The anion specificity of the sodium-potassium-chloride cotransporter in rabbit kidney outer medulla: studies on medullary plasma membranes

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Cryo-EM structure of the human NKCC1 transporter reveals mechanisms of ion coupling and specificity

The sodium-potassium-chloride transporter NKCC1 of the SLC12 family performs Na⁺ -dependent Cl^- - and K⁺ -ion uptake across plasma membranes. NKCC1 is important for regulating cell volume, hearing, blood pressure, and regulation of hyperpolarizing GABAergic and glycinergic signaling in the central nervous system. Here, we present a 2.6 Å resolution cryo-electron microscopy structure of human NKCC1 in the substrate-loaded (Na⁺ , K⁺ , and 2 Cl^- ) and occluded, inward-facing state that has also been observed for the SLC6-type transporters MhsT and LeuT. Cl^- binding at the Cl1 site together with the nearby K⁺ ion provides a crucial bridge between the LeuT-fold scaffold and bundle domains. Cl^- -ion binding at the Cl2 site seems to undertake a structural role similar to conserved glutamate of SLC6 transporters and may allow for Cl^- -sensitive regulation of transport. Supported by functional studies in mammalian cells and computational simulations, we describe a putative Na⁺ release pathway along transmembrane helix 5 coupled to the Cl2 site. The results provide insight into the structure-function relationship of NKCC1 with broader implications for other SLC12 family members.

Chronic sodium bromide treatment relieves autistic-like behavioral deficits in three mouse models of autism

Autism Spectrum Disorders (ASD) are neurodevelopmental disorders whose diagnosis relies on deficient social interaction and communication together with repetitive behavior. To date, no pharmacological treatment has been approved that ameliorates social behavior in patients with ASD. Based on the excitation/inhibition imbalance theory of autism, we hypothesized that bromide ions, long used as an antiepileptic medication, could relieve core symptoms of ASD. We evaluated the effects of chronic sodium bromide (NaBr) administration on autistic-like symptoms in three genetic mouse models of autism: Oprm1^-/-, Fmr1^-/- and Shank3^Δex13-16-/- mice. We showed that chronic NaBr treatment relieved autistic-like behaviors in these three models. In Oprm1^-/- mice, these beneficial effects were superior to those of chronic bumetanide administration. At transcriptional level, chronic NaBr in Oprm1 null mice was associated with increased expression of genes coding for chloride ions transporters, GABA_A receptor subunits, oxytocin and mGlu4 receptor. Lastly, we uncovered synergistic alleviating effects of chronic NaBr and a positive allosteric modulator (PAM) of mGlu4 receptor on autistic-like behavior in Oprm1^-/- mice. We evidenced in heterologous cells that bromide ions behave as PAMs of mGlu4, providing a molecular mechanism for such synergy. Our data reveal the therapeutic potential of bromide ions, alone or in combination with a PAM of mGlu4 receptor, for the treatment of ASDs.

Cryo-EM structures of DrNKCC1 and hKCC1: a new milestone in the physiology of cation-chloride cotransporters

New milestones have been reached in the field of cation-Cl- cotransporters with the recently released cryo-electron microscopy (EM) structures of the Danio rerio (zebrafish) Na+-K+-2Cl- cotransporter (DrNKCC1) and the human K+-Cl- cotransporter (hKCC1). In this review we provide a brief timeline that identifies the multiple breakthroughs in the field of solute carrier 12 transporters that led to the structure resolution of two of its key members. While cation-Cl- cotransporters share the overall architecture of carriers belonging to the amino acid-polyamine-organocation (APC) superfamily and some of their substrate binding sites, several new insights are gained from the two individual structures. A first major feature relates to the largest extracellular domain between transmembrane domain (TMD) 5 and TMD6 of KCC1, which stabilizes the dimer and forms a cap that likely participates in extracellular gating. A second feature is the conservation of the K+ and Cl- binding sites in both structures and evidence of an unexpected second Cl- coordination site in the KCC1 structure. Structural data are discussed in the context of previously published studies that examined the basic and kinetics properties of these cotransport mechanisms. A third characteristic is the evidence of an extracellular gate formed by conserved salt bridges between charged residues located toward the end of TMD3 and TMD4 in both transporters and the existence of an additional neighboring bridge in the hKCC1 structure. A fourth feature of these newly solved structures relates to the multiple points of contacts between the monomer forming the cotransporter homodimer units. These involve the TMDs, the COOH-terminal domains, and the large extracellular loop for hKCC1.

