Efflux Of Na Research Articles

Increasing salinity sequentially induces salt tolerance responses in Szarvasi-1 energy grass

Soil salinity causes severe physiological disorders, decline in biomass, and crop production worldwide becoming more critical with global climate change. Consequently, salt-tolerant varieties received major focus in all sectors of agriculture. Biomass plants such as Szarvasi-1 energy grass (Elymus elongatus subsp. ponticus cv. Szarvasi-1) may play an important role in energy production if they are tolerant to environmental stresses. In this study, Szarvasi-1 energy grass has been investigated to reveal its tolerance to 50–200 mM NaCl in hydroponics. Significant decline in stomatal conductance appeared at 100 mM NaCl treatment but fresh and dry weight and the maximal quantum efficiency of PSII decreased only at 200 mM NaCl. Relative water content and total chlorophyll concentration did not change compared to the control. Leaf water potential was maintained at the control level for one week NaCl exposure, decrease became significant only after two weeks. Malondialdehyde concentration did not refer to oxidative stress. In the element composition of the plants, remarkable increase was found only for Mo whereas Ca, K, S, P, Mn decreased compared to the control. K to Na ratio remained higher than one in the shoot even at 200 mM NaCl. Salt treatment caused temporal and concentration-dependent changes in the expression of genes in the phenylpropanoid pathway, Na transport, photosynthesis, and cellular protection and repair. Szarvasi-1 was found to be fairly tolerant to NaCl which induced a sequential response switching on vacuolar compartmentalization at 50 mM, Na efflux at 100 mM, and cellular protection and repair at 200 mM.

Cardiac deficiency of P21-activated kinase 1 promotes atrial arrhythmogenesis in mice following adrenergic challenge.

P21-activated kinase 1 (Pak1) signalling plays a vital and overall protective role in the heart. However, the phenotypes of Pak1 deficiency in the cardiac atria have not been well explored. In this study, Pak1 cardiac-conditional knock-out (cKO) mice were studied under baseline and adrenergic challenge conditions. Pak1 cKO mice show atrial arrhythmias including atrial fibrillation (AF) in vivo, detected during anaesthetized electrocardiography without evidence of interstitial fibrosis upon Masson's trichrome staining. Optical mapping of left atrial preparations from Pak1 cKO mice revealed a higher incidence of Ca2+ and action potential alternans under isoprenaline challenge and differences in baseline action potential and calcium transient characteristics. Type-2 ryanodine receptor (RyR2) channels from Pak1 cKO hearts had a higher open probability than those from wild-type. Reverse transcription-quantitative polymerase chain reaction and Western blotting indicated that pCamkIIδ and RyR2 are highly phosphorylated at baseline in the atria of Pak1 cKO mice, while the expression of Slc8a2 and Slc8a3 as a Na+-Ca2+ exchanger, controlling the influx of Ca2+ from outside of the cell and efflux of Na+ from the cytoplasm, are augmented. Chromatin immunoprecipitation study showed that pCreb1 interacts with Slc8a2 and Slc8a3. Our study thus demonstrates that deficiency of Pak1 promotes atrial arrhythmogenesis under adrenergic stress, probably through post-translational and transcriptional modifications of key molecules that are critical to Ca2+ homeostasis. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Exposure to pH 3.5 water has no effect on the gills of the Amazonian tambaqui (Colossoma macropomum).

