Regulation of Cl- secretion by alpha2-adrenergic receptors in mouse colonic epithelium

The anion exchanger PAT-1 (Slc26a6) does not participate in oxalate or chloride transport by mouse large intestine

The membrane-bound transport proteins responsible for oxalate secretion across the large intestine remain unidentified. The apical chloride/bicarbonate (Cl^-/HCO₃^-) exchanger encoded by Slc26a6, known as PAT-1 (putative anion transporter 1), is a potential candidate. In the small intestine, PAT-1 makes a major contribution to oxalate secretion but whether this role extends into the large intestine has not been directly tested. Using the PAT-1 knockout (KO) mouse, we compared the unidirectional absorptive ([Formula: see text]) and secretory ([Formula: see text]) flux of oxalate and Cl^- across cecum, proximal colon, and distal colon from wild-type (WT) and KO mice in vitro. We also utilized the non-specific inhibitor DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid) to confirm a role for PAT-1 in WT large intestine and (in KO tissues) highlight any other apical anion exchangers involved. Under symmetrical, short-circuit conditions the cecum and proximal colon did not transport oxalate on a net basis, whereas the distal colon supported net secretion. We found no evidence for the participation of PAT-1, or indeed any other DIDS-sensitive transport mechanism, in oxalate or Cl^- by the large intestine. Most unexpectedly, mucosal DIDS concurrently stimulated [Formula: see text] and [Formula: see text] by 25-68% across each segment without impacting net transport. For the colon, these changes were directly proportional to increased transepithelial conductance suggesting this response was the result of bidirectional paracellular flux. In conclusion, PAT-1 does not contribute to oxalate or Cl^- transport by the large intestine, and we urge caution when using DIDS with mouse colonic epithelium.

Biogenic Amines: Signals Between Commensal Microbiota and Gut Physiology

There is increasing interest in the interactions among the gut microbiota, gut, and brain, which is often referred to as the "microbiota-gut-brain" axis. Biogenic amines including dopamine, norepinephrine, serotonin, and histamines are all generated by commensal gut microorganisms and are suggested to play roles as signaling molecules mediating the function of the "microbiota-gut-brain" axis. In addition, such amines generated in the gut have attracted attention in terms of possible clues into the etiologies of depression, anxiety, and even psychosis. This review covers the latest research related to the potential role of microbe-derived amines such as catecholamine, serotonin, histamine, as well as other trace amines, in modulating not only gut physiology but also brain function of the host. Further attention in this field can offer not only insight into expanding the fundamental roles and impacts of the human microbiome, but also further offer new therapeutic strategies for psychological disorders based on regulating the balance of resident bacteria.

Early life adversity in piglets induces long-term upregulation of the enteric cholinergic nervous system and heightened, sex-specific secretomotor neuron responses

BACKGROUND

Early life adversity (ELA) is a risk factor for the later-life onset of gastrointestinal (GI) diseases such as irritable bowel syndrome (IBS); however, the mechanisms are poorly understood. Here, we utilized a porcine model of ELA, early weaning stress (EWS), to investigate the influence of ELA on the development and function of the enteric nervous system (ENS).

METHODS

Female and castrated male (Male-C) piglets were weaned from their sow either at 15 days of age (EWS) or 28 days of age (late weaning control, LWC). At 60 and 170 days of age, ileal mucosa-submucosa preparations were mounted in Ussing chambers and veratridine- and corticotropin releasing factor (CRF)-releasing factor-evoked short circuit current (Isc ) responses were recorded as indices of secretomotor neuron function. Enteric neuron numbers and the expression of select neurotransmitters and their receptors were also measured.

KEY RESULTS

Compared with LWC pigs, female, but not Male-C EWS, pigs exhibited heightened veratridine-induced Isc responses at 60 and 170 days of age that were inhibited with tetrodotoxin and atropine. Ileum from EWS pigs had higher numbers of enteric neurons that were choline acetyltransferase positive. Markers of increased cholinergic signaling (increased acetylcholinesterase) and downregulated mucosal muscarinic receptor 3 gene expression were also observed in EWS pigs.

CONCLUSIONS & INFERENCES

This study demonstrated that EWS in pigs induces lasting and sex-specific hypersensitivity of secretomotor neuron function and upregulation of the cholinergic ENS. These findings may represent a mechanistic link between ELA and lifelong susceptibility to GI diseases such as IBS.

