Ionic and osmotic regulation of the haemolymph of the dragonfly, Libellula quadrimaculata (Odonata : Libellulidae)

Larvae of the widespread dragonfly, Libellula quadrimaculata, were adapted to a series of salt solutions, and the osmotic pressure, and sodium, potassium and chloride concentrations in the haemolymph measured. The regulation of potassium is extremely efficient over the range 0–50 m-mole/l. external concentration. Above this, larvae die. Sodium and chloride are regulated to a lesser extent, the larvae being able to withstand considerable changes in the concentration of these ions in the haemolymph. However, at higher external concentrations, the haemolymph concentration of these ions is maintained below that of the external medium. The osmotic pressure is regulated in parallel with sodium concentration over most of the range tested. However, in higher salinities, the osmotic pressure of the haemolymph does not fall below that of the external medium. This is seen as a strategy to limit the amount of drinking in saline media. Overall, the osmoregulatory system of L. quadrimaculata resembles that of brackish-water insects, rather than that of the more strictly freshwater dragonflies that have been studied.     

Rectale Chloridepithelien und osmoregulatorische Salzaufnahme durch den Enddarm von Zygopteren und Anisopteren Libellenlarven

The larvae of Coenagrion puella possess 3, and the larvae of Aeshna cyanea up to 486 rectal chloride epithelia which in both species are organized as transporting epithelia. Combined applications of the histochemical chloride precipitation technique, energy-dispersive micro-analysis of X-rays, autoradiography, and scintillation counting on A. cyanea revealed that the chloride epithelia adsorb chloride from the external solution. By use of radioactive sodium and chloride in hypotonic concentrations applied on normal and anus-sealed larvae it was demonstrated that the rectum of both species is the main pathway for salt uptake into the haemolymph. The stepwise increase in external osmolarity by the addition of mannitol results in a concomitant reduction of chloride uptake into the haemolymph. These results suggest that the rectal chloride epithelia are involved in hyperosmotic regulation by the absorption of salt from the external medium.     

Effect of elevated temperature on osmotic and ionic regulation in a salt-tolerant caddisfly from Death Valley,California

The effects of temperature (8–10 or 20°C) on regulation of haemolymph osmotic and ionic concentrations were investigated over a range of salinities (0–25‰) in fifth-instar larvae of the Death Valley caddisfly Limnephilus assimilis. At low temperatures, levels of chloride and sodium in the haemolymph are regulated over a wide range of salinities corresponding to the salinities at which larvae occur in nature and at which they can complete development into adults. In contrast, haemolymph osmolality is constant at low salinities (<14‰) but approaches conformity with the medium at higher salinities. High temperature reduces the larva's ability to maintain low chloride concentrations in its haemolymph and also leads to a reduction in haemolymph osmotic pressure; thus, at high temperatures ions account for more of the haemolymph osmotic concentration than at low temperatures. These data suggest that the absence of larvae from thermal pools and from all Death Valley waters in summer can be explained by the effects of high water temperatures on hydromineral regulation.     

Transepithelial potential in the Magadi tilapia, a fish living in extreme alkalinity

