Regulation of active sodium and potassium transport in the distal colon of the rat. Role of the aldosterone and glucocorticoid receptors

Dietary Na+ depletion up-regulates NKCC1 expression and enhances electrogenic Cl- secretion in rat proximal colon

The corticosteroid hormone, aldosterone, markedly enhances K+ secretion throughout the colon, a mechanism critical to its role in maintaining overall K+ balance. Previous studies demonstrated that basolateral NKCC1 was up-regulated by aldosterone in the distal colon specifically to support K+ secretion-which is distinct from the more well-established role of NKCC1 in supporting luminal Cl- secretion. However, considerable segmental variability exists between proximal and distal colonic ion transport processes, especially concerning their regulation by aldosterone. Furthermore, delineating such region-specific effects has important implications for the management of various gastrointestinal pathologies. Experiments were therefore designed to determine whether aldosterone similarly up-regulates NKCC1 in the proximal colon to support K+ secretion. Using dietary Na+ depletion as a model of secondary hyperaldosteronism in rats, we found that proximal colon NKCC1 expression was indeed enhanced in Na+-depleted (i.e., hyperaldosteronemic) rats. Surprisingly, electrogenic K+ secretion was not detectable by short-circuit current (ISC) measurements in response to either basolateral bumetanide (NKCC1 inhibitor) or luminal Ba2+ (non-selective K+ channel blocker), despite enhanced K+ secretion in Na+-depleted rats, as measured by 86Rb+ fluxes. Expression of BK and IK channels was also found to be unaltered by dietary Na+ depletion. However, bumetanide-sensitive basal and agonist-stimulated Cl- secretion (ISC) were significantly enhanced by Na+ depletion, as was CFTR Cl- channel expression. These data suggest that NKCC1-dependent secretory pathways are differentially regulated by aldosterone in proximal and distal colon. Development of therapeutic strategies in treating pathologies related to aberrant colonic K+/Cl- transport-such as pseudo-obstruction or ulcerative colitis-may benefit from these findings.

Traditional uses, chemical compounds, pharmacological activities and clinical studies on the traditional Chinese prescription Yi-Gan San

ETHNOPHARMACOLOGICAL RELEVANCE

A widely used traditional prescription, Yi-Gan San (YGS) is a remedy for neurodegenerative disorders. The formulation consists of seven Chinese medicinal materials in specific proportions, namely Uncariae Ramulus cum Uncis (Uncaria rhynchophylla (Miq.) Miq. ex Havil.), Bupleuri Radix (Bupleurum chinense DC.), Angelicae Sinensis Radix (Angelica sinensis (Oliv.) Diels), Chuanxiong Rhizoma (Ligusticum wallichii Franch.), Poria (Poria cocos (Schw.) Wolf), Atractylodis Macrocephalae Rhizoma (Atractylodes macrocephala Koidz.) and Glycyrrhizae Radix et Rhizoma (Glycyrrhiza uralensis Fisch.). Using YGS has been shown to alleviate various behavioural and psychological symptoms of dementia (BPSD).

AIM OF THIS REVIEW

The goal of this review is to give up-to-date information about the traditional uses, chemistry, pharmacology and clinical efficacy of YGS based on the scientific literature and to learn the current focus and provide references in the next step.

MATERIALS AND METHODS

The database search room was accessed using the search terms "Yi-Gan San" and "Yokukansan" to obtain results from resources such as Web of Science, PubMed, Google Scholar and Sci Finder Scholar. We not only consulted the literature of fellow authors for this review but also explored classical medical books.

RESULTS

YGS has been used to cure neurosis, sleeplessness, night weeping and restlessness in infants. Its chemical components primarily consist of triterpenes, flavonoids, phenolics, lactones, alkaloids and other types of compounds. These active ingredients displayed diverse pharmacological activities to ameliorate BPSD by regulating serotonergic, glutamatergic, cholinergic, dopaminergic, adrenergic, and GABAergic neurotransmission. In addition, YGS showed neuroprotective, antistress, and anti-inflammatory effects. The majority of cases of neurodegenerative disorders are treated with YGS, including Alzheimer's disease and dementia with Lewy bodies.

CONCLUSIONS

Based on previous studies, YGS has been used as a traditional prescription in East Asia, such as Japan, Korea and China, and it has diverse chemical compounds and multiple pharmacological activities. Nevertheless, few experimental studies have focused on chemical and quantitative YGS studies, suggesting that further comprehensive research on its chemicals and quality assessments is critical for future evaluations of drug efficacy.

Local aldosterone synthesis in the large intestine of mouse: An ex vivo incubation study

Objective

To investigate the regulation of local aldosterone synthesis by physiological stimulants in the murine gut.

Methods

Male mice were fed for 14 days with normal, high (1.6%) or low (0.01%) sodium diets. Tissue liver receptor homolog-1 and aldosterone in the colon and caecum were detected using an enzyme-linked immunosorbent assay (ELISA). Released corticosterone and aldosterone in tissue incubation experiments after stimulation with angiotensin II (Ang II) and dibutyryl-cAMP (DBA; the second messenger of adrenocorticotropic hormone) were assayed using an ELISA. Tissue aldosterone synthase (CYP11B2) protein levels were measured using an ELISA and Western blots.

Results

In incubated colon tissues, aldosterone synthase levels were increased by a low-sodium diet; and by Ang II and DBA in the normal diet group. Release of aldosterone into the incubation buffer was increased from the colon by a low-sodium diet and decreased by a high-sodium diet in parallel with changes in aldosterone synthase levels. In mice fed a normal diet, colon incubation with both Ang II and DBA increased the release of aldosterone as well as its precursor corticosterone.

Conclusion

Local aldosterone synthesis in the large intestine is stimulated by a low-sodium diet, dibutyryl-cAMP and Ang II similar to the adrenal glands.

From salt to hypertension, what is missed?

Excess salt intake is viewed as a major contributor to hypertension and cardiovascular disease, and dietary salt restriction is broadly recommended by public health guidelines. However, individuals can have widely varying physiological responses to salt intake, and a tailored approach to evaluation and intervention may be needed. The traditional sodium related concepts are challenging to assess clinically for two reasons: (1) spot and 24-hour urine sodium are frequently used to evaluate salt intake, but are more suitable for population study, and (2) some adverse effects of salt may be blood pressure-independent. In recent years, previously unknown mechanisms of sodium absorption and storage have been discovered. This review will outline the limitations of current methods to assess sodium balance and discuss new potential evaluation methods and treatment targets.

Flupirtine enhances NHE-3-mediated Na+ absorption in rat colon via an ENS-dependent mechanism

Recent studies in our lab have shown that the KV7 channel activator, flupirtine, inhibits colonic epithelial Cl- secretion through effects on submucosal neurons of the enteric nervous system (ENS). We hypothesized that flupirtine would also stimulate Na+ absorption as a result of reduced secretory ENS input to the epithelium. To test this hypothesis, unidirectional 22Na+ fluxes were measured under voltage-clamped conditions. Pharmacological approaches using an Ussing-style recording chamber combined with immunofluorescence microscopy techniques were used to determine the effect of flupirtine on active Na+ transport in the rat colon. Flupirtine stimulated electroneutral Na+ absorption in partially seromuscular-stripped colonic tissues, while simultaneously inhibiting short-circuit current (ISC; i.e., Cl- secretion). Both of these effects were attenuated by pretreatment with the ENS inhibitor, tetrodotoxin. The Na+/H+ exchanger isoform 3 (NHE-3)-selective inhibitor, S3226, significantly inhibited flupirtine-stimulated Na+ absorption, whereas the NHE-2-selective inhibitor HOE-694 did not. NHE-3 localization near the apical membranes of surface epithelial cells was also more apparent in flupirtine-treated colon versus control. Flupirtine did not alter epithelial Na+ channel (ENaC)-mediated Na+ absorption in distal colonic tissues obtained from hyperaldosteronaemic rats and had no effect in the normal ileum but did stimulate Na+ absorption in the proximal colon. Finally, the parallel effects of flupirtine on ISC (Cl- secretion) and Na+ absorption were significantly correlated with each other. Together, these data indicate that flupirtine stimulates NHE-3-dependent Na+ absorption, likely as a result of reduced stimulatory input to the colonic epithelium by submucosal ENS neurons.NEW & NOTEWORTHY We present a novel mechanism regarding regulation of epithelial ion transport by enteric neurons. Activation of neuronal KV7 K+ channels markedly stimulates Na+ absorption and inhibits Cl- secretion across the colonic epithelium. This may be useful in developing new treatments for diarrheal disorders, such as irritable bowel syndrome with diarrhea (IBS-D).

