A putative H(+)-K(+)-ATPase is selectively expressed in surface epithelial cells of rat distal colon

Segmental differences in upregulated apical potassium channels in mammalian colon during potassium adaptation

Rat proximal and distal colon are net K⁺ secretory and net K⁺ absorptive epithelia, respectively. Chronic dietary K⁺ loading increases net K⁺ secretion in the proximal colon and transforms net K⁺ absorption to net K⁺ secretion in the distal colon, but changes in apical K⁺ channel expression are unclear. We evaluated expression/activity of apical K⁺ (BK) channels in surface colonocytes in proximal and distal colon of control and K⁺-loaded animals using patch-clamp recording, immunohistochemistry, and Western blot analyses. In controls, BK channels were more abundant in surface colonocytes from K⁺ secretory proximal colon (39% of patches) than in those from K⁺-absorptive distal colon (12% of patches). Immunostaining demonstrated more pronounced BK channel α-subunit protein expression in surface cells and cells in the upper 25% of crypts in proximal colon, compared with distal colon. Dietary K⁺ loading had no clear-cut effects on the abundance, immunolocalization, or expression of BK channels in proximal colon. By contrast, in distal colon, K⁺ loading 1) increased BK channel abundance in patches from 12 to 41%; 2) increased density of immunostaining in surface cells, which extended along the upper 50% of crypts; and 3) increased expression of BK channel α-subunit protein when assessed by Western blotting (P < 0.001). Thus apical BK channels are normally more abundant in K⁺ secretory proximal colon than in K⁺ absorptive distal colon, and apical BK channel expression in distal (but not proximal) colon is greatly stimulated as part of the enhanced K⁺ secretory response to dietary K⁺ loading.

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.

The renal H+, K+-ATPases as therapeutic targets

The kidney is an important regulatory organ responsible for maintaining constant blood volume and composition despite wide variations in the intake of food and water. Throughout the nephron, the functional unit of the kidney, there is a wide variety of proteins that function to add additional waste products and to recover needed materials from the lumen filtrate. The collecting duct of the nephron is the primary renal location for the H+, K+-ATPases, a group of ion pumps that function in both acid/base balance and potassium homeostasis. This review summarizes the present understanding of the structure and functions for the different subtypes of the H+, K+-ATPases under specific physiologic conditions. The obstacles in determining the pharmacologic properties of the different subtypes are considered and future directions for the inhibition and/or stimulation of the H+, K+-ATPases are evaluated.

Bicarbonate secretion: a neglected aspect of colonic ion transport

Understanding of the mechanism of colonic electrolyte transport has markedly increased over the past three decades. This article provides a brief summary of the critical features of Na, Cl, and K transport in the large intestine and how these processes may be altered in diarrhea. Less understood is the mechanism of colonic HCO3 secretion. Recent progress in the regulation of HCO3 secretion in the distal colon is summarized with emphasis on the interrelationship between Cl-dependent, short-chain fatty acid (SCFA)-dependent, and cAMP-induced HCO3 secretion. cAMP down-regulates Cl-dependent HCO3 secretion, while SCFA stimulates HCO3 secretion but also inhibits both Cl-dependent and cAMP-induced HCO3 secretion. As SCFAs are the primary anions in stool, it is likely that SCFA-dependent HCO3 secretion is the primary mechanism of HCO3 secretion in the mammalian colon. Future studies will undoubtedly provide increased understanding of the mechanism of HCO3 secretion in health and disease.

Identification and regulation of the gastric H+/K+-ATPase in the rat heart

Previous reports indicate that H+/K+-adenosine triphosphatase (ATPase) might be expressed in the heart.

AIMS

The objectives of the present study were to explore the presence of H+/K+-ATPase protein and gene expression in the rat heart and to investigate whether the enzyme could contribute to potassium transport across the sarcolemma.

METHODS AND RESULTS

We performed reverse transcription-polymerase chain reaction (RT-PCR) on mRNA from myocardium and isolated cardiomyocytes using primers specific for the gastric H+/K+-ATPase alpha-subunit. The PCR products were sequenced and the predicted gastric H+/K+-ATPase sequence was verified. Western blots from myocardium detected a 34-kDa band and a 94-kDa band, indicating the beta-subunit and alpha-subunit of the gastric H+/K+-ATPase, respectively. Immunocytochemistry detected significant immunoreactivity of the beta-subunit in cardiomyocytes. H+/K+-ATPase-dependent potassium transport was assessed by 86Rb+-uptake in isolated cardiomyocytes. Both ouabain and the selective H+/K+-ATPase inhibitor Schering 28080 reduced 86Rb+-uptake at maximum specific inhibition, by 70 and 25%, respectively; the effects were additive. Competitive RT-PCR analysis indicated a significant upregulation of the myocardial H+/K+-ATPase in heart failure after myocardial infarction.

CONCLUSION

The gastric isoform of H+/K+-ATPase is expressed in rat cardiac myocytes, both at transcript and protein levels. Functional studies indicate that the enzyme could contribute to potassium and pHi regulation in cardiomyocytes.

Bacterial colonization leads to the colonic secretion of RELMbeta/FIZZ2, a novel goblet cell-specific protein

BACKGROUND & AIMS

Goblet cells are highly polarized exocrine cells found throughout the small and large intestine that have a characteristic morphology due to the accumulation of apical secretory granules. These granules contain proteins that play important physiologic roles in cellular protection, barrier function, and proliferation. A limited number of intestinal goblet cell-specific proteins have been identified. In this study, we investigate the expression and regulation of RELMbeta, a novel colon-specific gene.

