scholarly journals Cellular NH4+/K+ transport pathways in mouse medullary thick limb of Henle. Regulation by intracellular pH.

1992 ◽  
Vol 99 (3) ◽  
pp. 435-461 ◽  
Author(s):  
D Kikeri ◽  
A Sun ◽  
M L Zeidel ◽  
S C Hebert
Keyword(s):  
Intracellular Ph ◽  
Feedback Signal ◽  
Thick Ascending Limb ◽  
Transport Pathway ◽  
Cytosolic Ph ◽  
Transport Pathways ◽  
Buffering Power ◽  
Cd1 Mice ◽  
Apical Membranes ◽  
K Transport

Fluorescence and electrophysiological methods were used to determine the effects of intracellular pH (pHi) on cellular NH4+/K+ transport pathways in the renal medullary thick ascending limb of Henle (MTAL) from CD1 mice. Studies were performed in suspensions of MTAL tubules (S-MTAL) and in isolated, perfused MTAL segments (IP-MTAL). Steady-state pHi measured using 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) averaged 7.42 +/- 0.02 (mean +/- SE) in S-MTAL and 7.26 +/- 0.04 in IP-MTAL. The intrinsic cellular buffering power of MTAL cells was 29.7 +/- 2.4 mM/pHi unit at pHi values between 7.0 and 7.6, but below a pHi of 7.0 the intrinsic buffering power increased linearly to approximately 50 mM/pHi unit at pHi 6.5. In IP-MTAL, NH4+ entered cells across apical membranes via both Ba(2+)-sensitive pathway and furosemide-sensitive Na+:K+(NH4+):2Cl- cotransport mechanisms. The K0.5 and maximal rate for combined apical entry were 0.5 mM and 83.3 mM/min, respectively. The apical Ba(2+)-sensitive cell conductance in IP-MTAL (Gc), which reflects the apical K+ conductance, was sensitive to pHi over a pHi range of 6.0-7.4 with an apparent K0.5 at pHi approximately 6.7. The rate of cellular NH4+ influx in IP-MTAL due to the apical Ba(2+)-sensitive NH4+ transport pathway was sensitive to reduction in cytosolic pH whether pHi was changed by acidifying the basolateral medium or by inhibition of the apical Na+:H+ exchanger with amiloride at a constant pHo of 7.4. The pHi sensitivities of Gc and apical, Ba(2+)-sensitive NH4+ influx in IP-MTAL were virtually identical. The pHi sensitivity of the Ba(2+)-sensitive NH4+ influx in S-MTAL when exposed to (apical+basolateral) NH4Cl was greater than that observed in IP-MTAL where NH4Cl was added only to apical membranes, suggesting an additional effect of intracellular NH4+/NH3 on NH4+ influx. NH4+ entry via apical Na+:K+ (NH4+):2Cl- cotransport in IP-MTAL was somewhat more sensitive to reductions in pHi than the Ba(2+)-sensitive NH4+ influx pathway; NH4+ entry decreased by 52.9 +/- 13.4% on reducing pHi from 7.31 +/- 0.17 to 6.82 +/- 0.14. These results suggest that pHi may provide a negative feedback signal for regulating the rate of apical NH4+ entry, and hence transcellular NH4+ transport, in the MTAL. A model incorporating these results is proposed which illustrates the role of both pHi and basolateral/intracellular NH4+/NH3 in regulating the rate of transcellular N H4+ transport in the MTAL.

Secretion of H+ and K+ by bullfrog gastric mucosa: characterization of K+ transport pathway

1980 ◽  
Vol 239 (6) ◽  
pp. G485-G492
Author(s):  
P. C. Sen ◽  
L. L. Tague ◽  
T. K. Ray
Keyword(s):  
Gastric Mucosa ◽  
Nutrient Solution ◽  
Rana Catesbeiana ◽  
Transport Pathway ◽  
Mucosal Side ◽  
Apical Membranes ◽  
K Transport ◽  
Co2 Addition

