Effects of ethacrynic acid and furosemide on phosphorylation reactions of kidney mitochondria. Inhibition of the adenine nucleotide translocase.

Previous reports that ethacrynic acid and furosemide diminish mitochondrial P : O ratios and reduce (Na+ + K+)-ATPase activity suggested that these diuretics may inhibit mitochondrial phosphorylation reactions. This possibility was initially studied by determining the effects of ethacrynic acid and furosemide on [32P]ATP exchange activity of rat kidney mitochondria. Concentrations of both drugs at 10(-4) M or greater, significantly inhibited [32P]ATP exchange. To investigate the mechanism of this inhibition, the effects of ethacrynic acid and furosemide on the ATPase activity of intract mitochondria and sonicated submitochondrial particles were determined. Both diuretics inhibited ATPase activity of intact mitochondria at 10(-4) M. In contrast, ATPase of submitochondrial particles was significantly less susceptible to inhibition by the diuretics. These results suggested that ethacrynic acid anf furosemide inhibit adenine nucleotide transport across the mitochondrial membrane. This was directly tested by determining the effects of the diretics on the mitochondrial adenine nucleotide translocase. At 5-10(-4) M, both ethacrynic acid and furosemide significantly inhibited adenine nucleotide transport. These findings suggest that ethacrynic acid and furosemide may diminish renal tubular solute reabsorption by direct inhibition of adenine nucleotide transport across the mitochondrial inner membrane.     

Effects of ethacrynic acid and furosemide on phosphory-lation reactions of kidney mitochondria. Inhibition of the adenine nucleotide translocase

Previous reports that ethacrynic acid and furosemide diminish mitochondrial P : O ratios and reduce (Na⁺ + K⁺)-ATPase activity suggested that these diuretics may inhibit mitochondrial phosphorylation reactions. This possibility was initially studied by determining the effects of ethacrynic acid and furosemide on [³²P]ATP exchange activity of rat kidney mitochondria. Concentrations of both drugs at 10⁻⁴ M or greater, significantly inhibited [³²P]ATP exchange. To investigate the mechanism of this inhibition, the effects of ethacrynic acid and furosemide on the ATPase activity of intact mitochondria and sonicated submitochondrial particles were determined. Both diuretics inhibited ATPase activity of intact mitochondria at 10⁻⁴ M. In contrast, ATPase of submitochondrial particles was significantly less susceptible to inhibition by the diuretics. These results suggested that ethacrynic acid and furosemide inhibit adenine nucleotide transport across the mitochondrial membrane. This was directly tested by determining the effects of the diuretics on the mitochondrial adenine nucleotide translocase. At 5 · 10⁻⁴ M, both ethacrynic acid and furosemide significantly inhibited adenine nucleotide transport. These findings suggest that ethacrynic acid and furosemide may diminish renal tubular solute reabsorption by direct inhibition of adenine nucleotide transport across the mitochondrial inner membrane.     

Effect of diuretics on intestinal transport of electrolytes, glucose, and amino acid.

The jejunal mucosal membrane of albino mice was used to study the electrical properties and ion transport. The membrane was bathed in Krebs-Ringer solution with or without glucose.When ethacrynic acid (EA), furosemide, or amiloride was added to the bathing fluid of both sides, a transient increase followed by a decrease of both potential difference (PD) and short circuit current (Isc) were observed. In glucose-containing bathing medium, EA inhibited both net Na and Cl flux and residual flux; however, EA had little effect on both Na and Cl flux in glucose-free bathing medium. Studies using everted intestinal sac technique showed that EA inhibited both glucose and L-tyrosine across the mucosal membrane against concentration gradients. Furosemide and amiloride were less potent than EA in inhibiting the Na and Cl flux when the bathing solution contained glucose. But these two compounds had no effect on glucose and L-tyrosine transport across the intestinal mucosa. Furthermore, they did inhibit Cl flux even in the condition of glucose-free bathing medium. It is postulated that all three diuretics act on the brush-border membrane of the intestine. EA probably inhibits the Na-glucose cotransporting system; furosemide and amiloride inhibit the simple diffusion process of Na entry of Cl exit by decreasing the conductance of the membrane.     

