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1.
Hubert Felle 《生物化学与生物物理学报:生物膜》1980,602(1):181-195
Green thallus cells of the aquatic liverwort, Riccia fluitans, are rapidly depolarized in the presence of 1–20 μM NH4Cl and 5–100 μM CH3NH3Cl, respectively. Simultaneously, the membrane conductance is increased from 0.41 to 1.2 S · m?2. Uptake of [14C]methylamine is stimulated by increasing [K+]o and inhibited by increasing [Na+]o or [H+]o, is highly voltage sensitive, and saturates at low amine concentrations.Double-reciprocal plots of (a) maximal membrane depolarization and (b) methylamine uptake vs. external amine concentration give apparent Km values of 2 ± 1 μM ammonia and 25–50 μM methylamine; Km values for changes in conductance and membrane current are greater and voltage dependent. Whereas the amine transport into the cell is strongly inhibited by CN?, the amine efflux is stimulated.The current-voltage characteristics of the ammonia transport are represented by a sigmoid curve with an equilibrium potential of ?60 mV, and this is understood as a typical carrier curve with a saturation current of about 70 mA · m?2. It is further concluded that the evidently carrier-mediated transport is competitive for the two amines tested, and that ammonia and methylamine are transported in the protonated form as NH4+ and CH3NH3+ into the cytoplasm. 相似文献
2.
A. Venegoni A. Moroni S. Gazzarrini Maria Teresa Marr 《Plant biology (Stuttgart, Germany)》1997,110(5):369-377
Previous data in Egeria densa leaves demonstrated a strong inhibitory effect of Cs+ on passive K+ influx and on K+-induced, ATP-dependent electrogenic proton extrusion. In this paper we analyzed, using the same material, the effects of Cs+ on ammonium (NH4+) and methylammonium (CH3NH3+) transport in order to elucidate whether a common transport system for K+ and NH4+ could be demonstrated. The effects of Cs+ on NH4+- and CH3NH3+-induced titratable H+ extrusion (–ΔH+) and on transmembrane electrical potential difference (Em) in E. densa leaves were analyzed in parallel. All experiments were run either in the absence or presence of fusicoccin, corresponding to low or high H+-ATPase activity and membrane hyperpolarization and leading, in this material, to respectively active or passive transport of K+. The results suggest the presence in E. densa leaves of two distinct pathways for NH4+ uptake: one in common with NH4+ and (with lower affinity) CH3NH3+, insensitive to Cs+, and a second system, operating at higher H+-ATPase activity and Em hyperpolarization, strongly inhibited by Cs+ and impermeable to CH3NH3+. In agreement with this hypothesis, Xenopus laevis oocytes injected with the KAT1 RNA of Arabidopsis thaliana were permeable to K+ and NH4+, but not to CH3NH3+. 相似文献
3.
Summary Sulphate uptake by rabbit ileal brush border membrane vesicles was stimulated by a transmembrane sodium gradient ([Na+]
o
>[Na+]
i
), but not by a similar potassium gradient.35SO
4
2–
influx (J
oi
SO4
) from outside (o) to inside (i) these vesicles was a hyperbolic function of [SO
4
2–
]
o
and the affinity constant for anion transport was strongly influenced by [Na+]
o
(100mm Na+,K
t
SO4
=0.52mm SO
4
2–
; 10mm Na+,K
t
SO4
=4.32mm SO
4
2–
).J
t
SO4
was a sigmoidal function of [Na+]
o
at pH 7.4 for both low (0.2m) and high (4.0mm) [SO
4
2–
]
o
. The Na+-dependency ofJ
t
SO4
was examined at pH 6.0, 7.4, and 8.0 (same pH inside and outside). At pH 6.0 and 7.4 sigmoidal Na+-dependentJ
t
SO4
exhibited nonlinear Eadie-Hofstee plots indicative of a transport mechanism capable of binding a variable number of sodium ions over the [Na+]
o
range used. Hill plots of anion transport under these conditions displayed slopes near unity at low [Na+]
o
and slopes approximating 2.0 at higher cation concentrations. At pH 8.0, Na+-dependentJ
t
SO4
was hyperbolic and showed linear Eadie-Hofstee and Hill plots, the latter with a single slope near 1.0. When a H+ gradient was imposed across the vesicle wall (pH
i
=8.0, pH
o
=6.0), Na+-dependentJ
t
SO4
was hyperbolic and significantly increased at each [Na+]
o
over values observed using bilateral pH 8.0. In contrast, a H+ gradient oriented in the opposite direction (pH
