Fluid absorption in isolated perfused colonic crypts

Electroacupuncture Improving Intestinal Barrier Function in Rats with Irritable Bowel Syndrome Through Regulating Aquaporins

BACKGROUND

Intestinal mucosal barrier dysfunction plays a crucial role in the pathogenesis of irritable bowel syndrome with diarrhea (IBS-D). In order to explore the mechanism of electroacupuncture (EA) treatment on intestinal mucosal barrier, this study observed the effect of EA on aquaporins (AQPs), tight junctions (TJs), NF-κB pathway and the gut microbiota in IBS-D rats.

METHODS

The IBS-D model was established by acetic acid enema combined with chronic restraint method. The effects of EA on the treatment of IBS-D were examined by the abdominal withdrawal reflex score, Bristol's fecal character score, fecal water content, small intestine propulsion rate and HE staining. AQPs, TJs and inflammation-related molecular mechanisms were explored. The fecal samples were applied for 16S rRNA sequencing to assess the effect of EA intervention to the intestinal bacterial abundance.

RESULTS

EA reduced intestinal sensitization, restored intestinal motility and improved inflammatory cell infiltration. Furthermore, EA improved intestinal inflammation and flora environment significantly, inhibited NF-κB signaling and inflammatory factors (IL-1β and TNF-α). It can also increase the gene and protein expression of AQPs (AQP1, AQP3, and AQP8) and the gene levels of TJs (ZO-1 and Occludin).

CONCLUSION

EA has an inhibitory effect on the NF-κB signaling pathway, and regulates the proteins of AQP1, AQP3, AQP8, and TJs to restore the balance of water metabolism and intestinal permeability in IBS-D, which also restored the function of the intestinal mucosa by regulating the intestinal flora.

Nutritional Considerations in Colorectal Surgery in Diverting Ileostomy Patients: A Review

Colorectal surgery often results in a temporary or permanent ileostomy construction. The general nutritional status and intake of patients with an ileostomy have not received much attention and scientific evidence is lacking. Nutritional complications associated with ileostomy formation and colonic exclusion include fluid (dehydration) and electrolyte abnormalities (mainly hyponatremia), impaired renal function occurring from plasma volume depletion, and reduced energy absorption due to the role of the large bowel in energy assimilation. People with ileostomies frequently avoid specific foods, due to concerns of possible malfunction or food blockages of their stoma, which may produce a negative effect on their overall dietary intake and nutritional status. The present article reviews the existing literature on nutritional considerations for those with an ileostomy and discusses measures to optimize overall nutritional status of this category of patients.

The effect of claudin-15 deletion on cationic selectivity and transport in paracellular pathways of the cecum and large intestine

The large intestine plays a pivotal role in water and electrolyte balance. Paracellular transport may play a role in ion transport mechanisms in the cecum and large intestine; however, these molecular mechanisms and their physiological roles have not been fully studied. Claudin-15 forms a cation channel in tight junctions in the small intestine, but its role in the cecum and large intestine has not been investigated. This study aimed to explore the physiological role of claudin-15 in the cecum and large intestine using claudin-15 (Cldn15) KO mice. Electrical conductance, short-circuit current, Na⁺ flux, and dilution potential were assessed in isolated tissue preparations mounted in Ussing chambers. The induced short-circuit current of short-chain fatty acids, which are fermentative products in the intestinal tract, was also measured. Compared to wild type mice, the electrical conductance and paracellular Na⁺ flux was decreased in the cecum, but not the middle large intestine, while in both the cecum and the middle large intestine, paracellular Na⁺ permeability was decreased in Cldn15 KO mice. These results suggest that claudin-15 is responsible for Na⁺ permeability in the tight junctions of the cecum and large intestine and decreased Na⁺ permeability in the cecum may cause impaired absorption function.