We investigated the transepithelial potential (TEP) and its responses to changes in the external medium in Alcolapia grahami, a small cichlid fish living in Lake Magadi, Kenya. Magadi water is extremely alkaline (pH = 9.92) and otherwise unusual: titratable alkalinity (290 mequiv L(-1), i.e. HCO(3) (-) and CO(3) (2-)) rather than Cl(-) (112 mmol L(-1)) represents the major anion matching Na(+) = 356 mmol L(-1), with very low concentrations of Ca(2+) and Mg(2+) (<1 mmol L(-1)). Immediately after fish capture, TEP was +4 mV (inside positive), but stabilized at +7 mV at 10-30 h post-capture when experiments were performed in Magadi water. Transfer to 250% Magadi water increased the TEP to +9.5 mV, and transfer to fresh water and deionized water decreased the TEP to -13 and -28 mV, respectively, effects which were not due to changes in pH or osmolality. The very negative TEP in deionized water was attenuated in a linear fashion by log elevations in [Ca(2+)]. Extreme cold (1 vs. 28°C) reduced the positive TEP in Magadi water by 60%, suggesting blockade of an electrogenic component, but did not alter the negative TEP in dilute solution. When fish were transferred to 350 mmol L(-1) solutions of NaHCO(3), NaCl, NaNO(3), or choline Cl, only the 350 mmol L(-1) NaHCO(3) solution sustained the TEP unchanged at +7 mV; in all others, the TEP fell. Furthermore, after transfer to 50, 10, and 2% dilutions of 350 mmol L(-1) NaHCO(3), the TEPs remained identical to those in comparable dilutions of Magadi water, whereas this did not occur with comparable dilutions of 350 mmol L(-1) NaCl-i.e. the fish behaves electrically as if living in an NaHCO(3) solution equimolar to Magadi water. We conclude that the TEP is largely a Na(+) diffusion potential attenuated by some permeability to anions. In Magadi water, the net electrochemical forces driving Na(+) inwards (+9.9 mV) and Cl(-) outwards (+3.4 mV) are small relative to the strong gradient driving HCO(3) (-) inwards (-82.7 mV). Estimated permeability ratios are P (Cl)/P (Na) = 0.51-0.68 and [Formula: see text] = 0.10-0.33. The low permeability to HCO(3) (-) is unusual, and reflects a unique adaptation to life in extreme alkalinity. Cl(-) is distributed close to Nernst equilibrium in Magadi water, so there is no need for lower P (Cl). The higher P (Na) likely facilitates Na(+) efflux through the paracellular pathway. The positive electrogenic component is probably due to active HCO(3) (-) excretion.     

The effect of enternal sodium concentration upon sodium fluxes in Chironomus dorsalis (Meig.) and Camptochironomus tentans (Fabr.), and the effect of other ions on sodium influx in C. tentans.

In comparison with other freshwater animals, the sodium uptake mechanism in fourth instar larvae of both C. tentans and C. dorsalis has a moderate affinity for sodium. In both species half maximum influx (Km) occurs at about 0.57 mM-Na+ and is unaltered by salt depletion. Maximum influx is achieved in steady-state C. tentans at 1.9 mM-Na+, and in steady-state C. dorsalis at 3.0 mM-Na+. Both of these values increase on depletion. Efflux also appears to be saturable at higher external sodium concentrations. In C. tentans, sodium may be transported independently of chloride, although it seems likely that sodium movement is enhanced by chloride. Sulphate strongly inhibits sodium influx. Nitrate apparently inhibits sodium influx at low concentrations, but this inhibition is progressively overcome at external sodium concentrations approaching 4 mM. A number of cations interfere with sodium influx in depleted C. tentans, notably H+, Li+ and, to a lesser extent NH4+. It is suggested that these ions compete with sodium for carrier sites. Potassium is apparently transported independently of sodium.     

The anal portion as a salt-excreting organ in a seawater mosquito larva,A?des togoi Theobald

Summary Larvae of the marine mosquito,Aëdes togoi Theobald, tolerate environmental salinities ranging from fresh water to 300% sea water. When they were transferred from fresh water to sea water, sodium concentration in the haemolymph increased for the first 2 days and decreased to the seawater-adapted level within 4 days. When transferred from sea water to fresh water, the sodium concentration decreased markedly for the first 2 days and attained the freshwater adapted level after 4 days. When the larvae in sea water were ligated near the anus, they died within 3 days, showing an increased sodium level in the haemolymph. The larvae ligated at the neck lost considerable body weight and died within 4 days.When the anal portion, a terminal portion of the hindgut, was catheterized, the larvae maintained in sea water showed an increase in haemolymph sodium. The anal portion epithelium of the larvae adapted to 100 and 150% sea water demonstrated a strong positive reaction to the histochemical assay for chloride ions, whereas the reaction was negative or weakly positive in freshwater adapted larvae. In the larvae with the anal papillae ligated, a slight increase in haemolymph sodium occurred while in sea water. The anal papillae were weakly positive to chloride ions. Unlike salt-water mosquito larvae of the other species, in which the rectum is considered to be involved in hyperosmotic urine production and the anal papillae appear to be the extrarenal organ, the anal portion inA. togoi larvae seems to play an important role in excretion of excess ions when placed in hyperosmotic media.     