i
=6.0, pH
o
=8.0) led to Na+-dependentJ
t
SO4
that was sigmoidal and significantly lower at each [Na+]
o
than values found using bilateral pH 6.0. Electrogenicity ofJ
t
SO4
at pH 8.0 for both high and low [Na+]
o
was demonstrated by using a valinomycin-induced transmembrane electrical potential difference. At pH 6.0, electrogenicJ
t
SO4
occurred only at low [Na+]
o
(5mm); anion transfer was electroneutral at 50mm Na+. A model is proposed for proton regulation of sodium sulphate cotransport where flux stoichiometry is controlled by [H+]
i
and sodium binding affinity is modified by [H+]
o
. Preliminary experiments with rabbit proximal tubular brush border membrane vesicles disclosed similarJ
t
SO4
kinetic properties and a common transport mechanism may occur in both tissues. 相似文献
4.
Charles Levinson 《The Journal of membrane biology》1992,126(3):277-284
Summary After swelling in hyposmotic solution, Ehrlich ascites tumor cells shrink towards their original volume. Upon restoration of isosmolality (300 mOsm) the cells initially shrink but subsequently recover volume. This regulatory volume increase (RVI) is completely blocked when [Na+]
o
or [Cl–]
o
is reduced by 50% in the presence of normal [K+]
o
. With normal [NaCl]
o
but less than 2 mm [K+]
o
, not only is volume recovery blocked but the cells lose KCl and shrink. When [K+]
o
is increased to 5 mm there is a rapid net uptake of K+ and Cl– which results in volume recovery. This suggests that the reswelling phase requires the simultaneous presence of Na+, K+, and Cl–. Although ouabain has no effect on volume recovery, bumetanide completely blocks RVI by inhibiting a cotransport pathway that mediates the net uptake of Na+, K+ and Cl– in the ratio of 1Na1K2Cl. Na+ that accumulates is then replaced by K+ via the Na/K pump.I wish to thank my colleague, Dr. Thomas C. Smith for advice and helpful comments during the course of these studies. The excellent technical assistance provided by Rebecca Corcoran-Merrill is gratefully acknowledged.This investigation was supported by Grant CA 32927 from the National Cancer Institute, U.S. Public Health Service. 相似文献
5.
The transport stoichiometry of the electrogenic sodium-bicarbonate cotransporter (SLC4A5 or NBCe2) in mouse choroid plexus was examined. Whole-cell recording methods measured the currents carried by the NBCe2, using experimental solutions determined to minimise the contributions of the other ion conductances present. Increases in outward current were observed when 21.2 mM was added to the bath solution in the presence of Na+, but not N-methyl-d-glucamine. This -induced current was completely abolished by 500 μM 4,4′-diisothiocyanostilbene-2,2′-disulphonic acid. The reversal potential for the -induced current was −95.1 ± 7.1 mV (n = 11), a value which corresponds to a NBCe2 transport stoichiometry of 3 with 1 Na+. The NBCe2, with this stoichiometry, will mediate the efflux of and Na+ from the cell into the cerebrospinal fluid at the apical membrane of the choroid plexus. 相似文献
6.
In the presence of NH4Cl and hypotonic solutions, Rana balcanica red cells respond by increasing their volume. The stimulation of cellular volume by hypotonicity is more rapid than that of NH4Cl, while the maximum value is less than that observed in the presence of NH4Cl. Depending on the cause of swelling, (net uptake of NH4Cl or decrease in external osmolality) cells show specific responses. The NH4Cl treatment causes a significant increase in intracellular Na+, from 5·14±0·78 to 29·84±0·47 mmoles l−1 cell, while hypotonicity leads to a significant decrease of this cation, to 3·85±0·25 mmoles l−1 cell in relation to the control, after 30 min of incubation of Rana balcanica erythrocytes. In addition, amiloride significantly reverses the NH4Cl effect with respect to intracellular Na+. Both treatments cause a significant K+ loss in comparison with controls. Two glycolytic enzymes glyceraldehyde phosphate dehydrogenase (GAPDH) and pyruvate kinase (PK) of Rana balcanica haemolysate were found to respond to the NH4Cl effect by significantly decreasing their activity. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
7.