Human Colonoid-Myofibroblast Coculture for Study of Apical Na+/H+ Exchangers of the Lower Cryptal Neck Region

Cation and anion transport in the colonocyte apical membrane is highly spatially organized along the cryptal axis. Because of lack of experimental accessibility, information about the functionality of ion transporters in the colonocyte apical membrane in the lower part of the crypt is scarce. The aim of this study was to establish an in vitro model of the colonic lower crypt compartment, which expresses the transit amplifying/progenitor (TA/PE) cells, with accessibility of the apical membrane for functional study of lower crypt-expressed Na⁺/H⁺ exchangers (NHEs). Colonic crypts and myofibroblasts were isolated from human transverse colonic biopsies, expanded as three-dimensional (3D) colonoids and myofibroblast monolayers, and characterized. Filter-grown colonic myofibroblast-colonic epithelial cell (CM-CE) cocultures (myofibroblasts on the bottom of the transwell and colonocytes on the filter) were established. The expression pattern for ion transport/junctional/stem cell markers of the CM-CE monolayers was compared with that of nondifferentiated (EM) and differentiated (DM) colonoid monolayers. Fluorometric pH_i measurements were performed to characterize apical NHEs. CM-CE cocultures displayed a rapid increase in transepithelial electrical resistance (TEER), paralleled by downregulation of claudin-2. They maintained proliferative activity and an expression pattern resembling TA/PE cells. The CM-CE monolayers displayed high apical Na⁺/H⁺ exchange activity, mediated to >80% by NHE2. Human colonoid-myofibroblast cocultures allow the study of ion transporters that are expressed in the apical membrane of the nondifferentiated colonocytes of the cryptal neck region. The NHE2 isoform is the predominant apical Na⁺/H⁺ exchanger in this epithelial compartment.

Colonic Fluid and Electrolyte Transport 2022: An Update

Colonic epithelial cells are responsible for maintaining a delicate balance between luminal secretion and the absorption of fluids and ions. This review aims to discuss and update the model of colonic electrolyte secretion and absorption via the cystic fibrosis transmembrane regulator (CFTR), epithelial sodium channel (ENaC), Na-K-Cl cotransporters (NKCC1 and 2), Na-H exchangers (NHE1–4), colonic H,KATPase, and several other key components involved in multi-level transepithelial ion transport. Developments in our understanding of the activity, regulation, localization, and relationships of these ion transporters and their interactions have helped forge a more robust understanding of colonic ion movement that accounts for the colonic epithelium’s role in mucosal pH modulation, the setting of osmotic gradients pivotal for fluid retention and secretion, and cell death regulation. Deviations from homeostatic ion transport cause diarrhea, constipation, and epithelial cell death and contribute to cystic fibrosis, irritable bowel syndrome (IBS), ulcerative colitis, and cancer pathologies. Signal transduction pathways that regulate electrolyte movement and the regulatory relationships between various sensors and transporters (CFTR as a target of CaSR regulation and as a regulator of ENaC and DRA, for example) are imperative aspects of a dynamic and comprehensive model of colonic ion homeostasis.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

SLC9 Gene Family: Function, Expression, and Regulation

The Slc9 family of Na⁺ /H⁺ exchangers (NHEs) plays a critical role in electroneutral exchange of Na⁺ and H⁺ in the mammalian intestine as well as other absorptive and secretory epithelia of digestive organs. These transport proteins contribute to the transepithelial Na⁺ and water absorption, intracellular pH and cellular volume regulation as well as the electrolyte, acid-base, and fluid volume homeostasis at the systemic level. They also influence the function of other membrane transport mechanisms, affect cellular proliferation and apoptosis as well as cell migration, adherence to the extracellular matrix, and tissue repair. Additionally, they modulate the extracellular milieu to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺ /H⁺ exchange is inhibited in selected gastrointestinal diseases, either by intrinsic factors (e.g., bile acids, inflammatory mediators) or infectious agents and associated bacterial toxins. Disrupted NHE activity may contribute not only to local and systemic electrolyte imbalance but also to the disease severity via multiple mechanisms. In this review, we describe the cation proton antiporter superfamily of Na⁺ /H⁺ exchangers with a particular emphasis on the eight SLC9A isoforms found in the digestive tract, followed by a more integrative description in their roles in each of the digestive organs. We discuss regulatory mechanisms that determine the function of Na⁺ /H⁺ exchangers as pertinent to the digestive tract, their regulation in pathological states of the digestive organs, and reciprocally, the contribution of dysregulated Na⁺ /H⁺ exchange to the disease pathogenesis and progression. © 2018 American Physiological Society. Compr Physiol 8:555-583, 2018.

Human Enteroids as a Model of Upper Small Intestinal Ion Transport Physiology and Pathophysiology

BACKGROUND & AIMS

Human intestinal crypt-derived enteroids are a model of intestinal ion transport that require validation by comparison with cell culture and animal models. We used human small intestinal enteroids to study neutral Na(+) absorption and stimulated fluid and anion secretion under basal and regulated conditions in undifferentiated and differentiated cultures to show their functional relevance to ion transport physiology and pathophysiology.