The transport of K+ and H+ (both expressed as mueq/h) by in vitro chambered bullfrog (Rana catesbeiana) gastric mucosa have been studied under a variety of conditions such as anoxia, addition of p-chloromercuribenzene sulfonic acid (PCMBS) into the secretory solution, inclusion of ouabain in the nutrient solution, addition of thiocyanate (SCN-) into the mucosal solution, and replacement of nutrient chloride (Cl-) with sulfate (SO4(2-)), or gluconate (Gl). Anoxia reversibly reduced the H+ transport close to zero within 15 min and gradually reduces the K+ transport throughout the 2-h period of anoxia. The presence of 2.5 X 10(-4) M mucosal PCMBS in the histamine-stimulated mucosa increases the K+ transport, which is promptly reduced by changing the gas phase to 95% N2-5% CO2. Addition of ouabain to the nutrient solution of the histamine-stimulated mucosa with PCMBS on the mucosal side significantly (P < 0.05) reduces the K+ transport within 60 min. Addition of SCN- to the mucosal solution of a histamine-stimulated mucosa with regular nutrient or O, K+ nutrient and 10, K+ mucosal solution reduces the H+ transport to near zero within 60 min. This SCN- inhibition can be reversed by elevating secretory K+. Substitution of nutrient Cl- with SO4(2-) or Gl in the histamine-stimulated mucosa reversibly inhibits H+ transport and reduces K+ transport to a low level (0.7 +/- 0.05). Our data suggest that the K+ transport across the apical membranes of gastric cells is to a large extent a passive carrier-mediated process, and the transport of both K+ and Cl- are coupled at the apical membrane.

scholarly journals Effects of ammonium on intracellular pH in rat medullary thick ascending limb: mechanisms of apical membrane NH4+ transport.

1994 ◽  
Vol 103 (5) ◽  
pp. 917-936 ◽  
Author(s):  
B A Watts ◽  
D W Good
Keyword(s):  
Steady State ◽  
Intracellular Ph ◽  
Apical Membrane ◽  
Ammonium Transport ◽  
Thick Ascending Limb ◽  
Transport Pathway ◽  
Critical Determinant ◽  
Medullary Thick Ascending Limb ◽  
Intracellular Alkalinization

The renal medullary thick ascending limb (MTAL) actively reabsorbs ammonium ions. To examine the effects of NH4+ transport on intracellular pH (pHi) and the mechanisms of apical membrane NH4+ transport, MTALs from rats were isolated and perfused in vitro with 25 mM HCO3(-)-buffered solutions (pH 7.4). pHi was monitored using the fluorescent dye BCECF. In the absence of NH4+, the mean pHi was 7.16. Luminal addition of 20 mM NH4+ caused a rapid intracellular acidification (dpHi/dt = 11.1 U/min) and reduced the steady state pHi to 6.67 (delta pHi = 0.5 U), indicating that apical NH4+ entry was more rapid than entry of NH3. Luminal furosemide (10(-4) M) reduced the initial rate of cell acidification by 70% and the fall in steady state pHi by 35%. The residual acidification observed with furosemide was inhibited by luminal barium (12 mM), indicating that apical NH4+ entry occurred via both furosemide (Na(+)-NH4(+)-2Cl- cotransport) and barium-sensitive pathways. The role of these pathways in NH4+ absorption was assessed under symmetric ammonium conditions. With 4 mM NH4+ in perfusate and bath, mean steady state pHi was 6.61 and net ammonium absorption was 12 pmol/min/mm. Addition of furosemide to the lumen abolished net ammonium absorption and caused pHi to increase abruptly (dpHi/dt = 0.8 U/min) to 7.0. Increasing luminal [K+] from 4 to 25 mM caused a similar, rapid cell alkalinization. The pronounced cell alkalinization observed with furosemide or increasing [K+] was not observed in the absence of NH4+. In symmetric 4 mM NH4+ solutions, addition of barium to the lumen caused a slow intracellular alkalinization and reduced net ammonium absorption only by 14%. Conclusions: (a) ammonium transport is a critical determinant of pHi in the MTAL, with NH4+ absorption markedly acidifying the cells and maneuvers that inhibit apical NH4+ uptake (furosemide or elevation of luminal [K+]) causing intracellular alkalinization; (b) most or all of transcellular ammonium absorption is mediated by apical membrane Na(+)-NH4(+)-2Cl- cotransport; (c) NH4+ also permeates a barium-sensitive apical membrane transport pathway (presumably apical membrane K+ channels) but this pathway does not contribute significantly to ammonium absorption under physiologic (symmetric ammonium) conditions.