Tambaqui (Colossoma macropomum) are a model species for tropical fish physiology, but details are lacking about their ionoregulatory response to acid waters. To provide specifics, we measured unidirectional Na+ fluxes in low pH waters. Sodium influx ([Formula: see text]) was uninhibited during acute exposure to pH 4.5 and 3.5, and Na efflux ([Formula: see text]) rose only slightly at pH 3.5; net Na+ flux ([Formula: see text]) remained positive at all pH. Similarly, during 24h transfer to pH 3.5 [Formula: see text], [Formula: see text], and [Formula: see text] were unchanged at all times. Taking a closer look at the mechanism of Na+ transport in the gills of tambaqui we found that [Formula: see text] was uninhibited by HMA, a Na+/H+-exchanger blocker, and Benzamil, a Na+-channel inhibitor, casting doubt on their role in Na+ uptake in this fish. Measurement of Na+/K+-ATPase (NKA) and H+-ATPase (VHA) activity showed that neither changed at low pH compared to measurements at pH 6.5. Western blot analysis of ATPase expression saw no changes in amount of NKA and VHA at low pH, and immunohistochemistry showed expression of both NKA and VHA on lamellae and interlamellar region of tambaqui gills and that both proteins co-localized to the same gill cells. Location of expression also did not change in low pH water. Amazingly, tambaqui seem unaffected by pH 3.5 water, making them one of the most acid-tolerant fish species examined so far. In addition, they appear to share key ionoregulatory traits with other fish of the order Characiformes, which suggest a common origin for the ionoregulatory attributes.

Melatonin improves K+ and Na+ homeostasis in rice under salt stress by mediated nitric oxide

Rice (Oryza sativa L.) productivity is greatly affected by soil salinity and melatonin (MLT) has long been recognized as a positive molecule that can alleviate the damage caused by salt. Here, the role of nitric oxide (NO) in the regulation of salt tolerance by MLT was investigated in rice. MLT pretreatment increased the fresh and dry weight of rice seedlings under salt stress. Its beneficial effects include less relative electrolyte leakage (REL) and better K+/Na+ homeostasis. MLT increased the activity of nitric oxide synthase (NOS). The polyamines (PAs) content and the utilization of arginine were also increased, thereby increasing NO content in salt-stressed rice seedlings. Pharmacological approach showed that NO, as a necessary downstream signaling molecule, was involved in the regulation of MLT on the K+/Na+ homeostasis of rice. Under salt stress, MLT improved the H+-pumps activities in plasma membrane (PM) and vacuole membrane (VM) in roots, MLT also increased the ATP content of rice roots by increasing the NO content of rice. Thus, the efflux of Na+ and the influx of K+ were promoted. When endogenous NO was scavenged, the regulation of K+/Na+ homeostasis by MLT was blocked. Therefore, MLT mediated K+/Na+ homeostasis of rice under salt stress by mediating NO.

Measurement of Differential Na(+) Efflux from Apical and Bulk Root Zones of Intact Barley and Arabidopsis Plants.

Rapid sodium cycling across the plasma membrane of root cells is widely thought to be associated with Na+ toxicity in plants. However, the efflux component of this cycling is not well understood. Efflux of Na+ from root cells is believed to be mediated by Salt Overly-Sensitive-1, although expression of this Na+/H+ antiporter has been localized to the vascular tissue and root meristem. Here, we used a chambered cuvette system in which the distal root of intact salinized barley and Arabidopsis thaliana plants (wild-type and sos1) were isolated from the bulk of the root by a silicone-acrylic barrier, so that we could compare patterns of 24Na+ efflux in these two regions of root. In barley, steady-state release of 24Na+ was about four times higher from the distal root than from the bulk roots. In the distal root, 24Na+ release was pronouncedly decreased by elevated pH (9.2), while the bulk-root release was not significantly affected. In A. thaliana, tracer efflux was about three times higher from the wild-type distal root than from the wild-type bulk root and also three to four times higher than both distal- and bulk-root fluxes of Atsos1 mutants. Elevated pH also greatly reduced the efflux from wild-type roots. These findings support a significant role of SOS1-mediated Na+ efflux in the distal root, but not in the bulk root.

