K+/NH4+ antiporter: a unique ammonium carrying transporter in the kidney inner medulla

Bioinorganic Chemistry of the Alkali Metal Ions

The common Group 1 alkali metals are indeed ubiquitous on earth, in the oceans and in biological systems. In this introductory chapter, concepts involving aqueous chemistry and aspects of general coordination chemistry and oxygen atom donor chemistry are introduced. Also, there are nuclear isotopes of importance. A general discussion of Group 1 begins from the prevalence of the ions, and from a comparison of their ionic radii and ionization energies. While oxygen and water molecule binding have the most relevance to biology and in forming a detailed understanding between the elements, there is a wide range of basic chemistry that is potentially important, especially with respect to biological chelation and synthetic multi-dentate ligand design. The elements are widely distributed in life forms, in the terrestrial environment and in the oceans. The details about the workings in animal, as well as plant life are presented in this volume. Important biometallic aspects of human health and medicine are introduced as well. Seeing as the elements are widely present in biology, various particular endogenous molecules and enzymatic systems can be studied. Sodium and potassium are by far the most important and central elements for consideration. Aspects of lithium, rubidium, cesium and francium chemistry are also included; they help in making important comparisons related to the coordination chemistry of Na(+) and K(+). Physical methods are also introduced.

Cellular and molecular basis of increased ammoniagenesis in potassium deprivation

Hypokalemia is associated with increased ammoniagenesis and stimulation of net acid excretion by the kidney in both humans and experimental animals. The molecular mechanisms underlying these effects remain unknown. Toward this end, rats were placed in metabolic cages and fed a control or K(+)-deficient diet (KD) for up to 6 days. Rats subjected to KD showed normal acid-base status and serum electrolytes composition. Interestingly, urinary NH(4)(+) excretion increased significantly and correlated with a parallel decrease in urine K(+) excretion in KD vs. control animals. Molecular studies showed a specific upregulation of the glutamine transporter SN1, which correlated with the upregulation of glutaminase (GA), glutamate dehydrogenase (GDH), and phosphoenolpyruvate carboxykinase. These effects occurred as early as day 2 of KD. Rats subjected to a combined KD and 280 mM NH(4)Cl loading (to induce metabolic acidosis) for 2 days showed an additive increase in NH(4)(+) excretion along with an additive increment in the expression levels of ammoniagenic enzymes GA and GDH compared with KD or NH(4)Cl loading alone. The incubation of cultured proximal tubule cells NRK 52E or LLC-PK(1) in low-K(+) medium did not affect NH(4)(+) production and did not alter the expression of SN1, GA, or GDH in NRK cells. These results demonstrate that K(+) deprivation stimulates ammoniagenesis through a coordinated upregulation of glutamine transporter SN1 and ammoniagenesis enzymes. This effect is developed before the onset of hypokalemia. The signaling pathway mediating these events is likely independent of KD-induced intracellular acidosis. Finally, the correlation between increased NH(4)(+) production and decreased K(+) excretion indicate that NH(4)(+) synthesis and transport likely play an important role in renal K(+) conservation during hypokalemia.

NH4+ modulates renal tubule amantadine transport independently of intracellular pH changes

A bicarbonate-dependent organic cation transporter, unique from rOCT1 and rOCT2, primarily mediates amantadine uptake into renal proximal tubules. We examined whether intracellular pH regulates bicarbonate-dependent amantadine transporter function in these tubules. NH(4)Cl treatment resulted in immediate intracellular alkalinization of tubules for up to 30s followed by gradual acidification that was maximal at 5min. Proximal tubule amantadine uptake was similarly inhibited (60%) by NH(4)Cl during both the early intracellular alkalinization and later acidification phases. Sodium propionate treatment resulted in immediate intracellular acidification of proximal tubules without inhibiting amantadine uptake. NH(4)Cl inhibition of bicarbonate-dependent amantadine uptake was dose-dependent, competitive and sex-dependent. NH(4)Cl, NH(4)NO(3), (NH(4))(2)SO(4) and (NH(4))(2)HPO(4) inhibited amantadine uptake into proximal tubules similarly. NH(4)Cl also stimulated efflux of amantadine and tetraethylammonium from preloaded proximal tubules, suggesting mediation of a facilitated process. These data suggest the potential for direct modulation of organic cation transporters by NH(4)(+) in rat kidney proximal tubules.