An apical K+-dependent HCO3− transport pathway opposes transepithelial HCO3− absorption in rat medullary thick ascending limb

2004 ◽  
Vol 287 (1) ◽  
pp. F57-F63 ◽  
Author(s):  
Bruns A. Watts ◽  
David W. Good
Keyword(s):  
Initial Rate ◽  
Apical Membrane ◽  
Channel Blockers ◽  
Thick Ascending Limb ◽  
Acid Base ◽  
Transport Pathway ◽  
Transport Pathways ◽  
Medullary Thick Ascending Limb ◽  
And Function

Absorption of HCO3− in the medullary thick ascending limb (MTAL) is mediated by apical membrane Na+/H+ exchange. The identity and function of other apical acid-base transporters in this segment have not been defined. The present study was designed to examine apical membrane HCO3−/OH−/H+ transport pathways in the rat MTAL and to determine their role in transepithelial HCO3− absorption. MTALs were perfused in vitro in Na+- and Cl−-free solutions containing 25 mM HCO3−, 5% CO2. Lumen addition of either 120 mM Cl− or 50 mM Na+ (50 μM EIPA present) had no effect on intracellular pH (pHi). Lumen Cl− addition also had no effect on pHi in the presence of 145 mM Na+ or in the nominal absence of HCO3−/CO2. Thus there was no evidence for apical Cl−/HCO3− (OH−) exchange, Na+-dependent Cl−/HCO3− exchange, or Na+-HCO3− cotransport. In contrast, in tubules studied in Na+- and Cl−-free solutions containing 25 mM HCO3−, 5% CO2 and 120 mM K+, removal of luminal K+ induced a rapid and pronounced decrease in pHi (ΔpHi = 0.56 ± 0.06 pH U). pHi recovered following lumen K+ readdition. The initial rate of net base efflux induced by lumen K+ removal was decreased 85% at the same pHi in the nominal absence of HCO3−/CO2, indicating a dependence on HCO3−/CO2 and arguing against apical K+/H+ exchange. A combination of the apical K+ channel blockers quinidine (0.1 mM) and glybenclamide (0.25 mM) had no effect on the lumen K+-induced pHi changes, arguing against electrically coupled K+ and HCO3− conductances. The effect of lumen K+ on pHi was inhibited by 1 mM H2DIDS. In addition, lumen addition of DIDS increased transepithelial HCO3− absorption from 10.7 ± 0.7 to 14.9 ± 0.7 pmol·min−1·mm−1 ( P < 0.001) and increased pHi slightly in MTAL studied in physiological solutions (25 mM HCO3− and 4 mM K+). Lumen DIDS stimulated HCO3− absorption in the absence and presence of furosemide. These results are consistent with an apical membrane K+-dependent HCO3− transport pathway that mediates coupled transfer of K+ and HCO3− from cell to lumen in the MTAL. This mechanism, possibly an apical K+-HCO3− cotransporter, functions in parallel with apical Na+/H+ exchange and opposes transepithelial HCO3− absorption.

Characterization of Na+-H+ antiport in type II alveolar epithelial cells

1987 ◽  
Vol 252 (5) ◽  
pp. C490-C498 ◽  
Author(s):  
E. P. Nord ◽  
S. E. Brown ◽  
E. D. Crandall
Keyword(s):  
Epithelial Cells ◽  
Intracellular Ph ◽  
Cell Types ◽  
Alveolar Epithelial Cells ◽  
Hill Coefficient ◽  
Type Ii ◽  
Transport Pathway ◽  
Cytosolic Ph ◽  
Alveolar Epithelial ◽  
Major Mechanism

The presence of a Na+-H+ exchange pathway in the plasma membrane of type II alveolar epithelial cells was explored using the pH-sensitive fluorescent probe 2,7-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) to monitor changes in cytosolic pH. Freshly prepared pneumocytes suspended in medium at pH 7.4 had an intracellular pH of 7.07 +/- 0.07. Acid-loaded cells equilibrated in sodium-free buffer showed rapid cytoplasmic alkalinization when exposed to sodium. This response to sodium was inhibited greater than 90% by 10(-4) M amiloride. The presence of the K+ ionophore, valinomycin, had no effect on the rate of Na+-dependent alkalinization, indicating the electroneutrality of the system. Li+ partially supported the alkalinization process, but other monovalent cations, notably K+, Rb+, and Cs+, were without effect. Kinetic analysis for Na+ at the external binding site yielded KNat (dissociation constant) = 62 +/- 3 mM. Hill equation analysis of the data derived a Hill coefficient (n) = 1.2 +/- 0.1 for Na+, consistent with a 1:1 stoichiometry for Na+ and H+ for the transporter. The Ki for amiloride inhibition of proton efflux at the external locus was 0.45 microM. These findings define the transport pathway as Na+-H+ antiport, with kinetic parameters somewhat similar to those described for other cell types. Antiport activity was detected at intracellular pH (pHi) values of 6.8 or below, with no activity observed at pHi 7.0-7.2. It is suggested that Na+-H+ exchange is a major mechanism whereby pneumocytes recover from an acid load and that this transport pathway may play an important role in vectorial reabsorption of Na+ from the alveolar air spaces.