Aldosterone up-regulates basolateral Na+ -K+ -2Cl- cotransporter-1 to support enhanced large-conductance K+ channel-mediated K+ secretion in rat distal colon

Na⁺ -K⁺ -2Cl^- cotransporter-1 (NKCC1) facilitates basolateral K⁺ and Cl^- uptake, supporting their efflux across mucosal membranes of colonic epithelial cells. NKCC1 activity has also been shown to be critical for electrogenic K⁺ secretion induced by aldosterone, which is known to stimulate large-conductance K⁺ (BK) channel expression in mucosal membranes. This study was aimed to (1) identify whether aldosterone enhances NKCC1 expression specifically to support BK-mediated K⁺ secretion and (2) to determine whether increased NKCC1 supports electrogenic Cl^- secretion in parallel to K⁺ secretion. Dietary Na⁺ depletion was used to induce secondary hyperaldosteronism in rats, or aldosterone was administered ex vivo to rat distal colonic mucosae. NKCC1-dependent electrogenic K⁺ or Cl^- secretion was measured as a function of short circuit current (I_SC ). qRT-PCR, western blot, and immunofluorescence analyses were performed using standard techniques. Aldosterone enhanced NKCC1 and BKα expression and electrogenic K⁺ secretion in the distal colon, which was inhibited by either serosal bumetanide (NKCC1 inhibitor) or mucosal iberiotoxin (IbTX; BK channel blocker), but not TRAM-34 (IK channel blocker). Expression of NKCC1 and BKα proteins was enhanced in crypt cells of hyper-aldosterone rats. However, neither NKCC1-dependent Cl^- secretion nor CFTR (apical Cl^- channel) expression was enhanced by aldosterone. We conclude that aldosterone enhances NKCC1 to support BK-mediated K⁺ secretion independently of Cl^- secretion in the distal colon. The regulation of NKCC1 expression/K⁺ secretion by aldosterone may be a therapeutic target in treating gastrointestinal disorders associated with alterations in colonic K⁺ transport, such as colonic pseudo-obstruction, and hyperkalemia associated with renal disease.

Penicillin G Induces H+, K+-ATPase via a Nitric Oxide-Dependent Mechanism in the Rat Colonic Crypt

Background/Aims:

The colonic H⁺, K⁺ ATPase (HKA2) is a heterodimeric membrane protein that exchanges luminal K⁺ for intracellular H⁺ and is involved in maintaining potassium homeostasis. Under homeostatic conditions, the colonic HKA2 remains inactive, since most of the potassium is absorbed by the small intestine. In diarrheal states, potassium is secreted and compensatory potassium absorption becomes necessary. This study proposes a novel mechanism whereby the addition of penicillin G sodium salt (penG) to colonic crypts stimulates potassium uptake in the presence of intracellular nitric oxide (NO), under sodium-free (0-Na⁺) conditions.

Methods:

Sprague Dawley rat colonic crypts were isolated and pHi changes were monitored through the ammonium prepulse technique. Increased proton extrusion in 0-Na⁺ conditions reflected heightened H⁺, K⁺ ATPase activity. Colonic crypts were exposed to penG, L-arginine (a NO precursor), and N-nitro l-arginine methyl ester (L-NAME, a NO synthase inhibitor).

Results:

Isolated administration of penG significantly increased H⁺, K⁺ ATPase activity from baseline, p 0.0067. Co-administration of arginine and penG in 0-Na⁺ conditions further upregulated H⁺, K⁺ ATPase activity, p <0.0001. Crypt perfusion with L-NAME and penG demonstrated a significant reduction in H⁺, K⁺ ATPase activity, p 0.0058.

Conclusion:

Overall, acute exposure of colonic crypts to penG activates the H⁺, K⁺ ATPase in the presence of NO. This study provides new insights into colonic potassium homeostasis.

Role of enteroendocrine L-cells in arginine vasopressin-mediated inhibition of colonic anion secretion

Key points

Arginine vasopressin (AVP) stimulates the release of enteroendocrine L‐cell derived hormones glucagon‐like peptide‐1 (GLP‐1) and peptide YY (PYY) in vitro from mouse and human colons.

This is mediated by the AVP receptor 1B, which is highly enriched in colonic L‐cells and linked to the elevation of L‐cell calcium and cAMP concentrations.

By means of Ussing chambers, we show that AVP reduced colonic anion secretion, although this was blocked by a specific neuropeptide Y receptor Y1 receptor antagonist, suggesting that L‐cell‐released PYY acts locally on the epithelium to modulate fluid balance.

In human serum samples, PYY concentrations were higher in samples with raised osmolality and copeptin (surrogate marker for AVP).

These findings describe, for the first time, the role of L‐cells in AVP regulated intestinal fluid secretion, potentially linking together hormonal control of blood volume and blood glucose levels, and thus adding to our understanding of the complex pathways involved in the gut hormonal response to different stimuli.

Abstract

Arginine vasopressin (AVP) regulates fluid balance and blood pressure via AVP receptor (AVPR)2 in the kidney and AVP receptor 1A in vascular smooth muscle. Its role in intestinal function has received less attention. We hypothesized that enteroendocrine L‐cells producing glucagon‐like peptide 1 (GLP‐1) and peptide YY (PYY) may be a target of AVP and contribute to the control of fluid balance. Avpr1b expression was assessed by quantitative RT‐PCR on flourescence‐activated cell sorting‐isolated L‐ and control cells and was enriched in colonic L‐cells. AVP stimulated GLP‐1 and PYY release from primary cultured murine and human colonic cells and was associated with elevated calcium and cAMP concentrations in L‐cells as measured in cultures from GLU‐Cre/ROSA26‐GCaMP3 and GLU‐Epac2camps mice. An antagonist of AVPR1B reduced AVP‐triggered hormone secretion from murine and human cells. In Ussing chambers, basolaterally applied AVP reduced colonic anion secretion and this effect was blocked by a specific neuropeptide Y receptor Y1 (NPY1R) antagonist. In human serum, PYY concentrations were higher in samples with raised osmolality or copeptin (a surrogate marker for AVP). In conclusion, we propose that AVP activates L‐cell AVPR1B, causing GLP‐1 and PYY secretion. PYY in turn reduces colonic anion secretion via epithelial NPY1R. Our data suggest L‐cells are active players in the hypothalamic control of intestinal fluid homeostasis, providing a potential link between the regulation of blood volume/pressure/osmolality and blood glucose.

Arginine vasopressin (AVP) stimulates the release of enteroendocrine L‐cell derived hormones glucagon‐like peptide‐1 (GLP‐1) and peptide YY (PYY) in vitro from mouse and human colons.

This is mediated by the AVP receptor 1B, which is highly enriched in colonic L‐cells and linked to the elevation of L‐cell calcium and cAMP concentrations.

By means of Ussing chambers, we show that AVP reduced colonic anion secretion, although this was blocked by a specific neuropeptide Y receptor Y1 receptor antagonist, suggesting that L‐cell‐released PYY acts locally on the epithelium to modulate fluid balance.