Migraine Cause and Treatment

Background: Research shows that migraine brains have hyperactive sensory organs and multiple sensory receptor connections. Hyper activity of these organs needs extra supply of nutrition to support increased electrical activity. Today’s medicines reduce or prevent the functioning of these neurons by blocking essential voltage dependent calcium or sodium channel instead of providing nutrients. We asked: if we provide support for extra electrical activity of migraineurs, would it prevent migraines without the use of medicines?Methods: We reviewed published literature and conducted research over 6 months studying 650 volunteer migraineurs in a migraine-research Facebook group. Participants were screened for migraine types, answered a questionnaire on medical conditions, medicines used, and lifestyle. They were provided instructions on the use of the migraine protocol and were evaluated weekly.Findings: Migraine frequency appears to be exacerbated by carbohydrate-rich and salt- and water-poor diets and may be worsened by medicines that block voltage gated calcium or sodium channels. Stopping these medicines, reducing carbohydrates and increasing saline in electrolytes appears to prevent and/or stop migraines.Conclusions: H2O and Na efflux from cells caused by glucose, electrolyte mineral (Na , Cl-, K ) ratio may be disrupted in carbohydrate heavy diets causing migraines. Changes to diet that include increased salt intake along with reduced carbohydrate intake appears to prevent glucose induced electrolyte changes which then decreases migraine frequency. In the present study, all participants who made these dietary changes were able to eliminate migraine medications and remained migraine free.

Catalysis and Selectivity of Na+ Permeation in Bacterial Sodium Channel NaVAb

The determination of a high-resolution 3D structure of voltage-gated sodium channel NaVAb opens the way to elucidating the mechanism of ion conductance and selectivity. To examine the selective permeation of Na+ over K+ through the selectivity filter of the channel, we performed large-scale molecular dynamics simulations of NaVAb in an explicit, hydrated lipid bilayer at 0mV successively in 150mM NaCl, 150mM KCl, and a combination of both, for a total simulation time of more than 70 microseconds. Although the cytoplasmic end of the pore is closed, reversible influx and efflux of Na+ and K+ through the selectivity filter occurred spontaneously during simulations, leading to equilibrium movement of these cations between the extracellular medium and the central cavity of the channel. Analysis of Na+ dynamics reveals a knock-on mechanism of ion permeation characterized by alternating occupancy of the channel by 2 and 3 Na+ ions, with a computed rate of translocation of (6±1) × 106 ions per second that is consistent with expectations from electrophysiological studies. The binding of Na+ is intimately coupled to conformational isomerization of the four E177 side chains lining the extracellular end of the selectivity filter. The reciprocal coordination of variable numbers of Na+ ions and carboxylate groups leads to their condensation into ionic clusters of variable charge and spatial arrangement. By stabilizing multiple ionic occupancy states while helping Na+ ions diffuse within the selectivity filter, the conformational flexibility of E177 side chains underpins the knock-on mechanism of Na+ permeation. K+ also forms ionic complexes with E177 but, contrary to Na+, K+ ions pass through the selectivity filter in single file. The analysis of competitive binding of Na+ and K+ in mixed-cation simulations provides detailed mechanistic insight into the molecular basis of ion selectivity in NaVAb.

Catalysis of Na + permeation in the bacterial sodium channel Na V Ab

Determination of a high-resolution 3D structure of voltage-gated sodium channel Na(V)Ab opens the way to elucidating the mechanism of ion conductance and selectivity. To examine permeation of Na(+) through the selectivity filter of the channel, we performed large-scale molecular dynamics simulations of Na(V)Ab in an explicit, hydrated lipid bilayer at 0 mV in 150 mM NaCl, for a total simulation time of 21.6 μs. Although the cytoplasmic end of the pore is closed, reversible influx and efflux of Na(+) through the selectivity filter occurred spontaneously during simulations, leading to equilibrium movement of Na(+) between the extracellular medium and the central cavity of the channel. Analysis of Na(+) dynamics reveals a knock-on mechanism of ion permeation characterized by alternating occupancy of the channel by 2 and 3 Na(+) ions, with a computed rate of translocation of (6 ± 1) × 10(6) ions⋅s(-1) that is consistent with expectations from electrophysiological studies. The binding of Na(+) is intimately coupled to conformational isomerization of the four E177 side chains lining the extracellular end of the selectivity filter. The reciprocal coordination of variable numbers of Na(+) ions and carboxylate groups leads to their condensation into ionic clusters of variable charge and spatial arrangement. Structural fluctuations of these ionic clusters result in a myriad of ion binding modes and foster a highly degenerate, liquid-like energy landscape propitious to Na(+) diffusion. By stabilizing multiple ionic occupancy states while helping Na(+) ions diffuse within the selectivity filter, the conformational flexibility of E177 side chains underpins the knock-on mechanism of Na(+) permeation.