Inorganic ion composition of haemolymph of the cecropia silkmoth: changes with diet and ontogeny

The levels' of sodium, potassium, magnesium, calcium, chloride, trehalose, and osmotic pressure in haemolymph were studied during ontogeny in the silkmoth Hyalophora cecropia, reared on either foliage or an artificial diet. Potassium and calcium in haemolymph changed little with ontogeny or diet, and averaged 35 and 10 m-equiv/l., respectively. The haemolymph levels of sodium and chloride were greater in larvae fed on artificial diet than in those on foliage, reflecting the levels in the respective diets; after cessation of feeding, the levels in haemolymph of the two groups approached common values (sodium 1–2 m-equiv/l.; chloride 20 m-equiv/l.). Magnesium was higher in haemolymph of foliage-fed larvae (100 m-equiv/l.) than in diet-fed larvae (65 m-equiv/l.), and in both groups declined after spinning to an average level of 40 m-equiv/l.Haemolymph from fifth instar Manduca sexta larvae reared on an artificial diet had ion levels very close to those in Cecropia similarly reared. Haemolymph of Danaus plexippus at three stages of development was also similar, but showed some quantitative differences from the other species.     

Calcium transport in intact human erthrocytes

Intact human erythrocytes can be readily loaded with calcium by incubation in hypersomotic media at alkaline pH. Erythrocyte calcium content increases from 15-20 to 120-150 nmol/g hemoglobin after incubation for 2 h at 20 degree C in a 400 mosmol/kg, pH 7.8 solution containing 100 mM sodium chloride, 90 mM tetramethylammonium chloride, 1 mM potassium chloride, and 10 mM calcium chloride. Calcium uptake is a time-dependent process that is associated with an augmented efflux of potassium. The ATP content in these cells remains at more than 60% of normal and is not affected by calcium. Calcium uptake is influenced by the cationic composition of the external media. The response to potassium is diphasic. With increasing potassium concentrations, the net accumulation of calcium initially increases, becoming maximal at 1 mM potassium, then diminishes, falling below basal levels at concentrations above 3 mM potassium. Ouabain inhibits the stimulatory effect of low concentrations of potassium. The inhibitory effects of higher concentrations of potassium are ouabain insensitive and independent of the external calcium concentration. Sodium also inhibits calcium uptake but this inhibition can be modified by altering the external concentration of calcium. The effux of calcium from loaded erythrocytes is not significantly altered by changes in osmolality, medium ion composition, or ouabain. It is concluded that hypertonicity increases the net uptake of calcium by increasing the influx of calcium and that some part of the sodium potassium transport system is involved in this influx process.     

A critical analysis of transepithelial potential in intact killifish (Fundulus heteroclitus) subjected to acute and chronic changes in salinity