In human serum samples, PYY concentrations were higher in samples with raised osmolality and copeptin (surrogate marker for AVP).

These findings describe, for the first time, the role of L‐cells in AVP regulated intestinal fluid secretion, potentially linking together hormonal control of blood volume and blood glucose levels, and thus adding to our understanding of the complex pathways involved in the gut hormonal response to different stimuli.

Insulin, CCAAT/enhancer-binding proteins and lactate regulate the human 11β-hydroxysteroid dehydrogenase type 2 gene expression in colon cancer cell lines

11β-Hydroxysteroid dehydrogenases (11beta-HSD) modulate mineralocorticoid receptor transactivation by glucocorticoids and regulate access to the glucocorticoid receptor. The isozyme 11beta-HSD2 is selectively expressed in mineralocorticoid target tissues and its activity is reduced in various disease states with abnormal sodium retention and hypertension, including the apparent mineralocorticoid excess. As 50% of patients with essential hypertension are insulin resistant and hyperinsulinemic, we hypothesized that insulin downregulates the 11beta-HSD2 activity. In the present study we show that insulin reduced the 11beta-HSD2 activity in cancer colon cell lines (HCT116, SW620 and HT-29) at the transcriptional level, in a time and dose dependent manner. The downregulation was reversible and required new protein synthesis. Pathway analysis using mRNA profiling revealed that insulin treatment modified the expression of the transcription factor family C/EBPs (CCAAT/enhancer-binding proteins) but also of glycolysis related enzymes. Western blot and real time PCR confirmed an upregulation of C/EBP beta isoforms (LAP and LIP) with a more pronounced increase in the inhibitory isoform LIP. EMSA and reporter gene assays demonstrated the role of C/EBP beta isoforms in HSD11B2 gene expression regulation. In addition, secretion of lactate, a byproduct of glycolysis, was shown to mediate insulin-dependent HSD11B2 downregulation. In summary, we demonstrate that insulin downregulates HSD11B2 through increased LIP expression and augmented lactate secretion. Such mechanisms are of interest and potential significance for sodium reabsorption in the colon.

Multiple mineralocorticoid response elements localized in different introns regulate intermediate conductance K+ (Kcnn4) channel expression in the rat distal colon

An elevated plasma aldosterone and an increased expression of the intermediate conductance K⁺ (IK/Kcnn4) channels are linked in colon. This observation suggests that the expression of Kcnn4 gene is controlled through the action of aldosterone on its cognate receptor (i.e., mineralocorticoid receptor; MR). In order to establish this, we performed chromatin immunoprecipitation (ChIP) assay to identify the MR response elements (MREs) in a region that spanned 20 kb upstream and 10 kb downstream of the presumed transcription start site (TSS) using chromatin from the colonic epithelial cells of normal and aldosterone-treated rats. MREs were immunoprecipitated in an approximately 5 kb region that spanned the first and second introns in the aldosterone rats. These regions were individually cloned in luciferase-expression vector lacking enhancer activity. These clones were tested for enhancer activity in vitro by transfecting in HEK293T and CaCo2 cells with MR and aldosterone treatment. At least four regions were found to be responsive to the MR and aldosterone. Two regions were identified to contain MREs using bioinformatics tools. These clones lost their enhancer activity after mutation of the presumptive MREs, and thus, established the functionality of the MREs. The third and fourth clones did not contain any bioinformatically obvious MREs. Further, they lost their activity upon additional sub-cloning, which suggest cooperativity between the regions that were separated upon sub-cloning. These results demonstrate the presence of intronic MREs in Kcnn4 and suggest a highly cooperative interaction between multiple intronic response elements.

Potassium balance in dialysis patients

The advent of dialytic therapy has enabled nephrologists to provide life-saving therapy, but potassium balance continues to be an ever present challenge in the ESRD population. Although a small percent of patients are chronically hypokalemic, hyperkalemia is by far the most common abnormality in dialysis patients. It is associated with increased all-cause mortality, cardiovascular mortality, and arrhythmogenic death. Although alterations of the dialysis bath may decrease predialysis potassium, potassium baths <2 mEq/l are associated with a higher risk of sudden cardiac death. Studies show that patients are aware of the risks of hyperkalemia, but adherence to a low potassium diet is suboptimal. ACEI, ARBs, and spironolactone may cause slight increases in potassium even in anuric patients, requiring increased surveillance. Fludrocortisone and potassium binders have not been proven to be beneficial in lowering interdialytic potassium levels. Frequent hemodialysis may be a viable option, and studies of prophylactic placement of implantable cardioverter/defibrillators are underway.

Yokukansan treatment of chronic renal failure patients receiving hemodialysis, with behavioral and psychological symptoms of dementia: an open-label study

OBJECTIVE

The efficacy and safety of yokukansan (YKS) for chronic renal failure (CRF) patients receiving hemodialysis with behavioral and psychological symptoms of dementia (BPSD) was evaluated.

METHODS

Twelve CRF patients receiving hemodialysis with BPSD were recruited and 7.5 g of YKS powder was added to ongoing therapy with antipsychotics. Neuropsychiatric Inventory (NPI) criteria and Barthel Index before and after 4-week YKS treatment were compared.

RESULTS

Analysis of the mean score for NPI revealed a significant improvement during the period of YKS administration (25.3 ± 17.6 versus 8.36 ± 4.46; p = 0.0069). The mean score for the Barthel Index showed no significant difference during the period of YKS administration. Mean level of serum potassium was still within the normal range. No subjects had severe adverse reactions necessitating discontinuation from the study.

CONCLUSION

Yokukansan significantly improved the symptoms of BPSD in CRF patients receiving hemodialysis without critical side effects.

Electrolyte and acid-base disorders in inflammatory bowel disease

INFLAMMATORY BOWEL DISEASE (IBD) IS A CHRONIC INFLAMMATORY INTESTINAL DISORDER ENCOMPASSING TWO MAJOR ENTITIES: Crohn's disease and ulcerative colitis. Intestinal inflammatory processes reduce the absorption of sodium, chloride and calcium, while they increase potassium secretion. In addition, mild to severe metabolic alkalosis may occur in IBD patients, mainly depending on the severity of the disease and the part of the gastrointestinal tract being affected. The aim of this review is the presentation of the electrolyte and acid-base disturbances in IBD and how the activity state of the disease and/or treatment may affect them.

High salt intake down-regulates colonic mineralocorticoid receptors, epithelial sodium channels and 11β-hydroxysteroid dehydrogenase type 2

Besides the kidneys, the gastrointestinal tract is the principal organ responsible for sodium homeostasis. For sodium transport across the cell membranes the epithelial sodium channel (ENaC) is of pivotal relevance. The ENaC is mainly regulated by mineralocorticoid receptor mediated actions. The MR activation by endogenous 11β-hydroxy-glucocorticoids is modulated by the 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2). Here we present evidence for intestinal segment specific 11β-HSD2 expression and hypothesize that a high salt intake and/or uninephrectomy (UNX) affects colonic 11β-HSD2, MR and ENaC expression. The 11β-HSD2 activity was measured by means of 3H-corticosterone conversion into 3H-11-dehydrocorticosterone in Sprague Dawley rats on a normal and high salt diet. The activity increased steadily from the ileum to the distal colon by a factor of about 3, an observation in line with the relevance of the distal colon for sodium handling. High salt intake diminished mRNA and protein of 11β-HSD2 by about 50% (p<0.001) and reduced the expression of the MR (p<0.01). The functionally relevant ENaC-β and ENaC-γ expression, a measure of mineralocorticoid action, diminished by more than 50% by high salt intake (p<0.001). The observed changes were present in rats with and without UNX. Thus, colonic epithelial cells appear to contribute to the protective armamentarium of the mammalian body against salt overload, a mechanism not modulated by UNX.