Abstract TP78: Down-regulation Of Vascular Na+/k+ ATPase In Oophorectomized Rats Feeding A High Salt Diet Is Associated With The Cerebral Aneurysm Formation

Background and Purpose— Hypertension is thought to be associated with a high incidence of stroke. However, not all patients with unruptured cerebral aneurysms are hypertensive. In the DOCA-salt rats the increase in body water-free Na+ storage associated with hypertension is suggested. We hypothesized that in oophorectomized rats fed a high salt diet, body water-free Na+ accumulation may be increased, leading to the formation of cerebral aneurysms. To address the relationship between the increase in the body Na+-to-water ratio that characterizes water-free Na+ accumulation and the formation of cerebral aneurysms, we focused on vascular Na efflux pump Na+/K+ ATPase. Methods— Thirteen-week old female Sprague-Dawley rats subjected to carotid artery ligation were fed a high-salt diet and divided into 3 groups; a group without- (HSD) and another with bilateral oophorectomy (HSD/OVX) and a 3rd group that underwent additional renal artery ligation (HSD/OVX/RL) to induce hypertension. Results— Compared to HSD rats, the incidence of cerebral aneurysms and the body Na+-to-water ratio were significantly higher in HSD/OVX- and HSD/OVX/RL rats independent of hypertension. In their aneurysmal wall, ATP1α2, a subtype of Na+/K+-ATPase were down-regulated and renin-angiotensin system- and inflammation related molecules were up-regulated. In HSD/OVX/RL rats treatment with low dose olmesartan up-regulated ATP1α2 without affecting blood pressure and reduced the body Na+-to-water ratio and the incidence of cerebral aneurysm formation. Conclusions— These results suggest that a reduction in the vascular Na efflux pump during excessive salt intake in oophorectomized rats may be associated with the increase in water-free Na+ accumulation directing to cerebral aneurysm formation.

NMR structures of the transmembrane domains of the α4β2 nAChR

The α4β2 nicotinic acetylcholine receptor (nAChR) is the predominant heteromeric subtype of nAChRs in the brain, which has been implicated in numerous neurological conditions. The structural information specifically for the α4β2 and other neuronal nAChRs is presently limited. In this study, we determined structures of the transmembrane (TM) domains of the α4 and β2 subunits in lauryldimethylamine-oxide (LDAO) micelles using solution NMR spectroscopy. NMR experiments and size exclusion chromatography-multi-angle light scattering (SEC-MALS) analysis demonstrated that the TM domains of α4 and β2 interacted with each other and spontaneously formed pentameric assemblies in the LDAO micelles. The Na+ flux assay revealed that α4β2 formed Na+ permeable channels in lipid vesicles. Efflux of Na+ through the α4β2 channels reduced intra-vesicle Sodium Green™ fluorescence in a time-dependent manner that was not observed in vesicles without incorporating α4β2. The study provides structural insight into the TM domains of the α4β2 nAChR. It offers a valuable structural framework for rationalizing extensive biochemical data collected previously on the α4β2 nAChR and for designing new therapeutic modulators.

