The colonic H+,K+-ATPase functions as a Na+-dependent K+(NH4+)-ATPase in apical membranes from rat distal colon

Increased colonic K+ excretion through inhibition of the H,K-ATPase type 2 helps reduce plasma K+ level in a murine model of nephronic reduction

Hyperkalemia is frequently observed in patients at the end-stage of chronic kidney disease (CKD), and has possible harmful consequences on cardiac function. Many strategies are currently used to manage hyperkalemia, one consisting of increasing fecal K⁺ excretion through the administration of cation-exchange resins. In this study, we explored another more specific method of increasing intestinal K⁺ secretion by inhibiting the H,K-ATPase type 2 (HKA2), which is the main colonic K⁺ reabsorptive pathway. We hypothetised that the absence of this pump could impede the increase of plasma K⁺ levels following nephronic reduction (N5/6) by favoring fecal K⁺ secretion. In N5/6 WT and HKA2KO mice under normal K⁺ intake, the plasma K⁺ level remained within the normal range, however, a load of K⁺ induced strong hyperkalemia in N5/6 WT mice (9.1 ± 0.5 mM), which was significantly less pronounced in N5/6 HKA2KO mice (7.9 ± 0.4 mM, p < 0.01). This was correlated to a higher capacity of HKA2KO mice to excrete K⁺ in their feces. The absence of HKA2 also increased fecal Na⁺ excretion by inhibiting its colonic ENaC-dependent absorption. We also showed that angiotensin-converting-enzyme inhibitor like enalapril, used to treat hypertension during CKD, induced a less severe hyperkalemia in N5/6 HKA2KO than in N5/6 WT mice. This study therefore provides the proof of concept that the targeted inhibition of HKA2 could be a specific therapeutic maneuver to reduce plasma K⁺ levels in CKD patients.

H,K-ATPase type 2 regulates gestational extracellular compartment expansion and blood pressure in mice

The modifications of the hemodynamic system and hydromineral metabolism are physiological features characterizing a normal gestation. Thus, the ability to expand plasma volume without increasing the level of blood pressure is necessary for the correct perfusion of the placenta. The kidney is essential in this adaptation by reabsorbing avidly sodium and fluid. In this study, we observed that the H,K-ATPase type 2 (HKA2), an ion pump expressed in kidney and colon and already involved in the control of the K⁺ balance during gestation, is also required for the correct plasma volume expansion and to maintain normal blood pressure. Indeed, compared with WT pregnant mice that exhibit a 1.6-fold increase of their plasma volume, pregnant HKA2-null mice (HKA2KO) only modestly expand their extracellular volume (×1.2). The renal expression of the epithelial Na channel (ENaC) α- and γ-subunits and that of the pendrin are stimulated in gravid WT mice, whereas the Na/Cl^- cotransporter (NCC) expression is downregulated. These modifications are all blunted in HKA2KO mice. This impeded renal adaptation to gestation is accompanied by the development of hypotension in the pregnant HKA2KO mice. Altogether, our results showed that the absence of the HKA2 during gestation leads to an "underfilled" situation and has established this transporter as a key player of the renal control of salt and potassium metabolism during gestation.

Feeding through your gills and turning a toxicant into a resource: how the dogfish shark scavenges ammonia from its environment

Nitrogen (N) appears to be a limiting dietary resource for elasmobranchs, required not only for protein growth but also for urea-based osmoregulation. Building on recent evidence that the toxicant ammonia can be taken up actively at the gills of the shark and made into the valuable osmolyte urea, we demonstrate that the uptake exhibits classic Michaelis–Menten saturation kinetics with an affinity constant (Km) of 379 µmol l−1, resulting in net N retention at environmentally realistic ammonia concentrations (100–400 µmol l−1) and net N loss through stimulated urea-N excretion at higher levels. Ammonia-N uptake rate increased or decreased with alterations in seawater pH, but the changes were much less than predicted by the associated changes in seawater PNH3, and more closely paralleled changes in seawater NH4+ concentration. Ammonia-N uptake rate was insensitive to amiloride (0.1 mmol l−1) or to a 10-fold elevation in seawater K+ concentration (to 100 mmol l−1), suggesting that the mechanism does not directly involve Na+ or K+ transporters, but was inhibited by blockade of glutamine synthetase, the enzyme that traps ammonia-N to fuel the ornithine–urea cycle. High seawater ammonia inhibited uptake of the ammonia analogue [14C]methylamine. The results suggest that branchial ammonia-N uptake may significantly supplement dietary N intake, amounting to about 31% of the nitrogen acquired from the diet. They further indicate the involvement of Rh glycoproteins (ammonia channels), which are expressed in dogfish gills, in normal ammonia-N uptake and retention.

Physiological and molecular responses of the spiny dogfish shark (Squalus acanthias) to high environmental ammonia: scavenging for nitrogen

In teleosts, a branchial metabolon links ammonia excretion to Na(+) uptake via Rh glycoproteins and other transporters. Ureotelic elasmobranchs are thought to have low branchial ammonia permeability, and little is known about Rh function in this ancient group. We cloned Rh cDNAs (Rhag, Rhbg and Rhp2) and evaluated gill ammonia handling in Squalus acanthias. Control ammonia excretion was <5% of urea-N excretion. Sharks exposed to high environmental ammonia (HEA; 1 mmol(-1) NH4HCO3) for 48 h exhibited active ammonia uptake against partial pressure and electrochemical gradients for 36 h before net excretion was re-established. Plasma total ammonia rose to seawater levels by 2 h, but dropped significantly below them by 24-48 h. Control ΔP(NH3) (the partial pressure gradient of NH3) across the gills became even more negative (outwardly directed) during HEA. Transepithelial potential increased by 30 mV, negating a parallel rise in the Nernst potential, such that the outwardly directed NH4(+) electrochemical gradient remained unchanged. Urea-N excretion was enhanced by 90% from 12 to 48 h, more than compensating for ammonia-N uptake. Expression of Rhp2 (gills, kidney) and Rhbg (kidney) did not change, but branchial Rhbg and erythrocytic Rhag declined during HEA. mRNA expression of branchial Na(+)/K(+)-ATPase (NKA) increased at 24 h and that of H(+)-ATPase decreased at 48 h, while expression of the potential metabolon components Na(+)/H(+) exchanger2 (NHE2) and carbonic anhydrase IV (CA-IV) remained unchanged. We propose that the gill of this nitrogen-limited predator is poised not only to minimize nitrogen loss by low efflux permeability to urea and ammonia but also to scavenge ammonia-N from the environment during HEA to enhance urea-N synthesis.