The pharmacologic treatment of short bowel syndrome: new tricks and novel agents

Short bowel syndrome (SBS) is a manifestation of massive resection of the intestines resulting in severe fluid, electrolyte, and vitamin/mineral deficiencies. Diet and parenteral nutrition play a large role in the management of SBS; however, pharmacologic options are becoming more readily available. These pharmacologic agents focus on reducing secretions and stimulating intestinal adaptation. The choice of medication is highly dependent on the patient's symptoms, remaining anatomy, and risk versus benefit profile for each agent. This article focuses on common and novel pharmacologic medications used in SBS, including expert advice on their indications and use.

Microbiome, HPA axis and production of endocrine hormones in the gut

Recent accumulating evidence indicates that the gut microbiome can affect the development and regulation of the hypothalamic-pituitary-adrenal axis and behavior, with central integrative systems being crucial in the successful physiological adaptation of the organism to external stressor. In contrast, host-derived hormones increase the bacterial proliferative capacity and pathogenicity. In the gut lumen, this type of cross-talk between microorganisms and the host is presumed to be performed continually through various kinds of luminal molecules, as numerous types of bacteria and host cells are in close proximity in the gastrointestinal tract of mammals.We herein focus on bidirectional signaling between the gut microbiome and the host in terms of commensal microbiota affecting the hypothalamic-pituitary-adrenal HPA axis response and behaviors and further discuss the role of gut luminal catecholamines and γ-aminobutyric acid, both of which are presumed to be involved in this signaling.

Regulation of transepithelial ion transport in the rat late distal colon by the sympathetic nervous system

The colorectum (late distal colon) is innervated by the sympathetic nervous system, and many colorectal diseases are related to disorders of the sympathetic nervous system. The sympathetic regulation of colorectal ion transport is rarely reported. The present study aims to investigate the effect of norepinephrine (NE) in the normal and catecholamine-depleted condition to clarify the regulation of the sympathetic adrenergic system in ion transport in the rat colorectum. NE-induced ion transport in the rats colorectum was measured by short-circuit current (I(sc)) recording; the expression of beta-adrenoceptors and NE transporter (NET) were quantified by real-time PCR, and western blotting. When the endogenous catecholamine was depleted by reserpine, the baseline I(sc) in the colorectum was increased significantly comparing to controls. NE evoked downward deltaI(sc) in colorectum of treated rats was 1.8-fold of controls. The expression of beta(2)-adrenoceptor protein in the colorectal mucosa was greater than the control, though the mRNA level was reduced. However, NET expression was significantly lower in catecholamine-depleted rats compared to the controls. In conclusion, the sympathetic nervous system plays an important role in regulating basal ion transport in the colorectum. Disorders of sympathetic neurotransmitters result in abnormal ion transport, beta-adrenoceptor and NET are involved in the process.

Farnesoid X receptor agonists attenuate colonic epithelial secretory function and prevent experimental diarrhoea in vivo

OBJECTIVE

Bile acids are important regulators of intestinal physiology, and the nuclear bile acid receptor, farnesoid X receptor (FXR), is emerging as a promising therapeutic target for several intestinal disorders. Here, we investigated a role for FXR in regulating intestinal fluid and electrolyte transport and the potential for FXR agonists in treating diarrhoeal diseases.

DESIGN

Electrogenic ion transport was measured as changes in short-circuit current across voltage-clamped T84 cell monolayers or mouse tissues in Ussing chambers. NHE3 activity was measured as BCECF fluorescence in Caco-2 cells. Protein expression was measured by immunoblotting and cell surface biotinylation. Antidiarrhoeal efficacy of GW4064 was assessed using two in vivo mouse models: the ovalbumin-induced diarrhoea model and cholera toxin (CTX)-induced intestinal fluid accumulation.

RESULTS

GW4064 (5 μmol/L; 24 h), a specific FXR agonist, induced nuclear translocation of the receptor in T84 cells and attenuated Cl(-) secretory responses to both Ca(2+) and cAMP-dependent agonists. GW4064 also prevented agonist-induced inhibition of NHE3 in Caco-2 cells. In mice, intraperitoneal administration of GW4064 (50 mg/mL) also inhibited Ca(2+) and cAMP-dependent secretory responses across ex vivo colonic tissues and prevented ovalbumin-induced diarrhoea and CTX-induced intestinal fluid accumulation in vivo. At the molecular level, FXR activation attenuated apical Cl(-) currents by inhibiting expression of cystic fibrosis transmembrane conductance regulator channels and inhibited basolateral Na(+)/K(+)-ATPase activity without altering expression of the protein.