Ussing's “Little Chamber”: 60 Years+ Old and Counting

GENERAL COMMENTARY article Front. Physiol., 03 March 2011Sec. Renal and Epithelial Physiology https://doi.org/10.3389/fphys.2011.00006

Osmotic adaptation in halotolerant yeast, Debaryomyces nepalensis NCYC 3413: role of osmolytes and cation transport

Debaryomyces nepalensis NCYC 3413, a food spoiling yeast isolated from rotten apple, has been previously demonstrated as halotolerant yeast. In the present study, we assessed its growth, change in cell size, and measured the intracellular polyol and cations (Na(+) or K(+)) accumulated during growth in the absence and presence of different concentrations of salts (NaCl and KCl). Cells could tolerate 2 M NaCl and KCl in defined medium. Scanning electron microscopic results showed linear decrease in mean cell diameter with increase in medium salinity. Cells accumulated high amounts of K(+) during growth at high concentrations of KCl. However, it accumulated low amounts of Na(+) and high amounts of K(+) when grown in the presence of NaCl. Cells grown in the absence of salt showed rapid influx of Na(+)/K(+) on incubation with high salt. On incubation with 2 M KCl, cells grown at 2 M NaCl showed an immediate efflux of Na(+) and rapid uptake of K(+) and vice versa. To withstand the salt stress, osmotic adjustment of intracellular cation was accompanied by intracellular accumulation of polyol (glycerol, arabitol, and sorbitol). Based on our result, we hypothesize that there exists a balanced efflux and synthesis of osmolytes when D. nepalensis was exposed to hypoosmotic and hyperosmotic stress conditions, respectively. Our findings suggest that D. nepalensis is an Na(+) excluder yeast and it has an efficient transport system for sodium extrusion.

Futile Na+ cycling at the root plasma membrane in rice (Oryza sativa L.): kinetics, energetics, and relationship to salinity tolerance.

Globally, over one-third of irrigated land is affected by salinity, including much of the land under lowland rice cultivation in the tropics, seriously compromising yields of this most important of crop species. However, there remains an insufficient understanding of the cellular basis of salt tolerance in rice. Here, three methods of 24Na+ tracer analysis were used to investigate primary Na+ transport at the root plasma membrane in a salt-tolerant rice cultivar (Pokkali) and a salt-sensitive cultivar (IR29). Futile cycling of Na+ at the plasma membrane of intact roots occurred at both low and elevated levels of steady-state Na+ supply ([Na+]ext=1 mM and 25 mM) in both cultivars. At 25 mM [Na+]ext, a toxic condition for IR29, unidirectional influx and efflux of Na+ in this cultivar, but not in Pokkali, became very high [>100 μmol g (root FW)−1 h−1], demonstrating an inability to restrict sodium fluxes. Current models of sodium transport energetics across the plasma membrane in root cells predict that, if the sodium efflux were mediated by Na+/H+ antiport, this toxic scenario would impose a substantial respiratory cost in IR29. This cost is calculated here, and compared with root respiration, which, however, comprised only ∼50% of what would be required to sustain efflux by the antiporter. This suggests that either the conventional ‘leak-pump’ model of Na+ transport or the energetic model of proton-linked Na+ transport may require some revision. In addition, the lack of suppression of Na+ influx by both K+ and Ca2+, and by the application of the channel inhibitors Cs+, TEA+, and Ba2+, questions the participation of potassium channels and non-selective cation channels in the observed Na+ fluxes.

Ion fluxes and hematological parameters of two teleosts from the Rio Negro, Amazon, exposed to hypoxia

The aim of this study was to describe the effect of hypoxia on whole body ion fluxes and hematological parameters in two Amazonian teleosts: Serrasalmus eigenmanni and Metynnis hypsauchen. The increase of Na+ and Cl- effluxes on M. hypsauchen exposed to hypoxia may be related to an increase of gill ventilation and effective respiratory surface area, to avoid a reduction in the oxygen uptake, and/or with the decrease of pHe, that could inhibit Na+ and Cl- transporters and, therefore, reduce influx of these ions. Effluxes of Na+ and Cl- were lower in hypoxia than in normoxia for S. eigenmanni, possibly because in hypoxia this species would reduce gill ventilation and oxygen uptake, which would lead to a decrease of gill ion efflux and, consequently, reducing ion loss. The increase on hematocrit (Ht) during hypoxia in M. hypsauchen probably was caused by an increase of the red blood cell volume (MCV). For S. eigenmanni the increase on glucose possibly results from the usage of glucose reserve mobilization. Metynnis hypsauchen showed to be more sensitive to hypoxia than Serrasalmus eigenmanni, since the first presented more significant alterations on these osmoregulatory and hematological parameters. Nevertheless, the alterations observed for both species are strategies adopted by fishes to preserve oxygen supply to metabolizing tissues during exposure to hypoxia.