Parameter estimation for mathematical models of a nongastric H+(Na+)-K(+)(NH4+)-ATPase

The role of nongastric H(+)-K(+)-ATPase (HKA) in ion homeostasis of macula densa (MD) cells is an open question. To begin to explore this issue, we developed two mathematical models that describe ion fluxes through a nongastric HKA. One model assumes a 1H(+):1K(+)-per-ATP stoichiometry; the other assumes a 2H(+):2K(+)-per-ATP stoichiometry. Both models include Na+ and NH4+ competitive binding with H+ and K+, respectively, a characteristic observed in vitro and in situ. Model rate constants were obtained by minimizing the distance between model and experimental outcomes. Both 1H(+)(1Na(+)):1K(+)(1NH4 (+))-per-ATP and 2H(+)(2Na(+)):2K(+)(2NH4 (+))-per-ATP models fit the experimental data well. Using both models, we simulated ion net fluxes as a function of cytosolic or luminal ion concentrations typical for the cortical thick ascending limb and MD region. We observed that (1) K+ and NH4+ flowed in the lumen-to-cytosol direction, (2) there was competitive behavior between luminal K+ and NH4+ and between cytosolic Na+ and H+, 3) ion fluxes were highly sensitive to changes in cytosolic Na+ or H+ concentrations, and 4) the transporter does mostly Na+ / K+ exchange under physiological conditions. These results support the concept that nongastric HKA may contribute to Na+ and pH homeostasis in MD cells. Furthermore, in both models, H+ flux reversed at a luminal pH that was <5.6. Such reversal led to Na+ / H+ exchange for a luminal pH of <2 and 4 in the 1:1-per-ATP and 2:2-per-ATP models, respectively. This suggests a novel role of nongastric HKA in cell Na+ homeostasis in the more acidic regions of the renal tubules.

H-K-ATPase type 2: relevance for renal physiology and beyond

H-K-ATPase type 2 (HKA2), also known as the "nongastric" or "colonic" H-K-ATPase, is broadly expressed, and its presence in the kidney has puzzled experts in the field of renal ion transport systems for many years. One of the most important and robust characteristics of this transporter is that it is strongly stimulated after dietary K(+) restriction. This result prompted many investigators to propose that it should play a role in allowing the kidney to efficiently retain K(+) under K(+) depletion. However, the apparent absence of a clear renal phenotype in HKA2-null mice has led to the idea that this transporter is an epiphenomenon. This review summarizes past and recent findings regarding the functional, structural and physiological characteristics of H-K-ATPase type 2. The findings discussed in this review suggest that, as in the famous story, the ugly duckling of the X-K-ATPase family is actually a swan.

Re-print of "Intestinal luminal nitrogen metabolism: role of the gut microbiota and consequences for the host"

Alimentary and endogenous proteins are mixed in the small intestinal lumen with the microbiota. Although experimental evidences suggest that the intestinal microbiota is able to incorporate and degrade some of the available amino acids, it appears that the microbiota is also able to synthesize amino acids raising the view that amino acid exchange between the microbiota and host can proceed in both directions. Although the net result of such exchanges remains to be determined, it is likely that a significant part of the amino acids recovered from the alimentary proteins are used by the microbiota. In the large intestine, where the density of bacteria is much higher than in the small intestine and the transit time much longer, the residual undigested luminal proteins and peptides can be degraded in amino acids by the microbiota. These amino acids cannot be absorbed to a significant extent by the colonic epithelium, but are precursors for the synthesis of numerous metabolic end products in reactions made by the microbiota. Among these products, some like short-chain fatty acids and organic acids are energy substrates for the colonic mucosa and several peripheral tissues while others like sulfide and ammonia can affect the energy metabolism of colonic epithelial cells. More work is needed to clarify the overall effects of the intestinal microbiota on nitrogenous compound metabolism and consequences on gut and more generally host health.

Colon luminal content and epithelial cell morphology are markedly modified in rats fed with a high-protein diet

Hyperproteic diets are used in human nutrition to obtain body weight reduction. Although increased protein ingestion results in an increased transfer of proteins from the small to the large intestine, there is little information on the consequences of the use of such diets on the composition of large intestine content and on epithelial cell morphology and metabolism. Rats were fed for 15 days with either a normoproteic (NP, 14% protein) or a hyperproteic isocaloric diet (HP, 53% protein), and absorptive colonocytes were observed by electron microscopy or isolated for enzyme activity studies. The colonic luminal content was recovered for biochemical analysis. Absorbing colonocytes were characterized by a 1.7-fold reduction in the height of the brush-border membranes (P = 0.0001) after HP diet consumption when compared with NP. This coincided in the whole colon content of HP animals with a 1.8-fold higher mass content (P = 0.0020), a 2.2-fold higher water content (P = 0.0240), a 5.2-fold higher protease activity (P = 0.0104), a 5.5-fold higher ammonia content (P = 0.0008), and a more than twofold higher propionate, valerate, isobutyrate, and isovalerate content (P < 0.05). The basal oxygen consumption of colonocytes was similar in the NP and HP groups, but ammonia was found to provoke a dose-dependent decrease of oxygen consumption in the isolated absorbing colonocytes. The activity of glutamine synthetase (which condenses ammonia and glutamate) was found to be much higher in colonocytes than in small intestine enterocytes and was 1.6-fold higher (P = 0.0304) in colonocytes isolated from HP animals than NP. Glutaminase activity remained unchanged. Thus hyperproteic diet ingestion causes marked changes both in the luminal environment of colonocytes and in the characteristics of these cells, demonstrating that hyperproteic diet interferes with colonocyte metabolism and morphology. Possible causal relationships between energy metabolism, reduced height of colonocyte brush-border membranes, and reduced water absorption are discussed.

Pharmacological profiles of the murine gastric and colonic H,K-ATPases

BACKGROUND

The H,K-ATPase, consisting of α and ß subunits, belongs to the P-type ATPase family. There are two isoforms of the α subunit, HKα₁ and HKα₂ encoded by different genes. The ouabain-resistant gastric HKα₁-H,K-ATPase is Sch28080-sensitive. However, the colonic HKα₂-H,K-ATPase from different species shows poor primary structure conservation of the HKα₂ subunit between species and diverse pharmacological sensitivity to ouabain and Sch28080. This study sought to determine the contribution of each gene to functional activity and its pharmacological profile using mouse models with targeted disruption of HKα₁, HKα₂, or HKbeta genes.

METHODS

Membrane vesicles from gastric mucosa and distal colon in wild-type (WT), HKα₁, HKα₂, or HKß knockout (KO) mice were extracted. K-ATPase activity and pharmacological profiles were examined.