CONCLUSIONS

These data reveal a novel antisecretory role for the FXR in colonic epithelial cells and suggest that FXR agonists have excellent potential for development as a new class of antidiarrheal drugs.

Non-genomic estrogen regulation of ion transport and airway surface liquid dynamics in cystic fibrosis bronchial epithelium

Male cystic fibrosis (CF) patients survive longer than females and lung exacerbations in CF females vary during the estrous cycle. Estrogen has been reported to reduce the height of the airway surface liquid (ASL) in female CF bronchial epithelium. Here we investigated the effect of 17β-estradiol on the airway surface liquid height and ion transport in normal (NuLi-1) and CF (CuFi-1) bronchial epithelial monolayers. Live cell imaging using confocal microscopy revealed that airway surface liquid height was significantly higher in the non-CF cells compared to the CF cells. 17β-estradiol (0.1–10 nM) reduced the airway surface liquid height in non-CF and CF cells after 30 min treatment. Treatment with the nuclear-impeded Estrogen Dendrimer Conjugate mimicked the effect of free estrogen by reducing significantly the airway surface liquid height in CF and non-CF cells. Inhibition of chloride transport or basolateral potassium recycling decreased the airway surface liquid height and 17β-estradiol had no additive effect in the presence of these ion transporter inhibitors. 17β-estradiol decreased bumetanide-sensitive transepithelial short-circuit current in non-CF cells and prevented the forskolin-induced increase in ASL height. 17β-estradiol stimulated an amiloride-sensitive transepithelial current and increased ouabain-sensitive basolateral short-circuit current in CF cells. 17β-estradiol increased PKCδ activity in CF and non-CF cells. These results demonstrate that estrogen dehydrates CF and non-CF ASL, and these responses to 17β-estradiol are non-genomic rather than involving the classical nuclear estrogen receptor pathway. 17β-estradiol acts on the airway surface liquid by inhibiting cAMP-mediated chloride secretion in non-CF cells and increasing sodium absorption via the stimulation of PKCδ, ENaC and the Na⁺/K⁺ATPase in CF cells.

Ursodeoxycholic acid attenuates colonic epithelial secretory function

Dihydroxy bile acids, such as chenodeoxycholic acid (CDCA), are well known to promote colonic fluid and electrolyte secretion, thereby causing diarrhoea associated with bile acid malabsorption. However, CDCA is rapidly metabolised by colonic bacteria to ursodeoxycholic acid (UDCA), the effects of which on epithelial transport are poorly characterised. Here, we investigated the role of UDCA in the regulation of colonic epithelial secretion. Cl(-) secretion was measured across voltage-clamped monolayers of T84 cells and muscle-stripped sections of mouse or human colon. Cell surface biotinylation was used to assess abundance/surface expression of transport proteins. Acute (15 min) treatment of T84 cells with bilateral UDCA attenuated Cl(-) secretory responses to the Ca(2+) and cAMP-dependent secretagogues carbachol (CCh) and forskolin (FSK) to 14.0 ± 3.8 and 40.2 ± 7.4% of controls, respectively (n = 18, P < 0.001). Investigation of the molecular targets involved revealed that UDCA acts by inhibiting Na(+)/K(+)-ATPase activity and basolateral K(+) channel currents, without altering their cell surface expression. In contrast, intraperitoneal administration of UDCA (25 mg kg(-1)) to mice enhanced agonist-induced colonic secretory responses, an effect we hypothesised to be due to bacterial metabolism of UDCA to lithocholic acid (LCA). Accordingly, LCA (50-200 μm) enhanced agonist-induced secretory responses in vitro and a metabolically stable UDCA analogue, 6α-methyl-UDCA, exerted anti-secretory actions in vitro and in vivo. In conclusion, UDCA exerts direct anti-secretory actions on colonic epithelial cells and metabolically stable derivatives of the bile acid may offer a new approach for treating intestinal diseases associated with diarrhoea.