Regulation of electroneutral NaCl absorption by the small intestine

Na(+) and Cl(-) movement across the intestinal epithelium occurs by several interconnected mechanisms: (a) nutrient-coupled Na(+) absorption, (b) electroneutral NaCl absorption, (c) electrogenic Cl(-) secretion by CFTR, and (d) electrogenic Na(+) absorption by ENaC. All these transport modes require a favorable electrochemical gradient maintained by the basolateral Na(+)/K(+)-ATPase, a Cl(-) channel, and K(+) channels. Electroneutral NaCl absorption is observed from the small intestine to the distal colon. This transport is mediated by apical Na(+)/H(+) (NHE2/3) and Cl(-)/HCO(3)(-) (Slc26a3/a6 and others) exchangers that provide the major route of NaCl absorption. Electroneutral NaCl absorption and Cl(-) secretion by CFTR are oppositely regulated by the autonomic nerve system, the immune system, and the endocrine system via PKAα, PKCα, cGKII, and/or SGK1. This integrated regulation requires the formation of macromolecular complexes, which are mediated by the NHERF family of scaffold proteins and involve internalization of NHE3. Through use of knockout mice and human mutations, a more detailed understanding of the integrated as well as subtle regulation of electroneutral NaCl absorption by the mammalian intestine has emerged.

Aldosterone increases Na+ -K+ -ATPase activity in skeletal muscle of patients with Conn's syndrome

OBJECTIVE

In Conn's syndrome, hypokalaemia normally results from renal potassium loss because of the effect of excess aldosterone on Na(+) -K(+) -ATPase in principal cells. Little is known about the effect of aldosterone on cellular potassium redistribution in skeletal muscle. Our study determined the effect of aldosterone on muscle Na(+) -K(+) -ATPase.

DESIGN

Muscle biopsies were taken from six patients immediately before and 1 month after adrenalectomy. Ten age-matched subjects with normal levels of circulating aldosterone served as controls.

RESULTS

  Average plasma aldosterone was significantly higher in presurgery (235·0 ± 51·1 pg/ml) than postsurgery (64·5 ± 25·1 pg/ml) patients. Similarly, Na(+) -K(+) -ATPase activity, relative mRNA expression of α(2) (not α(1) or α(3) ) and β(1) (not β(2) or β(3) ), and protein abundance of α(2) and β(1) subunits were greater in pre- than postsurgery samples (128·7 ± 12·3 vs 79·4 ± 13·3 nmol·mg/protein/h, 2·45 ± 0·31 vs 1·04 ± 0·17, 1·92 ± 0·22 vs1·02 ± 0·14, 2·17 ± 0·33 vs 0·98 ± 0·09 and 1·70 ± 0·17 vs 0·90 ± 0·17, respectively, all P<0·05). The activity and mRNA expression of the α(2) and β(1) subunits correlated well with plasma aldosterone levels (r = 0·71, r = 0·75 and r = 0·78, respectively, all P < 0·01).

CONCLUSIONS

  Our study provides the first evidence in human skeletal muscle that increased plasma aldosterone leads to increased Na(+) -K(+) -ATPase activity via increases in α(2) and β(1) subunit mRNAs and their protein expressions. The increased activity may contribute in part to the induction of hypokalaemia in patients with Conn's syndrome.

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.

Glycyrrhetinic acid food supplementation lowers serum potassium concentration in chronic hemodialysis patients

Hyperkalemia is a common life-threatening problem in hemodialysis patients. Because glycyrrhetinic acid (GA) inhibits the enzyme 11beta-hydroxy-steroid dehydrogenase II and thereby increases cortisol availability to the colonic mineralocorticoid receptor, it has the potential to lower serum potassium concentrations. To test this, 10 patients in a 6 month prospective, double-blind, placebo-controlled crossover study were given cookies or bread rolls supplemented with glycyrrhetinic acid or placebo. Twenty-four-hour blood pressure measurements were performed at baseline and week 6 and 12 of each treatment period. The ratio of plasma cortisol/cortisone was significantly increased in all patients on GA as compared to baseline or placebo, indicating appropriate enzyme inhibition. Nine of the 10 patients had a persistent decrease in predialysis serum potassium concentration. On GA, mean predialysis serum potassium was significantly lower than at baseline or on placebo. On placebo, serum potassium was significantly elevated above the upper limit of normal in 76% compared to 30% of measurements during GA treatment. Furthermore, on this treatment the frequency of severe hyperkalemia significantly decreased from 9% to 0.6%. No differences were found in parameters reflecting sodium retention. Although these studies show that prolonged GA supplementation persistently lowers serum potassium in dialysis patients, a long-term toxicity study will be mandatory before we recommend the routine use of this treatment.

Increased colonic sodium absorption in rats with chronic renal failure is partially mediated by AT1 receptor agonism

To test the hypothesis that colonic Na(+) transport is altered in the 5/6 nephrectomized rat model of chronic renal failure (CRF), we measured Na(+) fluxes across distal colon from control (CON), CRF, and CRF rats treated with the angiotensin II (ANG II) receptor antagonist losartan (+LOS). We also evaluated overall fluid and Na(+) balance and compared colonic protein and mRNA expression profiles for electroneutral [sodium-hydrogen exchanger (NHE)] and electrogenic Na(+) transport [epithelial sodium channel (ENaC)] in these groups. Consistent with a 60% enhancement in colonic Na(+) absorption in CRF, urinary Na(+) excretion increased by about 50% while serum Na(+) homeostasis was maintained. These CRF-induced changes in Na(+) handling were normalized by treatment with LOS. Net Na(+) absorption was also stimulated in in vitro tissues from CON rats following acute serosal addition of ANG II (10(-7) M), and this increase was blocked by AT(1) antagonism but not by an AT(2) antagonist. In CRF, colonic protein and mRNA expression variably increased for apical NHE2, NHE3, and ENaC alpha-, beta-, gamma-subunits, whereas expression of basolateral NHE1 and Na(+)-K(+)-ATPase (alpha-isoform) remained unaltered. Upregulation of the ENaC subunit mRNA was attenuated somewhat by LOS treatment. Previously, we showed that colonic AT(1) receptor protein is upregulated twofold in CRF, and here we find that AT(1) and AT(2) mRNA and AT(2) protein abundance is unchanged in CRF. We conclude that Na(+) absorption in CRF rat distal colon is increased due to elevated expression of proteins mediating electroneutral and electrogenic uptake and that it is partially mediated by AT(1) receptors.

Effects of chronic hypoxia on electrogenic transport and transport-related oxygen consumption in rat distal colon

The distal colon epithelium of rats submitted to chronic hypoxia shows higher short-circuit current (Isc) which, unlike non-hypoxic rat epithelium, has an amiloride-sensitive component despite low serum aldosterone levels. Isc and oxygen consumption (QO2) were simultaneously measured in mucosae from rats submitted to 0.5 atm for 10 days and from control rats in a modified Ussing chamber. Hypoxia increased Isc but not QO2. The slope of the regression line between Isc and QO2 reduction after ouabain addition was decreased in epithelia from hypoxic rats (P = 0.03). Chloride secretion blockade reduced Isc and QO2 in both groups, while sodium channel blockade did so only in the hypoxic group. Dual blockade in hypoxic rat epithelia caused correlated (P = 0.0025) additive decreases in Isc and QO2. Presented results suggest that chronic hypoxia induces an improved coupling between QO2 and electrogenic ion transport, and electrogenic sodium absorption despite low aldosterone levels.

Altered ENaC expression leads to impaired sodium absorption in the noninflamed intestine in Crohn's disease

BACKGROUND & AIMS

Crohn's disease (CD) is a chronic inflammatory bowel disease. In this study, we have investigated sodium absorption via epithelial sodium channels (ENaC) in the macroscopically noninflamed colon in active CD.