RESULTS

The colonic H,K-ATPase demonstrated slightly greater affinity for K(+) than the gastric H,K-ATPase. This K-ATPase activity was not detected in the colon of HKα₂ KO but was observed in HKß KO with properties indistinguishable from WT. Neither ouabain nor Sch28080 had a significant effect on the WT colonic K-ATPase activity, but orthovanadate abolished this activity. Amiloride and its analogs benzamil and 5-N-ethyl-N-isopropylamiloride inhibited K-ATPase activity of HKα₁-containing H,K-ATPase; the dose dependence of inhibition was similar for all three inhibitors. In contrast, the colonic HKα₂-H,K-ATPase was not inhibited by these compounds.

CONCLUSIONS

These data demonstrate that the mouse colonic H,K-ATPase exhibits a ouabain- and Sch28080-insensitive, orthovanadate-sensitive K-ATPase activity. Interestingly, pharmacological studies suggested that the mouse gastric H,K-ATPase is sensitive to amiloride.

GENERAL SIGNIFICANCE

Characterization of the pharmacological profiles of the H,K-ATPases is important for understanding the relevant knockout animals and for considering the specificity of the inhibitors.

Colonic potassium handling

Homeostatic control of plasma K+ is a necessary physiological function. The daily dietary K+ intake of approximately 100 mmol is excreted predominantly by the distal tubules of the kidney. About 10% of the ingested K+ is excreted via the intestine. K+ handling in both organs is specifically regulated by hormones and adapts readily to changes in dietary K+ intake, aldosterone and multiple local paracrine agonists. In chronic renal insufficiency, colonic K+ secretion is greatly enhanced and becomes an important accessory K+ excretory pathway. During severe diarrheal diseases of different causes, intestinal K+ losses caused by activated ion secretion may become life threatening. This topical review provides an update of the molecular mechanisms and the regulation of mammalian colonic K+ absorption and secretion. It is motivated by recent results, which have identified the K+ secretory ion channel in the apical membrane of distal colonic enterocytes. The directed focus therefore covers the role of the apical Ca2+ and cAMP-activated BK channel (KCa1.1) as the apparently only secretory K+ channel in the distal colon.

Discovery and validation of colonic tumor-associated proteins via metabolic labeling and stable isotopic dilution

The unique biology of a neoplasm is reflected by its distinct molecular profile compared with normal tissue. To understand tumor development better, we have undertaken a quantitative proteomic search for abnormally expressed proteins in colonic tumors from Apc(Min/+) (Min) mice. By raising pairs of Min and wild-type mice on diets derived from natural-abundance or (15)N-labeled algae, we used metabolic labeling to compare protein levels in colonic tumor versus normal tissue. Because metabolic labeling allows internal control throughout sample preparation and analysis, technical error is minimized as compared with in vitro labeling. Several proteins displayed altered expression, and a subset was validated via stable isotopic dilution using synthetic peptide standards. We also compared gene and protein expression among tumor and nontumor tissue, revealing limited correlation. This divergence was especially pronounced for species showing biological change, highlighting the complementary perspectives provided by transcriptomics and proteomics. Our work demonstrates the power of metabolic labeling combined with stable isotopic dilution as an integrated strategy for the identification and validation of differentially expressed proteins using rodent models of human disease.

PGE2 reduces net water and chloride absorption from the rat colon by targeting the Na+/H+ exchanger and the Na+ K+ 2Cl- cotransporter

An effect of PGE2 on water and chloride absorption was already established in a previous work. This study is an attempt to find the mechanism of action of the prostaglandin by investigating the involvement of three major transporters namely the Na+ -K+ ATPase, the Na+/H+ exchanger and the Na+ K+ 2Cl- cotransporter. Rats were injected with PGE2 and 15 min later, the colon was perfused in situ with Krebs Ringer buffer, and net water and chloride absorption were determined. When the involvement of the cotransporter and/or the exchanger was investigated, animals were injected with, respectively, furosemide and amiloride 10 min before PGE2. Superficial and crypt colonocytes were then isolated and the protein expression of the Na+ -K+ ATPase and the Na+ K+ 2Cl- was determined by western blot analysis. The effect of PGE2 on the pump activity in presence or absence of the transporters' inhibitors was also studied. PGE2 decreased net water and chloride absorption from the colon, increased the Na+ -K+ ATPase activity in superficial cells and reduced it in crypt cells. The prostaglandin was found to stimulate secretion in superficial cells by targeting the Na+ K+ 2Cl- symporter, and reduce absorption in crypt cells by targeting the Na+/H+ antiporter. Changes in the activity of the pump are secondary to changes in the activity of the other transporters.

Molecular mechanisms of renal ammonia transport

Acid-base homeostasis to a great extent relies on renal ammonia metabolism. In the past several years, seminal studies have generated important new insights into the mechanisms of renal ammonia transport. In particular, the theory that ammonia transport occurs almost exclusively through nonionic NH(3) diffusion and NH(4)(+) trapping has given way to a model postulating that a variety of proteins specifically transport NH(3) and NH(4)(+) and that this transport is critical for normal ammonia metabolism. Many of these proteins transport primarily H(+) or K(+) but also transport NH(4)(+). Nonerythroid Rh glycoproteins transport ammonia and may represent critical facilitators of ammonia transport in the kidney. This review discusses the underlying aspects of renal ammonia transport as well as specific proteins with important roles in renal ammonia transport.

Effects of amino acid-derived luminal metabolites on the colonic epithelium and physiopathological consequences

Depending on the amount of alimentary proteins, between 6 and 18 g nitrogenous material per day enter the large intestine lumen through the ileocaecal junction. This material is used as substrates by the flora resulting eventually in the presence of a complex mixture of metabolites including ammonia, hydrogen sulfide, short and branched-chain fatty acids, amines; phenolic, indolic and N-nitroso compounds. The beneficial versus deleterious effects of these compounds on the colonic epithelium depend on parameters such as their luminal concentrations, the duration of the colonic stasis, the detoxication capacity of epithelial cells in response to increase of metabolite concentrations, the cellular metabolic utilization of these metabolites as well as their effects on colonocyte intermediary and oxidative metabolism. Furthermore, the effects of metabolites on electrolyte movements through the colonic epithelium must as well be taken into consideration for such an evaluation. The situation is further complicated by the fact that other non-nitrogenous compounds are believed to interfere with these various phenomenons. Finally, the pathological consequences of the presence of excessive concentrations of these compounds are related to the short- and, most important, long-term effects of these compounds on the rapid colonic epithelium renewing and homeostasis.