Uptake of 36Cl and 22Na by the Brain-Cerebrospinal Fluid System: Comparison of the Permeability of the Blood-Brain and Blood-Cerebrospinal Fluid Barriers

Abstract: Transport and permeability properties of the blood-brain and blood-CSF barriers were determined by kinetic analysis of radioisotope uptake from the plasma into the CNS of the adult rat. Cerebral cortex and cerebellum uptake curves for ³⁶Cl and ²²Na were resolved into two components. The fast component (t_½ 0.02–0.05 h, fractional volume 0.04–0.08) is comprised of the vascular compartment and a small perivascular space whereas the slow component (t_½ 1.06–1.69 h, fractional volume 0.92–0.96) represents isotope movement across the blood-brain barrier into the brain extracellular and cellular compartments. Uptake curves of both ³⁶Cl and ²²Na into the CSF were also resolved into two components, a fast component (t_½ 0.18 h, fractional volume 0.24) and a slow component (t_½ 1.2 h, fractional volume 0.76). Evidence suggests that the fast component represents isotope movement across the blood-CSF barrier, i.e., the choroid plexuses, whereas the CSF slow component probably reflects isotope penetration primarily from the brain extracellular fluid into the CSF. The extracellular fluid volume of the cerebral cortex and cerebellum was estimated as ?13% from the initial slope of the curve of brain space versus CSF space curve for both ³⁶Cl and ²²Na. Like the choroid plexuses, the glial cell compartment of the brain appears to accumulate Cl from 2 to 6 times that predicted for passive distribution. The relative permeability of the blood-CSF and blood-brain barriers to ³⁶Cl, ²²Na, and [³H]mannitol was determined by calculating permeability surface-area products (PA). Analysis of the PA values for all three isotopes indicates that the effective permeability of the choroidal epithelium (blood/CSF barrier) is significantly greater than that of the capillary endothelium in the cerebral cortex and cerebellum (blood-brain barrier).     

Na-K-Cl协同转运蛋白研究进展

Na-K-Cl协同转运蛋白是一类膜蛋白,负责转运Na、K、Cl离子进出上皮细胞与非上皮细胞。Na-K-Cl介导的转运过程是电中性的,多数情况下是1Na:1K:2Cl(乌贼轴突中是2Na:1 K:3Cl),其活性被布美他尼(bumetanide)和呋塞米(furosemide)所抑制。迄今为止,Na-K-Cl协同转运蛋白被鉴定出来两个同源异构体:NKCC1和NKCC2。NKCC1存在于多个组织中,含有NKCC1的上皮大多数属于分泌上皮,而且会有Na-K-Cl协同转运蛋白位于基底膜外侧;NKCC2只存在于肾脏,位于上皮细胞致密班的顶膜上。Na-K-Cl协同转运蛋白的调控在不同的细胞和组织中是不同的。Na-K-Cl协同转运蛋白的活性会受激素刺激和细胞体积变化的影响;有些组织中,这种调控作用(尤其是NKCC1亚基)是通过特定的激酶使该转运蛋白自身发生氧化/硝化、磷酸化/去磷酸化来实现的;蛋白过表达在Na-K-Cl协同转运蛋白的激活中也起重要作用。     

Characterization of Na-K-ATPase in dog tracheal epithelium: enzymatic and ion transport measurements