Cl(-)-dependent secretory mechanisms in isolated rat bile duct epithelial units

Cholangiocytes absorb and secrete fluid, modifying primary canalicular bile. In several Cl(-)-secreting epithelia, Na(+)-K(+)-2Cl(-) cotransport is a basolateral Cl(-) uptake pathway facilitating apical Cl(-) secretion. To determine if cholangiocytes possess similar mechanisms independent of CO2/HCO, we assessed Cl(-)-dependent secretion in rat liver isolated polarized bile duct units (IBDUs) by using videomicroscopy. Without CO2/HCO, forskolin (FSK) stimulated secretion entirely dependent on Na(+) and Cl(-) and inhibited by Na(+)-K(+)-2Cl(-) inhibitor bumetanide. Carbonic anhydrase inhibitor ethoxyzolamide had no effect on FSK-stimulated secretion, indicating negligible endogenous CO2/HCO transport. In contrast, FSK-stimulated secretion was inhibited approximately 85% by K(+) channel inhibitor Ba(2+) and blocked completely by bumetanide plus Ba(2+). IBDU Na(+)-K(+)-2Cl(-) cotransport activity was assessed by recording intracellular pH during NH4Cl exposure. Bumetanide inhibited initial acidification rates due to NH entry in the presence and absence of CO2/HCO. In contrast, when stimulated by FSK, a 35% increase in Na(+)-K(+)-2Cl(-) cotransport activity occurred without CO2/HCO. These data suggest a cellular model of HCO-independent secretion in which Na(+)-K(+)-2Cl(-) cotransport maintains high intracellular Cl(-) concentration. Intracellular cAMP concentration increases activate basolateral K(+) conductance, raises apical Cl(-) permeability, and causes transcellular Cl(-) movement into the lumen. Polarized IBDU cholangiocytes are capable of vectorial Cl(-)-dependent fluid secretion independent of HCO. Bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransport, Cl(-)/HCO exchange, and Ba(2+)-sensitive K(+) channels are important components of stimulated fluid secretion in intrahepatic bile duct epithelium.

Ammonium in nervous tissue: transport across cell membranes, fluxes from neurons to glial cells, and role in signalling

Most, but not all, animal cell membranes are permeable to NH3, the neutral, minority form of ammonium which is in equilibrium with the charged majority form NH4+. NH4+ crosses many cell membranes via ion channels or on membrane transporters, and cultured mammalian astrocytes and glial cells of bee retina take up NH4+ avidly, in the latter case on a Cl(-)-cotransporter selective for NH4+ over K+. In bee retina, a flux of ammonium from neurons to glial cells is an essential component of energy metabolism, which involves a flux of alanine from glial cells to neurons. In mammalian brain, both glutamate and ammonium are taken up preferentially by astrocytes and form glutamine. Glutamine is transferred to neurons where it is deamidated to re-form glutamate; the maintenance of this cycle appears to require a substantial flux of ammonium from neurons to astrocytes. In addition to maintaining the glial cell content of fixed N (a "bookkeeping" function), ammonium is expected to participate in the regulation of glial cell metabolism (a signalling function): it will increase conversion of glutamate to glutamine, and, by activating phosphofructokinase and inhibiting the alpha-ketoglutarate dehydrogenase complex, it will tend to increase the formation of lactate.