METHODS

The regulation of RELMbeta messenger RNA expression was determined in LS174T, Caco-2, and HT-29 cell lines in response to stimulation with interleukin 13 and lipopolysaccharide. Quantitative reverse-transcription polymerase chain reaction, immunoblots, and immunohistochemistry were used to examine the expression of RELMbeta in BALB/c and C.B17.SCID mice housed in conventional, germ-free, and gnotobiotic environments.

RESULTS

Messenger RNA for RELMbeta is restricted to the undifferentiated, proliferating colonic epithelium. Immunohistochemistry shows that this protein is expressed in goblet cells located primarily in the distal half of the colon and cecum with lower levels detectable in the proximal colon. High levels of RELMbeta can be detected in the stool of mice and humans, where it exists as a homodimer under nonreducing conditions. Interestingly, the secretion of RELMbeta is dramatically reduced in germ-free mice. Furthermore, introduction of germ-free mice into a conventional environment results in enhanced expression and robust secretion of RELMbeta within 48 hours.

CONCLUSIONS

These studies define a new goblet cell-specific protein and provide the first evidence that colon-specific gene expression can be regulated by colonization with normal enteric bacteria.

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.

Expression of ATP1AL1, a non-gastric proton pump, in human colorectum

The expression of mRNAs encoding a human nongastric proton pump (ATP1AL1) in the colorectum was investigated. The real-time PCR gave significant levels of signals not only in the distal part of human colon and rectum, but also in the proximal part of the colon. ATP1AL1 mRNA was overexpressed in 12 out of 20 human colorectal adenocarcinomas compared with the level in the accompanying normal mucosa. It is noted that astonishing levels of the mRNA overexpression were found in 4 carcinomas, which were detected even by Northern blot. The very high levels of ATP1AL1 mRNA expression in some cancer tissues may be connected to an unknown specific pathophysiological condition.

Nongastric H-K-ATPase in rodent prostate: lobe-specific expression and apical localization

The molecular basis of active ion transport in secretory glands such as the prostate is not well characterized. Rat nongastric H-K-ATPase is expressed at high levels in distal colon surface cell apical membranes and thus is referred to as "colonic." Here we show that the ATPase is expressed in rodent prostate complex in a lobe-specific manner. RT-PCR and Western blot analyses indicate that rat nongastric H-K-ATPase alpha-subunit (alpha(ng)) mRNA and protein are present in coagulating gland (anterior prostate) and lateral and dorsal prostate and absent from ventral lobe, whereas Na-K-ATPase alpha-subunit is present in all lobes. RT-PCR analysis shows that Na-K-ATPase alpha(4) and alpha(3) and gastric H-K-ATPase alpha-subunit are not present in significant amounts in all prostate lobes. Relatively low levels of Na-K-ATPase alpha(2) were found in lateral, dorsal, and anterior lobes. alpha(ng) protein expression is anteriodorsolateral: highest in coagulating gland, somewhat lower in dorsal lobe, and even lower in lateral lobe. Na-K-ATPase protein abundance has the reverse order: expression in ventral lobe is higher than in coagulating gland. alpha(ng) protein abundance is higher in coagulating gland than distal colon membranes. Immunohistochemistry shows that in rat and mouse coagulating gland epithelium alpha(ng) protein has an apical polarization and Na-K-ATPase alpha(1) is localized in basolateral membranes. The presence of nongastric H-K-ATPase in rodent prostate apical membranes may indicate its involvement in potassium concentration regulation in secretions of these glands.

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.

Intersubunit interactions in human X,K-ATPases: role of membrane domains M9 and M10 in the assembly process and association efficiency of human, nongastric H,K-ATPase alpha subunits (ATP1al1) with known beta subunits

Na,K- and H,K-ATPase (X,K-ATPase) alpha subunits need association with a beta subunit for their maturation, but the authentic beta subunit of nongastric H,K-ATPase alpha subunits has not been identified. To better define alpha-beta interactions in these ATPases, we coexpressed human, nongastric H,K-ATPase alpha (AL1) and Na,K-ATPase alpha1 (alpha1NK) as well as AL1-alpha1 and alpha1-AL1 chimeras, which contain exchanged M9 and M10 membrane domains, together with each of the known beta subunits in Xenopus oocytes and followed their resistance to cellular and proteolytic degradation and their ER exit. We show that all beta subunits (gastric betaHK, beta1NK, beta2NK, beta3NK, or Bufo bladder beta) can associate efficiently with alpha1NK, but only gastric betaHK, beta2NK, and Bufo bladder beta can form stably expressed AL1-beta complexes that can leave the ER. The trypsin resistance and the forces of subunit interaction, probed by detergent resistance, are lower for AL1-beta complexes than for alpha1NK-beta complexes. Furthermore, chimeric alpha1-AL1 can be stabilized by beta subunits, but alpha1-AL1-gastric betaHK complexes are retained in the ER. On the other hand, chimeric AL1-alpha1 cannot be stabilized by any beta subunit. In conclusion, these results indicate that (1) none of the known beta subunits is the real partner subunit of AL1 but an as yet unidentified, authentic beta should have structural features resembling gastric betaHK, beta2NK, or Bufo bladder beta and (2) beta-mediated maturation of alpha subunits is a multistep process which depends on the membrane insertion properties of alpha subunits as well as on several discrete events of intersubunit interactions.