We investigated the in vivo salinity-dependent behavior of transepithelial potential (TEP) in Fundulus heteroclitus (3-9 g) using indwelling coelomic catheters, a technique which was validated against blood catheter measurements in a larger species (Opsanus beta; 35-70 g). In seawater (SW)-acclimated killifish, TEP was +23 mV (inside positive), but changed to -39 mV immediately after transfer to freshwater (FW). Acute transfer to dilute salinities produced a TEP profile, which rapidly attenuated as salinity increased (0, 2.5, 5 and 10% SW), with cross-over to positive values between 20 and 40% SW, and a linear increase thereafter (60, 80 and 100% SW). TEP response profiles were also recorded after acute transfer to comparable dilutions of 500 mmol L(-1) NaCl, NaNO3, Na gluconate, choline chloride, N-methyl-D-glutamate (NMDG) chloride, or 1,100 mosmol kg(-1) mannitol. These indicated high non-specific cation permeability and low non-specific anion permeability without influence of osmolality in SW-acclimated killifish. While there was a small electrogenic component in high salinity, a Na+ diffusion potential predominated at all salinities due to the low P Cl/P Na (0.23) of the gills. The very negative TEP in FW was attenuated in a linear fashion by log elevations in [Ca2+] such that P Cl/P Na increased to 0.73 at 10 mmol L(-1). SW levels of [K+] or [Mg2+] also increased the TEP, but none of these cations alone restored the positive TEP of SW-acclimated killifish. The very negative TEP in FW attenuated over the first 12 h of exposure and by 24-30 h reached +3 mV, representative of long-term FW-acclimated animals; this reflected a progressive increase in P Cl/P Na from 0.23 to 1.30, probably associated with closing of the paracellular shunt pathway. Thereafter, the TEP in FW-acclimated killifish was unresponsive to [Ca2+] (also to [K+], [Mg2+], or chloride salts of choline and NMDG), but became more positive at SW levels of [Na+]. Killifish live in a variable salinity environment and are incapable of gill Cl(-) uptake in FW. We conclude that the adaptive significance of the TEP patterns is that changeover to a very negative TEP in FW will immediately limit Na+ loss while not interfering with active Cl(-) uptake because there is none. Keeping the shunt permeability high for a few hours means that killifish can return to SW and instantaneously re-activate their NaCl excretion mechanism.     

Dependence of the red blood cell calcium pump on the membrane potential

(1) It is shown that the rate of calcium extrusion from intact human red cells is faster at a membrane potential of approximately +50 mV (inside) than at approximately -50 mV. (2) The positive potential applied was the chloride potential of KCl cells in a K-gluconate medium when the Ca2+ sensitive K+ channel was blocked by 0.3mM quinidine. The negative potential resulted from the high K+ permeability in Ca2+ loaded cells (the cells were loaded to a Ca2+ activity in the cell water of about 50 microM). (3) It is further demonstrated that the Ca2+ affinity of the pump ATPase is decreased both at the internal (high affinity) and external (low affinity) site by increasing the proton concentration. Acidification thus inhibits internally and stimulates externally. (4) An indirect effect of the membrane potential on the pump activity via the accompanying pH shifts on either side of the membrane could be ruled out by choosing Ca2+ concentrations which are fully activating at the internal Ca2+ binding site at pH 6.5 and not yet inhibitory at the external Ca2+ binding site at pH 8. (5) The result is compatible with the assumption that the human red cell Ca-pump is exchanging Ca2+ for protons, yet is electrogenic by virtue of a stoichiometry of 1H+:1Ca2+ for this exchange.     

Membrane potential of primitive red cells from chick embryo is a proton potential

The membrane potential of primitive red cells from 4- and 6-day old chick embryos has been determined using the fluorescent dye Dis-C3-(5). At day 4 the membrane potential Em was -44 mV for pH 7.4 and 20 degrees C and -36 mV at day 6. Both values are far removed from the equilibrium potential for chloride, which is about -14 mV at day 6. Changes in the external potassium, sodium or chloride concentration were without effect on the membrane potential, except at very high potassium concentrations, where a small but significant depolarization was observed at day 6. The measurements gave the same results in the absence or presence of the anion exchange blocking agent DIDS. Three pieces of evidence indicate that the membrane potential of primitive red cells is primarily caused by an electrogenic H+ conductance: 1) The measured membrane potential of -36 mV at day 6 is close to the previously determined proton equilibrium potential (Baumann and Haller, 1983) EH + of -36 mV. 2) Addition of the electrosilent Cl-/OH- exchanger tributyltin causes a significant depolarization of about 20 mV at day 4 and about 14 mV at day 6. 3) Measurement of hydrogen ion fluxes demonstrate a potential dependent proton conductance, which increases with depolarization. These results indicate that large qualitative differences exist with regard to the mechanisms involved in the generation of membrane potential and hydrogen distribution between red cell and plasma of embryonic and adult chicken.     