METHODS

Human intestinal tissue specimens were obtained from an endoscopic biopsy or surgical resections performed at Johns Hopkins Hospital. Crypts were isolated, enteroids were propagated in culture, induced to undergo differentiation, and transduced with lentiviral vectors. Crypt markers, surface cell enzymes, and membrane ion transporters were characterized using quantitative reverse-transcription polymerase chain reaction, immunoblot, or immunofluorescence analyses. We used multiphoton and time-lapse confocal microscopy to monitor intracellular pH and luminal dilatation in enteroids under basal and regulated conditions.

RESULTS

Enteroids differentiated upon withdrawal of WNT3A, yielding decreased crypt markers and increased villus-like characteristics. Na(+)/H(+) exchanger 3 activity was similar in undifferentiated and differentiated enteroids, and was affected by known inhibitors, second messengers, and bacterial enterotoxins. Forskolin-induced swelling was completely dependent on cystic fibrosis transmembrane conductance regulator and partially dependent on Na(+)/H(+) exchanger 3 and Na(+)/K(+)/2Cl(-) cotransporter 1 inhibition in undifferentiated and differentiated enteroids. Increases in cyclic adenosine monophosphate with forskolin caused enteroid intracellular acidification in HCO3(-)-free buffer. Cyclic adenosine monophosphate-induced enteroid intracellular pH acidification as part of duodenal HCO3(-) secretion appears to require cystic fibrosis transmembrane conductance regulator and electrogenic Na(+)/HCO3(-) cotransporter 1.

CONCLUSIONS

Undifferentiated or crypt-like, and differentiated or villus-like, human enteroids represent distinct points along the crypt-villus axis; they can be used to characterize electrolyte transport processes along the vertical axis of the small intestine. The duodenal enteroid model showed that electrogenic Na(+)/HCO3(-) cotransporter 1 might be a target in the intestinal mucosa for treatment of secretory diarrheas.

Differential DNA methylation patterns of homeobox genes in proximal and distal colon epithelial cells

Region and cell-type specific differences in the molecular make up of colon epithelial cells have been reported. Those differences may underlie the region-specific characteristics of common colon epithelial diseases such as colorectal cancer and inflammatory bowel disease. DNA methylation is a cell-type specific epigenetic mark, essential for transcriptional regulation, silencing of repetitive DNA and genomic imprinting. Little is known about any region-specific variations in methylation patterns in human colon epithelial cells. Using purified epithelial cells and whole biopsies (n= 19) from human subjects, we generated epigenome-wide DNA methylation data (using the HELP-tagging assay), comparing the methylation signatures of the proximal and distal colon. We identified a total of 125 differentially methylated sites (DMS) mapping to transcription start sites of protein-coding genes, most notably several members of the homeobox (HOX) family of genes. Patterns of differential methylation were validated with MassArray EpiTYPER. We also examined DNA methylation in whole biopsies, applying a computational technique to deconvolve variation in methylation within cell types and variation in cell-type composition across biopsies. Including inferred epithelial proportions as a covariate in differential methylation analysis applied to the whole biopsies resulted in greater overlap with the results obtained from purified epithelial cells compared with when the covariate was not included. Results obtained from both approaches highlight region-specific methylation patterns of HOX genes in colonic epithelium. Regional variation in methylation patterns has implications for the study of diseases that exhibit regional expression patterns in the human colon, such as inflammatory bowel disease and colorectal cancer.

An investigation into the relationship between small intestinal fluid secretion and systemic arterial blood pressure in the anesthetized rat

The effects of changes in the steady level of diastolic blood pressure on fluid flux across the jejunum has been investigated in the anesthetized rat during perfusion with a nutrient-free and Na⁺-free solution. Diastolic blood pressure was manipulated by intravenous infusions, during the jejunal perfusions, of vasodilators (vasoactive intestinal polypeptide, acetyl-β-methylcholine, and phentolamine) and a vasoconstrictor (arginine vasopressin), each of which acts through a different cellular mechanism. The outcome was that fluid flux was related by a parabolic relationship with diastolic blood pressure in which net secretion occurred over the range 40–100 mmHg, whereas net absorption was recorded at diastolic pressures exceeding 100 mmHg and below 40 mmHg. Against a background of normal absorption promoted by perfusion with 145 mmol L⁻¹ Na⁺/5 mmol L⁻¹ glucose solution, reductions in diastolic blood pressure markedly reduced the mean rate of fluid absorption by 58% overall, whereas the rate of glucose absorption remained unchanged. Our results were explained on the basis that vasodilatation led to increased capillary pressure and then to net filtration of fluid from the mesenteric capillary bed. Experiments in which Escherichia coli heat-stable toxin was added to the jejunal perfusate confirmed the absence of a secretory response, which was consistent with the absence of effect of the toxin on diastolic blood pressure.