Basolateral K-Cl cotransporter regulates colonic potassium absorption in potassium depletion

Active potassium absorption in the rat distal colon is electroneutral, Na(+)-independent, partially chloride-dependent, and energized by an apical membrane H,K-ATPase. Both dietary sodium and dietary potassium depletion substantially increase active potassium absorption. We have recently reported that sodium depletion up-regulates H,K-ATPase alpha-subunit mRNA and protein expression, whereas potassium depletion up-regulates H,K-ATPase beta-subunit mRNA and protein expression. Because overall potassium absorption is non-conductive, K-Cl cotransport (KCC) at the basolateral membrane may also be involved in potassium absorption. Although KCC1 has not been cloned from the colon, we established, in Northern blot analysis with mRNA from the rat distal colon using rabbit kidney KCC1 cDNA as a probe, the presence of an expected size mRNA in the rat colon. This KCC1 mRNA is substantially increased by potassium depletion but only minimally by sodium depletion. KCC1-specific antibody identified a 155-kDa protein in rat colonic basolateral membrane. Potassium depletion but not sodium depletion resulted in an increase in KCC1 protein expression in basolateral membrane. The increase of colonic KCC1 mRNA abundance and KCC1 protein expression in potassium depletion of the rat colonic basolateral membrane suggests that K-Cl cotransporter: 1) is involved in transepithelial potassium absorption and 2) regulates the increase in potassium absorption induced by dietary potassium depletion. We conclude that active potassium absorption in the rat distal colon involves the coordinated regulation of both apical membrane H,K-ATPase and basolateral membrane KCC1 protein.

Na(+)-K(+)-ATPase alpha(2)-isoform expression in guinea pig hearts during transition from compensation to decompensation

Disturbance in ionic gradient across sarcolemma may lead to arrhythmias. Because Na(+)-K(+)-ATPase regulates intracellular Na(+) and K(+) concentrations, and therefore intracellular Ca(2+) concentration homeostasis, our aim was to determine whether changes in the Na(+)-K(+)-ATPase alpha-isoforms in guinea pigs during transition from compensated (CLVH) to decompensated left ventricular hypertrophy (DLVH) were concomitant with arrhythmias. After 12- and 20-mo aortic stenosis, CLVH and DLVH were characterized by increased mean arterial pressure (30% and 52.7%, respectively). DLVH differed from CLVH by significantly increased end-diastolic pressure (34%), decreased sarco(endo)plasmic reticulum Ca(2+)-ATPase (-75%), and increased Na(+)/Ca(2+) exchanger (25%) mRNA levels and by the occurrence of ventricular arrhythmias. The alpha-isoform (mRNA and protein levels) was significantly lower in DLVH (2.2 +/- 0.2- and 1. 4 +/- 0.15-fold, respectively, vs. control) than in CLVH (3.5 +/- 0. 4- and 2.2 +/- 0.13-fold, respectively) and was present in sarcolemma and T tubules. Changes in the levels of alpha(1)- and alpha(3)-isoform in CLVH and DLVH appear physiologically irrelevant. We suggest that the increased level of alpha(2)-isoform in CLVH may participate in compensation, whereas its relative decrease in DLVH may enhance decompensation and arrhythmias.

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.

Lessons from genetically engineered animal models VIII. Absorption and secretion of ions in the gastrointestinal tract

Absorption and secretion of ions in gastrointestinal and other epithelial tissues require the concerted activities of ion pumps, channels, symporters, and exchangers, which operate in coupled systems to mediate transepithelial transport. Our understanding of the identities, membrane locations, and biochemical activities of epithelial ion transporters has advanced significantly in recent years, but major gaps and uncertainties remain in our understanding of their physiological functions. Increasingly, this problem is being addressed by the analysis of mutant mouse models developed by gene targeting. In this review, we discuss gene knockout studies of the secretory isoform of the Na(+)-K(+)-2Cl(-) cotransporter, isoforms 1, 2, and 3 of the Na(+)/H(+) exchanger, and the colonic H(+)-K(+)-ATPase. This approach is leading to a clearer understanding of the functions of these transporters in the living animal.

Colonic H-K-ATPase alpha- and beta-subunits express ouabain-insensitive H-K-ATPase

Active K absorption in the rat distal colon is energized by an apical H-K-ATPase, a member of the gene family of P-type ATPases. The H-K-ATPase alpha-subunit (HKcalpha) has been cloned and characterized (together with the beta-subunit of either Na-K-ATPase or gastric H-K-ATPase) in Xenopus oocytes as ouabain-sensitive (86)Rb uptake. In contrast, HKcalpha, when expressed in Sf9 cells without a beta-subunit, yielded evidence of ouabain-insensitive H-K-ATPase. Because a beta-subunit (HKcbeta) has recently been cloned from rat colon, this present study was initiated to determine whether H-K-ATPase and its sensitivity to ouabain are expressed when these two subunits (HKcalpha and HKcbeta) are transfected into a mammalian cell expression system. Transfection of HEK-293 cells with HKcalpha and HKcbeta cDNAs resulted in the expression of HKcalpha and HKcbeta proteins and their delivery to plasma membranes. H-K-ATPase activity was identified in crude plasma membranes prepared from transfected cells and was 1) saturable as a function of increasing K concentration with a K(m) for K of 0.63 mM; 2) inhibited by orthovanadate; and 3) insensitive to both ouabain and Sch-28080. In parallel transfection studies with HKcalpha and Na-K-ATPase beta1 cDNAs and with HKcalpha cDNA alone, there was expression of ouabain-insensitive H-K-ATPase activity that was 60% and 21% of that in HKcalpha/HKcbeta cDNA transfected cells, respectively. Ouabain-insensitive (86)Rb uptake was also identified in cells transfected with HKcalpha and HKcbeta cDNAs. These studies establish that HKcalpha cDNA with HKcbeta cDNA express ouabain-insensitive H-K-ATPase similar to that identified in rat distal colon.