On the mechanism of sodium ion translocation by methylmalonyl-CoA decarboxylase from Veillonella alcalescens

Veillonella alcalescens during lactate degradation developed an Na+ concentration gradient with 7-8 times higher external than internal Na+ concentrations in the logarithmic growth phase. The gradient declined to a factor of 1.9 in the late stationary phase. Methylmalonyl-CoA decarboxylase reconstituted into proteoliposomes performed an active electrogenic Na+ transport, creating delta psi of 60 mV, delta pNa+ of 50 mV, and delta mu Na+ of 110 mV. In the initial phase of the transport, the decarboxylase catalyzed the uptake of 2 Na+ ions malonyl-CoA molecule decarboxylated. During further development of the electrochemical Na+ gradient, this ratio gradually declined to zero, when decarboxylation continued without further increase of the internal Na+ concentration. The rate of malonyl-CoA decarboxylation declined initially during development of the membrane potential, but remained unchanged later on. Monensin abolished the Na+ gradient and increased the malonyl-CoA decarboxylation rate 2.8-fold. On dissipating the membrane potential with valinomycin, the internal Na+ concentration reached three times higher values than in its absence, and the decarboxylation rate increased 2.8-fold. Methylmalonyl-CoA decarboxylase catalyzed an exchange of internal and external Na+ ions in addition to net Na+ accumulation. The initial rate of Na+ influx was double that of malonyl-CoA decarboxylation. In the following, both rates decreased about twofold in parallel to values which remained constant during further development of the electrochemical Na+ gradient. Thus, Na+ influx and malonyl-CoA decarboxylation follow a stoichiometry of approximately 2:1, independent of the magnitude of the electrochemical Na+ gradient and are thus highly coupled events.     

The diffusion and electrogenic components of the membrane potential of hypoxic myocardium

The diffusion and electrogenic components of the resting potential of hypoxic ventricular muscle were separated by inhibition of the sodium pump with 10(-4) M ouabain. The response to varying external K concentrations (Ko) was studied. Arterially perfused rabbit hearts were submitted to 60 min hypoxia in Krebs solution containing 5 mM K throughout or to different external K concentrations during the last 20 min of hypoxia. For K concentrations between 1.5 and 10 mM, hypoxia did not change the resting potential except for a slight hyperpolarization in 7.5 mM K. The diffusion component of the resting potential did not differ from the resting potential at Ko less than 5 mM. An electrogenic potential of -3 to -6 mV was detectable at Ko values between 5 and 10 mM. The internal K concentration, Ki, was estimated from extrapolations to zero potential of the relation resting potential vs. Ko in normoxic and hypoxic hearts. These experiments revealed a decline of Ki of 16 mM with hypoxia. The variation of the diffusion potential with external K was fitted by a PNa:PK ratio five times lower than in normoxia. It has been concluded that an increase in K permeability and the persistence of electrogenic Na extrusion during hypoxia of rather short duration prevent membrane depolarization despite the myocardial K loss.     

The ionic basis of the resting potential of molluscan unstriated muscle

Summary The membrane potential (Vm) of unstriated, non-spiking fibres from the buccal retractor muscle of the opisthobranch molluscPhiline aperta is primarily determined by the distribution of the potassium ion across the membrane. In salines where potassium is varied and chloride remains constant or nearly so, the membrane potential varied with log external K⁺ with a slope of 50.6 (±2.3) mV per decade. In chloride-free salines the slope was 48.5 mV per decade. The experiments were conducted at temperatures of 18–20° C.A ten-fold reduction in external chloride concentration depolarised the fibres by around 10 mV, indicating that chloride permeability makes some contribution to Vm. In salines where [K]₀·[Cl]₀ is constant the Nernst slope was 55.8 mV per decade compared with the theoretical value of 58 mV.The experimental data suggest that the internal potassium concentration of the fibres is 247±31 mM and pNa/pK is 0.01, giving a predicted value of Vm in sea water of –72 mV. The membrane potential of 90 fibres measured in sea water was –74.2±1.3 mV. The membrane contains an electrogenic sodium pump which contributes 4–5 mV to the membrane potential.     