Soraya P. Shirazi-Beechey Jean-Pierre Gorvel R. Brian Beechey 《Journal of bioenergetics and biomembranes》1988,20(2):273-288
In the small intestine of the rabbit the process of Na+-dependent uptake of phosphate occurs only at the brush-border of duodenal enterocytes. Li+ can replace Na+. The process is activated when either K+, Cs+, Rb+, or choline is present in the intravesicular space. The presence of membrane-permeable anions is essential for maximum rates of phosphate transport. We conclude that the mechanism of the phosphate carrier is electrogenic at pH 6–8, probably two Na+ moving with each H2PO
4
–
. This. will lead to the development of a positive charge within the vesicle. The variation of theK
m
for H2PO
4
–
with pH is thought to be the consequence of the affinity of the carrier protein for H2PO
4
–
increasing as the pH increases. Polyclonal antibodies against membrane vesicles isolated from rabbit duodenum, jejunum, and ileum were prepared. The antibodies raised against the ileum and jejunum both activated the phosphate transport process, while the anti-duodenum antibody preparation inhibited phosphate transport. 相似文献
8.
Summary Rabbit erythrocytes are well known for possessing highly active Na+/Na+ and Na+/H+ countertransport systems. Since these two transport systems share many similar properties, the possibility exists that they represent different transport modes of a single transport molecule. Therefore, we evaluated this hypothesis by measuring Na+ transport through these exchangers in acid-loaded cells. In addition, selective inhibitors of these transport systems such as ethylisopropyl-amiloride (EIPA) and N-ethylmaleimide (NEM) were used. Na+/Na+ exchange activity, determined as the Na
o
+
-dependent22Na efflux or Na
i
+
-induced22Na entry was completely abolished by NEM. This inhibitor, however, did not affect the H
i
+
-induced Na+ entry sensitive to amiloride (Na+/H+ exchange activity). Similarly, EIPA, a strong inhibitor of the Na+/H+ exchanger, did not inhibit Na+/Na– countertransport, suggesting the independent nature of both transport systems. The possibility that the NEM-sensitive Na+/Na+ exchanger could be involved in Na+/H+ countertransport was suggested by studies in which the net Na+ transport sensitive to NEM was determined. As expected, net Na+ transport through this transport system was zero at different [Na+]
i
/[Na+]
o
ratios when intracellular pH was 7.2. However, at pH
i
=6.1, net Na+ influx occurred when [Na+]
i
was lower than 39mm. Valinomycin, which at low [K+]
o
was lower than 39mm. Valinomycin, which at low [K+]
o
clamps the membrane potential close to the K+ equilibrium potential, did not affect the net NEM-sensitive Na+ entry but markedly stimulated, the EIPA-and NEM-resistant Na+ uptake. This suggest that the net Na+ entry through the NEM-sensitive pathway at low pH
i
, is mediated by an electroneutral process possibly involving Na+/H+ exchange. In contrast, the EIPA-sensitive Na+/H+ exchanger is not involved in Na+/Na+ countertransport, because Na+ transport through this mechanism is not affected by an increase in cell Na– from 0.4 to 39mm. Altogether, these findings indicate that both transport systems: the Na+/Na+ and Na+/H+ exchangers, are mediated by distinct transport proteins. 相似文献
9.