The nongastric H+-K+-ATPases: molecular and functional properties

The Na-K/H-K-ATPase gene family is divided in three subgroups including the Na-K-ATPases, mainly involved in whole body and cellular ion homeostasis, the gastric H-K-ATPase involved in gastric fluid acidification, and the newly described nongastric H-K-ATPases for which the identification of physiological roles is still in its infancy. The first member of this last subfamily was first identified in 1992, rapidly followed by the molecular cloning of several other members. The relationship between each member remains unclear. The functional properties of these H-K-ATPases have been studied after their ex vivo expression in various functional expression systems, including the Xenopus laevis oocyte, the insect Sf9 cell line, and the human HEK 293 cells. All these H-K-ATPase alpha-subunits appear to encode H-K-ATPases when exogenously expressed in such expression systems. Recent data suggest that these H-K-ATPases could also transport Na+ in exchange for K+, revealing a complex cation transport selectivity. Moreover, they display a unique pharmacological profile compared with the canonical Na-K-ATPases or the gastric H-K-ATPase. In addition to their molecular and functional characterizations, a major goal is to correlate the molecular expression of these cloned H-K-ATPases with the native K-ATPases activities described in vivo. This appears to be more complex than anticipated. The discrepancies between the functional data obtained by exogenous expression of the nongastric H-K-ATPases and the physiological data obtained in native organs could have several explanations as discussed in the present review. Extensive studies will be required in the future to better understand the physiological role of these H-K-ATPases, especially in disease processes including ionic or acid-base disorders.

Colonic H-K-ATPase beta-subunit: identification in apical membranes and regulation by dietary K depletion

P-type ATPases require both alpha- and beta-subunits for functional activity. Although an alpha-subunit for colonic apical membrane H-K-ATPase (HKcalpha) has been identified and studied, its beta-subunit has not been identified. We cloned putative beta-subunit rat colonic H-K-ATPase (HKcbeta) cDNA that encodes a 279-amino-acid protein with a single transmembrane domain and sequence homology to other rat beta-subunits. Northern blot analysis demonstrates that this HKcbeta is expressed in several rat tissues, including distal and proximal colon, and is highly expressed in testis and lung. HKcbeta mRNA abundance is upregulated threefold compared with normal in distal colon but not proximal colon, testis, or lung of K-depleted rats. In contrast, Na-K-ATPase beta1 mRNA abundance is unaltered in distal colon of K-depleted rats. Na depletion, which also stimulates active K absorption in distal colon, does not increase HKcbeta mRNA abundance. Western blot analyses using a polyclonal antibody raised to a glutathione S-transferase-HKcbeta fusion protein established expression of a 45-kDa HKcbeta protein in both apical and basolateral membranes of rat distal colon, but K depletion increased HKcbeta protein expression only in apical membranes. Physical association between HKcbeta and HKcalpha proteins was demonstrated by Western blot analysis performed with HKcbeta antibody on immunoprecipitate of apical membranes of rat distal colon and HKcalpha antibody. Tissue-specific upregulation of this beta-subunit mRNA in response to K depletion, localization of its protein, its upregulation by K depletion in apical membranes of distal colon, and its physical association with HKcalpha protein provide compelling evidence that HKcbeta is the putative beta-subunit of colonic H-K-ATPase.

Does the colonic H,K-ATPase also act as an Na,K-ATPase?

We previously have demonstrated that the colonic P-ATPase alpha subunit cDNA encodes an H,K-ATPase when expressed in Xenopus laevis oocytes. Besides its high level of amino acid homology (75%) with the Na,K-ATPase, the colonic H,K-ATPase also shares a common pharmacological profile with Na,K-ATPase, because both are ouabain-sensitive and Sch 28080-insensitive. These features raise the possibility that an unrecognized property of the colonic H, K-ATPase would be Na+ translocation. To test this hypothesis, ion-selective microelectrodes were used to measure the intracellular Na+ activity of X. laevis oocytes expressing various combinations of P-ATPase subunits. The results show that expression in oocytes of the colonic H,K-ATPase affects intracellular Na+ homeostasis in a way similar to the expression of the Bufo marinus Na,K-ATPase; intracellular Na+ activity is lower in oocytes expressing the colonic H,K-ATPase or the B. marinus Na,K-ATPase than in oocytes expressing the gastric H,K-ATPase or a beta subunit alone. In oocytes expressing the colonic H,K-ATPase, the decrease in intracellular Na+ activity persists when diffusive Na+ influx is enhanced by functional expression of the amiloride-sensitive epithelial Na+ channel, suggesting that the decrease is related to increased active Na+ efflux. The Na+ decrease depends on the presence of K+ in the external medium and is inhibited by 2 mM ouabain, a concentration that inhibits the colonic H,K-ATPase. These data are consistent with the hypothesis that the colonic H,K-ATPase may transport Na+, acting as an (Na,H),K-ATPase. Despite its molecular and functional characterization, the physiological role of the colonic (Na,H),K-ATPase in colonic and renal ion homeostasis remains to be elucidated.