Critical role of gut microbiota in the production of biologically active, free catecholamines in the gut lumen of mice

There is increasing interest in the bidirectional communication between the mammalian host and prokaryotic cells. Catecholamines (CA), candidate molecules for such communication, are presumed to play an important role in the gut lumen; however, available evidence is limited because of the lack of actual data about luminal CA. This study evaluated luminal CA levels in the gastrointestinal tract and elucidated the involvement of gut microbiota in the generation of luminal CA by comparing the findings among specific pathogen-free mice (SPF-M), germ-free mice (GF-M), and gnotobiotic mice. Substantial levels of free dopamine and norepinephrine were identified in the gut lumen of SPF-M. The free CA levels in the gut lumen were lower in GF-M than in SPF-M. The majority of CA was a biologically active, free form in SPF-M, whereas it was a biologically inactive, conjugated form in GF-M. The association of GF-M with either Clostridium species or SPF fecal flora, both of which have abundant β-glucuronidase activity, resulted in the drastic elevation of free CA. The inoculation of E. coli strain into GF-M induced a substantial amount of free CA, but the inoculation of its mutant strain deficient in the β-glucuronidase gene did not. The intraluminal administration of DA increased colonic water absorption in an in vivo ligated loop model of SPF-M, thus suggesting that luminal DA plays a role as a proabsorptive modulator of water transport in the colon. These results indicate that gut microbiota play a critical role in the generation of free CA in the gut lumen.

ATP secretion in the male reproductive tract: essential role of CFTR

Extracellular ATP is essential for the function of the epididymis and spermatozoa, but ATP release in the epididymis remains uncharacterized. We investigated here whether epithelial cells release ATP into the lumen of the epididymis, and we examined the role of the cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-) and HCO(3)(-) conducting ion channel known to be associated with male fertility, in this process. Immunofluorescence labelling of mouse cauda epididymidis showed expression of CFTR in principal cells but not in other epithelial cells. CFTR mRNA was not detectable in clear cells isolated by fluorescence-activated cell sorting (FACS) from B1-EGFP mice, which express enhanced green fluorescent protein (EGFP) exclusively in these cells in the epididymis. ATP release was detected from the mouse epididymal principal cell line (DC2) and increased by adrenaline and forskolin. Inhibition of CFTR with CFTR(inh172) and transfection with CFTR-specific siRNAs in DC2 cells reduced basal and forskolin-activated ATP release. CFTR-dependent ATP release was also observed in primary cultures of mouse epididymal epithelial cells. In addition, steady-state ATP release was detected in vivo in mice, by measuring ATP concentration in a solution perfused through the lumen of the cauda epididymidis tubule and collected by cannulation of the vas deferens. Luminal CFTR(inh172) reduced the ATP concentration detected in the perfusate. This study shows that CFTR is involved in the regulation of ATP release from principal cells in the cauda epididymidis. Given that mutations in CFTR are a leading cause of male infertility, we propose that defective ATP signalling in the epididymis might contribute to dysfunction of the male reproductive tract associated with these mutations.

Epidermal growth factor chronically upregulates Ca(2+)-dependent Cl(-) conductance and TMEM16A expression in intestinal epithelial cells

Dysregulated epithelial fluid and electrolyte transport is a common feature of many intestinal disorders. However, molecular mechanisms that regulate epithelial transport processes are still poorly understood, thereby limiting development of new therapeutics. Previously, we showed that epidermal growth factor (EGF) chronically enhances intestinal epithelial secretory function. Here, we investigated a potential role for altered expression or activity of apical Cl(−) channels in mediating the effects of EGF. Cl(−) secretion across monolayers of T(84) colonic epithelia was measured as changes in short-circuit current. Protein expression/phosphorylation was measured by RT-PCR and Western blotting. Under conditions that specifically isolate apical Ca(2+)-activated Cl(−) channel (CaCC) currents, EGF pretreatment (100 ng ml(−1) for 15 min) potentiated carbachol (CCh)-induced responses to 173 ± 25% of those in control cells, when measured 24 h later (n = 26; P < 0.01). EGF-induced increases in CaCC currents were abolished by the transmembrane protein 16A (TMEM16A) inhibitor, T16A(inh)-A01 (10 μm). Furthermore, TMEM16A mRNA and protein expression was increased by EGF to 256 ± 38% (n = 7; P < 0.01) and 297 ± 46% (n = 9, P < 0.001) of control levels, respectively. In contrast, EGF did not alter CFTR expression or activity. EGF-induced increases in Cl(−) secretion, CaCC currents and TMEM16A expression were attenuated by a PKCδ inhibitor, rottlerin (20 μm), and a phosphatidylinositol 3-kinase (PI3K) inhibitor, LY290042 (25 μm). Finally, LY290042 inhibited EGF-induced phosphorylation of PKCδ. We conclude that EGF chronically upregulates Ca(2+)-dependent Cl(−) conductances and TMEM16A expression in intestinal epithelia by a mechanism involving sequential activation of PI3K and PKCδ. Therapeutic targeting of EGF receptor-dependent signalling pathways may provide new approaches for treatment of epithelial transport disorders.