Evidence from simultaneous intracellular- and surface-pH transients that carbonic anhydrase II enhances CO2 fluxes across Xenopus oocyte plasma membranes

The α-carbonic anhydrases (CAs) are zinc-containing enzymes that catalyze the interconversion of CO2 and HCO3 (-). Here, we focus on human CA II (CA II), a ubiquitous cytoplasmic enzyme. In the second paper in this series, we examine CA IV at the extracellular surface. After microinjecting recombinant CA II in a Tris solution (or just Tris) into oocytes, we expose oocytes to 1.5% CO2/10 mM HCO3 (-)/pH 7.50 while using microelectrodes to monitor intracellular pH (pHi) and surface pH (pHS). CO2 influx causes the familiar sustained pHi fall as well as a transient pHS rise; CO2 efflux does the opposite. Both during CO2 addition and removal, CA II increases the magnitudes of the maximal rate of pHi change, (dpHi/dt)max, and the maximal change in pHS, ΔpHS. Preincubating oocytes with the inhibitor ethoxzolamide eliminates the effects of CA II. Compared with pHS, pHi begins to change only after a delay of ~9 s and its relaxation has a larger (i.e., slower) time constant (τpHi > τpHS ). Simultaneous measurements with two pHi electrodes, one superficial and one deep, suggest that impalement depth contributes to pHi delay and higher τpHi . Using higher CO2/HCO3 (-) levels, i.e., 5%/33 mM HCO3 (-) or 10%/66 mM HCO3 (-), increases (dpHi/dt)max and ΔpHS, though not in proportion to the increase in [CO2]. A reaction-diffusion mathematical model (described in the third paper in this series) accounts for the above general features and supports the conclusion that cytosolic CA-consuming entering CO2 or replenishing exiting CO2-increases CO2 fluxes across the cell membrane.

Activation of AMPK inhibits cholera toxin stimulated chloride secretion in human and murine intestine

Increased intestinal chloride secretion through chloride channels, such as the cystic fibrosis transmembrane conductance regulator (CFTR), is one of the major molecular mechanisms underlying enterotoxigenic diarrhea. It has been demonstrated in the past that the intracellular energy sensing kinase, the AMP-activated protein kinase (AMPK), can inhibit CFTR opening. We hypothesized that pharmacological activation of AMPK can abrogate the increased chloride flux through CFTR occurring during cholera toxin (CTX) mediated diarrhea.

Chloride efflux was measured in isolated rat colonic crypts using real-time fluorescence imaging. AICAR and metformin were used to activate AMPK in the presence of the secretagogues CTX or forskolin (FSK). In order to substantiate our findings on the whole tissue level, short-circuit current (SCC) was monitored in human and murine colonic mucosa using Ussing chambers. Furthermore, fluid accumulation was measured in excised intestinal loops.

CTX and forskolin (FSK) significantly increased chloride efflux in isolated colonic crypts. The increase in chloride efflux could be offset by using the AMPK activators AICAR and metformin. In human and mouse mucosal sheets, CTX and FSK increased SCC. AICAR and metformin inhibited the secretagogue induced rise in SCC, thereby confirming the findings made in isolated crypts. Moreover, AICAR decreased CTX stimulated fluid accumulation in excised intestinal segments.

The present study suggests that pharmacological activation of AMPK effectively reduces CTX mediated increases in intestinal chloride secretion, which is a key factor for intestinal water accumulation. AMPK activators may therefore represent a supplemental treatment strategy for acute diarrheal illness.