METHODS

Sodium transport via ENaC was investigated in Ussing chambers using biopsy specimens of sigmoid colon from controls and active CD limited to the small intestine. ENaC messenger RNA expression and subcellular localization were studied by real-time polymerase chain reaction and confocal microscopy. Effects of proinflammatory cytokines on ENaC and signaling via mitogen-activated protein kinases were investigated in rat distal colon. Therapeutic inhibition of mitogen-activated protein kinases was studied in CD biopsy specimens.

RESULTS

Electrogenic sodium absorption via ENaC was strongly impaired in the macroscopically noninflamed CD colon because of reduced gamma-ENaC transcription, whereas subcellular localization of ENaC was not changed. In contrast to impaired epithelial sodium transport, epithelial barrier function was not altered in noninflamed CD colon, indicating that paracellular leak flux of ions did not contribute to decreased sodium absorption. Exposure of rat distal colon to tumor necrosis factor alpha led to reduced electrogenic sodium absorption because of impaired transcriptional gamma-ENaC induction, which resembled the changes found in CD. Tumor necrosis factor alpha effects were dependent on extracellular signal-regulated kinase 1/2 but not p38 or c-Jun-N-terminal kinase because inhibition of mitogen-activated protein kinase/extracellular regulated kinase (MEK)1/2 but not inhibition of p38 or c-Jun-N-terminal kinase prevented suppression of ENaC. Finally, therapeutic inhibition of MEK1/2 restored electrogenic sodium absorption in CD.

CONCLUSIONS

In CD, macroscopically noninflamed colon contributes to diarrhea via impaired ENaC-mediated sodium absorption. Inhibition of extracellular signal-regulated kinase might serve as a potential therapeutic strategy for CD diarrhea.

Sodium 4-phenylbutyrate upregulates ENaC and sodium absorption in T84 cells

Butyrate and other short-chain fatty acids (SCFA), produced by colonic bacterial flora, affect numerous epithelial cell functions. To better understand how SCFA regulate ion transport, we investigated the effects of 4-phenylbutyrate (4-PBA) on Na(+) absorption in T84 cells. Under standard cell culture conditions, the short circuit current did not display any amiloride-sensitive Na(+) absorption and was wholly representative of Cl(-) secretion. However, when T84 cells were grown in the presence of 5 mM 4-PBA, a gradual appearance of amiloride-sensitive Na(+) channel (ENaC) activity was observed that reached a plateau after 24 h. Quantitative RT-PCR and Western blot studies of ENaC subunit expression indicated that 4-PBA stimulated alpha and gamma subunits. Trichostatin A, an inhibitor of histone deacetylase, mimicked the effects of 4-PBA, suggesting that 4-PBA affects ENaC expression by inhibiting deacetylases. 4-PBA had no effect on ENaC expression in airway epithelial cells indicating tissue-specific effect. We conclude that butyrate plays an important role in regulating colonic Na(+) absorption by increasing ENaC transcription and activity.

The mechanisms of differential glucocorticoid and mineralocorticoid action in the brain and peripheral tissues

Mineralocorticoids and glucocorticoids are key categories of adrenocorticosteroid hormones that mediate distinct physiological responses. While the primary role of aldosterone, the major mineralocorticoid, is in regulating sodium homeostasis, the major role of the glucocorticoids is mediating the catabolic response to stress. Over the past two decades, these adrenocorticosteroid hormones have been the subject of considerable attention due to the paradox that despite exerting greatly different physiological effects, they act through very closely related receptors and a common DNA response element. This review will examine the research focused on the mechanisms of selective adrenocorticosteroid action. In general, it has been demonstrated that differential adrenocorticosteroid action is mediated at pre-receptor, receptor, and post-receptor levels, depending on the target tissue and physiological environment. The marked neuroendocrine pathophysiologies resulting from perturbations in this complex system make it imperative that further research into mechanisms of coordination of the three levels of adrenocorticosteroid control be conducted.

Induction of 11beta-hydroxysteroid dehydrogenase type 2 and hyperaldosteronism are essential for enhanced sodium absorption after total colectomy in rats

BACKGROUND

Patients who undergo total colectomy with ileopouch anal reconstruction often have persistent diarrhea and frequent bowel movements. Analysis of the intestinal adaptation after total colectomy may lead to developing novel therapies for postoperative diarrhea.

METHODS

Sprague-Dawley rats underwent total colectomy with ileoanal reconstruction and were sacrificed 4 and 8 weeks later. Mucosal response to aldosterone was evaluated with the use of ileal mucosa in an Ussing chamber by measuring short circuit current after in vitro stimulation with aldosterone. We investigated the expression of 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD 2) in intestinal epithelial cells. To examine the role of hyperaldosteronism, we also evaluated rats treated with a sodium-deficient diet or subcutaneous aldosterone infusion.

RESULTS

Aldosterone levels increased 80-fold after total colectomy. A comparable amount of aldosterone dramatically increased aldosterone-mediated, amiloride-sensitive short circuit current in the mucosa from colectomized rats, but not in control rats. We measured an increase in 11beta-HSD 2 messenger RNA and protein in the distal ileum from colectomized rats. Circulating aldosterone appears to be essential for these functional and molecular changes because similar results were obtained by using the mucosa from both dietary sodium-depleted and aldosterone-infused rats.

CONCLUSIONS

Induction of 11beta-HSD 2 is essential for enhanced mineralocorticoid action in the remnant ileum after total colectomy in rats.

Three distinct mechanisms of HCO3- secretion in rat distal colon

HCO(3)(-) secretion has long been recognized in the mammalian colon, but it has not been well characterized. Although most studies of colonic HCO(3)(-) secretion have revealed evidence of lumen Cl(-) dependence, suggesting a role for apical membrane Cl(-)/HCO(3)(-) exchange, direct examination of HCO(3)(-) secretion in isolated crypt from rat distal colon did not identify Cl(-)-dependent HCO(3)(-) secretion but did reveal cAMP-induced, Cl(-)-independent HCO(3)(-) secretion. Studies were therefore initiated to determine the characteristics of HCO(3)(-) secretion in isolated colonic mucosa to identify HCO(3)(-) secretion in both surface and crypt cells. HCO(3)(-) secretion was measured in rat distal colonic mucosa stripped of muscular and serosal layers by using a pH stat technique. Basal HCO(3)(-) secretion (5.6 +/- 0.03 microeq.h(-1).cm(-2)) was abolished by removal of either lumen Cl(-) or bath HCO(3)(-); this Cl(-)-dependent HCO(3)(-) secretion was also inhibited by 100 microM DIDS (0.5 +/- 0.03 microeq.h(-1).cm(-2)) but not by 5-nitro-3-(3-phenylpropyl-amino)benzoic acid (NPPB), a Cl(-) channel blocker. 8-Bromo-cAMP induced Cl(-)-independent HCO(3)(-) secretion (and also inhibited Cl(-)-dependent HCO(3)(-) secretion), which was inhibited by NPPB and by glibenclamide, a CFTR blocker, but not by DIDS. Isobutyrate, a poorly metabolized short-chain fatty acid (SCFA), also induced a Cl(-)-independent, DIDS-insensitive, saturable HCO(3)(-) secretion that was not inhibited by NPPB. Three distinct HCO(3)(-) secretory mechanisms were identified: 1) Cl(-)-dependent secretion associated with apical membrane Cl(-)/HCO(3)(-) exchange, 2) cAMP-induced secretion that was a result of an apical membrane anion channel, and 3) SCFA-dependent secretion associated with an apical membrane SCFA/HCO(3)(-) exchange.