Masako Chou Takafumi Matsunaga Y. Takada Noriyuki Fukunaga 《Extremophiles : life under extreme conditions》1999,3(2):89-95
NH4
+ transport system of a psychrophilic marine bacterium Vibrio sp. strain ABE-1 (Vibrio ABE-1) was examined by measuring the uptake of [14C]methylammonium ion (14CH3NH3
+) into the intact cells. 14CH3NH3
+ uptake was detected in cells grown in medium containing glutamate as the sole nitrogen source, but not in those grown in
medium containing NH4Cl instead of glutamate. Vibrio ABE-1 did not utilize CH3NH3
+ as a carbon or nitrogen source. NH4Cl and nonradiolabeled CH3NH3
+ completely inhibited 14CH3NH3
+ uptake. These results indicate that 14CH3NH3
+ uptake in this bacterium is mediated via an NH4
+ transport system and not by a specific carrier for CH3NH3
+. The respiratory substrate succinate was required to drive 14CH3NH3
+ uptake and the uptake was completely inhibited by KCN, indicating that the uptake was energy dependent. The electrochemical
potentials of H+ and/or Na+ across membranes were suggested to be the driving forces for the transport system because the ionophores carbonylcyanide
m-chlorophenylhydrazone and monensin strongly inhibited uptake activities at pH 6.5 and 8.5, respectively. Furthermore, KCl
activated 14CH3NH3
+ uptake. The 14CH3NH3
+ uptake activity of Vibrio ABE-1 was markedly high at temperatures between 0° and 15°C, and the apparent K
m value for CH3NH3
+ of the uptake did not change significantly over the temperature range from 0° to 25°C. Thus, the NH4
+ transport system of this bacterium was highly active at low temperatures.
Received: August 1, 1998 / Accepted: October 8, 1998 相似文献
10.
Human heart Na+ channels were expressed transiently in both mammalian cells and Xenopus oocytes, and Na+ currents measured using 150 mM intracellular Na+. The kinetics of decaying outward Na+ current in response to 1-s depolarizations in the F1485Q mutant depends on the predominant cation in the extracellular solution, suggesting an effect on slow inactivation. The decay rate is lower for the alkali metal cations Li+, Na+, K+, Rb+, and Cs+ than for the organic cations Tris, tetramethylammonium, N-methylglucamine, and choline. In whole cell recordings, raising [Na+]o from 10 to 150 mM increases the rate of recovery from slow inactivation at −140 mV, decreases the rate of slow inactivation at relatively depolarized voltages, and shifts steady-state slow inactivation in a depolarized direction. Single channel recordings of F1485Q show a decrease in the number of blank (i.e., null) records when [Na+]o is increased. Significant clustering of blank records when depolarizing at a frequency of 0.5 Hz suggests that periods of inactivity represent the sojourn of a channel in a slow-inactivated state. Examination of the single channel kinetics at +60 mV during 90-ms depolarizations shows that neither open time, closed time, nor first latency is significantly affected by [Na+]o. However raising [Na+]o decreases the duration of the last closed interval terminated by the end of the depolarization, leading to an increased number of openings at the depolarized voltage. Analysis of single channel data indicates that at a depolarized voltage a single rate constant for entry into a slow-inactivated state is reduced in high [Na+]o, suggesting that the binding of an alkali metal cation, perhaps in the ion-conducting pore, inhibits the closing of the slow inactivation gate. 相似文献
11.