The human non-gastric H,K-ATPase has a different cation specificity than the rat enzyme

The primary sequence of non-gastric H,K-ATPase differs much more between species than that of Na,K-ATPase or gastric H,K-ATPase. To investigate whether this causes species-dependent differences in enzymatic properties, we co-expressed the catalytic subunit of human non-gastric H,K-ATPase in Sf9 cells with the beta(1) subunit of rat Na,K-ATPase and compared its properties with those of the rat enzyme (Swarts et al., J. Biol. Chem. 280, 33115-33122, 2005). Maximal ATPase activity was obtained with NH(4)(+) as activating cation. The enzyme was also stimulated by Na(+), but in contrast to the rat enzyme, hardly by K(+). SCH 28080 inhibited the NH(4)(+)-stimulated activity of the human enzyme much more potently than that of the rat enzyme. The steady-state phosphorylation level of the human enzyme decreased with increasing pH, [K(+)], and [Na(+)] and nearly doubled in the presence of oligomycin. Oligomycin increased the sensitivity of the phosphorylated intermediate to ADP, demonstrating that it inhibited the conversion of E(1)P to E(2)P. All three cations stimulated the dephosphorylation rate dose-dependently. Our studies support a role of the human enzyme in H(+)/Na(+) and/or H(+)/NH(4)(+) transport but not in Na(+)/K(+) transport.

Evidence from knockout mice against physiologically significant aquaporin 8-facilitated ammonia transport

Aquaporin (AQP)8-facilitated transport of NH3has been suggested recently by increased NH3permeability in Xenopus oocytes and yeast expressing human or rat AQP8. We tested the proposed roles of AQP8-facilitated NH3transport in mammalian physiology by comparative phenotype studies in wild-type vs. AQP8-null mice. AQP8-facilitated NH3transport was confirmed in mammalian cell cultures expressing rat or mouse AQP8, in which the fluorescence of a pH-sensing yellow fluorescent protein was measured in response to ammonia (NH3/NH4+) gradients. Relative AQP8 single-channel NH3-to-water permeability was ∼0.03. AQP8-facilitated NH3and water permeability in a native tissue was confirmed in membrane vesicles isolated from testes of wild-type vs. AQP8-null mice, in which BCECF was used as an intravesicular pH indicator. A series of in vivo studies were done in mice, including 1) serum ammonia measurements before and after ammonia infusion, 2) renal ammonia clearance, 3) colonic ammonia absorption, and 4) liver ammonia accumulation and renal ammonia excretion after acute and chronic ammonia loading. Except for a small reduction in hepatic ammonia accumulation and increase in ammonia excretion in AQP8-null mice loaded with large amounts of ammonia, there were no significant differences in wild-type vs. AQP8-null mice. Our results support the conclusion that AQP8 can facilitate NH3transport but provide evidence against physiologically significant AQP8-facilitated NH3transport in mice.

Loss of acidification of anterior prostate fluids in Atp12a-null mutant mice indicates that nongastric H-K-ATPase functions as proton pump in vivo

The physiological functions of nongastric (colonic) H-K-ATPase (gene symbol Atp12a), unlike those of Na-K-ATPase and gastric H-K-ATPase, are poorly understood. It has been suggested that it pumps Na+ more efficiently than H+; however, so far, there is no direct evidence that it pumps H+ in vivo. Previously, we found that the nongastric H-K-ATPase alpha-subunit is expressed in apical membranes of rodent anterior prostate epithelium, in a complex with the Na-K-ATPase beta1-subunit. Here we report the effects of Atp12a gene ablation on polarization of the beta1-subunit and secretory function of the anterior prostate. In nongastric H-K-ATPase-deficient prostate, the Na-K-ATPase alpha-subunit resided exclusively in basolateral membranes; however, the beta1-subunit disappeared from apical membranes, demonstrating that beta1 is an authentic subunit of nongastric H-K-ATPase in vivo and that apical localization of beta1 in the prostate is completely dependent on its association with the nongastric H-K-ATPase alpha-subunit. A remarkable reduction in acidification of anterior prostate fluids was observed: pH 6.38 +/- 0.14 for wild-type mice and 6.96 +/- 0.10 for homozygous mutants. These results show that nongastric H-K-ATPase is required for acidification of luminal prostate fluids, thereby providing a strong in vivo correlate of previous functional expression studies demonstrating that it operates as a proton pump.

The non-gastric H,K-ATPase is oligomycin-sensitive and can function as an H+,NH4(+)-ATPase

We used the baculovirus/Sf9 expression system to gain new information on the mechanistic properties of the rat non-gastric H,K-ATPase, an enzyme that is implicated in potassium homeostasis. The alpha2-subunit of this enzyme (HKalpha2) required a beta-subunit for ATPase activity thereby showing a clear preference for NaKbeta1 over NaKbeta3 and gastric HKbeta. NH4(+), K+, and Na+ maximally increased the activity of HKalpha2-NaKbeta1 to 24.0, 14.2, and 5.0 micromol P(i) x mg(-1) protein x h(-1), respectively. The enzyme was inhibited by relatively high concentrations of ouabain and SCH 28080, whereas it was potently inhibited by oligomycin. From the phosphorylation level in the presence of oligomycin and the maximal NH4(+)-stimulated ATPase activity, a turnover number of 20,000 min(-1) was determined. All three cations decreased the steady-state phosphorylation level and enhanced the dephosphorylation rate, disfavoring the hypothesis that Na+ can replace H+ as the activating cation. The potency with which vanadate inhibited the cation-activated enzyme decreased in the order K+ > NH4(+) > Na+, indicating that K+ is a stronger E2 promoter than NH4(+), whereas in the presence of Na+ the enzyme is in the E1 form. For K+ and NH4(+), the E2 to E1 conformational equilibrium correlated with their efficacy in the ATPase reaction, indicating that here the transition from E2 to E1 is rate-limiting. Conversely, the low maximal ATPase activity with Na+ is explained by a poor stimulatory effect on the dephosphorylation rate. These data show that NH4(+) can replace K+ with similar affinity but higher efficacy as an extracellular activating cation in rat nongastric H,K-ATPase.

Expression of the ammonia transporter proteins Rh B glycoprotein and Rh C glycoprotein in the intestinal tract

Ammonia metabolism is important in multiple aspects of gastrointestinal physiology, but the mechanisms of ammonia transport in the gastrointestinal tract remain incompletely defined. The present study examines expression of the ammonia transporter family members Rh B glycoprotein (RhBG) and Rh C glycoprotein (RhCG) in the mouse gastrointestinal tract. Real-time RT-PCR amplification and immunoblot analysis identified mRNA and protein for both RhBG and RhCG were expressed in stomach, duodenum, jejunum, ileum, and colon. Immunohistochemistry showed organ and cell-specific expression of both RhBG and RhCG. In the stomach, both RhBG and RhCG were expressed in the fundus and forestomach, but not in the antrum. In the forestomach, RhBG was expressed by all nucleated squamous epithelial cells, whereas RhCG was expressed only in the stratum germinativum. In the fundus, RhBG and RhCG immunoreactivity was present in zymogenic cells but not in parietal or mucous cells. Furthermore, zymogenic cell RhBG and RhCG expression was polarized, with apical RhCG and basolateral RhBG immunoreactivity. In the duodenum, jejunum, ileum, and colon, RhBG and RhCG immunoreactivity was present in villous, but not in mucous or crypt cells. Similar to the fundic zymogenic cell, RhBG and RhCG expression in villous epithelial cells was polarized when apical RhCG and basolateral RhBG immunoreactivity was present. Thus the ammonia transporting proteins RhBG and RhCG exhibit cell-specific, axially heterogeneous, and polarized expression in the intestinal tract suggesting they function cooperatively to mediate gastrointestinal tract ammonia transport.