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.

Hydroxylase inhibition attenuates colonic epithelial secretory function and ameliorates experimental diarrhea

Hydroxylases are oxygen-sensing enzymes that regulate cellular responses to hypoxia. Transepithelial Cl(-) secretion, the driving force for fluid secretion, is dependent on O(2) availability for generation of cellular energy. Here, we investigated the role of hydroxylases in regulating epithelial secretion and the potential for targeting these enzymes in treatment of diarrheal disorders. Ion transport was measured as short-circuit current changes across voltage-clamped monolayers of T(84) cells and mouse colon. The antidiarrheal efficacy of dimethyloxallyl glycine (DMOG) was tested in a mouse model of allergic disease. Hydroxylase inhibition with DMOG attenuated Ca(2+)- and cAMP-dependent secretory responses in voltage-clamped T(84) cells to 20.2 ± 2.6 and 38.8 ± 6.7% (n=16; P≤0.001) of those in control cells, respectively. Antisecretory actions of DMOG were time and concentration dependent, being maximal after 18 h of DMOG (1 mM) treatment. DMOG specifically inhibited Na(+)/K(+)-ATPase pump activity without altering its expression or membrane localization. In mice, DMOG inhibited agonist-induced secretory responses ex vivo and prevented allergic diarrhea in vivo. In conclusion, hydroxylases are important regulators of epithelial Cl(-) and fluid secretion and present a promising target for development of new drugs to treat transport disorders.

Stress at the intestinal surface: catecholamines and mucosa-bacteria interactions

Psychological stress has profound effects on gastrointestinal function, and investigations over the past few decades have examined the mechanisms by which neural and hormonal stress mediators act to modulate gut motility, epithelial barrier function and inflammatory states. With its cellular diversity and large commensal bacterial population, the intestinal mucosa and its overlying mucous environment constitute a highly interactive environment for eukaryotic host cells and prokaryotic bacteria. The elaboration of stress mediators, particularly norepinephrine, at this interface influences host cells engaged in mucosal protection and the bacteria which populate the mucosal surface and gut lumen. This review will address growing evidence that norepinephrine and, in some cases, other mediators of the adaptation to stress modulate mucosal interactions with enteric bacteria. Stress-mediated changes in this delicate interplay may shift the microbial colonization patterns on the mucosal surface and alter the susceptibility of the host to infection. Moreover, changes in host-microbe interactions in the digestive tract may also influence ongoing neural activity in stress-responsive brain areas.

Strain-dependent differences in electrogenic secretion of electrolytes across mouse colon epithelium

Mice have proven to be powerful models for the study of human physiology and pathophysiology. With the advent of techniques for genomic manipulation, the possibilities for studying inherited diseases in this convenient laboratory mammal are increasing by the day. It has been reported that when knocking out or otherwise modifying genes of interest in mice, the phenotype obtained can vary markedly depending on the genetic background of the animals used in the study. The aim of this work was to study whether the genetic background can influence the characteristics of fluid and electrolyte transepithelial transport in the distal colon of three mouse strains most in use in our and other laboratories. Ussing chamber recordings revealed that the colons of C57Bl/6J, Sv 129 and Black Swiss animals have distinctive responses to the calcium agonists carbachol and histamine that are not explained by the presence of different types of muscarinic and histaminergic receptors in these tissues. We have also found differences in the cAMP-activated, KCNMA1-channel-dependent potassium secretion between the strains. We interpret this to indicate a unique distribution of KCNMA1 channels in lower parts of the crypt of Sv 129 colonic epithelium compared with that of C57Bl/6J and Black Swiss animals. The reported differences should be taken into account when choosing the genetic background of animals to be used for genetic modification.

