Functional expression of the colonic H+,K+-ATPase alpha-subunit. Pharmacologic properties and assembly with X+,K+-ATPase beta-subunits

Chimeric domain analysis of the compatibility between H(+), K(+)-ATPase and Na(+),K(+)-ATPase beta-subunits for the functional expression of gastric H(+),K(+)-ATPase

Gastric H(+),K(+)-ATPase consists of alpha-subunit with 10 transmembrane domains and beta-subunit with a single transmembrane domain. We constructed cDNAs encoding chimeric beta-subunits between the gastric H(+),K(+)-ATPase and Na(+),K(+)-ATPase beta-subunits and co-transfected them with the H(+),K(+)-ATPase alpha-subunit cDNA in HEK-293 cells. A chimeric beta-subunit that consists of the cytoplasmic plus transmembrane domains of Na(+),K(+)-ATPase beta-subunit and the ectodomain of H(+),K(+)-ATPase beta-subunit assembled with the H(+),K(+)-ATPase alpha-subunit and expressed the K(+)-ATPase activity. Therefore, the whole cytoplasmic and transmembrane domains of H(+),K(+)-ATPase beta-subunit were replaced by those of Na(+),K(+)-ATPase beta-subunit without losing the enzyme activity. However, most parts of the ectodomain of H(+),K(+)-ATPase beta-subunit were not replaced by the corresponding domains of Na(+), K(+)-ATPase beta-subunit. Interestingly, the extracellular segment between Cys(152) and Cys(178), which contains the second disulfide bond, was exchangeable between H(+),K(+)-ATPase and Na(+), K(+)-ATPase, preserving the K(+)-ATPase activity intact. Furthermore, the K(+)-ATPase activity was preserved when the N-terminal first 4 amino acids ((67)DPYT(70)) in the ectodomain of H(+),K(+)-ATPase beta-subunit were replaced by the corresponding amino acids ((63)SDFE(66)) of Na(+),K(+)-ATPase beta-subunit. The ATPase activity was abolished, however, when 4 amino acids ((76)QLKS(79)) in the ectodomain of H(+),K(+)-ATPase beta-subunit were replaced by the counterpart ((72)RVAP(75)) of Na(+),K(+)-ATPase beta-subunit, indicating that this region is the most N-terminal one that discriminates the H(+),K(+)-ATPase beta-subunit from that of Na(+), K(+)-ATPase.

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

Recent studies have suggested that the colonic H+,K+-ATPase (HKalpha2) can secrete either Na+ or H+ in exchange for K+. If correct, this view would indicate that the transporter could function as either a Na+ or a H+ pump. To investigate this possibility a series of experiments was performed using apical membranes from rat colon which were enriched in colonic H+,K+-ATPase protein. An antibody specific for HKalpha2 was employed to determine whether HKalpha2 functions under physiological conditions as a Na+-dependent or Na+-independent K+-ATPase in this same membrane fraction. K+-ATPase activity was measured as [gamma-32P]ATP hydrolysis. The Na+-dependent K+-ATPase accounted for approximately 80% of overall K+-ATPase activity and was characterized by insensitivity to Sch-28080 but partial sensitivity to ouabain. The Na+-independent K+-ATPase activity was insensitive to both Sch-28080 and ouabain. Both types of K+-ATPase activity substituted NH4+ for K+ in a similar manner. Furthermore, our results demonstrate that when incubated with native distal colon membranes, the blocking antibody inhibited dramatically Na+-dependent K+-ATPase activity. Therefore, these data demonstrate that HKalpha2 can function in native distal colon apical membranes as a Na+-dependent K+-ATPase. Elucidation of the role of the pump as a transporter of Na+ versus H+ or NH4+ versus K+ in vivo will require additional studies.

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.

H-K-ATPase in the RCCT-28A rabbit cortical collecting duct cell line

In the present study, we demonstrate that the rabbit cortical collecting duct cell line RCCT-28A possesses three distinct H-K-ATPase catalytic subunits (HKalpha). Intracellular measurements of RCCT-28A cells using the pH-sensitive dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) indicated that the mechanism accounting for recovery from an acid load exhibited both K+ dependence and sensitivity to Sch-28080 characteristic of H-K-ATPases. Recovery rates were 0.022 +/- 0.005 pH units/min in the presence of K+, 0.004 +/- 0.002 in the absence of K+, and 0.002 +/- 0.002 in the presence of Sch-28080. The mRNAs encoding the HKalpha1 subunit and the H-K-ATPase beta-subunit (HKbeta) were detected by RT-PCR. In addition, two HKalpha2 species were found by RT-PCR and 5' rapid amplification of cDNA ends (5'-RACE) in the rabbit renal cortex. One was homologous to HKalpha2 cDNAs generated from other species, and the second was novel. The latter, referred to as HKalpha2c, encoded an apparent 61-residue amino-terminal extension that bore no homology to reported sequences. Antipeptide antibodies were designed on the basis of this extension, and these antibodies recognized a protein of the appropriate mass in both rabbit renal tissue samples and RCCT-28A cells. Such findings constitute very strong evidence for expression of the HKalpha2c subunit in vivo. The results suggest that the rabbit kidney and RCCT-28A cells express at least three distinct H-K-ATPases.