CD63 interacts with the carboxy terminus of the colonic H+-K+-ATPase to decrease [corrected] plasma membrane localization and 86Rb+ uptake

The carboxy terminus (CT) of the colonic H(+)-K(+)-ATPase is required for stable assembly with the beta-subunit, translocation to the plasma membrane, and efficient function of the transporter. To identify protein-protein interactions involved in the localization and function of HKalpha(2), we selected 84 amino acids in the CT of the alpha-subunit of mouse colonic H(+)-K(+)-ATPase (CT-HKalpha(2)) as the bait in a yeast two-hybrid screen of a mouse kidney cDNA library. The longest identified clone was CD63. To characterize the interaction of CT-HKalpha(2) with CD63, recombinant CT-HKalpha(2) and CD63 were synthesized in vitro and incubated, and complexes were immunoprecipitated. CT-HKalpha(2) protein (but not CT-HKalpha(1)) coprecipitated with CD63, confirming stable assembly of HKalpha(2) with CD63. In HEK-293 transfected with HKalpha(2) plus beta(1)-Na(+)-K(+)-ATPase, suppression of CD63 by RNA interference increased cell surface expression of HKalpha(2)/NKbeta(1) and (86)Rb(+) uptake. These studies demonstrate that CD63 participates in the regulation of the abundance of the HKalpha(2)-NKbeta(1) complex in the cell membrane.

Adaptative increase of ornithine production and decrease of ammonia metabolism in rat colonocytes after hyperproteic diet ingestion

Chronic high-protein consumption leads to increased concentrations of NH(4)(+)/NH(3) in the colon lumen. We asked whether this increase has consequences on colonic epithelial cell metabolism. Rats were fed isocaloric diets containing 20 (P20) or 58% (P58) casein as the protein source for 7 days. NH(4)(+)/NH(3) concentration in the colonic lumen and in the colonic vein blood as well as ammonia metabolism by isolated surface colonic epithelial cells was determined. After 2 days of consumption of the P58 diet, marked increases of luminal and colonic vein blood NH(4)(+)/NH(3) concentrations were recorded when compared with the values obtained in the P20 group. Colonocytes recovered from the P58 group were characterized at that time and thereafter by an increased capacity for l-ornithine and urea production through arginase (P < 0.05). l-Ornithine was mostly used in the presence of NH(4)Cl for the synthesis of the metabolic end product l-citrulline. After 7 days of the P58 diet consumption, however, the ammonia metabolism into l-citrulline was found lower (P < 0.01) when compared with the values measured in the colonocytes recovered from the P20 group despite any decrease in the related enzymatic activities (i.e., carbamoyl-phosphate synthetase I and ornithine carbamoyl transferase). This decrease was found to coincide with a return of blood NH(4)(+)/NH(3) concentration in colonic portal blood to values close to the one recorded in the P20 group. In response to increased NH(4)(+)/NH(3) concentration in the colon, the increased capacity of the colonocytes to synthesize l-ornithine is likely to correspond to an elevated l-ornithine requirement for the elimination of excessive blood ammonia in the liver urea cycle. Moreover, in the presence of NH(4)Cl, colonocytes diminished their synthesis capacity of l-citrulline from l-ornithine, allowing a lower cellular utilization of this latter amino acid. These results are discussed in relationship with an adaptative process that would be related to both interorgan metabolism and to the role of the colonic epithelium as a first line of defense toward luminal NH(4)(+)/NH(3) concentrations.

The carboxy terminus of the colonic H+,K+-ATPase α-subunit is required for stable β subunit assembly and function

BACKGROUND

The present experiments were designed to study the importance of the carboxy-terminus of colonic H(+), K(+)-ATPase alpha-subunit (HKalpha(2)), for both function as well as integrity of assembly with beta1-Na(+), K(+)-ATPase.

METHODS

For this purpose, a mutation of 84 amino acids in the carboxy-terminus was created (DeltaHKalpha(2)) and HEK-293 cells were used as expression systems for functional studies using (86)Rb(+)-uptake, coimmunoprecipitation using specific antibodies and fluorescence microscopy using green fluorescent protein.

RESULTS

The results demonstrate that comparable levels of expression of HKalpha(2) and DeltaHKalpha(2) mRNA were observed when cells were cotransfected with beta1 subunit. However, the abundance of expression of full length HKalpha(2) protein exceeded that of the truncated protein DeltaHKalpha(2). Ouabain-sensitive (86)Rb(+)-uptake was present only in cells cotransfected with HKalpha(2)/beta(1), indicating that the mutation was incapable of sustaining functionality. Coimmunoprecipitation experiments demonstrated that HKalpha(2) protein was immunoprecipitated more abundantly than DeltaHKalpha(2) when coexpressed with beta1. The use of sucrose gradients and green fluorescence protein immunofluorescence demonstrated that while the DeltaHKalpha(2)/beta(1) complex was confined to the endoplasmic reticulum, the HKalpha(2)/beta(1) complex translocated to the plasma membrane.

CONCLUSION

Taken together, our results are consistent with the view that the carboxy-terminus of HKalpha(2) facilitates the proper folding of the HKalpha(2)/beta(1) complex allowing translocation of the heterodimer to the plasma membrane where potassium uptake occurs. Otherwise, the alpha/beta complex is destined for degradation.