Craig C. Freudenrich Elizabeth Murphy Shi Liul Melvyn Lieberman 《Molecular and cellular biochemistry》1992,114(1-2):97-103
Several aspects of Mg2+ homeostasis were investigated in cultured chicken heart cells using the fluorescent Mg2+ indicator, FURAPTRA. The concentration of cytosolic Mg2+ ([Mg2+]i) is 0.48 ± 0.03 mM (n = 31). To test whether a putative Na/Mg exchange mechanism controls [Mg2+]i below electrochemical equilibrium, we manipulated the Na+ gradient and assessed the effects on [Mg2+]i. When extracellular Na+ was removed, [Mg2+]i increased; this increase was not altered in Mg-free solutions, but was attenuated in Ca-free solutions. A similar increase in [Mg2+]i, which was dependent upon extracellular Ca2+, was observed when intracellular Na+ was raised by inhibiting the Na/K pump with ouabain. These results do not provide evidence for Na/Mg exchange in heart cells, but they suggest that Ca2+ can modulate [Mg2+]i. In addition, removing extracellular Na+ caused a decrease in intracellular pH (pHi), as measured by pH-sensitive microelectrodes, and this acidification was attenuated when Cat+ was also removed from the solution. These results suggest that Ca2+ and H+ interact intracellularly. Since changes in the Na+ gradient can also alter pHi, we questioned whether pH can modulate [Mg2+]i. pHi was manipulated by the NH4Cl prepulse method. NH4
+-evoked changes in pHi, as measured by the fluorescent indicator BCECF, were accompanied by opposite changes in [Mg2+]i; [Mg2+]i changed by –0.16 mM/unit pH. These NH4
+-evoked changes in [Mg2+]i were not caused by movements of Mg2+ or Ca2+ across the sarcolemma or by changes in cytosolic Ca2+. Additionally, pHi was manipulated by changing extracellular pH (pHo). When pHo was decreased from 7.4 to 6.3, pHi decreased by 0.64 units and [Mg2+]i increased by 0.12 mM; in contrast, when pHo was raised from 7.4 to 8.3, pHi increased by 0.6 units and [Mg2+]i did not change significantly. The results of our investigations suggest that Ca 2+ and H+ can modulate [Mg2+]i, probably by affecting cytosolic Mg2+ binding and/or subcellular Mg2+ transport and that such redistribution of intracellular Mg2+ may play an important role in Mg2+ homeostasis in cardiac cells. 相似文献
12.
Different transport mechanisms in plant and human AMT/Rh-type ammonium transporters 总被引:5,自引:0,他引:5
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Mayer M Schaaf G Mouro I Lopez C Colin Y Neumann P Cartron JP Ludewig U 《The Journal of general physiology》2006,127(2):133-144
The conserved family of AMT/Rh proteins facilitates ammonium transport across animal, plant, and microbial membranes. A bacterial homologue, AmtB, forms a channel-like structure and appears to function as an NH3 gas channel. To evaluate the function of eukaryotic homologues, the human RhCG glycoprotein and the tomato plant ammonium transporter LeAMT1;2 were expressed and compared in Xenopus oocytes and yeast. RhCG mediated the electroneutral transport of methylammonium (MeA), which saturated with Km = 3.8 mM at pHo 7.5. Uptake was strongly favored by increasing the pHo and was inhibited by ammonium. Ammonium induced rapid cytosolic alkalinization in RhCG-expressing oocytes. Additionally, RhCG expression was associated with an alkali-cation conductance, which was not significantly permeable to NH4+ and was apparently uncoupled from the ammonium transport. In contrast, expression of the homologous LeAMT1;2 induced pHo-independent MeA+ uptake and specific NH4+ and MeA+ currents that were distinct from endogenous currents. The different mechanisms of transport, including the RhCG-associated alkali-cation conductance, were verified by heterologous expression in appropriate yeast strains. Thus, homologous AMT/Rh-type proteins function in a distinct manner; while LeAMT1;2 carries specifically NH4+, or cotransports NH3/H+, RhCG mediates electroneutral NH3 transport. 相似文献
13.
