposure duration. Death did not mainly result from freezing in long exposure duration. Autumn larvae are more susceptible and adaptable than winter and spring larvae. Winter larvae have a slight freeze-tolerance trend. Our research showed that M. alternatus came into complex cold-hardening strategies under natural selection. Freeze avoidance is the primary strategy; with prolonged exposure duration to above SCP or < 0 °C, chill tolerance is more important; this is followed by freeze tolerance during harsh winters.     

Isolation, characterization and metal-ion-binding properties of the alpha-subunit from S-100a protein.

The present experiments were undertaken to investigate the role of the phosphoinositides phosphatidylinositol 4-phosphate (PtdIns-4-P) and phosphatidylinositol 4,5-biphosphate (PtdIns-4,5-P2) in the alpha 1-adrenergic stimulation of respiration in isolated hamster brown adipocytes. Exposure of isolated brown adipocytes to the alpha-adrenergic-receptor agonist phenylephrine provoked a breakdown of 30-50% of the PtdIns-4-P and PtdIns-4,5-P2 after prelabelling of the cells with [32P]Pi. Coincident with the breakdown of phosphoinositides was an accumulation of labelled phosphatidic acid, which continued for the duration of the cell incubation. The time course of phosphoinositide breakdown was defined more precisely by pulse-chase experiments. Under these conditions, phenylephrine caused radioactivity in phosphatidylinositol, PtdIns-4-P and PtdIns-4,5-P2 to fall by more than 50% within 30 s and to remain at the depressed value for the duration of the incubation (10 min). This phospholipid response to alpha-adrenergic stimulation was blocked by exposure of the cells to phorbol 12-myristate 13-acetate (PMA); likewise phenylephrine stimulation of respiration was prevented by PMA. beta-Adrenergic stimulation of respiration and inhibition of respiration by 2-chloroadenosine and insulin were, however, unaffected by treatment with PMA. On the assumption that PMA is acting in these cells as an activator of protein kinase C, these results suggest the selective interruption of alpha-adrenergic actions in brown adipocytes by activated protein kinase C. These findings suggest that breakdown of phosphoinositides is an early event in alpha-adrenergic stimulation of brown adipocytes which may be important for the subsequent stimulation of respiration. The results from the pulse-chase studies also suggest, however, that phenylephrine-stimulated breakdown of inositol phospholipids is a short-lived event which does not appear to persist for the entire period of exposure to the alpha 1-adrenergic ligand.     

The effect of respiratory activity on brain blood flow during exposure to +Gz acceleration

The effect of respiratory activity on cardiovascular system during the exposure to +Gz acceleration was studied. For simultaneous, beat to beat evaluation of central and global cerebral blood flow, the system based on modified impedance cardiography method was used. The mechanism of modulation of the central and peripheral (cerebral) venous pressures and blood flow by intrathoracic pressure changes, has been shown. The data obtained from these studies suggest that reconsideration of the respiration technique during Anti-GSM seems indispensable.     

Mitochondrial bioenergetics during the initiation of mercuric chloride-induced renal injury. I. Direct effects of in vitro mercuric chloride on renal mitochondrial function

Increasing data suggest that mitochondrial dysfunction may be an important early component of nephrotoxin-induced changes in renal cell function and viability. This study was designed to obtain more detailed information about the effects on several basic bioenergetic parameters of the direct interaction of Hg2+ with renal cortical mitochondria in vitro as a necessary prelude to studies of mitochondrial functional changes after treatment with mercuric chloride in vivo. Beginning at a threshold level of 2 nmol of Hg2+/mg of mitochondrial protein, Hg2+ induced marked stimulation of State 4 respiration, mild inhibition of State 3 respiration, and 2,4-dinitrophenol uncoupled respiration, a striking increase in atractyloside-insensitive ADP uptake and stimulation of both basal- and Mg2+-activated oligomycin-sensitive mitochondrial ATPase activity. These effects of Hg2+ could be prevented and reversed by the sulfhydryl reagent dithioerythritol and by albumin but were not affected by Mg2+. Detailed studies on the addition of HgCl2 to the preparation at different stages of the mitochondrial isolation procedure demonstrated that the presence of other proteins decreased mitochondrial Hg2+ binding, that the Hg2+ was not readily washed off the mitochondria by nonprotein-containing solutions, and that prolonged exposure of mitochondria to Hg2+ during the isolation procedure did not markedly alter its functional effects or their reversibility as assessed on the final mitochondrial preparation. These data provide an important basis for critically assessing the changes in function of mitochondria isolated after in vivo treatment with mercuric chloride.     

Influence of G-suit abdominal bladder inflation on gas exchange during +GZ stress

Available data relating duration of +GZ stress to blood gas exchange status is limited. Furthermore, studies focusing on pulmonary gas exchange during +GZ stress when abdominal restriction is imposed have yielded conflicting results. To examine the time course of blood gas changes occurring during exposure to +GZ stress in dogs and the influence of G-suit abdominal bladder inflation on this time course, seven spontaneously breathing pentobarbital-anesthetized adult mongrel dogs were exposed to 60 s of up to +5 GZ stress with and without G-suit abdominal bladder inflation. Arterial and mixed venous blood were sampled for blood gas analysis during the first and last 20 s of the exposure and at 3 min postexposure. Little change in blood gas status was seen at +3 GZ regardless of G-suit status. However, with G-suit inflation, arterial PO2 fell by a mean of 14.7 Torr during the first 20 s at +4 Gz (P less than 0.01, t test) and 20.6 Torr at +5 GZ (P less than 0.01). It continued to fall an additional 10 Torr during the next 40 s at both +4 and +5 GZ. Arterial PO2 was still 5-10 Torr below control values (P less than 0.05) 3 min postexposure. A second series of experiments paralleling the first focused on blood gas status during repeated exposure to acceleration. Blood gas status was assessed in five dogs during the late 20 s of two 60-s exposures separated by 3 min at 0 GZ. No significant differences between the initial and repeated exposures were detected. The data indicate that G-suit abdominal bladder inflation promotes increased venous admixture.     