Plasmalemmal vacuolar H+-ATPase is decreased in microvascular endothelial cells from a diabetic model

Angiogenesis requires invasion of extracellular matrix (ECM) proteins by endothelial cells and occurs in hypoxic and acidic environments that are not conducive for cell growth and survival. We hypothesize that angiogenic cells must exhibit a unique system to regulate their cytosolic pH in order to cope with these harsh conditions. The plasmalemmal vacuolar type H(+)-ATPase (pmV-ATPase) is used by cells exhibiting an invasive phenotype. Because angiogenesis is impaired in diabetes, we hypothesized that pmV-ATPase is decreased in microvascular endothelial cells from diabetic rats. The in vitro angiogenesis assays demonstrated that endothelial cells were unable to form capillary-like structures in diabetes. The proton fluxes were slower in cells from diabetic than normal model, regardless of the presence or absence of Na(+) and HCO(3) (-) and were suppressed by V-H(+)-ATPase inhibitors. Immunocytochemical data revealed that pmV-ATPases were inconspicuous at the plasma membrane of cells from diabetic whereas in normal cells were prominent. The pmV-ATPase activity was lower in cells from diabetic than normal models. Inhibition of V-H(+)-ATPase suppresses invasion/migration of normal cells, but have minor effects in cells from diabetic models. These novel observations suggest that the angiogenic abnormalities in diabetes involve a decrease in pmV-ATPase in microvascular endothelial cells.

Blood-brain barrier permeability to ammonia in liver failure: a critical reappraisal

In patients with acute liver failure (ALF), hyperammonemia is related to development of cerebral edema and herniation. The present review discusses the mechanisms for the cerebral uptake of ammonia. A mathematical framework is provided to allow a quantitative examination of whether published studies can be explained by the conventional view that cerebral uptake of ammonia is restricted to diffusion of the unprotonated form (NH(3)) (the diffusion hypothesis). An increase in cerebral blood flow (CBF) enhanced ammonia uptake more than expected, possibly due to recruitment or heterogeneity of brain capillaries. Reported effects of pH on ammonia uptake were in the direction predicted by the diffusion hypothesis, but often less pronounced than expected. The published effects of mannitol, cooling, and indomethacin in experimental animals and patients were difficult to explain by the diffusion hypothesis alone, unless dramatic changes of capillary surface area or permeability for ammonia were induced. Therefore we considered the possible role of membrane protein mediated transport of NH(4)(+) across the blood-brain barrier (BBB). Early tracer studies in Rhesus monkeys suggested that NH(4)(+) is responsible for 20% or even more of the transport of ammonia from plasma to brain. In other locations, such as in the thick ascending limb of Hendle's loop and in isolated astrocytes, transport protein mediated translocation of NH(4)(+) is predominant. Many of the ion-transporters involved in renal NH(4)(+) reabsorbtion are also present in brain capillary membranes and could mediate uptake of NH(4)(+). Astrocytic uptake of NH(4)(+) is associated with increased extracellular K(+), which is a potent cerebral vasodilator. Such interference between transport of NH(4)(+) and other cations could be clinically important because increased cerebral blood flow often precedes cerebral herniation in acute liver failure. We suggest that protein mediated transport of NH(4)(+) through the brain capillary wall is a realistic possibility that should be more intensely studied.

Vacuolar H+-ATPase in human breast cancer cells with distinct metastatic potential: distribution and functional activity

Tumor cells thrive in a hypoxic microenvironment with an acidic extracellular pH. To survive in this harsh environment, tumor cells must exhibit a dynamic cytosolic pH regulatory system. We hypothesize that vacuolar H(+)-ATPases (V-ATPases) that normally reside in acidic organelles are also located at the cell surface, thus regulating cytosolic pH and exacerbating the migratory ability of metastatic cells. Immunocytochemical data revealed for the first time that V-ATPase is located at the plasma membrane of human breast cancer cells: prominent in the highly metastatic and inconspicuous in the lowly metastatic cells. The V-ATPase activities in isolated plasma membranes were greater in highly than in lowly metastatic cells. The proton fluxes via V-ATPase evaluated by fluorescence spectroscopy in living cells were greater in highly than in lowly metastatic cells. Interestingly, lowly metastatic cells preferentially used the ubiquitous Na(+)/H(+) exchanger and HCO(3)(-)-based H(+)-transporting mechanisms, whereas highly metastatic cells used plasma membrane V-ATPases. The highly metastatic cells were more invasive and migratory than the lowly metastatic cells. V-ATPase inhibitors decreased the invasion and migration in the highly metastatic cells. Altogether, these data indicate that V-ATPases located at the plasma membrane are involved in the acquisition of a more metastatic phenotype.

Simple method of preparation of rat colonocyte apical membranes using polyethylene glycol precipitation

BACKGROUND

Although a number of methods are available for the preparation of brush border membranes (BBM) from the small intestine, very few studies have been performed on the isolation of colonic apical membranes (CAM). Cation precipitation is one of the methods used for the isolation of these membranes, which can result in altered lipid composition as a result of activation of phospholipase. We have earlier established a method for BBM isolation using polyethylene glycol (PEG) precipitation. In the present study, CAM from isolated colonocytes were prepared using PEG and the isolated membranes were characterized.

METHODS

The CAM were prepared from isolated rat colonocytes using PEG precipitation and differential centrifugation. Purity was assessed by enrichment of the marker enzymes and contamination by other subcellular particles. Lipid composition and sugar components were analyzed in the isolated membranes.

RESULTS

The CAM showed 11-12-fold enrichment in alkaline phosphatase and 9-10-fold enrichment in gamma-glutamyl transpeptidase as compared with the cell homogenate. The final membrane preparation also showed good vesicle formation as seen by electron microscopy. There was very little contamination of basolateral membranes, microsomes or lysosomes in the final membrane preparation. Analysis of sugars indicated high content of fucose and sialic acid as compared with hexoses. Lipid analysis indicated the presence of various neutral and phospholipids and a cholesterol/phospholipid ratio of 0.65 was seen.

CONCLUSION

This study has shown a simple method of CAM preparation using PEG precipitation, which is less time-consuming than other methods and comparatively pure.

Macula densa cell signaling

Macula densa cells are renal sensor elements that detect changes in distal tubular fluid composition and transmit signals to the glomerular vascular elements. This tubuloglomerular feedback mechanism plays an important role in regulating glomerular filtration rate and blood flow. Macula densa cells detect changes in luminal sodium chloride concentration through a complex series of ion transport-related intracellular events. NaCl entry via a Na:K:2Cl cotransporter and Cl exit through a basolateral channel lead to cell depolarization and increases in cytosolic calcium. Na/H exchange (NHE2) results in cell alkalization, whereas intracellular [Na] is regulated by an apically located H(Na)-K ATPase and not by the traditional basolateral Na:K ATPase. Communication from macula densa cells to the glomerular vascular elements involves ATP release across the macula densa basolateral membrane through a maxi-anion channel. The adaptation of multi-photon microscopy is providing new insights into macula densa-glomerular signaling.