The dog tracheal epithelium actively secretes Cl and absorbs Na. The possible dependency of this electrolyte transport on a Mg-dependent, Na-K-activated adenosine triphosphatase (Na-K-ATPase, EC 3.6.1.3) was examined. The characteristics of this enzyme system were investigated using homogenates of tracheal epithelium. The electrical properties and ion fluxes of this epithelium were determined in tissues mounted in Ussing chambers. Addition of Na and K produced an approximate 50% activation of basal Mg-ATPase activity. The apparent Km values for ATP, Na, K, and Mg were 0.4, 12.7, 1.9, and 1.6 mM, respectively. The total specific ATPase activity was 8.1 +/- 0.4 and that of the Mg-ATPase 4.3 +/- 0.1 mumol Pi. mg protein -1.h-1. Addition of ouabain (1 muM) or omission of K from the submucosal bathing solution reduced potential difference (PD) and short-circuit current (SCC) significantly. Relatively low concentrations (0.1 mM or less) of ethacrynic acid, furosemide, or 2,4-dinitrophenol (2,4-DNP) depressed SCC and PD significantly, i.e., at concentrations that were without effect on the Na-K-ATPase activity. Ethacrynic acid inhibited Cl secretion, whereas 2,4-DNP lowered both Na and Cl transport. These data demonstrate that 1) the tracheal mucosa of dogs contains a Na-K-ATPase at relatively high specific activity, 2) this enzyme is likely contained in the basal aspect of this membrane, 3) it appears to be essential for maintenance of Cl secretion, and 4) Cl secretion can be reduced (by ethacrynic acid, furosemide, and 2,4-DNP) without Na-K-ATPase inhibition.     

Intracellular ion concentrations in the frog cornea epithelium during stimulation and inhibition of Cl secretion

Summary The intracellular electrolyte concentrations in the isolated cornea of the American bullfrog were determined in thin freeze-dried cryosections using energy-dispersive X-ray microanalysis. Stimulation of Cl secretion by isoproterenol resulted in a significant increase in the intracellular Na concentration but did not change the intracellular Cl concentration. Similar results were obtained when Cl secretion was stimulated by the Ca ionophore A23187. Inhibition of Cl secretion by ouabain produced a large increase in the intracellular Na concentration and an equivalent fall in the K concentration. Again, no increase or decrease in the intracellular Cl concentration was detectable. Clamping of the transepithelial potential to ±50 mV resulted in parallel changes in the transepithelial current and intracellular Na concentration, but, with the exception of the outermost cell layer, in no changes of the Cl concentration. Only when Cl secretion was inhibited by bumetanide or furosemide, together with a decrease in the Na concentration, was a large fall in the Cl concentration observed. Application of loop diuretics also produced significant increases in the P concentration and dry weight, consistent with some shrinkage of the epithelial cells. The results suggest the existence of a potent regulatory mechanism which maintains a constant intracellular Cl concentration and, thereby, a constant epithelial cell volume. Through the operation of this system any variation in the apical Cl efflux is compensated for by an equal change in the rate of Cl uptake across the basolateral membrane. Cl uptake is sensitive to loop diuretics, directly coupled to an uptake of Na, and dependent on the Na and K concentration gradients across the basolateral membrane. Isoproterenol and A23187 seem to increase the Cl permeability of the apical membrane and thus stimulate Cl efflux. Ouabain inhibits Cl secretion by abolishing the driving Na concentration gradient for Cl uptake across the basolateral membrane.     

Salinity and hormone interactions in affecting growth,transpiration and ionic relations ofPhaseolus vulgaris

Addition of either abscisic acid (ABA) or kinetin at 10⁻⁶ M to salinized media (20–120mM NaCl) induced remarkable effects on growth ofPhaseolus vulgaris plants. Whereas ABA inhibited the plant growth and the rate of transpiration, kinetin induced stimulation of both parameters. Moreover, ABA increased proline and phosphorus concentrations in the salinized plants whilst kinetin decreased them. ABA induced stimulation of the transport of K, Ca and Cl from root to shoot, accumulation of K, Na and Cl in root cells and inhibits the transport of Na and accumulation of Ca. Kinetin appeared to inhibit the transport and accumulation of Na and Cl, transport of K, and stimulates the accumulation of K and Ca as well as the transport of Ca. The highest influence of both ABA and kinetin was mostly observed when these hormones were used in combination with the highest concentration of NaCl (120 mM) in the medium.     