Regulation of epithelial ion transport by aldosterone through changes in gene expression

The year 2003 marks the 50th year since the unfolding of the chemical structures of both aldosterone and DNA. Since the recognition in the early 1960's that aldosterone and its cousin cortisol act through DNA binding proteins that alter gene transcription, research on these corticosteroid hormones and their receptors has attracted fervent attention, both for their importance in endocrine physiology, and as model systems for understanding gene regulation. Recently, aldosterone has emerged as arguably the single most important physiological regulator of extracellular fluid volume and blood pressure in mammals, and has been implicated in a variety of disease states in humans. Moreover, its principal receptor, the mineralocorticoid receptor is increasingly recognized as an important therapeutic target for the treatment of hypertension and congestive heart failure, as well as an important model system for understanding aspects of gene regulation. This increased insight into the functional and pathophysiologic importance of aldosterone has been accompanied by increased insight into its cellular and molecular mechanisms of action. Aldosterone acts in a variety of epithelial and non-epithelial tissues to influence extracellular fluid volume, blood pressure, salt appetite, and can under the appropriate conditions cause cardiac fibrosis. This review will address the current view of aldosterone's molecular mechanism of action in epithelia focusing primarily on the classical MR and on a particular MR target gene, SGK1.

Acid-base effects on intestinal Cl- absorption and vesicular trafficking

In rat ileum and colon, apical membrane Cl(-)/HCO(3)(-) exchange and net Cl(-) absorption are stimulated by increases in Pco(2) or [HCO(3)(-)]. Because changes in Pco(2) stimulate colonic Na(+) absorption, in part, by modulating vesicular trafficking of the Na(+)/H(+) exchanger type 3 isoform to and from the apical membrane, we examined whether changes in Pco(2) affect net Cl(-) absorption by modulating vesicular trafficking of the Cl(-)/HCO(3)(-) exchanger anion exchanger (AE)1. Cl(-) transport across rat distal ileum and colon was measured in the Ussing chamber, and apical membrane protein biotinylation of these segments and Western blots of recovered proteins were performed. In colonic epithelial apical membranes, AE1 protein content was greater at Pco(2) 70 mmHg than at Pco(2) 21 mmHg but was not affected by pH changes in the absence of CO(2). AE1 was internalized when Pco(2) was reduced and exocytosed when Pco(2) was increased, and both mucosal wortmannin and methazolamide inhibited exocytosis. Wortmannin also inhibited the increase in colonic Cl(-) absorption caused by an increase in Pco(2). Increases in Pco(2) stimulated ileal Cl(-) absorption, but wortmannin was without effect. Ileal epithelial apical membrane AE1 content was not affected by Pco(2). We conclude that CO(2) modulation of colonic, but not ileal, Cl(-) absorption involves effects on vesicular trafficking of AE1.

Oral budesonide significantly improves water absorption in patients with ileostomy for Crohn disease

BACKGROUND

In addition to their anti-inflammatory effects, steroids influence electrolyte and water transport systems in the intestinal mucosa. This study analysed the effect of the topically acting glucocorticoid budesonide on ileostomy output in patients with Crohn disease.

METHODS

Oral budesonide (3 mg/three times daily for 8 days; n = 20) was compared to placebo (n = 20) in a double-blind design using matched-pair randomization according to ileal resection length in patients without detectable inflammatory activity. Under controlled hospital conditions, absolute output volumes were measured and response was defined as a reduction in intestinal output of > 25% compared to pretreatment conditions.

RESULTS

In the treatment group, we observed an absolute decrease in median intestinal output from 1,240 ml to 865 ml (30.2%), compared to 0.3% under placebo (from 950 ml to 947.5 ml). Response was documented in 60% (12/20 patients) in the treatment group compared to no response under placebo (P < 0.0001). While both treatment groups showed similar absolute median reductions (400 ml with ileal resection < or = 20 cm and 405 ml with ileal resection > 20 cm), the relative reduction (response rate) was lower in the subgroup of an ileal resection > 20 cm (36%) due to the greater increase in output secondary to the loss of ileum.

CONCLUSIONS

These data support the assumption that the absorptive capacity of the intestinal mucosa for water may be improved by topically acting steroids and suggest that this occurs independently of their anti-inflammatory effect.

Glycyrrhetinic acid decreases plasma potassium concentrations in patients with anuria

ABSTRACT. Licorice-associated hypertension is thought to be due to increased renal sodium retention. The active compound of licorice, glycyrrhetinic acid (GA), inhibits renal 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) and by that mechanism increases access of cortisol to the mineralocorticoid receptor that causes renal sodium retention and potassium loss. In addition, a direct vascular effect of 11beta-HSD activity has recently been incriminated to promote hypertension, a contention based on in vitro observations. This investigation was designed to establish whether this extrarenal effect of 11beta-HSD is relevant for BP regulation and potassium concentrations in plasma. In a prospective, double-blind, cross-over study, seven patients with anuria on chronic hemodialysis were randomly assigned after a baseline period of 2 wk to placebo or GA (1 g/d) for 2 wk, separated by a washout phase of 3 wk. The ratio of plasma cortisol/cortisone, determined by gas chromatography-mass spectrometry, increased in all patients after GA intake (F = 9.705; P < 0.004), which indicates inhibition of 11beta-HSD. Twenty-four-hour BP values did not change throughout the study. The increase of the plasma cortisol/cortisone ratio was paralleled by a decline in the plasma potassium concentration in every patient. The mean +/- SD plasma potassium concentration decreased from 5.5 +/- 0.6 mM/L at baseline to 4.9 +/- 0.7 and 4.5 +/- 0.8 mM/L after 1 and 2 wk on GA, respectively (F = 9.934, P < 0.003). Extrarenal 11beta-HSD activity influences serum potassium concentrations but does not regulate BP independently of renal sodium retention.

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.

Structure, promoter analysis, and chromosomal localization of the murine H(+)/K(+)-ATPase alpha 2 subunit gene

The H(+)/K(+)-ATPase alpha2 subunit (HK alpha 2) of distal colon and renal collecting ducts plays a critical role in potassium and acid-base homeostasis. The isolation and complete sequence of the murine HK alpha 2 gene are reported. The HK alpha 2 gene contains 23 exons and spans 23.5 kb of genomic DNA. The exon/intron organization is comparable to that of the human ATP1AL1 gene. Primer extension and 5'-rapid amplification of cDNA ends of distal colon RNA were used to map the transcription initiation site. Fluorescence in situ hybridization analysis localized the HK alpha 2 gene to murine chromosome 14C3. Sequence analysis of 7.2 kb of the 5'-flanking region revealed numerous consensus sites for transcription factors, including two potential glucocorticoid response elements. Transient transfection of promoter-luciferase constructs demonstrated strong basal HK alpha 2 promoter activity in renal collecting duct cells but not in fibroblasts or in a medullary thick ascending limb of Henle's loop cell line. Deletion analysis revealed that the proximal 0.2 kb of the promoter was sufficient to confer activity in collecting duct cells. These data should prove important in elucidation of the mechanisms controlling the differential, tissue-specific expression of the HK alpha 2 gene.

IFN-gamma downregulates expression of Na(+)/H(+) exchangers NHE2 and NHE3 in rat intestine and human Caco-2/bbe cells

Diarrhea associated with inflammatory bowel diseases has traditionally been attributed to stimulated secretion. The purpose of this study was to determine whether chronic stimulation of intestinal mucosa by interferon-gamma (IFN-gamma) affects expression and function of the apical membrane Na(+)/H(+) exchangers NHE2 and NHE3 in rat intestine and Caco-2/bbe (C2) cells. Confluent C2 cells expressing NHE2 and NHE3 were treated with IFN-gamma for 2, 24, and 48 h. Adult rats were injected with IFN-gamma intraperitoneally for 12 and 48 h. NHE2 and NHE3 activities were measured by unidirectional (22)Na influx across C2 cells and in rat brush-border membrane vesicles. NHE protein and mRNA were assessed by Western and Northern blotting. IFN-gamma treatment of C2 monolayers caused a >50% reduction in NHE2 and NHE3 activities and protein expression. In rats, region-specific, time- and dose-dependent reductions of NHE2 and NHE3 activities, protein expression, and mRNA were observed after exposure to IFN-gamma. Chronic exposure of intestinal epithelial cells to IFN-gamma results in selective downregulation of NHE2 and NHE3 expression and activity, a potential cause of inflammation-associated diarrhea.