Effect of different salinities on the coniform chloride cells of mayfly larvae

The larvae of Callibaetis coloradensis can tolerate a fairly wide range of salinities at hypotonic concentrations. However, they are more sensitive to increasing than to decreasing salt concentrations. Exposure to isotonic concentration results in profound degenerations of the chloride cells within 1 day. Long-term adaptation to diluted fresh water causes a significant increase in the number of chloride cells, whereas the gradual concentration of fresh water to finally 120 mM sodium chloride within a period of 15 days leads to approximately 50 per cent mortality and significantly reduces the number of chloride cells in the survivors. The same reciprocal relation between the number of chloride cells and the external salinity was found in larvae of C. floridans collected from fresh- and brackish-water habitats. These results suggest that the adaptive behaviour of the chloride cells is correlated with the osmoregulatory situation and enables these animals to live in habitats of different salinities.     

Osmoregulation in larvae of the New Zealand salt-water mosquito Opifex fuscus Hutton

An account is given of the survival of larvae of Opifex fuscus at different salinities and of changes in osmotic pressure and sodium and chloride concentrations in the haemolymph.     

Membrane potential measurements during rat liver regeneration

Membrane potential was measured in perfused rat liver and was shown to increase from ?33 ± 1.0 mV in livers from normal rats to ?50 ± 1.1 mV in livers from rats 12 hr after partial hepatectomy. The hyperpolarization of the membrane in regenerating liver was no longer evident after perfusion with 1 mM ouabain for 5 min. Ouabain had a small (4 mV) depolarizing effect on membrane potential in normal liver. The potential measured in normal and regenerating liver decreased as a function of the external potassium concentration above 5 mM; however, the potential was more electronegative in regenerating liver compared to normal liver at all values of external potassium concentration, and the differences in potential between the two kinds of cells did not decrease at higher concentrations of external potassium. Thus, a plot of membrane potential vs external potassium concentration resulted in approximately parallel curves for the two different cell types. We conclude that hyperpolarization of the liver cell membrane is an early event during rat liver regeneration and results from an electrogenic Na-K pump.     

Water balance and osmoregulation in Stenocara gracilipes,a wax-blooming tenebrionid beetle from the Namib Desert

Dehydration (10 days at 27 degrees C) of the Namib tenebrionid Stenocara gracilipes resulted in a rapid weight loss (17.5%), and a substantial decline in haemolymph volume (72%). Although the lipid content decreased significantly, metabolic water production was insufficient to maintain total body water (TBW). Rehydration (no food) resulted in increases in haemolymph volume, body weight (sub-normal), and TBW to normality. Haemolymph osmolality, sodium, potassium, chloride, amino acids, and sugars (trehalose and glucose), were all subject to osmoregulatory control during both dehydration and rehydration. Major osmolar effectors in this species are sodium, chloride, and amino acids, with most of the contribution to regulation of haemolymph osmolality coming from changes in the levels of these constituents. Changes in amino acid levels are not the result of interchange with soluble protein during dehydration (the possibility exists during extended rehydration, however). Despite faecal losses of sodium being low (8.2% of that removed from the haemolymph during dehydration), sodium concentrations do not return to normal during rehydration. Chloride concentrations increase supra-normally when access to water is allowed, and remain elevated throughout the rehydration period. Although faecal loss of potassium greatly exceeded the amount removed from the haemolymph (by approximately 1.8 times), haemolymph potassium levels were strongly regulated during rehydration. S. gracilipes demonstrates an exquisite capacity to regulate haemolymph osmolality under conditions of both acute water-shortage and -abundance. Together with an efficient water economy (drinking when fog-water is available, and a superb water conservation mechanism in the form of wax-bloom production), this must serve to contribute to long-term survival of this species in an otherwise harsh abode.     