Relationship between thinning of eggshells and reduction in number of eggs produced after administration of some saluretic drugs in domestic fowls

1. The effect of some diuretic drugs (acetazolamide, hydrochlorothiazide, chlorthalidone, furosemide, ethacrynic acid and amiloride) on eggshell formation and egg production in domestic fowls were investigated. All of the tested compounds significantly inhibited eggshell formation, furosemide being the most potent drug.2. The number of laid eggs was also reduced by several of the diuretics. There was a correlation between reduction of shell thickness and number of eggs (r = 0.77; P < 0.01).3. If the tested diuretics were subdivided into two groups according to their different modes of action, where furosemide, ethacrynic acid and amiloride represent sodium transport inhibitors, and acetazolamide, hydrochlorothiazide and chlorthalidone (contributory action of the two latter) are inhibitors of carbanhydrase, the highest correlation coefficient (r = 0.96) between reduction of shell thickness and egg production was found for sodium transport inhibitors. The corresponding correlation coefficient for carbanhydrase inhibitors (r = 0.44) was not significant.4. The probable mechanisms of action of the diuretics on eggshell formation and ovulation are discussed.     

Coupling of a loop diuretic-sensitive Na+ influx with the net loop diuretic-sensitive K+ efflux in mouse NIH 3T3 cells

Mouse 3T3 fibroblasts have a loop diuretic sensitive Na+ transport system, responsible for more than 50% of the total Na+ influx. This transport system is dependent on the simultaneous presence of all three ions; Na+, K+, (Rb+) and Cl- in the extracellular medium. The same requirement for these three ions was also found for the loop diuretic-sensitive K+ efflux. In addition, the sensitivities of Na+ influx and Rb+ efflux for the two loop diuretics, furosemide and bumetanide were found to be similar. The similar ionic requirement and sensitivity towards loop diuretics of the two fluxes, support the hypothesis, that this loop diuretic-sensitive Na+ influx in mouse 3T3 cells, is accompanied by the net loop diuretic-sensitive K+ efflux.     

KCl transport across and insect epithelium: I. Tracer fluxes and the effects of ion substitutions

Summary Models for active Cl transport across epithelia are often assumed to be universal although they are based on detailed studies of a relatively small number of epithelia from vertebrate animals. Epithelial Cl transport is also important in many invertebrates, but little is known regarding its cellular mechanisms. We used short-circuit current, tracer fluxes and ion substitutions to investigate the basic properties of Cl absorption by locust hindgut, an epithelium which is ideally suited for transport studies. Serosal addition of 1mm adenosine 35-cyclic monophosphate (cAMP), a known stimulant of Cl transport in this tissue, increased short-circuit current (I _sc) and net reabsorptive³⁶Cl flux (J _net ^Cl ) by 1000%. Cl absorption did not exhibit an exchange diffusion component and was highly selective over all anions tested except Br. Several predictions of Na- and HCO₃-coupled models for Cl transport were tested: Cl-dependentI _sc was not affected by sodium removal (<0.05mm) during the first 75 min. Also, a large stimulation ofJ _net ^Cl was elicited by cAMP when recta were bathed for 6 hr in nominally Na-free saline (<0.001 to 0.2mm) and there was no correlation between Cl transport rate and the presence of micromolar quantities of Na contamination. Increased unidirectional influx of³⁶Cl into rectal tissue during cAMP-stimulation was not accompanied by a comparable uptake of²²Na.J _net ^Cl was independent of exogenous CO₂ and HCO₃, but was strongly dependent on the presence of K. These results suggest that the major fraction of Cl transport across this insect epithelium occurs by an unusual K-dependent mechanism that does not directly require Na or HCO₃.     