Bicarbonate-dependent chloride transport drives fluid secretion by the human airway epithelial cell line Calu-3

Anion and fluid secretion are both defective in cystic fibrosis (CF); however, the transport mechanisms are not well understood. In this study, Cl(-) and HCO(3)(-) secretion was measured using genetically matched CF transmembrane conductance regulator (CFTR)-deficient and CFTR-expressing cell lines derived from the human airway epithelial cell line Calu-3. Forskolin stimulated the short-circuit current (I(sc)) across voltage-clamped monolayers, and also increased the equivalent short-circuit current (I(eq)) calculated under open-circuit conditions. I(sc) was equivalent to the HCO(3)(-) net flux measured using the pH-stat technique, whereas I(eq) was the sum of the Cl(-) and HCO(3)(-) net fluxes. I(eq) and HCO(3)(-) fluxes were increased by bafilomycin and ZnCl(2), suggesting that some secreted HCO(3)(-) is neutralized by parallel electrogenic H(+) secretion. I(eq) and fluid secretion were dependent on the presence of both Na(+) and HCO(3)(-). The carbonic anhydrase inhibitor acetazolamide abolished forskolin stimulation of I(eq) and HCO(3)(-) secretion, suggesting that HCO(3)(-) transport under these conditions requires catalysed synthesis of carbonic acid. Cl(-) was the predominant anion in secretions under all conditions studied and thus drives most of the fluid transport. Nevertheless, 50-70% of Cl(-) and fluid transport was bumetanide-insensitive, suggesting basolateral Cl(-) loading by a sodium-potassium-chloride cotransporter 1 (NKCC1)-independent mechanism. Imposing a transepithelial HCO(3)(-) gradient across basolaterally permeabilized Calu-3 cells sustained a forskolin-stimulated current, which was sensitive to CFTR inhibitors and drastically reduced in CFTR-deficient cells. Net HCO(3)(-) secretion was increased by bilateral Cl(-) removal and therefore did not require apical Cl(-)/HCO(3)(-) exchange. The results suggest a model in which most HCO(3)(-) is recycled basolaterally by exchange with Cl(-), and the resulting HCO(3)(-)-dependent Cl(-) transport provides an osmotic driving force for fluid secretion.

Gene transfer of CFTR to airway epithelia: low levels of expression are sufficient to correct Cl- transport and overexpression can generate basolateral CFTR

Gene transfer of CFTR cDNA to airway epithelia is a promising approach to treat cystic fibrosis (CF). Most gene transfer vectors use strong viral promoters even though the endogenous CFTR promoter is very weak. To learn whether expressing CFTR at a low level in a fraction of cells would correct Cl(-) transport, we mixed freshly isolated wild-type and CF airway epithelial cells in varying proportions and generated differentiated epithelia. Epithelia with approximately 20% wild-type cells generated approximately 70% the transepithelial Cl(-) current of epithelia containing 100% wild-type cells. These data were nearly identical to those previously obtained with CFTR expressed under control of a strong promoter in a CF epithelial cell line. We also tested high level CFTR expression using the very strong cytomegalovirus (CMV) promoter as well as the cytokeratin-18 (K18) promoter. In differentiated airway epithelia, the CMV promoter generated 50-fold more transgene expression than the K18 promoter, but the K18 promoter generated more transepithelial Cl(-) current at high vector doses. Using functional studies, we found that with marked overexpression, some CFTR channels were present in the basolateral membrane where they shunted Cl(-) flow, thereby reducing net transepithelial Cl(-) transport. These results suggest that very little CFTR is required in a fraction of CF epithelial cells to complement Cl(-) transport because transepithelial Cl(-) flow is limited at the basolateral membrane. Thus they suggest a broad leeway in promoter strength for correcting the CF gene transfer, although at very high expression levels CFTR may be mislocalized to the basolateral membrane.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