Na + regulation by combined nitrogen in Azolla pinnata– Anabaena azollae symbiotic association during salt toxicity

To study the regulation of Na + ion by combined-N sources in Azolla pinnata– Anabaena azollae, Na + influx, intracellular Na + and Na + efflux were investigated in the presence of different N-sources (N 2, NH 4 +, NO 3 − and urea) and various NaCl concentrations. Sodium influx by Azolla pinnata fronds was minimum in the presence of NO 3 −. Almost identical levels of intracellular sodium, although less than N 2 and NH 4 + incubated fronds were observed in the presence of NO 3 − and urea. Efflux of sodium was minimum in urea and NO 3 − grown fronds. A low residual sodium was observed in the fronds incubated in NO 3 − and urea supplemented media. Results suggest that nitrate and urea curtailed the entry of sodium, reduced salt toxicity maximally by maintaining the minimum level of sodium and also conserved energy due to slow influx and efflux of Na + within the fronds during salt shock and the process of adaptation.

Regulatory volume decrease in northern fur seal (Callorhinus ursinus) red blood cells

The cation transport and regulatory volume decrease (RVD) were investigated in the red blood cells (RBCs) of northern fur seals (Callorhinus ursinus). Extracellular Ca-dependent Na efflux was increased to threefold by hypotonicity. K–Cl cotransport activity was not detected by hypotonic medium, but measured only by nitrite or N-ethylmaleimide stimulation. RBCs were restored to their original volume after being swollen in hypoosmotic medium with Ca, though this recovery was inhibited by the addition of quinidine. Based on these results, Na/Ca exchange transporter played the major role in the regulatory volume decrease in the RBCs of northern fur seals.

Dopamine and Noradrenaline Efflux in the Medial Prefrontal Cortex During Serial Reversals and Extinction of Instrumental Goal-Directed Behavior

The prefrontal cortex (PFC) of the rat supports cognitive flexibility, the ability to spontaneously adapt goal-directed behavior in response to radically changing situational demands. We have shown previously that transient inactivation of the rat medial PFC (mPFC) impairs initial reversal learning in a spatial 2-lever discrimination task. Given the importance of dopamine (DA) for PFC function, we studied DA (and noradrenaline [NA]) efflux in the mPFC during reversal learning. We observed a higher and more extended increase in DA efflux in rats performing the first reversal compared with controls performing the previously acquired discrimination. The results of an additional experiment suggest that such a difference between the reversal- and control-induced DA increases was absent during a third reversal. During the extinction session, DA efflux did not increase from basal levels. Increases in NA efflux were less than in DA and did not differ between control and any condition. We conclude that prefrontal DA activity is increased during execution of instrumental discrimination tasks and that this increase is amplified during the acquisition of a first, but not of later reversals. These data corroborate our previous findings and indicate that DA is critically involved in this form of cognitive flexibility.