Role of ouabain and diuretics on sodium, potassium and chloride retention in perfused rat kidney

A novel in situ kidney perfusion technique is described in Sprague-Dawley rats. The procedure involves retrograde perfusion from the renal veins via the kidneys, and then through the renal arteries and dorsal aorta. Ouabain (15 mM) in perfusate increased Na retention by 92%, decreased K retention by 53% and produced no effect on Cl retention. Ethacrynic acid (1 mM) in perfusate decreased Na retention by 52%, increased K retention by 105% and decreased Cl retention by 27%. Furosemide (1.5 mM) in perfusate decreased Na retention by 52%, increased K retention by 47% and decreased Cl retention by 56%. The Na-K-ATPase pump localized at the peritubular side of the proximal tubule cell is ouabain sensitive and Mg dependent. An Na-K pump responsible for Na influx and K effux exists at the luminal side of the proximal tubule cell and is ethacrynic acid and furosemide sensitive.     

Incorporation of 14CO2 in photosynthetic pigments of Chlorella pyrenoidosa

Abscisic acid (ABA) caused a 7–8-fold increase in volume flow in excised bean root systems and this was coupled with an increase in ⁴²K, ³⁶Cl and ²⁴Na flux into the xylem. The transport of ⁴²K and ³⁶Cl increased by a factor larger than the stimulation of volume flow, resulting in an increase in the concentration of those ions in the xylem exudate. Carbonyclcyanide-m-chlorophenyl hydrazone, on the other hand, eliminated ABA-stimulated ⁴²K transport and caused a further inhibition of ⁴²K flux, thus providing additional support for the proposition that ABA stimulation may involve an energised process of ion transport. ABA also increased the accumulation of ²⁴Na and ³⁶Cl in bean root tissue, but not that of ⁴²K.     

3H]bumetanide binding to membranes isolated from dog kidney outer medulla. Relationship to the Na,K,Cl co-transport system

We have synthesized the radiolabeled "loop" diuretics [3H]bumetanide and [3H]benzmetanide (3-benzylamino-4-phenoxy-5-sulfamoylbenzoic acid) and have tested their potential as reversible labels of the Na,K,Cl co-transport system. These compounds bind with high affinity (Kd less than or equal to 30 nM, under optimal conditions) to membranes isolated from dog kidney; we found approximately 2 pmol/mg of sites in crude membranes from the outer medulla, and less than or equal to 0.5 pmol/mg in a similar preparation from kidney cortex. On sucrose gradient centrifugation, a peak of [3H]bumetanide binding activity (30 pmol/mg) is obtained at 37% (w/v) sucrose, distinct from the basolateral membranes in outer medulla and from brush borders in proximal tubule; our hypothesis is that this peak contains luminal membranes from the thick ascending limb of the loop of Henle. [3H]Bumetanide is displaced from its binding sites by various unlabeled loop diuretics at concentrations that have previously been shown to inhibit co-transport. Na+, K+, and Cl- (K1/2 congruent to 2, 1, and 1 mM, respectively) are required for [3H]bumetanide binding, and Cl- inhibits at higher concentrations. We interpret these data to demonstrate that the Na,K,Cl cotransport system is the site involved in [3H]bumetanide binding in kidney membranes.     

Acidosis-Induced Enhanced Activity of the Na-K Exchange Pump in the In Vivo Choroid Plexus: An Ontogenetic Analysis of Possible Role in Cerebrospinal Fluid pH Homeostasis