SCFA increase intestinal Na absorption by induction of NHE3 in rat colon and human intestinal C2/bbe cells

Short-chain fatty acids (SCFA), produced by colonic bacterial flora fermentation of dietary carbohydrates, promote colonic Na absorption through mechanisms not well understood. We hypothesized that SCFA promote increased expression of apical membrane Na/H exchange (NHE), serving as luminal physiological cues for regulating colonic Na absorptive capacity. Studies were performed in human colonic C2/bbe (C2) monolayers and in vivo. In C2 cells exposed to butyrate, acetate, proprionate, or the poorly metabolized SCFA isobutyrate, apical membrane NHE3 activity and protein expression increased in a time- and concentration-dependent manner, whereas no changes were observed for NHE2. In contrast, no significant changes in brush-border hydrolase or villin expression were noted. Analogous to the in vitro findings, rats fed the soluble fiber pectin exhibited a time-dependent increase in colonic NHE3, but not NHE2, protein, mRNA, and brush-border activity. These changes were region-specific, as no changes were observed in the ileum. We conclude that luminal SCFA are important physiological cues for regulating colonic Na absorptive function, allowing the colon to adapt to chronic changes in dietary carbohydrate and Na loads.

Diarrhea in ulcerative colitis. The role of altered colonic sodium transport

In normal human colon, water and sodium (Na+) absorption are directly related. Defective Na+ absorption may therefore be an important factor in the pathogenesis of diarrhea in ulcerative colitis (UC). Electrophysiological studies have revealed profound decreases in channel-mediated apical Na+ entry and Na(+)-K(+)-ATPase-mediated basolateral Na+ extrusion in surface epithelial cells in inflamed human distal colon. Recent molecular biological studies indicate that mucosal inflammation in UC leads to significant decreases in Na+ channel beta- and gamma-subunit expression in the apical membrane of surface colonocytes, with a marked reduction in the levels of beta- and gamma-subunit-specific mRNAs. In addition, basolateral expression of the Na(+)-K(+)-ATPase alpha 1-isoform is reduced along the surface cell-crypt cell axis in UC, although there is no change in the level of the corresponding mRNA. Diarrhea in ulcerative colitis is therefore related, at least in part, to a major defect in electrogenic Na+ absorption, which reflects changes in the levels of expression of critical subunits of both the apical Na+ channel and basolateral Na(+)-K(+)-ATPase.

Multiple aspects of mineralocorticoid selectivity

Aldosterone regulates renal sodium reabsorption through binding to the mineralocorticoid receptor (MR). Because the glucocorticoid receptor (GR) is expressed together with the MR in aldosterone target cells, glucocorticoid hormones bound to GR may also intervene to modulate physiological functions in these cells. In addition, each steroid can bind both receptors, and the MR has equal affinity for aldosterone and glucocorticoid hormones. Several cellular and molecular mechanisms intervene to allow specific aldosterone regulatory effects, despite the large prevalence of glucocorticoid hormones in the plasma. They include the local metabolism of the glucocorticoid hormones into inactive derivatives by the enzyme 11beta-hydroxysteroid dehydrogenase; the intrinsic properties of the MR that discriminate between ligands through differential contacts; the possibility of forming homo- or heterodimers between MR and GR, leading to differential transactivation properties; and the interactions of MR and GR with other regulatory transcription factors. The relative contribution of each of these successive mechanisms may vary among aldosterone target cells (epithelial vs. nonepithelial) and according to the hormonal context. All these phenomena allow fine tuning of cellular functions depending on the degree of cooperation between corticosteroid hormones and other factors (hormonal or tissue specific). Such interactions may be altered in pathophysiological situations.

Regulation of DRA and AE1 in rat colon by dietary Na depletion

Two distinct Cl/anion exchange activities (Cl/HCO(3) and Cl/OH) identified in apical membranes of rat distal colon are distributed in cell type-specific patterns. Cl/HCO(3) exchange is expressed only in surface cells, whereas Cl/OH exchange is localized in surface and crypt cells. Dietary Na depletion substantially inhibits Cl/HCO(3) but not Cl/OH exchange. We determined whether anion exchange isoforms (AE) and/or downregulated in adenoma (DRA) are expressed in and related to apical membrane anion exchanges by examining localization of AE isoform-specific and DRA mRNA expression in normal and Na-depleted rats. Amplification of AE cDNA fragments by RT-PCR with colonic mRNA as template indicates that AE1 and AE2 but not AE3 mRNAs are expressed. In situ hybridization study revealed that AE1 mRNA is expressed predominantly in surface but not crypt cells. In contrast, AE2 polypeptide is expressed in basolateral membranes and DRA protein is expressed in apical membranes of both surface and crypt cells. AE1 mRNA is only minimally present in proximal colon, and DRA mRNA abundance is similar in distal and proximal colon. Dietary Na depletion reduces AE1 mRNA abundance but did not alter DRA mRNA abundance. This indicates that AE1 encodes surface cell-specific aldosterone-regulated Cl/HCO(3) exchange, whereas DRA encodes aldosterone-insensitive Cl/OH exchange.

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.

Chloride secretion by the intestinal epithelium: molecular basis and regulatory aspects

Chloride secretion is the major determinant of mucosal hydration throughout the gastrointestinal tract, and chloride transport is also pivotal in the regulation of fluid secretion by organs that drain into the intestine. Moreover, there are pathological consequences if chloride secretion is either reduced or increased such as in cystic fibrosis and secretory diarrhea, respectively. With the molecular cloning of many of the proteins and regulatory factors that make up the chloride secretory mechanism, there have been significant advances in our understanding of this process at the cellular level. Similarly, emerging data have clarified the intercellular relationships that govern the extent of chloride secretion. The goal of our article is to review this area of investigation, with an emphasis on recent developments and their implications for the physiology and pathophysiology of chloride transport.

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.

Age- and tissue-specific induction of NHE3 by glucocorticoids in the rat small intestine

Of the two known apical isoforms of the Na(+)/H(+) exchanger (NHE) family, only the NHE3 gene is regulated by glucocorticoids. The aim of these studies was to investigate the mechanisms underlying the effects of methylprednisolone (MP) on expression of NHE3 in the proximal and distal small intestine of suckling and adult rats. Immunoblots showed that the glucocorticoid responsiveness in the proximal small intestine was greatest in suckling animals (NHE3/beta-actin: 0.43 +/- 0.09 control vs. 1.57 +/- 0.15 MP; P < 0. 001), and responsiveness decreased with age with no effect in adults (0.56 +/- 0.14 vs. 0.64 +/- 0.17). Distal small intestine was responsive only in adult rats (0.49 +/- 0.13 vs. 1.65 +/- 0.09; P < 0.001). This pattern was confirmed at the mRNA level and by (22)Na(+) uptake. Western blot and [(3)H]dexamethasone mesylate binding showed that the responsiveness of NHE3 to glucocorticoids is directly related to the expression of glucocorticoid receptor (GR) in the small intestine. These studies suggest that loss and gain of glucocorticoid responsiveness in the proximal and distal small intestine, respectively, are related to age- and segment-dependent expression of GR.