Evidence of a rudimentary colon in the elasmobranch, Leucoraja erinacea

The transition from aquatic to terrestrial life presented tetrapodamorphs with the challenge of maintaining water homeostasis and preventing desiccation on land. The colon evolved in terrestrial vertebrates to help maintain fluid balance. Although marine elasmobranchs lack a colon, their spiral intestine contains a subregion that histologically appears to be colon-like, possibly representing an evolutionary precursor to terrestrial digestive tracts. The distal-most region of the spiral intestine of elasmobranchs has no villi and a large number of acid mucins: hallmarks of water absorption in the colons of terrestrial animals. To determine if histologically distinct regions of the elasmobranch digestive tract correspond to functional differences, we compared water absorption in different subregions of the skate, Leucoraja erinacea digestive tract. Water absorption in stomach and spiral intestinal sacs was linear with time and not hydrostatic pressure-dependent. The histologically distinct distal portion of the spiral intestine had a threefold higher rate of water absorption than the proximal portion of the spiral intestine. In addition, the water-selective, colon-specific aquaporin 4 is expressed strongly in the distal spiral intestine epithelia, correlating with the region of the spiral intestine exhibiting the greatest rate of water absorption. We demonstrate that the distal spiral intestine is histologically and functionally distinct from the rest of the spiral intestine and represents a rudimentary colon within the vertebrate lineage.

Water channel proteins in the gastrointestinal tract

Water transport through the human digestive system is physiologically crucial for maintaining body water homeostasis and ensure digestive and absorptive functions. Within the gastrointestinal tract, water recirculates, being secreted with the digestive juices and then almost entirely absorbed by the small and large intestine. The importance of aquaporins (AQPs), transmembrane water channel proteins, in the rapid passage of water across plasma membranes in the gastrointestinal tract appears immediately evident. Several AQP isoforms are found in gastrointestinal epithelia, with AQP1, 3, 7, 10 and 11 being the most abundantly expressed in the whole gut. On the other hand, AQP4 and 8 are located selectively in the stomach and colon, respectively. Here we review AQP expression and localization at the tissue, cellular and subcellular level in gastrointestinal epithelia, and their modification in various gut diseases.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

Toxin mediated diarrhea in the 21 century: the pathophysiology of intestinal ion transport in the course of ETEC, V. cholerae and rotavirus infection

An estimated 4 billion episodes of diarrhea occur each year. As a result, 2–3 million children and 0.5–1 million adults succumb to the consequences of this major healthcare concern. The majority of these deaths can be attributed to toxin mediated diarrhea by infectious agents, such as E. coli, V. cholerae or Rotavirus. Our understanding of the pathophysiological processes underlying these infectious diseases has notably improved over the last years. This review will focus on the cellular mechanism of action of the most common enterotoxins and the latest specific therapeutic approaches that have been developed to contain their lethal effects.

Acetylcholine-induced activation of M3 muscarinic receptors stimulates robust matrix metalloproteinase gene expression in human colon cancer cells

Previously, we showed that ACh-induced proliferation of human colon cancer cells is mediated by transactivation of epidermal growth factor (EGF) receptors (EGFRs). In the present study, we elucidate the molecular mechanism underlying this action. ACh-induced proliferation of H508 colon cancer cells, which express exclusively M3 muscarinic receptors (M3Rs), was attenuated by anti-EGFR ligand binding domain antibody, a broad-spectrum matrix metalloproteinase (MMP) inhibitor, anti-MMP7 antibody, a diphtheria toxin analog that blocks release of an EGFR ligand [heparin-binding EGF-like growth factor (HBEGF)], and anti-HBEGF antibody. Conditioned media from ACh-treated H508 cells induced proliferation of SNU-C4 colon cancer cells that express EGFR but not M3R. These actions were attenuated by an EGFR inhibitor and by anti-EGFR and anti-HBEGF antibodies. In H508, but not SNU-C4, colon cancer cells, ACh caused a striking dose- and time-dependent increase in levels of MMP7 mRNA and MMP7 protein. Similarly, ACh induced robust MMP1 and MMP10 gene transcription. ACh-induced MMP1, MMP7, and MMP10 gene transcription was attenuated by atropine, anti-EGFR antibody, and chemical inhibitors of EGFR and ERK activation. In contrast, inhibitors of phosphatidylinositol 3-kinase and NF-kappaB activation did not alter MMP gene transcription. Collectively, these findings indicate that MMP7-catalyzed release of HBEGF mediates ACh-induced transactivation of EGFR and consequent proliferation of colon cancer cells. ACh-induced activation of EGFR and downstream ERK signaling also regulates transcriptional activation of MMP7, thereby identifying a novel feed-forward mechanism for neoplastic cell proliferation.