beta-Amino acid transport across the renal brush-border membrane is coupled to both Na and Cl

Sodium-dependent beta-alanine uptake into dog renal brush-border membrane vesicles was studied. Kinetic analysis indicated a single transport system, highly specific for beta-amino acids, with Km = 35 microM at 100 mM NaCl. Sodium-dependent beta-alanine transport was markedly anion-dependent, being highest in the presence of chloride (Cl greater than Br greater than SCN greater than NO3 approximately I greater than F) and virtually nonexistent in the presence of gluconate and other nonphysiological chloride substitutes. In addition, it was observed that beta-alanine uptake could be driven against a concentration gradient by a chloride gradient. Similar results were found for sodium. Taken together, these observations provide strong evidence that beta-alanine transport across the renal brush-border membrane is coupled to both sodium and chloride. Studies of the dependence of beta-alanine flux on chloride and sodium concentrations indicated that one chloride ion and multiple sodium ions were involved in the beta-alanine transport event. beta-Alanine flux on chloride found to involve the net transfer of positive charge, consistent with these stoichiometric assignments. The hallucinogen harmaline inhibited beta-alanine uptake in a 1:1 fashion, presumably by acting at a single site on the transport molecule. The ability of harmaline to inhibit beta-alanine uptake was decreased when the chloride concentration was lowered but was unchanged when the sodium concentration was decreased. These results indicate that harmaline does not compete with sodium for a binding site on the carrier as has been suggested for other sodium-coupled transport systems, and that instead, chloride may be required for harmaline binding to the beta-alanine transporter.     

Ion Transport in Suaeda maritima: Its Relation to Growth and Implications for the Pathway of Radial Transport of Ions across the Root

The ion relations of the halophytc Suaeda maritima are described.When plants grew in 340 mol m^–3 sodium chloride (—1•76MPa) leaf solute potentials decreased, and were sustained around—2•5 MPa Inorganic ion concentration (mostly of sodiumchloride) accounted for this. Comparable shoot ion concentrationsof potassium, nitrate and sulphate occurred when plants grewon different salinity types characterized by these ions. Netsodium transport and shoot sodium concentration increased dramaticallywith increases in external sodium chloride concentration upto 85 mol m^–3; thereafter, further increases in externalsodium chloride concentration had relatively little effect uponeither shoot sodium concentration or upon net transport of sodiumto the shoot. The net transport of sodium plus potassium onlydoubled when the external concentration of sodium plus potassiumincreased from 24 to 687 mol m^–3 Shoot ion concentrationswere remarkably constant with time, external concentration andsalinity type. The membrane flux rates and symplasmic ion concentrations neededto sustain the observed net transport of sodium (of some 10mmol g^–1 dry wt. of roots d^–1) are calculated fromanatomical and stereological data for the root system of thisspecies. The minimum net sodium chloride flux to load the symplasmwould be 260 nmol m^–2s^–1 if the whole cortical andepidermal plasmalemmal surface area were used uniformly, butthe flux rate required would be 3000 nmol m^–2s^–1if uptake took place only at the root surface. A flux rate ofat least 1000 nmol m^–2s^–1 is needed between symplasmand xylem. The symplasmic concentration of NaCl would be atleast 80 mol m^–3. It is argued (1), that both symplasmicand xylem loading are likely to be passive processes mediatedby ion channels rather than active carriers, (2), that net iontransport at 340 mol m^–3 sodium chloride is close to themaximum which is physiologically sustainable and (3), that growthof this halophyte is limited by NaCl supply from the root. Key words: Suaeda maritima, halophyte, salinity, roots, radial ion transport