JNK mitogen-activated protein kinase limits calcium-dependent chloride secretion across colonic epithelial cells

Neuroimmune agonists induce epithelial Cl(-) secretion through elevations in intracellular Ca2+ or cAMP. Previously, we demonstrated that epidermal growth factor receptor (EGFR) transactivation and subsequent ERK MAPK activation limits secretory responses to Ca2+-dependent, but not cAMP-dependent, agonists. Although JNK MAPKs are also expressed in epithelial cells, their role in regulating transport function is unknown. Here, we investigated the potential role for JNK in regulating Cl(-) secretion in T(84) colonic epithelial cells. Western blot analysis revealed that a prototypical Ca2+-dependent secretagogue, carbachol (CCh; 100 microM), induced phosphorylation of both the 46-kDa and 54-kDa isoforms of JNK. This effect was mimicked by thapsigargin (TG), which specifically elevates intracellular Ca2+, but not by forskolin (FSK; 10 microM), which elevates cAMP. CCh-induced JNK phosphorylation was attenuated by the EGFR inhibitor, tyrphostin-AG1478 (1 microM). Pretreatment of voltage-clamped T(84) cells with SP600125 (2 microM), a specific JNK inhibitor, potentiated secretory responses to both CCh and TG but not to FSK. The effects of SP600125 on CCh-induced secretion were not additive with those of the ERK inhibitor, PD98059. Finally, in apically permeabilized T(84) cell monolayers, SP600125 potentiated CCh-induced K+ conductances but not Na+/K+ATPase activity. These data demonstrate a novel role for JNK MAPK in regulating Ca2+ but not cAMP-dependent epithelial Cl(-) secretion. JNK activation is mediated by EGFR transactivation and exerts its antisecretory effects through inhibition of basolateral K+ channels. These data further our understanding of mechanisms regulating epithelial secretion and underscore the potential for exploitation of MAPK-dependent signaling in treatment of intestinal transport disorders.

Adrenergic activation of electrogenic K+ secretion in guinea pig distal colonic epithelium: involvement of beta1- and beta2-adrenergic receptors

Adrenergic stimulation of electrogenic K+ secretion in isolated mucosa from guinea pig distal colon required activation of two beta-adrenergic receptor subtypes (beta-AdrR). Addition of epinephrine (epi) or norepinephrine (norepi) to the bathing solution of mucosae in Ussing chambers increased short-circuit current (Isc) and transepithelial conductance (Gt), consistent with this cation secretion. A beta-adrenergic classification was supported by propranolol antagonism of this secretory response and the lack of effect by the alpha-AdrR antagonists BE2254 (alpha1-AdrR) and yohimbine (alpha2-AdrR). Subtype-selective antagonists CGP20712A (beta1-AdrR), ICI-118551 (beta2-AdrR), and SR59320A (beta3-AdrR) were relatively ineffective at inhibiting the epi-stimulated Isc response. In combination, CGP20712A and ICI-118551 inhibited the response, which supported a synergistic action by beta1-AdrR and beta2-AdrR. Expression of mRNA for both beta1-AdrR and beta2-AdrR was indicated by RT-PCR of RNA from colonic epithelial cells. Protein expression was indicated by immunoblot showing bands at molecular weights consistent with monomers and oligomers. Immunoreactivity (ir) for beta1-AdrR and beta2-AdrR was prominent in basolateral membranes of columnar epithelial cells in the crypts of Lieberkühn as well as intercrypt surface epithelium. Cells in the pericryptal sheath also had beta1-AdrR(ir) but did not have discernable beta2-AdrR(ir). The adrenergic sensitivity of K+ secretion measured by Isc and Gt was relatively low as indicated by EC(50)s of 41 +/- 7 nM for epi and 50 +/- 14 nM for norepi. Adrenergic activation of electrogenic K+ secretion required the involvement of both beta1-AdrR and beta2-AdrR, occurring with an agonist sensitivity reduced compared with reported values for either receptor subtype.

Clinical and experimental evidence of sympathetic neural dysfunction during inflammatory bowel disease

1. Inflammatory bowel diseases (IBD) alter the function of the enteric nervous system and the sensory innervation of the gastrointestinal (GI) tract. Less is known about whether IBD also affects the sympathetic nervous system (SNS). Given the importance of the SNS in regulating GI function and possibly immune system activation, the present review examines the evidence of sympathetic dysfunction during IBD and its possible consequences. 2. Sympathetic axons within the GI tract innervate several cell types, including vascular myocytes, enteric neurons and immune cells. The major neurotransmitters released from sympathetic varicosities are noradrenaline, neuropeptide Y and ATP or a related purine. 3. Clinical studies of IBD patients have provided evidence of an association between IBD and axonal or demyelinating neuropathy. Assays of autonomic function suggest that ulcerative colitis and Crohn's disease, the two major forms of IBD, have contrasting effects on sympathetic neural activity. 4. Animal models of IBD have been used to examine the effects of these diseases on sympathetic neurophysiology. A decrease in the release of noradrenaline from sympathetic varicosities in inflamed and uninflamed regions of the GI tract has consistently been reported. Recent findings suggest that the decrease in neurotransmitter release may be due to inhibition of N-type voltage-gated Ca(2+) current in post-ganglionic sympathetic neurons. 5. Interest in the role of the SNS in IBD is rapidly increasing. However, much work needs to be done to enhance understanding of how SNS function is altered during IBD and what contribution, if any, these changes make to pathogenesis.