Specificity in mineralocorticoid versus glucocorticoid action

The mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) share considerable structural and functional homology. Overlapping effects on epithelial sodium transport are observed in vivo; in vitro, both are able to bind and transactivate through a common hormone response element. This has led several investigators to suggest that specificity is conferred primarily by prereceptor mechanisms, and we have addressed this question using both in vitro and in vivo approaches. Although the MR has been regarded as less transcriptionally active than the GR in vitro, significant differences are observed when epithelial rather than fibroblast cell lines are used. These differences are mediated by the N-termini of the receptors. Activation of intracellular signaling pathways differentially modulates MR- versus GR-mediated transactivation. Although these studies identify mechanisms by which specificity may be achieved, they do not prove that this occurs in vivo. Such studies have been limited by an absence of MR-regulated genes. Known candidate aldosterone-responsive genes have been examined in the rat distal colon; the time course and the specificity of the response to a single parenteral dose of corticosteroid has been characterized. The epithelial sodium channel beta and gamma subunit genes are both up-regulated within 60 minutes by either MR or GR activation. Similar responses are observed for the serum and glucocorticoid-regulated kinase and channel-inducing factor genes. All four genes show clear and rapid up-regulation of their mRNA levels by aldosterone, which is paralleled by GR-mediated up-regulation of expression. While they are indeed aldosterone-responsive genes, genes that are uniquely aldosterone-regulated remain to be identified.

Salt- and angiotensin II-dependent variations in amiloride-sensitive rectal potential difference in mice

1. In the rectum and distal nephron, sodium reabsorption is mediated by the amiloride-sensitive epithelial sodium channel (ENaC). The ENaC-mediated sodium transport is electrogenic and creates an amiloride-sensitive transepithelial potential difference (PD). 2. We have evaluated the salt- and angiotensin (Ang)II-dependent variations in amiloride-sensitive rectal PD in mice and assessed their relationship with renal sodium handling. 3. Rectal PD was measured in vivo in mice maintained on a medium-, low- or high-sodium diet. On a medium-salt diet, the mean (+/- SEM) amiloride-sensitive PD was larger in the afternoon than in the morning (-26.1 +/- 0.9 and -11.2 +/- 0.7 mV, respectively; P = 0.001), indicating a circadian cyclicity. Rectal PD increased on a low-sodium diet and decreased on a high-sodium diet. 4. Amiloride-sensitive rectal PD correlated significantly with the urinary Na+/K+ ratio (P < 0.001) and with sodium reabsorption in the distal nephron as measured by the lithium clearance technique (P < 0.001). 5. In mice treated with an AngII AT1 receptor antagonist, amiloride-sensitive rectal PD was increased in the afternoon compared with controls (-32.8 +/- 2.0 vs -24.4 +/- 0.9, respectively; P < 0.001). 6. At high doses, AngII decreased the amiloride-sensitive rectal PD and this effect was blunted by an AT1 receptor antagonist. 7. These results show the presence of a salt-dependent daily cyclicity of sodium transport in the mouse rectum that follows circadian changes in sodium handling in the distal nephron. Angiotensin II appears to modulate this diurnal pattern of rectal amiloride-sensitive sodium transport.

Identification of a mineralocorticoid/glucocorticoid response element in the human Na/K ATPase alpha1 gene promoter

Sodium-potassium ATPase (Na/K ATPase) is a major target of mineralocorticoids. Both aldosterone and glucocorticoids activate the human Na/K ATPase alpha1 and beta1 genes transcriptionally. The mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) have been shown to bind the glucocorticoid response element (GRE); however, a specific element responsible for the activation of the MR is not known. Sequence analysis of the putative regulatory region of the Na/K ATPase alpha1 gene revealed the presence of a hormone response element that allows the MR to interact with it, at least as well as if not better than the GR. This response element is designated MRE/GRE. In this investigation, we demonstrated the MR and GR induced gene expression in COS-1 cells by cotransfecting with respective expression plasmids (RshMR and RshGR) along with a luciferase reporter. The synthetic MRE/GRE linked to a neutral promoter was activated by MR (6-fold); however, the GR induced a lower level of expression (3.8-fold), suggesting that the element may be preferably MR responsive. Mutations in the synthetic MRE/GRE could not induce the expression with MR, whereas GR had a small effect. Electrophoretic mobility shift analyses demonstrated a direct interaction of MR and GR with the MRE/GRE that was supershifted by an antiMR antibody and the complex was partially cleared by an antiGR antibody, respectively, whereas nonimmune serum had no effect. Footprinting analyses of the promoter region showed that a portion of the DNA containing this element is protected by recombinant MR and GR. Thus these data confirm that this MRE/GRE interacts with both MR and GR but interaction with receptors may be more MR-responsive than response elements previously described.

Differential regulation of NHE isoforms by sodium depletion in proximal and distal segments of rat colon

Dietary sodium depletion has multiple diverse effects on ion transport in the rat colon, including both the induction and inhibition of electroneutral NaCl absorption in proximal and distal colon of rat, respectively. To establish the mechanism of the differential regulation of Na+ absorption by sodium depletion, this study utilized 1) HOE-694, a dose-dependent inhibitor of Na+/H+ exchanger (NHE) isoforms, in studies of proton gradient-driven 22Na uptake (i.e., Na+/H+ exchange) by apical membrane vesicles (AMV); 2) Northern blot analyses of NHE isoform-specific mRNA abundance; and 3) Western blot analyses of NHE isoform-specific protein expression. HOE-694 inhibition studies establish that 25 microM HOE-694-sensitive (NHE2) and 25 microM HOE-694-insensitive (NHE3) Na+/H+ exchange activities are present in AMV of both proximal and distal colon of normal rats. In proximal colon, dietary sodium depletion enhanced both NHE2 and NHE3 isoform-specific Na+/H+ exchange activities, protein expression, and mRNA abundance. In contrast, in distal colon both NHE2 and NHE3 isoform-specific Na+/H+ exchange activities, protein expression, and mRNA abundance were inhibited by sodium depletion. NHE1 isoform-specific mRNA abundance in proximal or distal colon was not altered by sodium depletion. Differential effects by sodium depletion on Na+/H+ exchange in rat colon are tissue specific and isoform specific; sodium depletion both induces and inhibits apical Na+/H+ exchange at a pretranslational level.

Electrogenic Na+ transport in rat late distal colon by natural and synthetic glucocorticosteroids

The potency of in vitro-added corticosteroids to stimulate electrogenic Na+ absorption (JNa, the Na+ absorptive short-circuit current blockable by 10(-4) M amiloride) was determined in rat late distal colon. JNa was determined 8 h after steroid addition from the drop in short-circuit current caused by 10(-4) M amiloride. The concentration dependency of JNa was obtained for seven corticosteroids and compared with that established for aldosterone. Apparent mineralocorticoid potencies as determined from apparent Michaelis-Menten constant (Km) values were as follows: aldosterone 1. 2 nM >> RU-28362 20 nM = deoxycorticosterone 20 nM > deoxycortisol 36 nM >/= dexamethasone 37 nM >> corticosterone 170 nM > cortisol 210 nM. These steroids exhibited Vmax values of 9-13 micromol. h-1. cm-2 and similar concentration dependencies. Hill coefficients were between 1.6 and 2.1, suggesting cooperative effects between activated receptors. We conclude that corticosteroids exhibit graded mineralocorticoid potency instead of a sharp partition into exclusive groups of mineralocorticoid and nonmineralocorticoid hormones. The low apparent Km value of RU-28362 for mineralocorticoid action and the need for high concentrations of the mineralocorticoid antagonist mespirenone to block this response indicated that JNa in a native mammalian epithelium can be mediated by the glucocorticoid receptor. Glucocorticoid receptor-specific amounts of RU-28362 in combination with mineralocorticoid receptor-specific amounts of aldosterone or of the mineralocorticoid antagonist spironolactone showed cooperative action, suggesting a heterodimeric activation of JNa by the glucocorticoid receptor and mineralocorticoid receptor.