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.

Valine 904, tyrosine 898, and cysteine 908 in Na,K-ATPase alpha subunits are important for assembly with beta subunits

A 26-amino acid sequence in an extracellular loop of the Na,K-ATPase alpha subunit between membrane-spanning segments 7 and 8 has been shown to bind to the beta subunit of Na,K-ATPase and to promote alphabeta assembly (Lemas, M. V., Hamrick, M., Takeyasu, K., and Fambrough, D. M. (1994) J. Biol. Chem. 269, 8255-8259) When this 26-amino acid sequence of the rat Na,K-ATPase alpha3 subunit was replaced by the corresponding sequence of the rat gastric H,K-ATPase alpha subunit, the chimeric alpha subunit assembled preferentially with the rat gastric H,K-ATPase beta subunit (Wang, S.-G., Eakle, K. A., Levenson, R., and Farley, R. A. (1997) Am. J. Physiol. 272, C923-C930). In the present study, these 26 amino acids (Asn886-Ala911) of rat Na,K-ATPase alpha3 were replaced by the corresponding amino acids Asn908-Ala933 of rat distal colon H, K-ATPase. Site-directed mutagenesis of the chimeric alpha subunits and Na,K-ATPase alpha3 showed that Val904, Tyr898, and Cys908 in the Na,K-ATPase alpha3 subunit are key residues in alphabeta subunit interactions. The V904Q mutation in Na,K-ATPase alpha3 reduced the Bmax for ouabain binding and the ATPase activity of alpha3beta1 complexes by approximately 95%, and Y898R reduced the Bmax and ATPase activity by approximately 60%. The complementary mutations Q904V and R898Y increased the amount of ouabain bound by yeast membranes expressing the chimera with the colon H,K-ATPase sequence. The amount of ouabain bound by complexes assembled between Na, K-ATPase alpha3 containing the Y898R,C908G mutations and gastric H, K-ATPase beta was less than 10% of wild type Na,K-ATPase alpha3 expressed with the same beta subunit. The R898Y,G908C mutations in the chimeric alpha subunits also increased ouabain binding.

A tyrosine-based signal regulates H-K-ATPase-mediated potassium reabsorption in the kidney

Isoforms of the H-K-ATPase participate in active K resorption in the renal collecting tubule. The cytoplasmic tail of the beta-subunit of the gastric H-K-ATPase includes a 4 amino acid motif which is highly homologous to tyrosine-based endocytosis signals. We have generated transgenic mice expressing an H-K-ATPase beta-subunit in which the tyrosine residue in this sequence has been mutated to alanine. Mice expressing the mutated protein manifest constitutive hypersecretion of gastric acid, demonstrating that the beta-subunit tyrosine-based motif is required for the regulated endocytosis of the H-K pump and hence the cessation of gastric acid output. To test the possibility that the tyrosine-based sequence in the tail of the H-K-ATPase beta-subunit plays a role in regulating the function of renal H-K-ATPases, we examined renal K clearance in normal and in transgenic mice. Blood pressure, urine volume, glomerular filtration rate (GFR), plasma Na, and Na excretion are similar in control and transgenic mice. However, plasma K concentrations are significantly higher in transgenic mice (4.76 +/- 0.13 meq/l in transgenic and 4. 12 +/- 0.04 meq/l in control; n = 9, P < 0.05) and K excretion is lower in the transgenic animals (fractional excretion of K was 26.2 +/- 3.62% in transgenic and 50.1 +/- 4.78% in control; n = 9, P < 0. 01). These data suggest that the tyrosine-based signal in the cytoplasmic tail of the H-K-ATPase beta-subunit functions in the kidney as it does in the stomach to internalize H-K pump and thus inactivate pump function. Its elimination may result in the constitutive presence of the pump at the cell surface and lead to excessive urinary K reabsorption.