Role of calnexin in the ER quality control and productive folding of CFTR; differential effect of calnexin knockout on wild-type and DeltaF508 CFTR

Cystic fibrosis (CF) is caused by the mutation in CF transmembrane conductance regulator (CFTR), a cAMP-dependent Cl(-) channel at the plasma membrane of epithelium. The most common mutant, DeltaF508 CFTR, has competent Cl(-) channel function, but fails to express at the plasma membrane since it is retained in the endoplasmic reticulum (ER) by the ER quality control system. Here, we show that calnexin (CNX) is not necessary for the ER retention of DeltaF508 CFTR. Our data show that CNX knockout (KO) does not affect the biosynthetic processing, cellular localization or the Cl(-) channel function of DeltaF508 CFTR. Importantly, cAMP-induced Cl(-) current in colonic epithelium from CNX KO/DeltaF508 CFTR mice was comparable with that of DeltaF508 CFTR mice, indicating that CNX KO failed to rescue the ER retention of DeltaF508 CFTR in vivo. Moreover, we show that CNX assures the efficient expression of WT CFTR, but not DeltaF508 CFTR, by inhibiting the proteasomal degradation, indicating that CNX might stimulate the productive folding of WT CFTR, but not DeltaF508 CFTR, which has folding defects.

EGF receptor transactivation and PI3-kinase mediate stimulation of ERK by alpha(2A)-adrenoreceptor in intestinal epithelial cells: a role in wound healing

Intestinal cells express alpha(2A)-adrenoreceptors that stimulate sodium and peptide absorption and promote cell proliferation. Involved mechanisms are poorly understood and are not fully related to inhibition of cAMP production. Previous study using a clone of CaCo2 cells expressing the human alpha(2A)-adrenoreceptor (CaCo2-3B) showed that alpha(2)-adrenoreceptor agonists cause extracellular signal-regulated kinase (ERK) phosphorylation. Present work examines the signaling pathway triggering ERK activation and investigates the consequence of alpha(2A)-adrenoreceptor stimulation on cell migration. Treatment of CaCo2-3B with the alpha(2)-adrenoreceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino) quinoxaline (UK14304) induces not only ERK, but also Akt phosphorylation. Both effects are strongly attenuated by inhibition or desensitization of epidermal growth factor (EGF) receptor, matrix metalloproteinase (MMP) blockade, heparin-binding-EGF neutralization or phosphatidylinositol 3-kinase (PI3-kinase) inhibitors. Conditioned medium from UK14304-treated CaCo2-3B stimulates ERK in parental CaCo2 by a mechanism sensitive to EGF receptor and PI3-kinase inhibitors. Exposure of CaCo2-3B to UK14304 accelerates the wound healing. This effect is abolished by heparin-binding-EGF neutralization but not by mitomycin C, indicating that it results probably from increased cell spreading and/or migration. In conclusion, alpha(2A)-adrenoreceptor activates ERK and Akt in intestinal cells by a common pathway which depends on PI3-kinase activation and results from EGF receptor transactivation, via an autocrine/paracrine pathway implying MMP activation and heparin-binding-EGF shedding. Therefore, alpha(2A)-adrenoreceptor could have a positive role in intestinal regeneration in vivo.

mRNA expression and binding sites for alpha2-adrenergic receptor subtypes in muscle layers of the ileum and spiral colon of dairy cows

OBJECTIVE

To measure maximum binding capacity (B(max)) and levels of mRNA expression for alpha(2)-adrenergic receptor (AR) subtypes in ileal and colonic muscle layers of healthy dairy cows.

SAMPLE POPULATION

Ileal and colonic muscle specimens from 6 freshly slaughtered cows.