Paracellular calcium transport across renal and intestinal epithelia

2014 ◽  
Vol 92 (6) ◽  
pp. 467-480 ◽  
Author(s):  
R. Todd Alexander ◽  
Juraj Rievaj ◽  
Henrik Dimke
Keyword(s):  
Thick Ascending Limb ◽  
Transport Pathway ◽  
Transport Pathways ◽  
Transcellular Transport ◽  
Renal Epithelium ◽  
Intestinal Epithelia ◽  
Physiological Processes ◽  
Renal Tubular Reabsorption ◽  
Renal Tubular ◽  
Paracellular Shunt

Calcium (Ca2+) is a key constituent in a myriad of physiological processes from intracellular signalling to the mineralization of bone. As a consequence, Ca2+ is maintained within narrow limits when circulating in plasma. This is accomplished via regulated interplay between intestinal absorption, renal tubular reabsorption, and exchange with bone. Many studies have focused on the highly regulated active transcellular transport pathways for Ca2+ from the duodenum of the intestine and the distal nephron of the kidney. However, comparatively little work has examined the molecular constituents creating the paracellular shunt across intestinal and renal epithelium, the transport pathway responsible for the majority of transepithelial Ca2+ flux. More specifically, passive paracellular Ca2+ absorption occurs across the majority of the intestine in addition to the renal proximal tubule and thick ascending limb of Henle’s loop. Importantly, recent studies demonstrated that Ca2+ transport through the paracellular shunt is significantly regulated. Therefore, we have summarized the evidence for different modes of paracellular Ca2+ flux across renal and intestinal epithelia and highlighted recent molecular insights into both the mechanism of secondarily active paracellular Ca2+ movement and the identity of claudins that permit the passage of Ca2+ through the tight junction of these epithelia.

Cation and anion transport pathways in volume regulatory response of human lymphocytes to hyposmotic media

1985 ◽  
Vol 248 (5) ◽  
pp. C480-C487 ◽  
Author(s):  
B. Sarkadi ◽  
R. Cheung ◽  
E. Mack ◽  
S. Grinstein ◽  
E. W. Gelfand ◽  
...  
Keyword(s):  
Human Lymphocytes ◽  
Regulatory Volume Decrease ◽  
Anion Transport ◽  
Red Cells ◽  
K Channel ◽  
Transport Pathway ◽  
Transport Pathways ◽  
Calmodulin Antagonist ◽  
Cl Transport ◽  
K Transport

The regulatory volume decrease of osmotically swollen human peripheral blood lymphocytes can be inhibited by agents acting on volume-activated K+- or Cl--transport pathways. Quinine, cetiedil, and 3,3'-dipropylthiadicarbocyanine were found to block the volume-induced K+ transport by interaction with sites on the outside face of the membrane, perhaps by competition with external K+. Drugs known to influence calmodulin action inhibit both volume-induced K+ and Cl- transport to varying degrees. Those inhibitors, particularly of K+ transport, are correlated with their calmodulin-antagonist activity. Penetrating sulfhydryl (SH) reagents (in contrast to nonpenetrating ones) are potent inhibitors of both volume-induced K+ and Cl- movements, indicating the presence of functionally important SH groups located within the membrane or at the cytoplasmic face. A number of agents, such as dipyridamole and oligomycin C, are specific inhibitors of the volume-activated anion pathway. In all respects studied, the inhibition characteristics of the volume-activated K+ pathway of lymphocytes resemble those of the Ca2+-activated K+ channel of red cells. In contrast, the volume-induced anion permeability differs from the primary anion-transport pathway of red cells.