Abstract: Changes in cellular [K] and [Na] in the choroidal epithelium (as a reflection of Na-K pump activity) were analyzed in Sprague-Dawley rats subjected to acute systemic acidosis. In the lateral and 4th ventricle choroid plexus (CP) of adult rats in which metabolic acidosis was induced for 1 h, cell [K] and [Na] increased and decreased by 35 and 15 m m /kg water, respectively, indicating marked stimulation of the Na-K exchange pump in the CSF-facing membrane; in contrast, this striking response of the CP to acidosis could not be elicited in immature animals (1 week old). Since the effects of respiratory acidosis on CP cell [K] and [Na] were similar to those of metabolic acidosis, the reduction in plasma pH (rather than in [HCO₃]) is likely the mechanism underlying the enhanced turnover of Na and K across the CP in adults. The concentration of Na and K in the cerebral cortex, medulla, and CSF was generally not altered during acute acid-base distortions in both mature and immature animals. The striking difference in the response of CNS tissue protected by the blood-CSF barrier (i.e., CP) and the blood-brain barrier (BBB) to systemic acidosis emphasizes a unique role, presumably homeostatic, for the plexus. Since propranolol substantially attenuated the acidosis-induced changes in choroidal cell [K] and [Na], it is possible that there is β-receptor modulation of the Na,K-ATPase (Na-K pump) in the CP. We postulate that the generally observed enhanced electropositivity in the CSF in systemic acidosis is brought about, at least in part, by facilitation of Na-K pumping in the CP, although induced changes in membrane permeability may also be a factor.     

Cerebrovascular Permeability Coefficients to Sodium, Potassium, and Chloride

CSF and regional brain concentrations of 42K, 22Na, 36Cl, and [14C]mannitol were determined 3-45 min after intravenous injection of the tracers in pentobarbital-anesthetized rats. Rapid influx of 36Cl and 22Na into ventricular CSF immediately established concentration gradients from CSF to brain extracellular fluid. The CSF contribution to brain uptake of tracers was greatest in periventricular brain regions, where brain 36Cl concentrations were up to ninefold higher than concentrations in regions distant from ventricular CSF. Acetazolamide (20 mg kg-1 i.p.), an inhibitor of CSF formation, decreased 36Cl uptake into CSF and into periventricular brain regions but not into frontal cortex. 36Cl uptake into brain was unidirectional for 10 min after intravenous injection, and, during that period, diffusion from ventricular CSF did not contribute to uptake in the frontal cortex. Therefore, cerebrovascular permeability coefficients could be calculated from tracer concentrations in frontal cortex at 10 min and equaled, in cm s-1, 13.5 X 10(-7) for 42K, 1.4 X 10(-7) for 22Na, 0.9 X 10(-7) for 36Cl, and 1.5 X 10(-7) for [14C]mannitol. The low cerebrovascular permeabilities to K, Na, and Cl, comparable to those of some cell membranes, and the permselectivity (K much greater than Na greater than Cl) suggest that a significant fraction of ion transport across cerebral capillaries is transcellular, i.e., across the endothelial cell membrane.     

Relationship between thinning of eggshells and reduction in number of eggs produced after administration of some saluretic drugs in domestic fowls.

1. The effect of some diuretic drugs (acetazolamide, hydrochlorothiazide, chlorthalidone, furosemide, ethacrynic acid and amiloride) on eggshell formation and egg production in domestic fowls were investigated. All of the tested compounds significantly inhibited eggshell formation, furosemide being the most potent drug. 2. The number of laid eggs was also reduced by several of the diuretics. There was a correlation between reduction of shell thickness and number of eggs (r = 0.77; P less than 0.01). 3. If the tested diuretics were subdivided into two groups according to their different modes of action, where furosemide, ethacrynic acid and amiloride represent sodium transport inhibitors, and acetazolamide, hydrochlorothiazide and chlorthalidone (contributory action of the two latter) are inhibitors of carbanhydrase, the highest correlation coefficient (r = 0.96) between reduction of shell thickness and egg production was found for sodium transport inhibitors. The corresponding correlation coefficient for carbanhydrase inhibitors (r = 0.44) was not significant. 4. The probable mechanisms of action of the diuretics on eggshell formation and ovulation are discussed.     