Inhibition of a K+ conductance by the phosphatase inhibitor calyculin A in rat distal colon

Basal membrane permeability of epithelial cells from the lower third and the middle of rat colonic crypts is dominated by a K+ conductance as shown by ion replacement experiments. Calyculin A, an inhibitor of protein phosphatases, induced a depolarization of these cells. The depolarization was concomitant with an inhibition of membrane current. The current inhibited by calyculin A had a reversal potential identical with the theoretical K+ equilibrium potential indicating that the drug inhibits a basal K+ conductance. The efficiency of calyculin A was comparable with that of other well-known K+ channel blockers such as Ba2+, tetraethylammonium or quinine. In the intact tissue, calyculin A exerted an inhibitory action on forskolin-induced anion secretion, an effect which may be explained by the decrease in the driving force for Cl- exit after inhibition of cellular K+ conductance. Together with previous results, these data suggest an inhibition of epithelial K+ conductance by phosphorylation.

The Na(+)-K(+)-ATPase beta 1 subunit is associated with the HK alpha 2 protein in the rat kidney

The Na-K-ATPase beta 1 subunit acts as the beta subunit for the HK alpha 2 protein in the rat kidney. The colonic H(+)-K(+)-ATPase is a member of the P-type ATPases, and has been shown to contribute to potassium transport by the mammalian kidney and colon. The P-type ATPases often consist of an alpha subunit that contains the catalytic site and a beta subunit that participates in regulation of enzyme activity and targeting of the enzyme to the plasma membrane. The cDNA of the alpha subunit (HK alpha 2) has been cloned and the HK alpha 2 protein has been isolated from the rat kidney and colon. However, a unique beta subunit for the colonic H(+)-K(+)-ATPase has not been described. To determine if one of the known beta subunits present in the kidney might act as the beta subunit for the colonic H(+)-K(+)-ATPase, microsomes enriched in the colonic H(+)-K(+)-ATPase were isolated using an HK alpha 2-specific antibody (AS 31.7) and the Minimac magnetic separation system. Immunoblots of rat kidney microsomal protein isolated with antibody AS 31.7 were probed with antibodies directed against the gastric HK beta subunit, Na(+)-K(+)-ATPase alpha 1, and Na(+)-K(+)-ATPase beta 1 subunits. A band of the appropriate size was detected with Na(+)-K(+)-ATPase beta 1-specific antibodies, but not those directed against HK beta 1. These data suggest that Na(+)-K(+)-ATPase beta 1 could be the beta subunit for the colonic H(+)-K(+)-ATPase in the kidney.

Increased sensitivity to K+ deprivation in colonic H,K-ATPase-deficient mice

Previous studies using isolated tissues suggest that the colonic H, K-ATPase (cHKA), expressed in the colon and kidney, plays an important role in K+ conservation. To test the role of this pump in K+ homeostasis in vivo, we generated a cHKA-deficient mouse and analyzed its ability to retain K+ when fed a control or K+-free diet. When maintained on a control diet, homozygous mutant (cHKA-/-) mice exhibited no deficit in K+ homeostasis compared to wild-type (cHKA+/+ greater, similar mice. Although fecal K+ excretion in cHKA-/- mice was double that of cHKA+/+ mice, fecal K+ losses were low compared with urinary K+ excretion, which was similar in both groups. When maintained on a K+-free diet for 18 d, urinary K+ excretion dropped over 100-fold, and to similar levels, in both cHKA-/- and cHKA+/+ mice; fecal K+ excretion was reduced in both groups, but losses were fourfold greater in cHKA-/- than in cHKA+/+ mice. Because of the excess loss of K+ in the colon, cHKA-/- mice exhibited lower plasma and muscle K+ than cHKA+/+ mice. In addition, cHKA-/- mice lost twice as much body weight as cHKA+/+ mice. These results demonstrate that, during K+ deprivation, cHKA plays a critical role in the maintenance of K+ homeostasis in vivo.

Differential localization of colonic H(+)-K(+)-ATPase isoforms in surface and crypt cells

Two distinct colonic H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase) isoforms can be identified in part on the basis of their sensitivity to ouabain. The colonic H(+)-K(+)-ATPase alpha-subunit (HKc alpha) was recently cloned, and its message and protein are present in surface (and the upper 20% of crypt) cells in the rat distal colon. These studies were performed to establish the spatial distribution of the ouabain-sensitive and ouabain-insensitive components of both H(+)-K(+)-ATPase activity in apical membranes prepared from surface and crypt cells and K(+)-dependent intracellular pH (pHi) recovery from an acid load both in isolated perfused colonic crypts and in surface epithelial cells. Whereas H(+)-K(+)-ATPase activity in apical membranes from surface cells was 46% ouabain sensitive, its activity in crypt apical membranes was 96% ouabain sensitive. Similarly, K(+)-dependent pHi recovery in isolated crypts was completely ouabain sensitive, whereas in surface cells K(+)-dependent pHi recovery was insensitive to ouabain. These studies provide compelling evidence that HKc alpha encodes the colonic ouabain-insensitive H(+)-K(+)-ATPase and that a colonic ouabain-sensitive H(+)-K(+)-ATPase isoform is present in colonic crypts and remains to be cloned and identified.