Vasopressin regulates apical and basolateral Na(+)-H+ antiporters in mouse medullary thick ascending limbs

1992 ◽  
Vol 262 (2) ◽  
pp. F241-F247 ◽  
Author(s):  
A. M. Sun ◽  
D. Kikeri ◽  
S. C. Hebert
Keyword(s):  
Steady State ◽  
Intracellular Ph ◽  
Arginine Vasopressin ◽  
Basolateral Membrane ◽  
Thick Ascending Limb ◽  
Basolateral Membranes ◽  
Medullary Thick Ascending Limb ◽  
Apical Membranes

We assessed in isolated perfused mouse medullary thick ascending limb (MTAL) segments Na(+)-H+ antiporter activity in both apical and basolateral membranes and the effects of arginine vasopressin (AVP) on the activities of these antiporters under isotonic conditions using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein to monitor intracellular pH (pHi). When the apical Na(+)-H+ antiporter was inhibited in the absence of AVP with removal of luminal Na+ plus addition of 0.5 mM amiloride, a small but significant increase in pHi was observed after luminal NH4Cl-induced acidification of MTAL cells to pHi less than 6.7. This increase in pHi was dependent on basolateral Na+ and inhibited with 0.5 mM basolateral amiloride, consistent with the function of a basolateral Na(+)-H+ antiporter. Basolateral AVP (100 microU/ml) enhanced the rate of pHi recovery due to the basolateral Na(+)-H+ antiporter by more than twofold. In contrast, AVP decreased the apical Na(+)-H+ antiporter activity by 50%. In the absence of AVP, addition of 0.5 mM amiloride to the luminal perfusate reduced steady-state pHi by 0.40 +/- 0.07 units, whereas exposure of the basolateral membrane to the same concentration of amiloride had no effect on pHi (delta pHi = 0.01 +/- 0.01 units). AVP reduced the magnitude of cell acidification on exposure of apical membranes to amiloride (delta pHi = 0.16 +/- 0.03) but increased the pHi response to basolateral amiloride (delta pHi = 0.09 +/- 0.00). Thus Na(+)-H+ antiporters are present on both apical and basolateral membranes of the mouse MTAL in the absence of AVP. AVP stimulates the basolateral, while inhibiting the apical, Na(+)-H+ antiporter.(ABSTRACT TRUNCATED AT 250 WORDS)

scholarly journals The effect of N-ethylmaleimide on K+ and Cl- transport pathways in the lamprey erythrocyte membrane: activation of K+/Cl- cotransport

1991 ◽  
Vol 159 (1) ◽  
pp. 325-334 ◽  
Author(s):  
K. Kirk
Keyword(s):  
Erythrocyte Membrane ◽  
Mammalian Species ◽  
Individual Case ◽  
Mean Value ◽  
Transport Pathway ◽  
Transport Pathways ◽  
Physiological Conditions ◽  
Cl Transport ◽  
Dependent Component ◽  
K Transport

The effect of the sulphydryl reagent N-ethylmaleimide on the K+ and Cl- transport pathways of the lamprey erythrocyte membrane was found to be quite complex. N-Ethylmaleimide inhibited the Ba(2+)-sensitive pathway that mediates most of the ouabain-resistant influx of K+ into the cell under physiological conditions but stimulated a Cl(−)-dependent, B(2+)-resistant K+ transport pathway that was inhibited by compounds that inhibit Cl(−)-dependent K+ transport in the human erythrocyte. N-Ethylmaleimide (in most cases) reduced the total influx of Cl- into the lamprey erythrocyte but (in all cases) introduced a K(+)-dependent component into the measured Cl- uptake; this was explained in terms of N-ethylmaleimide having inhibited the pathway primarily responsible for Cl- influx under physiological conditions but having stimulated a second, K(+)-dependent Cl- transport pathway. Although the magnitude of the K+ and Cl- fluxes stimulated by N-ethylmaleimide varied widely between cells from different lampreys, there was, in each individual case, a close similarity between the magnitude of the Cl(−)-dependent K+ influx (calculated from the 86Rb+ uptake) and the K(+)-dependent Cl- influx; the mean value for the ratio of the former to the latter was 1.01 +/− 0.03 (N = 5). The results are therefore consistent with the sulphydryl reagent having activated a K+/Cl- cotransport system similar to that present in erythrocytes from many mammalian species. This raises the possibility that the lamprey red cell may be a uniquely suitable system in which to study the characteristics of Cl- transport by this pathway.