HCl-induced cell edema in rabbit esophageal epithelium: a bumetanide-sensitive process

BACKGROUND & AIMS

The morphology of acid damage to esophageal epithelium is characterized by marked cell (swelling) edema. This observation suggests that, in the process of acid damage, an increase in osmotic forces develops within the cell that accounts for the increase in cell water. The aim of this study was to document that esophageal cells swell at acidic pH and to explore the nature of the osmolytes and mechanisms responsible for it.

METHODS

Cell edema was assessed in sections of rabbit esophageal epithelium by correlating morphological change with change in tissue wet weight after immersion in acidic solutions for up to 4 hours.

RESULTS

At pH < or = 2 for 2 hours, tissues gained weight and showed cell edema on both light and electron microscopy. In addition to being time- and pH-dependent, cell edema was dependent on bathing solution osmolytes, specifically Na+, K+, and Cl-, and could be inhibited by tissue pretreatment with bumetanide (or ethacrynic acid).

CONCLUSIONS

HCl exposure can spontaneously produce cell edema in esophageal epithelium. The phenomenon is pH- and time-dependent and requires acid stimulation of osmolyte absorption through a bumetanide-sensitive process compatible with an NaK2Cl cotransporter in the epithelial cell membrane.

Na+/K+/2Cl- cotransport in medullary thick ascending limb cells: kinetics and bumetanide binding

We examined the properties of Na+/K+/2Cl- cotransport in cultured mouse mTAL cells with respect to its kinetics, the contribution of K/K exchange to K fluxes mediated by the cotransporter, and [3H]bumetanide binding and turnover numbers in media with varying osmolality. The addition of bumetanide, the replacement of external Na+ or the replacement of external Cl- resulted in an almost identical (approx. 50%) decrease in K+ influx, suggesting that Na(+)-dependent, Cl(-)-dependent, BS K+ influx was a measure of Na+/K+/2Cl- cotransport. The kinetics of the BS K+ influx revealed a high affinity for external Na+ (apparent Km 7 mM) and external K+ (apparent Km 1.3 mM), but a very low affinity for external Cl- (apparent Km 67 mM with a two-site model). Of interest was the finding that none of the K+ (86Rb+) efflux was sensitive to bumetanide, suggesting the absence of cotransport mediated K/K exchange in this cell type. Specific [3H]bumetanide binding was a saturable function of free bumetanide concentration with a Kd of 0.20 microM and maximum binding (Bmax) of 0.63 pmol/mg, or about 53,000 sites per cell. Simultaneous transport and bumetanide binding assays yielded a turnover number of 255 min-1. The omission of external Na+, K+ or Cl- reduced specific [3H]bumetanide binding to values indistinguishable from zero. Changing medium osmolarity resulted in a co-ordinate change in BS K+ influx and bumetanide binding, with a monotonic increase in both transport and bumetanide binding with increase in osmolality from 200 to 400 mosmol/kg. About 85% of the cotransporter sites were located on the apical side, as in the intact mTAL tubule. The simultaneous measurement of BS ion transport and [3H]bumetanide binding in the mTAL cell may provide valuable insights into the regulation of Na+/K+/2Cl- cotransport in this nephron segment.