Human nongastric H+-K+-ATPase: transport properties of ATP1al1 assembled with different beta-subunits

To investigate whether nongastric H+-K+-ATPases transport Na+ in exchange for K+ and whether different beta-isoforms influence their transport properties, we compared the functional properties of the catalytic subunit of human nongastric H+-K+-ATPase, ATP1al1 (AL1), and of the Na+-K+-ATPase alpha1-subunit (alpha1) expressed in Xenopus oocytes, with different beta-subunits. Our results show that betaHK and beta1-NK can produce functional AL1/beta complexes at the oocyte cell surface that, in contrast to alpha1/beta1 NK and alpha1/betaHK complexes, exhibit a similar apparent K+ affinity. Similar to Na+-K+-ATPase, AL1/beta complexes are able to decrease intracellular Na+ concentrations in Na+-loaded oocytes, and their K+ transport depends on intra- and extracellular Na+ concentrations. Finally, controlled trypsinolysis reveals that beta-isoforms influence the protease sensitivity of AL1 and alpha1 and that AL1/beta complexes, similar to the Na+-K+-ATPase, can undergo distinct K+-Na+- and ouabain-dependent conformational changes. These results provide new evidence that the human nongastric H+-K+-ATPase interacts with and transports Na+ in exchange for K+ and that beta-isoforms have a distinct effect on the overall structural integrity of AL1 but influence its transport properties less than those of the Na+-K+-ATPase alpha-subunit.

The gamma-Na+,K+-ATPase subunit assembles selectively with alpha1/beta1-Na+,K+-ATPase but not with the colonic H+,K+-ATPase

BACKGROUND

The ubiquitous Na+-pump (Na+,K+-ATPase) assembles as a heterodimer of composition alpha/beta in some nephron segments, while in other segments it may exist as a heterotrimer of composition alpha/beta/gamma. The gamma-subunit has been reported to increase the affinity of the Na+-pump for adenosine 5'-triphosphate (ATP), and decrease affinity for both Na+ and K+. The alpha-subunit of the colonic H+,K+-ATPase (cHK) shares 75% sequence similarity with alpha1-Na+,K+-ATPase (alpha1) and assembles with beta1-Na+,K+-ATPase (beta1) in distal colon and renal medulla. Differences in pharmacological properties have been ascribed to when heterologously expressed function has been compared to function in vitro. The purpose of this study was to determine if cHK might associate with the gamma-subunit of the Na+,K+-ATPase (gamma) as a possible explanation for these variations in function.

METHODS

An antibody specific for the gamma was used in coimmunoprecipitation experiments to determine if the gamma assembles stably in vitro with cHK and beta1 in rat renal medulla or distal colon.

RESULTS

Our results demonstrate that the gamma-subunit assembles specifically with the Na+-pump, but not with cHK. Furthermore, the gamma-subunit assembly was specific for rat kidney and was not observed in distal colon.

CONCLUSION

Since the gamma-subunit did not assemble with the cHK/beta1 complex, gamma-subunit assembly cannot explain those variations in ex vivo and in vitro pharmacologic properties ascribed to cHK.

Novel regulation of cell [Na(+)] in macula densa cells: apical Na(+) recycling by H-K-ATPase

Na-K-ATPase is the nearly ubiquitous enzyme that maintains low-Na(+), high-K(+) concentrations in cells by actively extruding Na(+) in exchange for K(+). The prevailing paradigm in polarized absorbing epithelial cells, including renal nephron segments and intestine, has been that Na-K-ATPase is restricted to the basolateral membrane domain, where it plays a prominent role in Na(+) absorption. We have found, however, that macula densa (MD) cells lack functionally and immunologically detectable amounts of Na-K-ATPase protein. In fact, these cells appear to regulate their cytosolic [Na(+)] via another member of the P-type ATPase family, the colonic form of H-K-ATPase, which is located at the apical membrane in these cells. We now report that this constitutively expressed apical MD colonic H-K-ATPase can function as a Na(H)-K-ATPase and regulate cytosolic [Na(+)] in a novel manner. This apical Na(+)-recycling mechanism may be important as part of the sensor function of MD cells and represents a new paradigm in cell [Na(+)] regulation.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

In rat inner medullary collecting duct, NH uptake by the Na,K-ATPase is increased during hypokalemia

In rat terminal inner medullary collecting duct (tIMCD), the Na,K-ATPase mediates NH uptake, which increases secretion of net H(+) equivalents. K(+) and NH compete for a common binding site on the Na,K-ATPase. Therefore, NH uptake should increase during hypokalemia because interstitial K(+) concentration is reduced. We asked whether upregulation of the Na,K-ATPase during hypokalemia also increases basolateral NH uptake. To induce hypokalemia, rats ate a diet with a low K(+) content. In tIMCD tubules from rats given 3 days of dietary K(+) restriction, Na,K-ATPase beta(1)-subunit (NK-beta(1)) protein expression increased although NK-alpha(1) protein expression and Na,K-ATPase activity were unchanged relative to K(+)-replete controls. However, after 7 days of K(+) restriction, both NK-alpha(1) and NK-beta(1) subunit protein expression and Na,K-ATPase activity increased. The magnitude of Na,K-ATPase-mediated NH uptake across the basolateral membrane (J) was determined in tIMCD tubules perfused in vitro from rats after 3 days of a normal or a K(+)-restricted diet. J was the same in tubules from rats on either diet when measured at the same extracellular K(+) concentration. However, in either treatment group, increasing K(+) concentration from 10 to 30 mM reduced J >60%. In conclusion, with 3 days of K(+) restriction, NH uptake by Na,K-ATPase is increased in the tIMCD primarily from the reduced interstitial K(+) concentration.

Colonic H(+)-K(+)-ATPase in K(+) conservation and electrogenic Na(+) absorption during Na(+) restriction

Upregulation of the colonic H(+)-K(+)- ATPase (cHKA) during hyperaldosteronism suggests that it functions in both K(+) conservation and electrogenic Na(+) absorption in the colon when Na(+)-conserving mechanisms are activated. To test this hypothesis, wild-type (cHKA(+/+)) and cHKA-deficient (cHKA(-/-)) mice were fed Na(+)-replete and Na(+)-restricted diets and their responses were analyzed. In both genotypes, Na(+) restriction led to reduced plasma Na(+) and increased serum aldosterone, and mRNAs for the epithelial Na(+) channel (ENaC) beta- and gamma-subunits, channel-inducing factor, and cHKA were increased in distal colon. Relative to wild-type controls, cHKA(-/-) mice on a Na(+)-replete diet had elevated fecal K(+) excretion. Dietary Na(+) restriction led to increased K(+) excretion in knockout but not in wild-type mice. The amiloride-sensitive, ENaC-mediated short-circuit current in distal colon was significantly reduced in knockout mice maintained on either the Na(+)-replete or Na(+)-restricted diet. These results demonstrate that cHKA plays an important role in K(+) conservation during dietary Na(+) restriction and suggest that cHKA-mediated K(+) recycling across the apical membrane is required for maximum electrogenic Na(+) absorption.