Intracellular pH regulation in colonocytes of rat proximal colon

The regulation of intracellular pH (pH(i)) in colonocytes of the rat proximal colon has been investigated using the pH-sensitive dye BCECF and compared with the regulation of pH(i) in the colonocytes of the distal colon. The proximal colonocytes in a HEPES-buffered solution had pH(i)=7.24+/-0.04 and removal of extracellular Na(+) lowered pH(i) by 0.24 pH units. Acid-loaded colonocytes by an NH(3)/NH(4)(+) prepulse exhibited a spontaneous recovery that was partially Na(+)-dependent and could be inhibited by ethylisopropylamiloride (EIPA). The Na(+)-dependent recovery rate was enhanced by increasing the extracellular Na(+) concentration and was further stimulated by aldosterone. In an Na(+)- and K(+)-free HEPES-buffered solution, the recovery rate from the acid load was significantly stimulated by addition of K(+) and this K(+)-dependent recovery was partially blocked by ouabain. The intrinsic buffer capacity of proximal colonocytes at physiological pH(i) exhibited a nearly 2-fold higher value than in distal colonocytes. Butyrate induced immediate colonocyte acidification that was smaller in proximal than in distal colonocytes. This acidification was followed by a recovery phase that was both EIPA-sensitive and -insensitive and was similar in both groups of colonocytes. In a HCO(3)(-)/CO(2)-containing solution, pH(i) of the proximal colonocytes was 7.20+/-0.04. Removal of external Cl(-) caused alkalinization that was inhibited by DIDS. The recovery from an alkaline load induced by removal of HCO(3)(-)/CO(2) from the medium was Cl(-)-dependent, Na(+)-independent and blocked by DIDS. Recovery from an acid load in EIPA-containing Na(+)-free HCO(3)(-)/CO(2)-containing solution was accelerated by addition of Na(+). Removal of Cl(-) inhibited the effect of Na(+). In summary, the freshly isolated proximal colonocytes of rats express Na(+)/H(+) exchanger, H(+)/K(+) exchanger ((H(+)-K(+))-ATPase) and Na(+)-dependent Cl(-)/HCO(3)(-) exchanger that contribute to acid extrusion and Na(+)-independent Cl(-)/HCO(3)(-) exchanger contributing to alkali extrusion. All of these are likely involved in the regulation of pH(i) in vivo. Proximal colonocytes are able to maintain a more stable pH(i) than distal cells, which seems to be facilitated by their higher intrinsic buffer capacity.

Ammonium carriers in medullary thick ascending limb

Absorption of NH(4)(+) by the medullary thick ascending limb (MTAL) is a key event in the renal handling of NH(4)(+), leading to accumulation of NH(4)(+)/NH(3) in the renal medulla, which favors NH(4)(+) secretion in medullary collecting ducts and excretion in urine. The Na(+)-K(+)(NH(4)(+))-2Cl(-) cotransporter (BSC1/NKCC2) ensures approximately 50-65% of MTAL active luminal NH(4)(+) uptake under basal conditions. Apical barium- and verapamil-sensitive K(+)/NH(4)(+) antiport and amiloride-sensitive NH(4)(+) conductance account for the rest of active luminal NH(4)(+) transport. The presence of a K(+)/NH(4)(+) antiport besides BSC1 allows NH(4)(+) and NaCl absorption by MTAL to be independently regulated by vasopressin. At the basolateral step, the roles of NH(3) diffusion coupled to Na(+)/H(+) exchange or Na(+)/NH(4)(+) exchange, which favors NH(4)(+) absorption, and of Na(+)/K(+)(NH(4)(+))-ATPase, NH(4)(+)-Cl(-) cotransport, and NH(4)(+) conductance, which oppose NH(4)(+) absorption, have not been quantitatively defined. The increased ability of the MTAL to absorb NH(4)(+) during chronic metabolic acidosis involves an increase in BSC1 expression, but fine regulation of MTAL NH(4)(+) transport probably requires coordinated effects on various apical and basolateral MTAL carriers.

Impact of K(+) homeostasis on net acid secretion in rat terminal inner medullary collecting duct: role of the Na,K-ATPase

For the past 50 years, the mechanism of ammonium (NH(4)(+)) transport along the collecting duct has been thought to occur through active H(+) section in parallel with the nonionic diffusion of ammonia (NH(3)). This model is supported by two basic experimental observations. First, NH(4)(+) secretion generally correlates with the NH(3) concentration gradient between the interstitium and the collecting duct lumen. This NH(3) gradient is generated through both luminal acidification, which reduces luminal NH(3) concentration, and through countercurrent multiplication, which increases interstitial NH(3) concentration. The result is secretion of NH(3) into the collecting duct lumen down its concentration gradient. Second, because NH(4)(+) permeability is low relative to that of NH(3), there is significant secretion of NH(3) into the collecting duct lumen with minimal back-diffusion of NH(4)(+). However, our laboratory, as well as others, has shown that this model is an oversimplification of the mechanism of NH(4)(+) transport along the collecting duct. NH(4)(+) is transported through a variety of K(+) transport pathways including Na,K-ATPase. K(+) and NH(4)(+) compete for a common extracellular binding site on Na, K-ATPase. During hypokalemia, interstitial K(+) concentration is reduced, which augments NH(4)(+) uptake by the Na(+) pump. In K(+) restriction, Na,K-ATPase-mediated NH(4)(+) uptake provides an important source of H(+) for net acid secretion and for the titration of luminal buffers in the terminal inner medullary collecting duct. This pathway contributes to the increase in NH(4)(+) excretion and metabolic alkalosis observed during hypokalemia.