Cyclic AMP-dependent regulation of K+ transport in the rat distal colon

1. The effect of agonists of the cyclic AMP pathway and of 293B, a chromanole-derived K+ channel blocker, on K+ transport in the rat distal colon was studied by measuring unidirectional fluxes, uptake, and efflux of Rb+ in mucosa-submucosa preparations and by patch-clamp of crypt epithelia from isolated crypts. 2. 293B concentration-dependently inhibited basal and forskolin-stimulated short-circuit current. In isolated crypts 293B blocked a basal K+ conductance but had no effect on cyclic AMP-evoked depolarization induced by the opening of apical Cl- channels. When the effect of cyclic AMP on Cl- conductance was prevented by substituting Cl- with gluconate, an inhibition of total cellular K+ conductance by forskolin and a membrane-permeable cyclic AMP analogue was unmasked. 3. Unidirectional ion flux measurements revealed that 293B suppressed the increase in JRbsm induced by forskolin. This, together with the inhibition of cyclic AMP-induced anion secretion indicates that the drug blocks K+ channels, presumably both in the apical and the basolateral membrane. Forskolin caused not only inhibition of K+ absorption, but also stimulation of K+ secretion. The inhibition was diminished, but not blocked, in the presence of inhibitors of the apical H(+)-K(+)-ATPase, vanadate and ouabain. Forskolin stimulated serosal, bumetanide-sensitive Rb+ uptake, whereas mucosal, ouabain/vanadate-sensitive uptake remained unaffected. 4. Efflux experiments revealed that forskolin caused a redistribution of cellular K+ efflux reducing the ratio of basolateral versus apical Rb+ efflux. 5. These results suggest that intracellular cyclic AMP exerts its effects on K+ transport by several mechanisms: an increase in the driving force for K+ efflux due to the depolarization induced by opening of Cl- channels, a stimulation of the basolateral uptake of K+ via the Na(+)-K(+)-Cl(-)-cotransporter, and a decrease of the ratio of basolateral versus apical K+ conductance leading to an enhanced efflux of K+ into the lumen and a reduced K+ efflux to the serosal compartment.

Quantitative RT-PCR analysis of mRNAs encoding a colonic putative H, K-ATPase alpha subunit along the rat nephron: effect of K+ depletion

The rat nephron displays two ouabain-sensitive K-ATPases: one, which is present in proximal tubules and thick ascending limbs of normal rats, is specifically activated by K+ and is down-regulated by K+ depletion, whereas the other one appears in collecting ducts of K+-depleted rats and is activated by either Na+ or K+. To determine which of these two ATPases is similar to colonic-type H,K-ATPase, we quantitated by reverse transcriptase-polymerase chain reaction (RT-PCR) the mRNAs encoding the colonic H,K-ATPase alpha subunit in microdissected nephron segments. In normal rats, statistically significant amounts of colonic H,K-ATPase mRNAs were detected exclusively in cortical thick ascending limbs and cortical collecting ducts (200-500 copies/mm). Because these levels of expression were low (1-1.2 copies/target cell), they probably have no physiological relevance. In rats fed a K+-depleted diet for 2 weeks, expression of colonic H,K-ATPase was markedly enhanced in cortical and medullary collecting ducts (5000-12,000 copies/mm or 30-40 copies per cell), whereas it remained low in all other nephron segments. Thus, colonic H,K-ATPase alpha subunit is specifically expressed in cortical and outer medullary collecting ducts of K+-depleted rats where it likely accounts for the ouabain-sensitive K-ATPase activity.

Active potassium transport across guinea-pig distal colon: action of secretagogues

1. Adrenaline (5 microM) stimulated a K+ secretory current by 2.2 mu equiv h-1 cm-2 in isolated guinea-pig distal colonic epithelium. This secretory activity was inhibited entirely by addition of the loop diuretic bumetanide to the serosal solution. On-going K+ uptake via the absorptive pathway was unaltered by these changes. 2. Prostaglandin E2 (PGE2, 2 microM) stimulated electrogenic K+ secretion and Cl- secretion by 3.0 and 3.6 mu equiv h-1 cm-2, respectively. Serosal addition of bumetanide completely inhibited this K+ secretion but blocked only approximately 70% of Cl- secretion. The bumetanide-insensitive Cl- secretory current was dependent on the presence of Cl- and HCO3- in the bathing solutions. 3. Stimulation of electrogenic K+ secretion by PGE2 occurred with a half-maximal concentration of 4 nM, an affinity approximately 300 times higher than that for stimulation of Cl- secretion by PGE2. 4. Forskolin (10 microM) stimulated Cl- secretion by 4.9 mu equiv h-1 cm-2. The apparent K+ secretory rate was increased by only 1.5 mu equiv h-1 cm-2. A bumetanide-insensitive short-circuit current (ISC) was apparent and of the same size as that stimulated by PGE2. 5. Addition of the Ca2+ ionophore A23187 (10 microM), in the presence of indomethacin (1 microM) to reduce prostaglandin production, inhibited the K+ absorptive pathway by 40% and concurrently stimulated a small rate of electrogenic K+ secretion. 6. Active K+ absorption was inhibited by the addition of ouabain, omeprazole or SCH28080 to the mucosal solution. Both omeprazole and SCH28080 also stimulated a small negative ISC, consistent with electrogenic K+ secretion. 7. Association of K+ absorption, K+ secretion and Cl- secretion is indicated by similarities in transport mechanism and by secretagogue regulation. In particular, maximal rates of K+ secretory current require uptake via apical membrane K+ pumps. Such interrelations support a common cellular locus for these ion transport pathways.