ATP1AL1, a member of the non-gastric H,K-ATPase family, functions as a sodium pump

The human ATP1AL1-encoded protein (an alpha subunit of the human non-gastric H,K-ATPase) has previously been shown to assemble with the gastric H,K-ATPase beta subunit (gH,Kbeta) to form a functionally active ionic pump in HEK 293 cells. This pump has been found to be sensitive to both SCH 28080 and ouabain. However, the 86Rb+-influx mediated by the ATP1AL1-gH,Kbeta heterodimer in HEK 293 cells is at least 1 order of magnitude larger than the maximum ouabain-sensitive proton efflux detected in the same cells. In this study we find that the intracellular Na+ content in cells expressing ATP1AL1 and gH,Kbeta is two times lower than that in control HEK 293 cells in response to incubation for 3 h in the presence of 1 microM ouabain. Moreover, analysis of net Na+ efflux in HEK 293 expressing the ATP1AL1-gH,Kbeta heterodimer reveals the presence of Na+ extrusion activity that is not sensitive to 1 microM ouabain but can be inhibited by 1 mM of this drug. In contrast, ouabain-inhibitable Na+ efflux in control HEK 293 cells is similarly sensitive to either 1 microM or 1 mM ouabain. Finally, 86Rb+ influx through the ATP1AL1-gH,Kbeta complex is comparable to the 1 mM ouabain-sensitive Na+ efflux in the same cells. The data presented here suggest that the enzyme formed by ATP1AL1 and the gastric H,K-ATPase beta subunit in HEK 293 cells mediates primarily Na+,K+ rather than H+,K+ exchange.

Dietary K+ restriction upregulates total and Sch-28080-sensitive bicarbonate absorption in rat tIMCD

In tubules from the terminal segment of the inner medullary collecting duct (tIMCD) from rats with chronic metabolic acidosis, our laboratory has shown that bicarbonate absorption (JtCO2) is inhibited by removal of K+ from the luminal fluid or by the addition of Sch-28080 to the perfusate. The present study asked whether total and/or Sch-28080-sensitive JtCO2 is regulated by changes in systemic K+ homeostasis. Rat tIMCD tubules were perfused in vitro in symmetrical, HCO-3/CO2-buffered solutions containing 10 mM KCl + 6 mM NH4Cl. Total and Sch-28080-sensitive JtCO2 were measured in rats with varying K+ intake. In K+-replete rats, baseline JtCO2 was 2.1 +/- 0.3 pmol . mm-1 . min-1 (n = 6). In rats fed a K+-deficient diet for 3 days, JtCO2 was 5.4 +/- 0.7 pmol . mm-1 . min-1 (n = 16, P < 0. 05). To determine the mechanism for the increase in HCO-3 absorption observed with K+ restriction, the Sch-28080-sensitive component of JtCO2 was measured in each treatment group. Following the addition of Sch-28080 (10 microM) to the perfusate, a 40% reduction in JtCO2 was observed in K+-restricted rats. JtCO2 was not reduced following the addition of Sch-28080 in rats with normal K+ intake. Because Sch-28080-sensitive JtCO2 was increased in K+-restricted rats, Sch-28080-sensitive JtCO2 was studied further in tIMCD tubules from rats in this treatment group. In K+-restricted rats, JtCO2 decreased by 20% following the addition of 5 mM ouabain to the perfusate. This ouabain-induced decline in JtCO2 was observed both in the presence and in the absence of Sch-28080. We conclude that total and Sch-28080-sensitive net acid secretion is increased with dietary K+ restriction. However, since approximately 50% of JtCO2 is insensitive to both Sch-28080 and ouabain, future studies will be necessary to define other mechanisms of luminal acidification in the rat tIMCD.

Functional expression of putative H+-K+-ATPase from guinea pig distal colon

A guinea pig cDNA encoding the putative colonic H+-K+-ATPase alpha-subunit (T. Watanabe, M. Sato, K. Kaneko, T. Suzuki, T. Yoshida, and Y. Suzuki; GenBank accession no. D21854) was functionally expressed in HEK-293, a human kidney cell line. The cDNA for the putative colonic H+-K+-ATPase was cotransfected with cDNA for either rabbit gastric H+-K+-ATPase or Torpedo Na+-K+-ATPase beta-subunit. In both expressions, Na+-independent, K+-dependent ATPase (K+-ATPase) activity was detected in the membrane fraction of the cells, with a Michaelis-Menten constant for K+ of 0.68 mM. The expressed K+-ATPase activity was inhibited by ouabain, with its IC50 value being 52 microM. However, the activity was resistant to Sch-28080, an inhibitor specific for gastric H+-K+-ATPase. The ATPase was not functionally expressed in the absence of the beta-subunits. Therefore, it is concluded that the cDNA encodes the catalytic subunit (alpha-subunit) of the colonic H+-K+-ATPase. Although the beta-subunit of the colonic H+-K+-ATPase has not been identified yet, both gastric H+-K+-ATPase and Na+-K+-ATPase beta-subunits were found to act as a surrogate for the colonic beta-subunit for the functional expression of the ATPase. The present colonic H+-K+-ATPase first expressed in mammalian cells showed the highest ouabain sensitivity in expressed colonic H+-K+-ATPases so far reported (rat colonic in Xenopus oocytes had an IC50 = 0.4-1 mM; rat colonic in Sf9 cells had no ouabain sensitivity).