PROCEDURES

Ileal and colonic muscle layers were obtained by scraping the mucosa and submucosa from full-thickness tissue specimens. Level of mRNA expression for alpha(2)-AR subtypes was measured by real-time reverse transcriptase-PCR analysis and expressed relative to the mean mRNA expression of glyceraldehyde phosphate dehydrogenase, ubiquitin, and 18S ribosomal RNA. Binding studies were performed with tritiated RX821002 ((3)H-RX821002) and subtype-selective ligands as competitors.

RESULTS

mRNA expression for alpha(2AD)-, alpha(2B)-, and alpha(2C)-AR subtypes was similar in ileal and colonic muscle layers. The mRNA expression for alpha(2AD)-AR was significantly greater than that for alpha(2B)- and alpha(2C)-AR subtypes, representing 92%, 6%, and 2%, respectively, of the total mRNA. Binding competition of (3)H-RX821002 with BRL44408, imiloxan, and MK-912 was best fitted by a 1-site model. The B(max) of alpha(2AD)- and alpha(2C)-AR sub-types was greater than that of alpha(2B)-AR. The B(max) and level of mRNA expression were only correlated (r = 0.8) for alpha(2AD)-AR. Ratio of B(max) to mRNA expression for alpha(2C)-AR was similar to that for alpha(2B)-AR, but significantly greater than for alpha(2AD)-AR.

CONCLUSIONS AND CLINICAL RELEVANCE

Subtypes of alpha(2)-AR in bovine intestinal muscle layers are represented by a mixture of alpha(2AD)- and alpha(2C)-ARs and of alpha(2B)-AR at a lower density. Information provided here may help in clarification of the role of AR subtypes in alpha(2)-adrenergic mechanisms regulating bovine intestinal motility.

Two-pore-domain potassium channels support anion secretion from human airway Calu-3 epithelial cells

Potassium channels are required for the absorption and secretion of fluids and electrolytes in epithelia. Calu-3 cells possess a secretory phenotype, and are a model human airway submucosal gland serous cell. Short-circuit current (I(sc)) recordings from Calu-3 cells indicated that basal anion secretion was reduced by apical application of the K+ channel inhibitors bupivicaine, lidocaine, clofilium, and quinidine. Application of riluzole resulted in a large increase in I(sc), inhibited by apical application of either bupivicane or the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel blocker DPC. These results suggested that one or more members of the two-pore-domain K+ (K(2P)) channel family could influence anion secretion. Using RT-PCR, we found that Calu-3 cells express mRNA transcripts for TASK-2 (KCNK5), TWIK-1 (KCNK1), TWIK-2 (KCNK6) and TREK-1 (KCNK2). TASK-2, TWIK-2 and TREK-1 protein were detected by Western blotting, while immunolocalization of polarized cells confirmed protein expression of TREK-1 and TWIK-2 at the plasma cell membrane. TASK-2 protein staining was localized to intracellular vesicles, located beneath the apical membrane. While the pro-secretory role of basolateral K+ channels is well established, we suggest that apically located K2P channels, not previously described in airway epithelial cells, also play an important role in controlling the rate of transepithelial anion secretion.

Membrane cholesterol content modulates activation of BK channels in colonic epithelia

Changes in the level of membrane cholesterol regulate a variety of signaling processes including those mediated by acylated signaling molecules that localize to lipid rafts. Recently several types of ion channels have been shown to have cholesterol-dependent activity and to localize to lipid rafts. In this study, we have investigated the role of cholesterol in the regulation of ion transport in colonic epithelial cells. We observed that methyl-beta-cyclodextrin (MbetaCD), a cholesterol-sequestering molecule, activated transepithelial short circuit current (Isc), but only from the basolateral side. Similar results were obtained with a cholesterol-binding agent, filipin, and with the sphingomyelin-degrading enzyme, sphingomyelinase. Experiments with DeltaF508CFTR mutant mice indicated that raft disruption affected CFTR-mediated anion secretion, while pharmacological studies showed that this effect was due to activation of basolateral large conductance Ca2+-activated K+ (BK) channels. Sucrose density gradient centrifugation studies demonstrated that BK channels were normally present in the high-density fraction containing the detergent-insoluble cytoskeleton, and that following treatment with MbetaCD, BK channels redistributed into detergent-soluble fractions. Our evidence therefore implicates novel high-density cholesterol-enriched plasma membrane microdomains in the modulation of BK channel activation and anion secretion in colonic epithelia.