The regulation of immunological responses to parasitic infections and the development of tolerance.

It is well established that specific unresponsiveness to immunization can be induced by prolonged exposure to antigenic proteins. More generally, many parasitic infections, such as the helminth worms and the Leishmania parasites, appear to be able to persist in some of their human hosts over long periods of time, via what appears to be an ability to induce defective or inappropriate T-cell responses (= tolerance). Recent research has suggested that cytokines, produced by specific subsets of CD4+ T-cells (characterized by cytokine secretory profiles and growth properties), have an important, and often complex, role in promoting or inhibiting host protective immunity to parasitic infections. By examination of the population dynamics of the stimulation and regulation of cellular responses to infection, via the use of simple mathematical models, we show that nonlinear interactions between CD4+ T-cell subsets and their secreted cytokines can result in either host protection or immunological unresponsiveness, depending on the magnitude and duration of exposure to parasitic infection. Analyses also identify a possible mechanism to explain the stimulation of two separate peaks of enhanced T-cell-mediated responses over a wide range of levels of antigenic exposure.     

The effects of cold stress on respiration of diaphragm muscle

The mechanism of the stimulating effect of short-term cold exposure of animal on tissue respiration has been studied. Oxygen consumption by a piece of the rat diaphragm muscle, fixed in a frame and put into a polarographic cell, was measured with an oxygen electrode. It is found that 13 min exposure of the rat to an air temperature of +2°C induces (1) an increase in respiration rate in the diaphragm tissue, (2) a decrease in stimulation of respiration by dinitrophenol (DNP) and (3) a decrease in the sensitivity of respiration to amytal. The maximal respiration rate observed in the presence of 40 M DNP is unaffected. Acclimation for two weeks of rats at +2°C does not influence the measured parameters. Effects similar to those produced by short-term cold exposure can be obtained by treatment of the cold acclimated animals with norepinephrine. In non-acclimated rats, norepinephrine is ineffective. The effects of cold exposure (or norepinephrine) can be reproduced on the diaphragm from a non-treated animal by the addition of 0.2 mM oleate or 10 mM pyruvate to the incubation medium. All effects of cold exposure or norepinephrinein vivo, or oleate and pyruvatein vitro are inhibited by oubain or replacement of Na⁺ by Li⁺ in the incubation mixture. The role of fatty acids and Na,K ATP-ase in the thermoregulatory responses of the tissue respiration is discussed.     

Differential Regulation of Mitochondrial Pyruvate Carrier Genes Modulates Respiratory Capacity and Stress Tolerance in Yeast

Mpc proteins are highly conserved from yeast to humans and are necessary for the uptake of pyruvate at the inner mitochondrial membrane, which is used for leucine and valine biosynthesis and as a fuel for respiration. Our analysis of the yeast MPC gene family suggests that amino acid biosynthesis, respiration rate and oxidative stress tolerance are regulated by changes in the Mpc protein composition of the mitochondria. Mpc2 and Mpc3 are highly similar but functionally different: Mpc2 is most abundant under fermentative non stress conditions and important for amino acid biosynthesis, while Mpc3 is the most abundant family member upon salt stress or when high respiration rates are required. Accordingly, expression of the MPC3 gene is highly activated upon NaCl stress or during the transition from fermentation to respiration, both types of regulation depend on the Hog1 MAP kinase. Overexpression experiments show that gain of Mpc2 function leads to a severe respiration defect and ROS accumulation, while Mpc3 stimulates respiration and enhances tolerance to oxidative stress. Our results identify the regulated mitochondrial pyruvate uptake as an important determinant of respiration rate and stress resistance.     

Genetic selection of mutations in the high affinity K+ transporter HKT1 that define functions of a loop site for reduced Na+ permeability and increased Na+ tolerance

Potassium is an important macronutrient required for plant growth, whereas sodium (Na+) can be toxic at high concentrations. The wheat K+ uptake transporter HKT1 has been shown to function in yeast and oocytes as a high affinity K+-Na+ cotransporter, and as a low affinity Na+ transporter at high external Na+. A previous study showed that point mutations in HKT1, which confer enhancement of Na+ tolerance to yeast, can be isolated by genetic selection. Here we report on the isolation of mutations in new domains of HKT1 showing further large increases in Na+ tolerance. By selection in a Na+ ATPase deletion mutant of yeast that shows a high Na+ sensitivity, new HKT1 mutants at positions Gln-270 and Asn-365 were isolated. Several independent mutations were isolated at the Asn-365 site. N365S dramatically increased Na+ tolerance in yeast compared with all other HKT1 mutants. Cation uptake experiments in yeast and biophysical characterization in Xenopus oocytes showed that the mechanisms underlying the Na+ tolerance conferred by the N365S mutant were: reduced inhibition of high affinity Rb+ (K+) uptake at high Na+ concentrations, reduced low affinity Na+ uptake, and reduced Na+ to K+ content ratios in yeast. In addition, the N365S mutant could be clearly distinguished from less Na+-tolerant HKT1 mutants by a markedly decreased relative permeability for Na+ at high Na+ concentrations. The new mutations contribute to the identification of new functional domains and an amino acid in a loop domain that is involved in cation specificity of a plant high affinity K+ transporter and will be valuable for molecular analyses of Na+ transport mechanisms and stress in plants.