Erythrocyte K-Cl cotransport: properties and regulation

Erythrocytes possess a Cl-dependent, Na-independent K transport system cotransporting K and Cl in a 1:1 stoichiometry that is membrane potential independent. This K-Cl cotransporter is stimulated by cell swelling, acidification, Mg depletion, and thiol modification. Cell shrinkage, elevation of cellular divalent ions, thiol alkylation, phosphatase inhibitors, and derivatives of certain loop diuretics and stilbenes are inhibitory. Thus regulation of K-Cl cotransport at the membrane and cytoplasmic levels is highly complex. Basal K-Cl cotransport decreases with cellular maturation, whereas its modes of stimulation and inhibition are variable between species. The physiological inactivation appears to be prevented in low-K animal erythrocytes. In certain human hemoglobinopathies, K-Cl cotransport may be the cause of cellular dehydration and volume decrease. K-Cl cotransport occurs also in nonerythroid cells, such as in epithelial and liver cells of other species. At the threshold of molecular characterization, this comprehensive review places our present understanding of the mechanisms modulating K-Cl cotransport physiologically and pathophysiologically into kinetic and thermodynamic perspectives.

T84 cells: anion selectivity demonstrates expression of Cl- conductance affected in cystic fibrosis

The T84 cell line possesses an adenosine 3',5'-cyclic monophosphate (cAMP)-activated Cl- conductance and expresses high levels of the cystic fibrosis (CF) gene product, implicating it as a good model for CF research. To evaluate whether T84 Cl- conductance properties are consistent with those described in CF target epithelial, we used transepithelial measurements (verified by selective permeabilization of the basal membrane) to determine the apparent anion selectivity properties of the apical and basolateral membranes of stimulated and unstimulated T84 cells. Unstimulated epithelial cells were almost electrically inert, having a low transepithelial voltage (Vt; -6 mV, apical surface negative), a small equivalent short-circuit current (Isc,(eq.) 2.2 microA/cm2), a very high transepithelial resistance (Rt; 2,500 omega.cm2), and poor anion permselectivity properties at both membrane surfaces (0.8 less than PX/PCl- less than 1.1), where X is NO3-, Br-, I-, or gluconate. When stimulated with forskolin (10(-6) M), Vt increased 8-fold, Isc(eq) increased 30-fold, Rt fell to one-third of unstimulated values, and the apical surface became highly anion selective, i.e., NO3- (1.4) greater than Br- (1.2) greater than Cl- (1.0) greater than I- (0.7) greater than gluconate (0.0), where numbers in parentheses are PX/PCl-. I- was less permeable than Cl- and probably directly inhibits the anion conductance, since Rt was substantially greater after I- substitution than after substitution with the impermeable anion gluconate. Bumetanide (10(-4) M) significantly attenuated the response of Vt to anion substitutions at the basal membrane surface, indicating that the effects of substitution were predominantly on the Na(+)-K(+)-2Cl- cotransporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of DIDS inhibition of swelling-activated K-Cl cotransport in low K sheep erythrocytes

The inhibitory effect of various stilbene disulfonates was examined on the swelling-activated Cl-dependent K transport (K-Cl cotransport) in low K sheep erythrocytes. Both diisothiocyanatostilbenes H2DIDS and DIDS were found to be potent inhibitors. The DIDS concentration yielding 50% inhibition (IC50) of KCl cotransport was 60 microM in the absence of external K and 3 microM at physiological K concentration. Other stilbene derivatives, such as SITS (4-acetamido-4' isothiocyanatostilbene-2,2'-disulfonic acid), were only effective in the presence of external K, whereas DNDS (4,4'-dinitrostilbene-2,2'-disulfonic acid) and ISA (4-sulfophenyl isothiocyanate) had only slight effects at a concentration of 1 mM. The augmenting effect of external K is due to a second K site, distinguishable from the K transport site by its much higher affinity. No inhibition occurred in the absence of external Cl, whether or not external Rb(K) was present. Additionally, DIDS inhibited K-Cl cotransport activated by thiol alkylation with N-ethylmaleimide (NEM) as well as by Mg depletion in the presence of A23187 and a chelator. We conclude that allosteric sites affect the stilbene binding. When these sites are saturated, changes in external K or Cl concentration do not affect the affinity for DIDS (noncompetitive inhibition).