The Na-K-Cl Cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of a wide variety of epithelial and nonepithelial cells. The transport process mediated by Na-K-Cl cotransporters is characterized by electroneutrality (almost always with stoichiometry of 1Na:1K:2Cl) and inhibition by the loop diuretics bumetanide, benzmetanide, and furosemide. Presently, two distinct Na-K-Cl cotransporter isoforms have been identified by cDNA cloning and expression; genes encoding these two isoforms are located on different chromosomes and their gene products share approximately 60% amino acid sequence identity. The NKCC1 (CCC1, BSC2) isoform is present in a wide variety of tissues; most epithelia containing NKCC1 are secretory epithelia with the Na-K-Cl cotransporter localized to the basolateral membrane. By contrast, NKCC2 (CCC2, BSC1) is found only in the kidney, localized to the apical membrane of the epithelial cells of the thick ascending limb of Henle's loop and of the macula densa. Mutations in the NKCC2 gene result in Bartter's syndrome, an inherited disease characterized by hypokalemic metabolic alkalosis, hypercalciuria, salt wasting, and volume depletion. The two Na-K-Cl cotransporter isoforms are also part of a superfamily of cation-chloride cotransporters, which includes electroneutral K-Cl and Na-Cl cotransporters. Na-K-Cl cotransporter activity is affected by a large variety of hormonal stimuli as well as by changes in cell volume; in many tissues this regulation (particularly of the NKCCl isoform) occurs through direct phosphorylation/dephosphorylation of the cotransport protein itself though the specific protein kinases involved remain unknown. An important regulator of cotransporter activity in secretory epithelia and other cells as well is intracellular [Cl] ([Cl]_i), with a reduction in [Cl]_i being the apparent means by which basolateral Na-K-Cl cotransport activity is increased and thus coordinated with that of stimulated apical Cl channels in actively secreting epithelia.     

The Effect of Abscisic Acid on the Uptake and Distribution of Ions in Intact Seedlings of Phaseolus vulgaris cv. Redland Pioneer

Abscisic acid (ABA) was found to increase the accumulation of ³⁶Cl, total Cl, ²²Na and total Na⁺ in roots of intact bean seedlings. After an initial promotion. ABA inhibited longdistance transport of these ions from the root to the shoot. However, it consistently inhibited both uptake and transport of ⁴²K and total K⁺ in intact bean seedlings. A promotion of net ³⁶Cl influx (ψ_oc) and its accumulation in the root (Q*_v) concomitant decrease in transport index (long-distance transport as percentage of total influx) showed that ABA stimulates -³⁶Cl transport at the tonoplast. It inhibited H⁴ extrusion and net ⁸⁶Rb influx which agrees with a cation exchange theory K⁺/Rb⁺ transport.     

Na-K-Cl协同转运蛋白研究进展

Na-K-Cl协同转运蛋白是一类膜蛋白,负责转运Na、K、Cl离子进出上皮细胞与非上皮细胞。Na-K-Cl介导的转运过程是电中性的,多数情况下是1Na：1K：2C1（乌贼轴突中是2Na：1K：3C1）,其活性被布美他尼（bumetanide）和呋塞米（furosemide）所抑制。迄今为止,Na-K-Cl协同转运蛋白被鉴定出来两个同源异构体：NKCCl和NKCC2。NKCCl存在于多个组织中,合有NKCCl的上皮大多数属于分泌上皮,而且会有Na-K-Cl协同转运蛋白位于基底膜外侧;NKCC2只存在于肾脏,位于上皮细胞致密斑的顶膜上。Na-K-Cl协同转运蛋白的调控在不同的细胞和组织中是不同的。Na-K-Cl协同转运蛋白的活性会受激素刺激和细胞体积变化的影响;有些组织中,这种调控作用（尤其是NKCCl亚基）是通过特定的激酶使该转运蛋白自身发生氧化／硝化、磷酸化／去磷酸化来实现的;蛋白过表达在Na-K-Cl协同转运蛋白的激活中也起重要作用。