Basolateral regulation of pHi in isolated snake renal proximal tubules in presence and absence of bicarbonate

Intracellular pH (pHi) and its basolateral regulation were studied in isolated proximal-proximal and distal-proximal segments of garter snake (Thamnophis spp.) renal tubules with oil-filled lumens in HEPES-buffered and in HEPES-HCO-3-buffered media (pH 7.4 at 25 degrees C). pHi was measured with the pH-sensitive fluorescent dye 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) under resting conditions and in response to NH4Cl pulse. Resting pHi (approximately 7.1-7.2) and its response to and rate of recovery (dpHi/dt) from an NH4Cl pulse were not affected by the presence or absence of HCO-3 in either segment. Rate of recovery was depressed by Na+ removal in distal-proximal segments only and only in HEPES buffer. It was not affected by removal of Cl- or of both Na+ and Cl- or by reduction in membrane potential through addition of Ba2+ (5 mM) or high K+ (75 mM) in either segment in either HEPES or HEPES-HCO-3 buffer. The Na+/H+ exchange inhibitor ethylisopropylamiloride (EIPA) (100 microM) and the anion exchange inhibitor DIDS (100 microM) reduced dpHi/dt in the distal-proximal segments only and only in HEPES-HCO-3 buffer. The H+-ATPase inhibitor bafilomycin (1 microM), H+-K+-ATPase and K+/NH+4 exchange inhibitor Schering 28080 (10-100 microM), organic cation efflux inhibitor tetrapentylammonium (25 microM-20 mM), and K+ channel blocker tetraethylammonium (20 mM) had no effect on dpHi/dt in either segment. These data do not clearly support basolateral regulation of pHi in snake proximal renal tubules by commonly recognized Na+-dependent or Na+-independent acid or base transporters.

Chloride-dependent transport of NH4+ into bee retinal glial cells

Mammalian astrocytes convert glutamate to glutamine and bee retinal glial cells convert pyruvate to alanine. To maintain such amination reactions these glial cells may take up NH4+/NH3. We have studied the entry of NH4+/NH3 into bundles of glial cells isolated from bee retina by using the fluorescent dye BCECF to measure pH. Ammonium caused intracellular pH to decrease by a saturable process: the rate of change of pH was maximal for an ammonium concentration of about 5 mM. This acidifying response to ammonium was abolished by the loop diuretic bumetanide (100 microM) and by removal of extracellular Cl-. These results strongly suggest that ammonium enters the cell by contransport of NH4+ with Cl-. Removal of extracellular Na+ did not abolish the NH(4+)-induced acidification. The NH(4+)-induced pH change was unaffected when nearly all K+ conductance was blocked with 5 mM Ba2+ showing that NH4+ did not enter through Ba(2+)-sensitive ion channels. Application of 2 mM NH4+ led to a large increase in total intracellular proton concentration estimated to exceed 13.5 mEq/L. As the cell membrane appeared to be permeable to NH3, we suggest that when NH4+ entered the cells, NH3 left, so that protons were shuttled into the cell. This shuttle, which was strongly dependent on internal and external pH, was quantitatively modelled. In retinal slices, 2 mM NH4+ alkalinized the extracellular space: this alkalinization was reduced in the absence of bath Cl-. We conclude that NH4+ enters the glial cells in bee retina on a cotransporter with functional similarities to the NH4+(K+)-Cl- cotransporter described in kidney cells.