Cl- channels in basolateral renal medullary membrane vesicles: IV. Analogous channel activation by Cl- or cAMP-dependent protein kinase

We examined the interactions of cAMP-dependent protein kinase and varying aqueous Cl- concentrations in modulating the activity of Cl- channels obtained by fusing basolaterally enriched renal outer medullary vesicles into planar lipid bilayers. Under the present experimental conditions, the cis and trans solutions face the extracellular and intracellular aspects of these Cl- channels, respectively. Raising the trans Cl- concentration from 2 to 50 mM increased the channel open-time probability, raised the unit channel conductance, and affected the voltage-independent determinant (delta G) of channel activity but not the gating charge (Winters, C.J., Reeves, W.B., Andreoli, T.E. 1990. J. Membrane Biol. 118:269-278). With 2 mM trans KCl, trans addition of the catalytic subunit of PKA (C-PKA) plus ATP increased channel open-time probability and altered the voltage-independent determinant of channel activity without affecting either unit channel conductance or gating charge. The effect was ATP specific, did not occur with (C-PKA plus ATP) addition to cis solutions, and was abolished by denaturing C-PKA. Finally, (C-PKA plus ATP) activation of channel activity was not detected with relatively high (50 mM) trans Cl- concentrations. These data indicate that (C-PKA plus ATP) might modulate Cl- channel activity by phosphorylation at or near the Cl(-)-sensitive site on the intracellular face of these channels.

Characterization of a Na(+)-K(+)-2Cl- cotransport system in oocytes from Xenopus laevis

In order to characterize the transport systems mediating K+ uptake into oocytes, flux studies employing 86Rb were performed on Xenopus oocytes stripped of follicular cells by pretreatment with Ca2(+)-Mg2(+)-free Barth's medium. Total Rb+ uptake consisted of an ouabain-sensitive and an ouabain-insensitive flux. In the presence of 100 mmol/l NaCl and 0.1 mmol/l ouabain the ouabain-insensitive flux amounted to 754.7 +/- 59.9 pmol/oocyte per h (n = 30 cells, i.e., 10 cells each from three different animals). In the absence of Na+ (Na+ substituted by N-methylglucamine) or when Cl- was replaced by NO3- the ouabain-insensitive flux was reduced to 84.4 +/- 42.9 and 79.2 +/- 12.1 pmol/oocyte per h, respectively (n = 50 cells). Furthermore, this Na(+)- and Cl(-)-dependent flux was completely inhibited by 10(-4) mol/l bumetanide, a specific inhibitor of the Na(+)-K(+)-2Cl- cotransport system. These results suggest that K+ uptake via a bumetanide-sensitive Na(+)-K(+)-2Cl- cotransport system represents a major K+ pathway in oocytes.

Solubilization and partial purification of the rabbit parotid Na/K/Cl-dependent bumetanide binding site

We demonstrate that the high affinity bumetanide binding site of the rabbit parotid acinar cell can be extracted from a basolateral membrane fraction using relatively low concentrations (0.07%, wt/vol; 1 mg membrane protein/ml) of the nonionic detergent Triton X-100. This extracted site cannot be sedimented by ultracentrifugation at 100,000 x g x 1 hr. Bumetanide binding to this site retains the ionic characteristics of bumetanide binding to native membranes but shows a fivefold increase in binding affinity (Kd = 0.57 +/- 0.15 microM vs. Kd = 3.3 +/- 0.7 microM for native membranes). Inactivation of the extracted bumetanide binding site observed at detergent/protein ratios greater than 1 can be prevented or (partially) reversed by the addition of exogenous lipid (0.2% soybean phosphatidylcholine). When the 0.07% Triton extract is fractionated by sucrose density gradient centrifugation in 0.24% Triton X-100, 0.2% exogenous lipid and 200 mM salt, the high affinity bumetanide binding site sediments as a single band with S20,w = 8.8 +/- 0.8 S. This corresponds to a molecular weight approximately 200 kDa for the bumetanide binding protein-detergent-lipid complex and represents a sevenfold purification of this site relative to the starting membrane fraction. In contrast to previous attempts to purify Na/K/Cl cotransport proteins and their associated bumetanide binding sites, the present method avoids harsh detergent treatment as well as direct covalent modification (inactivation) of the transporter itself. As a consequence, one can follow the still active protein through a series of extraction and purification steps by directly monitoring its bumetanide binding properties.