Nongenomic Renal Effects of Aldosterone

There is increasing evidence for a dependency on aldosterone in the development of hypertension.1 How aldosterone influences the physiology that determines blood pressure or for that matter promotes the pathophysiology of high blood pressure turns out to be less than straightforward. There is the genomic pathway that begins with the binding of aldosterone to the classical mineralocorticoid receptor (MR) and ends with sodium reabsorption at the distal nephron. But aldosterone can convey nongenomic influences as well, actions that have received less attention, possibly in part because they seem counterintuitive to our genomic view of biology. Adding to the complexity of aldosterone’s actions is the fact that there are also nonepithelial (nondistal nephron) targets of aldosterone, such as the heart, the vasculature, the kidney, and other sites. In this issue of Hypertension , Schmidt et al2 describe a rapid (thus nongenomic) effect of administered aldosterone to increase resistance in renal vasculature (a nonepithelial target) in healthy human subjects. The findings follow on the heels of a rather large body of work on the genomic actions of aldosterone. Aldosterone’s nongenomic effects were first recognized more than a decade before an aldosterone-inducible gene was identified in the late 1990s.3 A nongenomic action characteristically occurs almost immediately, usually within several minutes,4 before sufficient time has elapsed for gene …

Low unidirectional sodium influx into root cells restricts net sodium accumulation in Thellungiella halophila, a salt-tolerant relative of Arabidopsis thaliana

Thellungiella halophila is a useful model species for research into plant salt tolerance. It is closely related to Arabidopsis thaliana, but shows considerably higher salt tolerance. Comparative analysis of ion homeostasis in the two species allows the identification of ion transport pathways that are critical for salt tolerance and provides the basis for future studies into their molecular features. Previous studies indicated that salt tolerance in T. halophila is accompanied by low accumulation of Na in the leaves. Kinetic analysis of net ion uptake over three days confirmed lower Na uptake and K loss in T. halophila compared with A. thaliana. Differential net Na uptake rates were still apparent after 6 weeks of salt treatment. To assess the contribution of unidirectional Na fluxes to net Na uptake, kinetic studies of (22)Na fluxes were carried out in both species. The results show that unidirectional root Na influx is significantly lower in salt-grown T. halophila plants than in A. thaliana exposed to the same level of salinity (100 mM). Quantitative comparison of unidirectional influx and net Na accumulation suggests that both species operate efficient Na efflux, which partly compensates for Na influx. Kinetic analysis of (22)Na efflux indicated higher root Na efflux in A. thaliana than in T. halophila. Thus A. thaliana appears to spend more energy on Na export while nevertheless accumulating more Na than T. halophila. It is proposed that limitation of Na influx is the main mechanism by which T. halophila secures low net Na accumulation in saline conditions. This strategy provides the basis for a positive balance between growth and net Na uptake rates, which is essential for survival in high salt.

Gill area, permeability and Na+,K+-ATPase activity as a function of size and salinity in the blue crab,Callinectes sapidus

Juvenile blue crabs, Callinectes sapidus, extensively utilize oligohaline and freshwater regions of the estuary. With a presumptively larger surface-area-to-body weight ratio, juvenile crabs could experience osmo- and ionoregulatory costs well in excess of that of adults. To test this hypothesis, crabs ranging over three orders of magnitude in body weight were acclimated to either sea water (1,000 mOsm) or dilute sea water (150 mOsm), and gill surface area, water and sodium permeabilities (calculated from the passive efflux of 3H2O and 22Na+), gill Na+, K+ -ATPase activity and expression were measured. Juveniles had a relatively larger gill surface area; weight-specific gill surface area decreased with body weight. Weight-specific water and sodium fluxes also decreased with weight, but not to the same extent as gill surface area; thus juveniles were able to decrease gill permeability slightly more than adults upon acclimation to dilute media. Crabs < 5 g in body weight had markedly higher activities of gill Na+ ,K+ -ATPase than crabs > 5 g in both posterior and anterior gills. Acclimation to dilute medium induced increased expression of Na+, K+ -ATPase and enzyme activity, but the increase was not as great in juveniles as in larger crabs. The increased weight-specific surface area for water gain and salt loss for small crabs in dilute media presents a challenge that is incompletely compensated by reduced permeability and increased affinity of gill Na+, K+ -ATPase for Na+. Juveniles maintain osmotic and ionic homeostasis by the expression and utilization of extremely high levels of gill Na+, K+ -ATPase, in posterior, as well as in anterior, gills.