Expression of HKalpha2 protein is increased selectively in renal medulla by chronic hypokalemia

Our laboratory has demonstrated by Northern analysis that chronic hypokalemia increases HKalpha2 (i.e., alpha-subunit of the colonic H+-K+-ATPase) mRNA abundance in the rat. To determine whether the increase in mRNA correlated with an increase in HKalpha2 protein, an antibody was raised against a synthetic peptide derived from amino acids 686-698 of the HKalpha2 sequence. The anti-HKalpha2 antibody hybridized to rat distal colon membranes which migrated at approximately 100 kDa (expected mobility of HKalpha2). HKalpha2 protein was not detected in plasma membranes from rat whole kidney or stomach (100 microg) derived from control animals. The antibody was then used to investigate changes in expression of HKalpha2 in renal cortex, renal medulla, and distal colon in two pathophysiological conditions: 1) chronic hypokalemia (LK) and 2) chronic metabolic acidosis (CMA). In LK rats there was a marked, but selective, increase in the abundance of HKalpha2 protein in membranes prepared from renal medulla. Nevertheless, a corresponding increase in HKalpha2 protein abundance was not observed in membranes prepared from the distal colon of LK rats. HKalpha2 protein abundance in CMA was indistinguishable from controls. Moreover, chronic hypokalemia had no effect on expression of alpha1-Na+-K+-ATPase or HKalpha1 in kidney or distal colon under any experimental condition. Therefore, HKalpha2 protein is tissue- and site-specifically upregulated in response to chronic hypokalemia but not by CMA. Furthermore, this regulatory response is localized to the renal medulla.

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.

The alpha-subunit of the colonic H+,K+-ATPase assembles with beta1-Na+,K+-ATPase in kidney and distal colon

Previous experiments from our laboratory (Codina, J., Kone, B. C., Delmas-Mata, J. T., and DuBose, T. D., Jr. (1996) J. Biol. Chem. 271, 29759-29763) demonstrated that the alpha-subunit of the colonic H+, K+-ATPase (HKalpha2) requires coexpression with a beta-subunit to support H+/K+ transport in a heterologous expression system (Xenopus laevis oocytes). In these studies, HKalpha2 formed stable and functional alpha.beta complexes when coexpressed with either the rat beta1-subunit of the Na+,K+-ATPase or the beta-subunit of the gastric H+,K+-ATPase, suggesting that different beta-subunits may interact with HKalpha2. The present studies tested this hypothesis by development and application of a specific antibody against HKalpha2 peptide. Subsequently, immunoprecipitation experiments were performed to determine if HKalpha2 co-precipitates with the same beta-subunit in organs known to express HKalpha2 protein. The data demonstrate that HKalpha2 assembles with beta1-Na+,K+-ATPase in the renal medulla and in distal colon.

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.

K depletion stimulates in vivo HCO3 reabsorption in surviving rat distal tubules

To evaluate whether K depletion enhances in vivo bicarbonate reabsorption (JtCO2) in surviving distal tubules (DT), we compared DT JtCO2 in five-sixths nephrectomized rats (Nx) with and without dietary K depletion (Nx-K). Furthermore, to identify possible mechanisms of increased JtCO2, we perfused inhibitors of proton secretion in both Nx and Nx-K rats. JtCO2 (102 +/- 8 pmol.min-1.mm-1) was significantly increased in Nx-K vs. Nx rats (65 +/- 7 pmol.min-1.mm-1, P < 0.05) but unaffected by 10(-6) M losartan perfusion (94 +/- 6 pmol.min-1.mm-1, P = not significant). Although 10(-5) M Sch-28080 also had no significant effect, 5 x 10(-9) M concanamycin A perfusion significantly decreased JtCO2 in Nx-K rats to 65 +/- 8 pmol.min-1. mm-1 (P < 0.05). Morphometric evaluation and H(+)-ATPase immunogold labeling of Nx-K A-type intercalated cells revealed cellular hypertrophy, elaborated apical microplicae, and enhanced H(+)-ATPase apical polarization. Accordingly, these combined studies confirm that K depletion enhances JtCO2 in surviving DT by stimulating H(+)-ATPase activity, independent of the AT1 receptor.