Multihormonal regulation of nephron epithelia: achieved through combinational mode?

1995 ◽  
Vol 269 (4) ◽  
pp. R739-R748 ◽  
Author(s):  
C. De Rouffignac
Keyword(s):  
Sodium Chloride ◽  
Hormonal Regulation ◽  
Biological Effects ◽  
Basolateral Membrane ◽  
Positive Interactions ◽  
Thick Ascending Limb ◽  
Transport Pathways ◽  
Receptor Complexes ◽  
Nephron Segment ◽  
Urinary Concentrating Ability

The kidney is the main organ regulating composition of body fluids. A considerable number of hormones control the activity of renal cells to maintain hydromineral equilibrium. It becomes more and more difficult to interpret this multihormonal control in terms of regulatory processes. To illustrate this complexity, the hormonal regulation of electrolyte transport in the nephron thick ascending limb is taken as an example. This nephron segment is largely responsible for two kidney functions: the urinary-concentrating ability (by its capacity to deliver hypertonic sodium chloride into the medullary interstitium) and regulation of magnesium excretion into final urine. Six hormones are presently identified as acting on the transport of both sodium chloride and magnesium ions in this nephron segment. Therefore, the pertinent question is how the thick ascending limb and, hence, the kidney, is capable of regulating water balance independently from magnesium balance. It is proposed that the hormones act in combination: circulating levels of the individual hormones acting on these cells may determine the configuration of the paracellular and transcellular transport pathways of the epithelium either in the “sodium” or “magnesium” mode. The configuration would depend on the distribution and activity of the receptor at the surface of the basolateral membrane in contact with the circulating hormones. This distribution along with stimulation of respective signal transduction pathways would lead to the final biological effects. It is already known that the distribution of cell receptors may vary according to factors such as age, nutritional variability, hormonal status, degree of desensitization of the receptors, etc. The modulation of hormonal responses would depend therefore on the degree of coupling of hormone-receptor complexes to different intracellular transduction pathways and on the resulting negative and/or positive interactions between these pathways.

Investigation of Na+ and K+ Transport in Halophytes: Functional Analysis of the HmHKT2;1 Transporter from Hordeum maritimum and Expression under Saline Conditions

2019 ◽  
Vol 60 (11) ◽  
pp. 2423-2435 ◽  
Author(s):  
Dorsaf Hmidi ◽  
Dorsaf Messedi ◽  
Claire Corratg�-Faillie ◽  
Th�o Marhuenda ◽  
C�cile Fizames ◽  
...  
Keyword(s):  
Absorption Efficiency ◽  
Extracellular Loop ◽  
Transmembrane Segment ◽  
Transport Pathway ◽  
Reverse Transcription Pcr ◽  
Encoding Gene ◽  
K Transport ◽  
Different Levels ◽  
Adaptation To Salinity

Abstract Control of K+ and Na+ transport plays a central role in plant adaptation to salinity. In the halophyte Hordeum maritimum, we have characterized a transporter gene, named HmHKT2;1, whose homolog HvHKT2;1 in cultivated barley, Hordeum vulgare, was known to give rise to increased salt tolerance when overexpressed. The encoded protein is strictly identical in two H. maritimum ecotypes, from two biotopes (Tunisian sebkhas) affected by different levels of salinity. These two ecotypes were found to display distinctive responses to salt stress in terms of biomass production, Na+ contents, K+ contents and K+ absorption efficiency. Electrophysiological analysis of HmHKT2;1 in Xenopus oocytes revealed distinctive properties when compared with HvHKT2;1 and other transporters from the same group, especially a much higher affinity for both Na+ and K+, and an Na+–K+ symporter behavior in a very broad range of Na+ and K+ concentrations, due to reduced K+ blockage of the transport pathway. Domain swapping experiments identified the region including the fifth transmembrane segment and the adjacent extracellular loop as playing a major role in the determination of the affinity for Na+ and the level of K+ blockage in these HKT2;1 transporters. The analysis (quantitative reverse transcription-PCR; qRT-PCR) of HmHKT2;1 expression in the two ecotypes submitted to saline conditions revealed that the levels of HmHKT2;1 transcripts were maintained constant in the most salt-tolerant ecotype whereas they decreased in the less tolerant one. Both the unique functional properties of HmHKT2;1 and the regulation of the expression of the encoding gene could contribute to H. maritimum adaptation to salinity.

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