The rat distal colon P-ATPase alpha subunit encodes a ouabain-sensitive H+, K+-ATPase

The functional properties and the pharmacological profile of the recently cloned cDNA colonic P-ATPase alpha subunit (Crowson, M.S., and Shull, G.E. (1992) J. Biol. Chem. 267, 13740-13748) were investigated by using the Xenopus oocyte expression system. Xenopus oocytes were injected with alpha subunit cRNAs from Bufo marinus bladder or rat distal colon and/or with beta subunit cRNA from B. marinus bladder. Two days after injection, K+ uptake was measured by using 86 Rb+ as a K+ surrogate, and pH measurements were performed by means of ion-selective microelectrodes. Co-injection of alpha and beta subunit cRNAs led to a large increase in 86Rb+ uptake, an intracellular alkalinization, and an extracellular medium acidification, as compared to alpha or beta injection alone. These results indicate that the colonic P-ATPase alpha subunit, like the bladder alpha subunit, acts as a functional H+,K+-ATPase, and that co-expression of alpha and beta subunits is required for the function. External K+ activation of the 86Rb+ uptake had a K1/2 of approximately 440 microM for the bladder isoform (consistent with the previously reported value (Jaisser, F., Horisberger, J.D., Geering, K., and Rossier, B.C. (1993) J. Cell. Biol. 123, 1421-1431) and a K1/2 of approximately 730 microM for the colonic isoform. Sch28080 was ineffective to reduce 86Rb+ uptake whereas ouabain inhibited the activity expressed from rat colon alpha subunit with a Ki of 970 microM when measured at the Vmax of the enzyme. We conclude that, when expressed in Xenopus oocytes, the rat colon P-ATPase alpha subunit encodes a ouabain-sensitive H+,K+-ATPase.

Immunocytochemical localization of H(+)-K(+)-ATPase in the rat colon

The presence and distribution of gastric-type H(+)-K(+)-ATPase were investigated in the rat colon using a monoclonal antibody raised against hog gastric H(+)-K(+)-ATPase. Rat stomach was used as positive control. Rat kidney and ileum, in both of which H(+)-K(+)-ATPase has been reported in the past, were also studied. In stomach, very strong staining was found confined to the parietal cells, and a strong band at M(r) approximately 94 kDa on the immunoblots. In colon a moderate staining was found in the supranuclear region of the epithelial cells, with similar intensity and distribution of staining of the surface and deep mucosa of the crypts, throughout the length of the colon. Another monoclonal antibody, specific to the 31 kDa subunit of H(+)-ATPase, used as a negative control, or omission of the primary antibody, resulted in lack of any staining in either colon or stomach. On immunoblots of homogenates of colonic mucosa, no specific band could be identified, either due to very low expression of the H(+)-K(+)-ATPase or loss of antigenicity of the epitope during the processing steps. No positive staining was observed in rat kidney and ileum, suggesting that they contain isoforms that are structurally different.

Proton and potassium transport by H+/K(+)-ATPases

1. H+/K(+)-ATPases are members of the P-type ATPase multigene family. The prototypical H+/K(+)-ATPase is the protein that acidifies gastric luminal contents. The physiological and pharmacological significance of this pump has led to a detailed investigation of its biochemistry and molecular and cell biology. 2. Recently, a number of closely related H+/K(+)-ATPase isoforms have been discovered. These isoforms are present in organs other than the stomach, including the colon and kidney, where they contribute to acid-base and potassium homeostasis. The structure, expression and physiological roles of the gastric H+/K(+)-ATPase and other isoforms are reviewed.

Amiloride-sensitive sodium channels in confluent M-1 mouse cortical collecting duct cells

Confluent M-1 cells show electrogenic Na+ absorption and possess an amiloride-sensitive Na(+)-conductance (Korbmacher et al., J. Gen. Physiol. 102:761-793, 1993). In the present study, we further characterized this conductance and identified the underlying single channels using conventional patch clamp technique. Moreover, we isolated poly(A)+ RNA from M-1 cells to express the channels in Xenopus laevis oocytes, and to check for the presence of transcripts related to the epithelial Na+ channel recently cloned from rat colon (Canessa et al., Nature 361:467-470, 1993). Patch clamp experiments were performed in 6-13-day-old confluent M-1 cells at 37 degrees C. In whole-cell experiments application of 10(-5) M amiloride caused a hyperpolarization of 24.9, SEM +/- 2.2 mV (n = 35) and a reduction of the inward current by 107 +/- 10 pA (n = 51) at a holding potential of -60 mV. Complete removal of bath Na+ had similar effects, indicating that the amiloride-sensitive component of the inward current is a Na+ current. The effect of amiloride was concentration-dependent with half-inhibition at 0.22 microM. The Na+ current saturated with increasing extracellular Na+ concentrations with an apparent Km of 24 mM. Na+ replacement for Li+ demonstrated a higher apical membrane conductance for Li+ than for Na+. In excised inside-out (i/o) or outside-out (o/o) patches from the apical membrane, we observed single-channels which showed slow kinetics and were reversibly inhibited by amiloride. Their average conductance for Na+ was 6.8 +/- 0.5 pS (n = 15) and for Li+ 11.2 +/- 1.0 pS (n = 14). They had no measurable conductance for K+. In o/o patches, channel activity was slightly voltage dependent with an open probability (NPo) of 0.46 +/- 0.14 and 0.16 +/- 0.05 at a holding potential of -100 and 0 mV, respectively (n = 8, P < 0.05). Using the two-microelectrode voltage-clamp technique, we assayed defolliculated stage V-VI Xenopus oocytes for an amiloride-sensitive inward current 1-6 days after injection with H2O or with 20-50 ng of M-1 poly(A)+ RNA. In poly(A)+ RNA-injected oocytes held at -60 or -100 mV application of amiloride (2 microM) reduced the Na-inward current by 25.5 +/- 4.6 nA (n = 25) while it had no effect in H2O-injected oocytes (n = 19). Northern blot analysis of M-1 poly(A+) RNA revealed the presence of transcripts related to the three known subunits of the rat colon Na+ channel (Canessa et al., Nature 367:463-467, 1994). We conclude that the channel in M-1 cells is closely related to the amiloride-sensitive epithelial Na+ channel in the rat colon and that the M-1 cell line provides a useful tool to investigate the biophysical and molecular properties of the corresponding channel in the cortical collecting duct.