S. Y. Chow Y. C. Yen-Chow H. S. White L. Hertz D. M. Woodbury 《Neurochemical research》1991,16(12):1275-1283
Inastrocytes, as [K+]o was increased from 1.2 to 10 mM, [K+]i and [Cl–]i were increased, whereas [Na+]i was decreased. As [K+]o was increased from 10 to 60 mM, intracellular concentration of these three ions showed no significant change. When [K+]o was increased from 60 to 122 mM, an increase in [K+]i and [Cl–]i and a decrease in [Na+]i were observed.Inneurons, as [K+]o was increased from 1.2 to 2.8 mM, [Na+]i and [Cl–]i were decreased, whereas [K+]i was increased. As [K+]o was increased from 2.8 to 30 mM, [K+]i, [Na+]i and [Cl–]i showed no significant change. When [K+]o was increased from 30 to 122 mM, [K+]i and [Cl–]i were increased, whereas [Na+]i was decreased. Inastrocytes, pHi increased when [K+]o was increased. Inneurons, there was a biphasic change in pHi. In lower [K+]o (1.2–2.8 mM) pHi decreased as [K+]o increased, whereas in higher [K+]o (2.8–122 mM) pHi was directly related to [K+]o. In bothastrocytes andneurons, changes in [K+]o did not affect the extracellular water content, whereas the intracellular water content increased as the [K+]o increased. Transmembrane potential (Em) as measured with Tl-204 was inversely related to [K+]o between 1.2 and 90 mM, a ten-fold increase in [K+]o depolarized the astrocytes by about 56 mV and the neurons about 52 mV. The Em values measured with Tl-204 were close to the potassium equilibrium potential (Ek) except those in neurons at lower [K+]o. However, they were not equal to the chloride equilibrium potential (ECl) at [K+]o lower than 30 mM in both astrocytes and neurons. Results of this study demonstrate that alteration of [K+]o produced different changes in [K+]i, [Na+]i, [Cl–]i, and pHi in astrocytes and neurons. The data show that astrocytes can adapt to alterations in [K+]o, in such a way to maintain a more suitable environment for neurons. 相似文献
14.
The Membrane Potential of Acetabularia mediterranea 总被引:8,自引:1,他引:7
H. D. W. Saddler 《The Journal of general physiology》1970,55(6):802-821
The cytoplasm of an Acetabularia cell is normally at a potential of about -170 mv relative to the external solution; the vacuole is also at this potential. Although there is strict flux equilibrium for all ions, the potential is more negative than the Nernst potentials of any of the permeating ions. Darkness, CCCP, low temperature, and reducing [Cl-]o by a factor of 25 all rapidly depolarize the membrane and inhibit Cl- influx. Some of these treatments do not inhibit the effluxes of K+ and Na+. Increasing [K+]o also depolarizes the membrane both under normal conditions and at low temperature; in the latter case the membrane is partially depolarized in normal seawater (low [K+]o) and in high [K+]o positive potentials of up to +15 mv are attained. It is concluded that the membrane potential is controlled by the electrogenic influx of Cl-, and also, at least in some circumstances, by the diffusion of K+. In addition, it is suggested that electrogenic efflux of H+ may be important in transient nonequilibrium situations. An Appendix deals with the interpretation of simple nonsteady-state tracer kinetic data. 相似文献
15.
The first analysis of chloride fluxes and compartmentation in a non-excised plant system is presented, examining ten ecologically pertinent conditions. The short-lived radiotracer couple 38Cl/39Cl was used as a Cl– tracer in intact barley (Hordeum vulgare L. cv. Klondike) seedlings, which were cultured and investigated under four external [Cl–], from abundant (0.1 mM) to potentially toxic (100 mM). Chloride–nitrogen interactions were investigated by varying N source (NO3
– or NH4
+) and strength (0.1 or 10 mM), in order to examine, at the subcellular compartmentation level, the antagonism, previously documented at the influx level, between Cl– and NO3
–, and the potential role of Cl– as a counterion for NH4
+ under conditions in which cytosolic [NH4
+] is excessive. Cytosolic [Cl–] increased with external [Cl–] from 6 mM to 360 mM. Cl– influx, fluxes to vacuole and shoot, and, in particular, efflux to the external medium, also increased along this gradient. Efflux reached 90% of influx at the highest external [Cl–]. Half-times of cytosolic Cl– exchange decreased between high-affinity and low-affinity influx conditions. The relationship between cytosolic [Cl–] and shoot flux indicated the presence of a saturable low-affinity transport system (SLATS) responsible for xylem loading of Cl–. N source strongly influenced Cl– flux to the vacuole, and moderately influenced Cl– influx and shoot flux, whereas efflux and half-time were insensitive to N source. Cytosolic pool sizes were not strongly or consistently influenced by N source, indicating the low potential for Cl– to act as a counterion to hyperaccumulating NH4
+. We discuss our results in relation to salinity responses in cereals.Abbreviations [Cl–]cyt
cytosolic chloride concentration
- [Cl–]o
external chloride concentration 相似文献
16.