Inactivation of the rabbit parotid Na/K/Cl cotransporter by N-ethylmaleimide

The inactivation of the rabbit parotid Na/K/Cl cotransporter by the irreversible sulfhydryl reagent N-ethylmaleimide (NEM) is studied by monitoring its effect on high affinity bumetanide binding to the carrier. NEM reduces the number of bumetanide binding sites with no significant change in the affinity of those remaining. NEM also reduces KCl-dependent 22Na flux via the cotransporter by the same factor as the reduction in bumetanide binding sites. Both bumetanide and its analogue furosemide can protect against the effect of NEM. The concentration range over which this protection occurs is in good agreement with affinities of these two compounds for the high affinity bumetanide binding site (2.6 and 8.5 microM, respectively), indicating an association of this site with the site of action of NEM. Also consistent with this hypothesis are the observations that (i) sodium and potassium, both of which are required for high affinity bumetanide binding, increase the rate of inactivation of binding by NEM and (ii) chloride, at concentrations previously shown to competitively inhibit bumetanide binding, protects the cotransporter against NEM. The effects of NEM on bumetanide binding are mimicked by another highly specific sulfhydryl reagent, methyl methanethiolsulfonate. The apparent rate constant for inactivation of high affinity bumetanide binding by NEM is a hyperbolic function of NEM concentration consistent with a model in which the inactivation reaction is first order in [NEM] and proceeds through an intermediate adsorptive complex. The data indicate that the presence of a reduced sulfhydryl group at or closely related to the bumetanide binding site is essential for the operation of the parotid Na/K/Cl cotransporter.

Activation of K+ channels in renal medullary vesicles by cAMP-dependent protein kinase

ADH, acting through cAMP, increases the potassium conductance of apical membranes of mouse medullary thick ascending limbs of Henle. The present studies tested whether exposure of renal medullary apical membranes in vitro to the catalytic subunit of cAMP-dependent protein kinase resulted in an increase in potassium conductance. Apical membrane vesicles prepared from rabbit outer renal medulla demonstrated bumetanide- and chloride-sensitive 22Na+ uptake and barium-sensitive, voltage-dependent 86Rb+ influx. When vesicles were loaded with purified catalytic subunit of cAMP-dependent protein kinase (150 mU/ml), 1 mM ATP, and 50 mM KCl, the barium-sensitive 86Rb+ influx increased from 361 +/- 138 to 528 +/- 120 pM/mg prot.30 sec (P less than 0.01). This increase was inhibited completely when heat-stable protein kinase inhibitor (1 microgram/ml) was also present in the vesicle solutions. The stimulation of 86Rb+ uptake by protein kinase required ATP rather than ADP. It also required opening of the vesicles by hypotonic shock, presumably to allow the kinase free access to the cytoplasmic face of the membranes. We conclude that cAMP-dependent protein kinase-mediated phosphorylation of apical membranes from the renal medulla increases the potassium conductance of these membranes. This mechanism may account for the ADH-mediated increase in potassium conductance in the mouse mTALH.

Purification of a putative Na+/D-glucose cotransporter from pig kidney brush border membranes on a phlorizin affinity column

Phlorizin, a potent inhibitor of the Na+/D-glucose cotransporter, was derivatised to 3-aminophlorizin and subsequently coupled to Affi-Gel 15. Affinity chromatography of pig kidney brush border membranes solubilised in Triton X-100 allowed the purification of a 60 kDa protein on this resin. We consider this protein to be the Na+/D-glucose cotransporter, or part of it, for the following reasons: (i) binding of this protein to Affi-Gel 15 specifically requires phlorizin covalently attached to the resin and is lowered when phlorizin is replaced by phloretin; (ii) binding of the 60 kDa protein to a phlorizin affinity column requires the presence of Na+; (iii) polyclonal as well as monoclonal antibodies against the 60 kDa protein inhibit binding of phlorizin to brush border membranes from rabbit and pig kidney.