Bufo marinus bladder H-K-ATPase carries out electroneutral ion transport

Bufo marinus bladder H-K-ATPase belongs to the Na-K-ATPase and H-K-ATPase subfamily of oligomeric P-type ATPases and is closely related to rat and human nongastric H-K-ATPases. It has been demonstrated that this ATPase transports K(+) into the cell in exchange for protons and sodium ions, but the stoichiometry of this cation exchange is not yet known. We studied the electrogenic properties of B. marinus bladder H-K-ATPase expressed in Xenopus laevis oocytes. In a HEPES-buffered solution, K(+) activation of the H-K-ATPase induced a slow-onset inward current that reached an amplitude of approximately 20 nA after 1-2 min. When measurements were performed in a solution containing 25 mM HCO at a PCO(2) of 40 Torr, the negative current activated by K(+) was reduced. In noninjected oocytes, intracellular alkalization activated an inward current similar to that due to B. marinus H-K-ATPase. We conclude that the transport activity of the nongastric B. marinus H-K-ATPase is not intrinsically electrogenic but that the inward current observed in oocytes expressing this ion pump is secondary to intracellular alkalization induced by proton transport.

Ion pump sorting in polarized renal epithelial cells

The plasma membranes of renal epithelial cells are divided into distinct apical and basolateral domains, which contain different inventories of ion transport proteins. Without this polarity vectorial ion and fluid transport would not be possible. Little is known of the signals and mechanisms that renal epithelial cells use to establish and maintain polarized distributions of their ion transport proteins. Analysis of ion pump sorting reveals that multiple complex signals participate in determining and regulating these proteins' subcellular localizations.

Sch-28080 depletes intracellular ATP selectively in mIMCD-3 cells

Two H(+)-K(+)-ATPase isoforms are present in kidney: the gastric, highly sensitive to Sch-28080, and the colonic, partially sensitive to ouabain. Upregulation of Sch-28080-sensitive H(+)-K(+)-ATPase, or "gastric" H(+)-K(+)-ATPase, has been demonstrated in hypokalemic rat inner medullary collecting duct cells (IMCDs). Nevertheless, only colonic H(+)-K(+)-ATPase mRNA and protein abundance increase in this condition. This study was designed to determine whether Sch-28080 inhibits transporters other than the gastric H(+)-K(+)-ATPase. In the presence of bumetanide, Sch-28080 (200 microM) and ouabain (2 mM) inhibited (86)Rb(+) uptake (>90%). That (86)Rb(+) uptake was almost completely abolished by Sch-28080 indicates an effect of this agent on the Na(+)-K(+)-ATPase. ATPase assays in membranes, or lysed cells, demonstrated sensitivity to ouabain but not Sch-28080. Thus the inhibitory effect of Sch-28080 was dependent on cell integrity. (86)Rb(+)-uptake studies without bumetanide demonstrated that ouabain inhibited activity by only 50%. Addition of Sch-28080 (200 microM) blocked all residual activity. Intracellular ATP declined after Sch-28080 (200 microM) but recovered after removal of this agent. In conclusion, high concentrations of Sch-28080 inhibit K(+)-ATPase activity in mouse IMCD-3 (mIMCD-3) cells as a result of ATP depletion.

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.

Ouabain-sensitive H,K-ATPase functions as Na,K-ATPase in apical membranes of rat distal colon

Na,K-ATPase activity has been identified in the apical membrane of rat distal colon, whereas ouabain-sensitive and ouabain-insensitive H,K-ATPase activities are localized solely to apical membranes. This study was designed to determine whether apical membrane Na,K-ATPase represented contamination of basolateral membranes or an alternate mode of H,K-ATPase expression. An antibody directed against the H, K-ATPase alpha subunit (HKcalpha) inhibited apical Na,K-ATPase activity by 92% but did not alter basolateral membrane Na,K-ATPase activity. Two distinct H,K-ATPase isoforms exist; one of which, the ouabain-insensitive HKcalpha, has been cloned. Because dietary sodium depletion markedly increases ouabain-insensitive active potassium absorption and HKcalpha mRNA and protein expression, Na, K-ATPase and H,K-ATPase activities and protein expression were determined in apical membranes from control and sodium-depleted rats. Sodium depletion substantially increased ouabain-insensitive H, K-ATPase activity and HKcalpha protein expression by 109-250% but increased ouabain-sensitive Na,K-ATPase and H,K-ATPase activities by only 30% and 42%, respectively. These studies suggest that apical membrane Na,K-ATPase activity is an alternate mode of ouabain-sensitive H,K-ATPase and does not solely represent basolateral membrane contamination.

H+,K+-ATPase

The H+,K+-ATPases comprise a group of integral membrane proteins that belong to the X+,K+-ATPase subfamily of P-type cation-transporting ATPases. Although these H+,K+-ATPase isoforms share approximately 60-70% amino acid identity, they exhibit discrete kinetic and pharmacological properties when expressed in heterologous systems. HK alpha2 has been categorized by its insensitivity to Sch-28080, an inhibitor of the gastric H+,K+-ATPase, and partial sensitivity to ouabain, an inhibitor of the Na+,K+-ATPase. This functional profile contrasts with the pharmacological sensitivities ascribed to HK alpha2 in transport studies in rat isolated medullary collecting ducts perfused in vitro and in mouse medullary collecting duct cell lines. HK alpha2 mRNA and protein abundance appears to be both tissue and site-specifically upregulated in response to chronic hypokalemia. This regulatory response has been localized to the outer and inner medulla. To reconcile these expressed sensitivities to those reported in vitro in isolated tubules and cells in culture, it would be necessary to invoke modification of the pharmacologic insensitivity of the colonic H+,K+-ATPase to Sch-28080. Although a 'unique' beta-subunit has been reported recently, this beta-subunit (beta(c)) is identical at the amino acid level to the recently cloned beta3-Na+,K+-ATPase. Moreover, while HK alpha2 can assemble indiscriminately with any X+,K+-ATPase beta-subunit, HK alpha2 has been reported to assemble stably with beta1-Na+,K+-ATPase in the renal medulla and in the distal colon. It remains conceivable that subunit assembly could be tissue specific and might respond to different physiological and pathophysiological stimuli. Furthermore, recent studies have suggested that the H+,K+-ATPase is both Na+-dependent and localized to the apical membrane in the distal colon. Therefore, future studies will need to resolve these discrepancies by determining if a unique, yet undiscovered H+,K+-ATPase isoform exists in kidney, or if post-translational modifications of the alpha- and/or beta-subunits could account for these functional diversities.