Involvement of the cytoskeleton in the effect of PGE2 on ion transport in the rat distal colon

This work aimed at studying the effect of PGE2 on water and chloride absorption from the rat distal colon and at investigating the involvement of the cytoskeleton in the modulation of colonic transporters. PGE2 increased significantly net water and chloride absorption. It increased also the activity of the Na+K+-ATPase and the expression of the Na+K+2Cl- cotransporter. The increase in pump activity was ascribed to its phosphorylation by PKA or PKC when activated upon binding of PGE2 to its receptors, and was deemed responsible for the increase in Cl- absorption. Cytochalasin B (CytoB), a disrupter of microfilaments, decreased net water and chloride absorption in presence or absence of PGE2. Furthermore it down-regulated both pump and cotransporter, and lowered Na+K+-ATPase activity. It was suggested that an intact actin cytoskeleton is required for the basal and the PGE2-elicited trafficking of both transporters. On the other hand, colchicine, an inhibitor of microtubule polymerization, had no effect on the absorption of water and chloride but abrogated the stimulatory effect of PGE2. Colchicine exerted a similar effect to that of cytochlasin on the expression of both pump and cotransporter in presence or absence of PGE2 except for the basal activity of the pump which was not altered by microtubule disruption. It was concluded that both microfilament and microtubular networks are involved in the basal and PGE2-elicited increase in colonic ion absorption.

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

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

NHE2 is the main apical NHE in mouse colonic crypts but an alternative Na+-dependent acid extrusion mechanism is upregulated in NHE2-null mice

The mechanism of apical Na(+)-dependent H(+) extrusion in colonic crypts is controversial. With the use of confocal microscopy of the living mouse distal colon loaded with BCECF or SNARF-5F (fluorescent pH sensors), measurements of intracellular pH (pH(i)) in epithelial cells at either the crypt base or colonic surface were reported. After cellular acidification, the addition of luminal Na(+) stimulated similar rates of pH(i) recovery in cells at the base of distal colonic crypts of wild-type or Na(+)/H(+) exchanger isoform 2 (NHE2)-null mice. In wild-type crypts, 20 microM HOE694 (NHE2 inhibitor) blocked 68-75% of the pH(i) recovery rate, whereas NHE2-null crypts were insensitive to HOE694, the NHE3-specific inhibitor S-1611 (20 microM), or the bicarbonate transport inhibitor 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS; 1 mM). A general NHE inhibitor, 5-(N-ethyl-N-isopropyl)amiloride (EIPA; 20 microM), inhibited pH(i) recovery in NHE2-null mice (46%) but less strongly than in wild-type mice (74%), suggesting both EIPA-sensitive and -insensitive compensatory mechanisms. Transepithelial Na(+) leakage followed by activation of basolateral NHE1 could confound the outcomes; however, the rates of Na(+)-dependent pH(i) recovery were independent of transepithelial leakiness to lucifer yellow and were unchanged in NHE1-null mice. NHE2 was immunolocalized on apical membranes of wild-type crypts but not NHE2-null tissue. NHE3 immunoreactivity was near the colonic surface but not at the crypt base in NHE2-null mice. Colonic surface cells from wild-type mice demonstrated S1611- and HOE694-sensitive pH(i) recovery in response to luminal sodium, confirming a functional role for both NHE3 and NHE2 at this site. We conclude that constitutive absence of NHE2 results in a compensatory increase in a Na(+)-dependent, EIPA-sensitive acid extruder distinct from NHE1, NHE3, or SITS-sensitive transporters.

Calcium-sensing receptor abrogates secretagogue- induced increases in intestinal net fluid secretion by enhancing cyclic nucleotide destruction

The calcium-sensing receptor (CaSR) provides a fundamental mechanism for diverse cells to detect and respond to modulations in the ionic and nutrient compositions of their extracellular milieu. The roles for this receptor are largely unknown in the intestinal tract, where epithelial cells are normally exposed to large variations in extracellular solutes. Here, we show that colonic CaSR signaling stimulates the degradation of cyclic nucleotides by phosphodiesterases and describe the ability of receptor activation to reverse the fluid and electrolyte secretory actions of cAMP- and cGMP-generating secretagogues, including cholera toxin and heat stable Escherichia coli enterotoxin STa. Our results suggest a paradigm for regulation of intestinal fluid transport where fine tuning is accomplished by the counterbalancing effects of solute activation of the CaSR on neuronal and hormonal secretagogue actions. The reversal of cholera toxin- and STa endotoxin-induced fluid secretion by a small-molecule CaSR agonist suggests that these compounds may provide a unique therapy for secretory diarrheas.