The H+-coupled transporter hPepT1 (SLC15A1) mediates the transport of di/tripeptides and many orally-active drugs across the brush-border membrane of the small intestinal epithelium. Incubation of Caco-2 cell monolayers (15 min) with the dietary phosphodiesterase inhibitors caffeine and theophylline inhibited Gly-Sar uptake across the apical membrane. Pentoxifylline, a phosphodiesterase inhibitor given orally to treat intermittent claudication, also decreased Gly-Sar uptake through a reduction in capacity (Vmax) without any effect on affinity (Km). The reduction in dipeptide transport was dependent upon both extracellular Na+ and apical pH but was not observed in the presence of the selective Na+/H+ exchanger NHE3 (SLC9A3) inhibitor S1611. Measurement of intracellular pH confirmed that caffeine was not directly inhibiting hPepT1 but rather having an indirect effect through inhibition of NHE3 activity. NHE3 maintains the H+-electrochemical gradient which, in turn, acts as the driving force for H+-coupled solute transport. Uptake of β-alanine, a substrate for the H+-coupled amino acid transporter hPAT1 (SLC36A1), was also inhibited by caffeine. The regulation of NHE3 by non-nutrient components of diet or orally-delivered drugs may alter the function of any solute carrier dependent upon the H+-electrochemical gradient and may, therefore, be a site for both nutrient-drug and drug-drug interactions in the small intestine. 相似文献
17.
Seiji Miyauchi 《生物化学与生物物理学报:生物膜》2010,1798(6):1164-4037
Pyroglutamate, also known as 5-oxoproline, is a structural analog of proline. This amino acid derivative is a byproduct of glutathione metabolism, and is reabsorbed efficiently in kidney by Na+-coupled transport mechanisms. Previous studies have focused on potential participation of amino acid transport systems in renal reabsorption of this compound. Here we show that it is not the amino acid transport systems but instead the Na+-coupled monocarboxylate transporter SLC5A8 that plays a predominant role in this reabsorptive process. Expression of cloned human and mouse SLC5A8 in mammalian cells induces Na+-dependent transport of pyroglutamate that is inhibitable by various SLC5A8 substrates. SLC5A8-mediated transport of pyroglutamate is saturable with a Michaelis constant of 0.36 ± 0.04 mM. Na+-activation of the transport process exhibits sigmoidal kinetics with a Hill coefficient of 1.8 ± 0.4, indicating involvement of more than one Na+ in the activation process. Expression of SLC5A8 in Xenopuslaevis oocytes induces Na+-dependent inward currents in the presence of pyroglutamate under voltage-clamp conditions. The concentration of pyroglutamate necessary for induction of half-maximal current is 0.19 ± 0.01 mM. The Na+-activation kinetics is sigmoidal with a Hill coefficient of 2.3 ± 0.2. Ibuprofen, a blocker of SLC5A8, suppressed pyroglutamate-induced currents in SLC5A8-expressing oocytes; the concentration of the blocker necessary for causing half-maximal inhibition is 14 ± 1 μM. The involvement of SLC5A8 can be demonstrated in rabbit renal brush border membrane vesicles by showing that the Na+-dependent uptake of pyroglutamate in these vesicles is inhibitable by known substrates of SLC5A8. The Na+ gradient-driven pyroglutamate uptake was stimulated by an inside-negative K+ diffusion potential induced by valinomycin, showing that the uptake process is electrogenic. 相似文献
18.