Activation of H+-ATPase by hypotonicity: a novel regulatory mechanism for H+ secretion in IMCD cells

The effect of hypotonicity on H+-ATPase activity was examined in cultured inner medullary collecting duct (mIMCD-3) cells. mIMCD-3 cells were grown to confluence, loaded with 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and assayed for H+-ATPase activity measured as the Na+- and K+-independent intracellular pH (pHi) recovery following an acid load. Exposure of mIMCD-3 cells to a hypotonic solution (150 mosmol/kgH2O) increased pHi recovery by approximately 350% (P < 0.0001). This effect was inhibited by diethylstilbestrol (an inhibitor of H+-ATPase) and was not dependent on external K+, indicating lack of involvement of H+-K+-ATPase. H+-ATPase activation was acute, independent of cell calcium, and was not secondary to Cl- channel activation. The magnitude of H+-ATPase upregulation was dependent on the osmolarity of the media, with maximum stimulation at 150 mosmol/kgH2O. H+-ATPase upregulation in hypotonicity was significantly blocked in the presence of staurosporine or calphostin C or in cells pretreated with phorbol 12-myristate 13-acetate (PMA), indicating involvement of protein kinase C. Hypotonicity inhibited the Na+/H+ exchanger activity in mIMCD-3 cells, indicating that its stimulatory effect is specific to H+-ATPase. In conclusion, a novel regulatory mechanism of H+-ATPase by hypotonicity is described. The increased H+-ATPase activity in hypotonicity may be responsible for increased HCO-3 reabsorption and maintained acid-base homeostasis in hyposmolar states.

Ouabain reduces net acid secretion and increases pHi by inhibiting NH4+ uptake on rat tIMCD Na(+)-K(+)-ATPase

In the rat terminal inner medullary collecting duct (tIMCD), Na+ pump inhibition reduces transepithelial net acid secretion (JtAMM) [JH = total CO2 absorption (JtCO2)+ total ammonia secretion] and increases resting intracellular pH (pHi). The increase in pHi and reduction in JH that follow ouabain addition do not occur in the absence of NH4+ nor when NH4+ is substituted with another weak base. The purpose of this study was to explore the mechanism of the NH4(+)-dependent reduction in JtCO2 and increase in pHi that follow ouabain addition. We hypothesized that NH4+ enters the tIMCD cell through the Na(+)-K(+)-ATPase with proton release in the cytosol. To test this hypothesis, tIMCDs were dissected from deoxycorticosterone-treated rats and perfused in vitro with symmetrical physiological saline solutions containing 6 mM NH4Cl. Since K+ and NH4+ compete for a common binding site on the Na+ pump, increasing extracellular K+ should limit NH4+ (and hence net H+) uptake by the Na+ pump. Upon increasing extracellular K+ concentration from 3 to 12 mM, the NH4(+)-dependent, ouabain-induced increase in pHi and reduction in JtCO2 were attenuated. In the presence but not in the absence of NH4+, reducing Na+ pump activity by limiting Na+ entry reduced JtCO2 and attenuated ouabain-induced alkalinization. Ouabain-induced alkalinization was not dependent on the presence of HCO3-/CO2 and was not reproduced with BaCl2 or bumetanide addition. Therefore, ouabain-induced alkalinization is not mediated by the Na(+)-K(+)-2Cl- cotransporter or a HCO3- transporter and is not mediated by changes in membrane potential. In conclusion, on the basolateral membrane of the tIMCD cell, NH4+ uptake is mediated by the Na(+)-K(+)-ATPase. These data provide an explanation for the reduction in net acid secretion in the tIMCD observed following administration of amiloride or with dietary K+ loading.

Cloning and functional expression of a human kidney Na+:HCO3- cotransporter

Several modes of HCO3- transport occur in the kidney, including Na+-independent Cl/HCO3- exchange (mediated by the AE family of Cl-/HCO3- exchangers), sodium-dependent Cl-/HCO3- exchange, and Na+:HCO3- cotransport. The functional similarities between the Na+-coupled HCO3- transporters and the AE isoforms (i.e. transport of HCO3- and sensitivity to inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) suggested a strategy for cloning the other transporters based on structural similarity with the AE family. An expressed sequence tag encoding part of a protein that is related to the known anion exchangers was identified in the GenBankTM expressed sequence tag data base and used to design an oligonucleotide probe. This probe was used to screen a human kidney cDNA library. Several clones were identified, isolated, and sequenced. Two overlapping cDNA clones were spliced together to form a 7.6-kilobase cDNA that contained the entire coding region of a novel protein. Based on the deduced amino acid sequence, the cDNA encodes a protein with a Mr of 116,040. The protein has 29% identity with human brain AE3. Northern blot analysis reveals that the 7.6-kilobase mRNA is highly expressed in kidney and pancreas, with detectable levels in brain. Functional studies in transiently transfected HEK-293 cells demonstrate that the cloned transporter mediates Na+:HCO3- cotransport.