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.

A novel N-terminal splice variant of the rat H+-K+-ATPase alpha2 subunit. Cloning, functional expression, and renal adaptive response to chronic hypokalemia

The H+-K+-ATPase of renal collecting duct mediates K+ conservation during chronic hypokalemia. K+ deprivation promotes H+-K+-ATPase alpha2 (HKalpha2) gene expression in the medullary collecting duct, the principal site of active K+ reabsorption, suggesting that this isozyme contributes to renal K+ reclamation. We report here that alternative transcriptional initiation and mRNA splicing give rise to distinct N-terminal variants of the HKalpha2 subunit. Sequence analysis and in vitro translation revealed that HKalpha2a corresponds to the known HKalpha2 cDNA (Crowson, M. S., and Shull, G. E. (1992) J. Biol. Chem. 267, 13740-13748), whereas HKalpha2b represents a novel variant truncated by 108 amino acids at its N terminus. HKalpha2b mRNA contains a complex 5'-untranslated region with eight upstream open reading frames, features implicated in translational regulation of other genes. Heterologous expression of HKalpha2b with and without the gastric H+-K+-ATPase beta subunit in HEK 293 cells indicated that this variant encodes a K+ uptake mechanism that is relatively Sch 28080-resistant, partially sensitive to ouabain, and appears to require coexpression with the gastric H+-K+-ATPase beta subunit for optimal functional activity. Northern analysis demonstrated that both subtypes (HKalpha2b > HKalpha2a) are expressed abundantly in distal colon and modestly in proximal colon and kidney. Moreover, the abundance of the two mRNAs increases coordinately among the renal zones, but not in colon, with chronic K+ deprivation. These results demonstrate the potential for complex control of HKalpha2 gene expression by transcriptional and posttranscriptional mechanisms not recognized in other members of the Na+-K+-ATPase/H+-K+-ATPase family.

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

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

Adaptation to low-K+ media increases H(+)-K(+)-ATPase but not H(+)-ATPase-mediated pHi recovery in OMCD1 cells

Studies in rat and rabbit outer medullary collecting duct of inner stripe origin (OMCDis) suggest that both H(+)-ATPase and H(+)-K(+)-ATPase participate in H+ secretion. However, the relative contributions of these transporters, and, in particular, that of H(+)-K(+)-ATPase to K+ absorption have not been defined precisely. The present study was designed to delineate more clearly the response of these two transporters to hypokalemia and acidosis in a newly developed mouse OMCD1 cell line. In cells grown in normal K+ (5 mM) media, intracellular pH (pHi) recovery was similar either in the presence or absence of K+ in the perfusate (delta pHi/min = 0.014 +/- 0.001 vs. 0.017 +/- 0.003, not significant). The inhibitory effects of Sch-28080 (10 microM) and bafilomycin A1 (10 nM) on pHi recovery were evident only in the presence and absence of K+ in the perfusate, respectively. In cells grown in low-K+ (2.5 mM) media to simulate chronic hypokalemia, pHi recovery was significantly faster than in cells grown in normal K+ media (delta pHi/min = 0.045 +/- 0.01 vs. 0.014 +/- 0.001, P < 0.01) and was inhibited specifically by Sch-28080, not by bafilomycin A1. In contrast, in cells preconditioned to low pH (7.0) to simulate chronic acidosis, the enhanced pHi recovery was abolished by bafilomycin A1 but not by Sch-28080. 86Rb+ uptake, when used as a K+ congener, was inhibited by Sch-28080. The K(m) for 86Rb+ uptake (H(+)-K(+)-ATPase activity) and the 50% inhibitory concentration for Sch-28080 were 270 and 5.0 microM, respectively. These studies provide evidence that, in morphologically homogeneous OMCD1 cells, 1) both H(+)-K(+)-ATPase and H(+)-ATPase participate in pHi regulation, 2) the H(+)-K(+)-ATPase is selectively upregulated by preconditioning in low-K+ media, and 3) conversely, preconditioning in low-pH media stimulates only the H(+)-ATPase. Thus, in OMCDis, the H(+)-K(+)-ATPase and H(+)-ATPase respond selectively and independently to chronic hypokalemia and acidosis, respectively.