Horst Helbig Christoph Korbmacher Michael Wiederholt 《The Journal of membrane biology》1987,99(3):173-186
Summary Using intracellular microelectrode technique, we investigated the changes in membrane voltage (V) of cultured bovine pigmented ciliary epithelial cells induced by different extracellular solutions. (1)V in 213 cells under steady-state conditions averaged –46.1±0.6 mV (sem). (2) Increasing extracellular K+ concentration ([K+]
o
) depolarizedV. Addition of Ba2+ could diminish this response. (3) Depolarization on doubling [K+]
o
was increased at higher [K+]
o
(or low voltage). (4) Removing extracellular Ca2+ decreasedV and reduced theV amplitude on increasing [K+]
o
. (5)V was pH sensitive. Extra-and intracellular acidification depolarizedV; alkalinization induced a hyperpolarization.V responses to high [K+]
o
were reduced at acidic extracellular pH. (6) Removing K
o
+
depolarized, K
o
+
readdition after K+ depletion transiently hyperpolarizedV. These responses were insensitive to Ba2+ but were abolished in the presence of ouabain or in Na+-free medium. (7) Na+ readdition after Na+ depletion transiently hyperpolarizedV. This reaction was markedly reduced in the presence of ouabain or in K+-free solution but unchanged by Ba2+. It is concluded that in cultured bovine pigmented ciliary epithelial cells K+ conductance depends on Ca2+, pH and [K+]
o
(or voltage). An electrogenic Na+/K+-transport is present, which is stimulated during recovery from K+ or Na+ depletion. This transport is inhibited by ouabain and in K+-or Na+-free medium. 相似文献
19.
Primary cultures of both mouse astrocytes and neurons accumulate more125I– than36Cl– from the medium. The average cell/medium ratio of125I– of astrocytes (1.01) is greater than that of neurons (0.74), whereas the ratio of36Cl– of neurons (0.47) is greater than that of astrocytes (0.25). The equilibrium potentials of both125I– and36Cl– calculated from the cell/medium ratios in astrocytes and neurons are significantly lower than their corresponding resting transmembrane potentials which suggest that both iodide and chloride are actively transported into both cell types. With respect to different transport inhibitors, thiocyanate is more effective in inhibiting125I– uptake whereas furosemide is more effective in inhibiting36Cl– uptake. Radioiodide uptake by mouse astrocytes was directly proportional to the [Na+]o but was not significantly affected by changes of [Cl–]o or [HCO
3
–
]o, except that it is low in bicarbonate-free medium. Radiochloride uptake by astrocytes was inversely related to [Cl–]o and [HCO
3
–
]o and was not affected [Na+]o, except that it was low in sodium-free medium. Radioiodide uptake by neurons was directly related to [Na+]o between 60 and 140 mM and inversely related to [HCO
3
–
]o between 10 and 40 mM, but it was not affected by [Cl–]o. Radiochloride uptake by neurons was directly related to [Cl–]o and to [Na+]o between 60 and 140 mM and was not affected by [HCO
3
–
]o. However, in sodium-free medium both125I– and36Cl– uptakes into neurons were higher than those in [Na+]o between 5 and 60 mM. These results indicate that uptake of125I– and36Cl– into astrocytes and neurons are different in their ion dependence and that they are under separate regulation.Special issue dedicated to Dr. Paola S. Timiras 相似文献
20.
A ouabain sensitive inward current occurs in Xenopus oocytes in Na+ and K+ -free solutions. Several laboratories have investigated the properties of this current and suggested that acidic extracellular pH (pHo) produces a conducting pathway through the Na+/K+ pump that is permeable to H+ and blocked by [Na+]o. An alternative suggestion is that the current is mediated by an electrogenic H+-ATPase. Here we investigate the effect of pHo and [Na+]o on both transient and steady-state ouabain-sensitive current. At alkaline or neutral pHo the relaxation rate of pre-steady-state current is an exponential function of voltage. Its U-shaped voltage dependence becomes apparent at acidic pHo, as predicted by a model in which protonation of the Na+/K+ pump reduces the energy barrier between the internal solution and the Na+ occluded state. The model also predicts that acidic pHo increases steady-state current leak through the pump. The apparent pK of the titratable group(s) is 6, suggesting that histidine is involved in induction of the conductance pathway. 22Na efflux experiments in squid giant axon and current measurements in oocytes at acidic pHo suggest that both Na+ and H+ are permeant. The acid-induced inward current is reduced by high [Na+]o, consistent with block by Na+. A least squares analysis predicts that H+ is four orders of magnitude more permeant than Na+, and that block occurs when 3 Na+ ions occupy a low affinity binding site (K
0.5=130±30 mM) with a dielectric coefficient of 0.23±0.03. These data support the conclusion that the ouabain-sensitive conducting pathway is a result of passive leak of both Na+ and H+ through the Na+/K+ pump. 相似文献