Ionic dependence of bumetanide binding to the rabbit parotid Na/K/Cl cotransporter

The Na/K/Cl-dependent component of the binding of the loop diuretic bumetanide to basolateral membrane vesicles from the rabbit parotid is studied. A Scatchard analysis indicates that this binding is due to a single high-affinity site with KD = 3.2 +/- 0.3 microM (n = 9) at 100 mM sodium, 100 mM potassium and 5 mM chloride. When KCl-dependent 22Na transport and tracer [3H]-bumetanide binding are monitored simultaneously as a function of (unlabeled) bumetanide concentration it is found that the K0.5 for bumetanide inhibition of both processes are identical indicating that the high-affinity bumetanide binding site studied here is identical with a bumetanide-inhibitory site on the Na/K/Cl cotransport system previously identified in this preparation (R.J. Turner. J.N. George and B.J. Baum, J. Membrane Biol. 94:143-152, 1986). High-affinity bumetanide binding exhibits a hyperbolic dependence on both [Na] and [K] consistent with Na/bumetanide and K/bumetanide binding stoichiometries of 1:1 and K0.5 values of approximately 33 mM for sodium and 23 mM for potassium. In contrast, the dependence on [Cl] is biphasic, with bumetanide binding increasing from 0 to 5 mM chloride and decreasing toward baseline levels thereafter. Scatchard analysis of this latter inhibitory effect of chloride indicates a competitive interaction with bumetanide in agreement with earlier indications that bumetanide inhibits Na/K/Cl cotransport at a chloride site. However, studies of the effects of various anions on bumetanide binding and 22Na transport show a poor correlation between the specificities of these two processes, suggesting that the inhibitory chloride site is not a chloride transport site.

Sodium cotransport systems in epithelial secretion

1. After considering the direct coupling and indirect coupling to the sodium gradient in sodium-dependent secretion across epithelia, the properties of the Na-K-Cl cotransporter involved in active chloride secretion and active chloride absorption are summarized. 2. A comparison between cellular mechanism of secretion and absorption shows that the direction of transepithelial transport is determined mainly by the intracellular localization of sodium cotransport systems and sodium-independent leaks. 3. Sodium cotransport systems, performing a similar function in various epithelia or species, may provide a powerful model to study their function at a molecular level.

Ammonium transport in medullary thick ascending limb of rabbit kidney: involvement of the Na+,K+,Cl(-)-cotransporter

In order to investigate the question whether ammonium reabsorption in the thick ascending limb of Henle's loop (TALH) proceeds via the Na+,K+,Cl(-)-cotransporter, plasma membrane vesicles were prepared from TALH cells isolated from rabbit kidney outer medulla and the effect of NH+4 on their transport properties was investigated. It was found that, in the presence of a 78-mmol/liter NaCl gradient, 5 mmol/liter NH+4 inhibited bumetanide-sensitive rubidium flux by 86%; a similar decrease was observed for 5 mmol/liter, K+. Inhibition of bumetanide-sensitive rubidium uptake by NH+4 was competitive and an apparent Ki of 1.9 mmol/liter was found. Bumetanide-sensitive sodium uptake measured in the presence of a 83 mmol/liter KCl gradient was not inhibited by 5 mmol/liter NH+4. A 100-mmol/liter NH4Cl gradient was, however, capable of stimulating bumetanide-sensitive sodium uptake to the same extent as a KCl gradient. These data suggest that NH+4 is accepted by the K+ site of the Na+,K+,Cl-cotransport system and that the transporter can function in a Na+,NH+4,2Cl mode. Since the affinity of the transporter for NH+4 lies in the concentration range found in the TALH lumen in vivo, it is concluded that Na+,NH+4,2Cl-cotransport can contribute to the NH+4 reabsorption in this tubular segment.