Functional expression and segmental localization of rat colonic K-adenosine triphosphatase

A putative cDNA for the colonic K-ATPase has recently been cloned (Crowson, M.S., and G. E. Shull. 1992. J. Biol. Chem. 267:13740-13748). Considerable evidence exists that there are two K-ATPases and active K absorptive processes in the rat distal colon: one that is ouabain sensitive and the other ouabain insensitive. The present study used the baculovirus expression system to express K-ATPase activity in insect Spodoptera frugiperda (Sf 9) cells and a polyclonal antibody (M-1), developed against a fusion protein produced from the 327 nucleotide fragment from 5' coding region of the putative K-ATPase cDNA, to identify the specific localization of the K-ATPase protein. K-ATPase activity (28.7 +/- 1.2 nmol inorganic phosphate/mg protein min) was expressed in plasma membranes isolated from Sf 9 cells infected with baculovirus containing recombinant DNA with the putative K-ATPase cDNA. Km for K for the K-ATPase was 1.2 mM. The expressed K-ATPase activity was not inhibited by ouabain (1 mM); while the Ki for vanadate inhibition was 8.3 microM. Western blot analysis with the M-1 antibody identified a 100-kD protein in apical membranes prepared from distal, but not proximal, rat colon. Immunohistochemical studies with M-1 antibody localized K-ATPase only in the apical membrane of surface cells, while an mAb (c464.6) against Na,K-ATPase localized basolateral membranes of both surface and crypt cells of rat distal colon. In conclusion, the putative K-ATPase cDNA encodes an ouabain-insensitive K-ATPase that is present only in the apical membrane of surface cells of rat distal colon.

Cell-specific expression of epithelial sodium channel alpha, beta, and gamma subunits in aldosterone-responsive epithelia from the rat: localization by in situ hybridization and immunocytochemistry

A highly selective, amiloride-sensitive, epithelial sodium channel from rat colon (rENaC), composed of three homologous subunits termed alpha, beta, and gamma rENaC, has been cloned by functional expression and was proposed to mediate electrogenic sodium reabsorption in aldosterone-responsive epithelia. To determine whether rENaC could account for sodium absorption in vivo, we studied the cellular localization of the sodium channel messenger RNA subunits by in situ hybridization and their cellular and subcellular distribution by immunocytochemistry in the kidney, colon, salivary, and sweat glands of the rat. In the kidney, we show that the three subunit mRNAs are specifically co-expressed in the renal distal convoluted tubules (DCT), connecting tubules (CNT), cortical collecting ducts (CCD), and outer medullary collecting ducts (OMCD), but not in the inner medullary collecting ducts (IMCD). We demonstrate co-localization of alpha, beta, and gamma subunit proteins in the apical membrane of a majority of cells of CCD and OMCD. Our data indicate that alpha, beta, and gamma subunit mRNAs and proteins are co-expressed in the distal nephron (excepting IMCD), a localization that correlates with the previously described physiological expression of amiloride-sensitive electrogenic sodium transport. Our data, however, suggest that another sodium transport protein mediates electrogenic amiloride-sensitive sodium reabsorption in IMCD. We also localized rENaC to the surface epithelial cells of the distal colon and to the secretory ducts of the salivary gland and sweat gland, providing further evidence consistent with the hypothesis that the highly selective, amiloride-sensitive sodium channel is physiologically expressed in aldosterone-responsive cells.

Primary sequence and functional expression of a novel beta subunit of the P-ATPase gene family

The cortical collecting tubule (CCT) of the mammalian kidney reabsorbs sodium and potassium, processes that are mediated by Na/K-ATPase and H/K-ATPase. CCT is also an important site for proton secretion, which is driven, in part, by H/K-ATPase. Na/K-ATPase and H/K-ATPase are members of the ion-motive P-ATPase gene family. They are closely related plasma membrane proteins which consist of alpha beta heterodimers. The urinary bladder of the toad Bufo marinus is the amphibian counterpart of mammalian CCT. We have previously characterized a ouabain-resistant Na/K-ATPase [see ref. 17], from TBM cells, a clonal cell line derived from the toad bladder, which expresses transepithelial sodium transport. In the present study, we report the primary sequence and functional expression of a novel beta subunit (beta bladder = beta b1) isolated from a toad bladder epithelial cell cDNA library. The deduced polypeptide is 299 amino acids in length and has a predicted molecular mass of 33 kDa. The beta b1 protein exhibits 35% amino acid identity to the previously characterized beta 1 of B. marinus Na/K-ATPase and 39% identity with beta 3 of B. marinus Na/K-ATPase. It shares 38% identity with the mammalian beta gastric H/K-ATPase and 52% with the mammalian beta 2 Na/K-ATPase. Northern blot analysis shows that a 1.4 x 10(3)-base mRNA is expressed at a high level in bladder epithelial cells and eye and at a trace level in kidney; it is not detectable in significant amounts in the stomach, colon and small intestine.(ABSTRACT TRUNCATED AT 250 WORDS)