Role of calcium and other trace elements in the gastrointestinal physiology

Calcium is an essential ion in both marine and terrestrial organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the regulation of neuronal function. The Ca²⁺ balance is maintained by three organ systems, including the gastrointestinal tract, bone and kidney.

Since first being cloned in 1993 the Ca²⁺-sensing receptor has been expressed along the entire gastrointestinal tract, until now the exact function is only partly elucidated. As of this date it still remains to be determined if the Ca²⁺-sensing receptor is involved in calcium handling by the gastrointestinal tract. However, there are few studies showing physiological effects of the Ca²⁺-sensing receptor on gastric acid secretion and fluid transport in the colon. In addition, polyamines and amino acids have been shown to activate the Ca²⁺-sensing receptor and also act as allosteric modifiers to signal nutrient availability to intestinal epithelial cells. Activation of the colonic Ca²⁺-sensing receptor can abrogate cyclic nucleotide-mediated fluid secretion suggesting a role of the receptor in modifying secretory diarrheas like cholera. For many cell types changes in extracellular Ca²⁺ concentration can switch the cellular behavior from proliferation to terminal differentiation or quiescence. As cancer remains predominantly a disease of disordered balance between proliferation, termination and apoptosis, disruption in the function of the Ca²⁺-sensing receptor may contribute to the progression of neoplastic disease. Loss of the growth suppressing effects of elevated extracellular Ca²⁺ have been demonstrated in colon carcinoma, and have been correlated with changes in the level of CaSR expression.

PGE2 exerts dose-dependent opposite effects on net water and chloride absorption from the rat colon

This work investigated the effect of different doses of PGE2 on net water and Cl- absorption from the rat colon, using an in situ perfusion technique. PGE2 exerted opposite effects at different concentrations. Net water and Cl- absorption was significantly reduced at low doses with a minimum at 0.4 microg/100g BW, and significantly elevated at high doses with an observed maximal effect at 21 microg/100g BW. At low doses, PGE2 increased in superficial cells, the activity of the Na+-K+ ATPase and the protein expression of the Na+K+2Cl- cotransporter, but reduced them in crypt cells. Thus, the reduction in net water and Cl- absorption was ascribed to an increase in secretion by surface cells that masked absorptive processes. At high doses, PGE2 increased significantly the activity of the Na+-K+ ATPase in superficial cells only, and was without any effect on the protein expression of the cotransporter and the pump in both surface and crypt cells. The observed increase in net water and Cl- absorption was attributed in this case to an increase in absorptive processes with no effect on secretion.

Aquaporin-8 is involved in water transport in isolated superficial colonocytes from rat proximal colon

Water is an essential nutrient because it must be introduced from exogenous sources to satisfy metabolic demand. Under physiologic conditions, the colon can absorb and secrete considerable amounts of water even against osmotic gradients, thus helping to maintain the body fluid balance. Here we describe studies on both aquaporin (AQP) expression and function using cells isolated from the superficial and lower crypt regions of the rat proximal colon. The expression of AQP-3, -4, and -8 in isolated colonocytes was determined by semiquantitative RT-PCR and by immunoblotting. The localization of AQP-8 in the colon was evaluated by immunohistochemistry. A stopped-flow light scattering method was used to examine osmotic water movement in isolated colonocytes. Moreover, the contribution of AQP-8 to overall water movement through isolated colonocytes was studied using RNA interference technology. Colonocytes from the proximal colon express AQP-3, -4, and -8 with increasing concentrations from the lower crypt cells toward those on the surface. Osmotic water permeability was higher in surface than in crypt colonocytes (P < 0.05); it was significantly inhibited by the water channel blocker dimethyl sulfoxide, and reversed by beta-mercaptoethanol (P < 0.05). Immunohistochemistry revealed a strong AQP-8 labeling in the apical membrane of the superficial colonocytes. Inhibition of aquaporin-8 expression by small interfering RNA significantly decreased osmotic water permeability (approximately 38%; P < 0.05). Current results indicate that aquaporin-8 may play a major role in water movement through the colon by acting on the apical side of the superficial cells.

Role of short-chain fatty acids in colonic HCO(3) secretion

Luminal isobutyrate, a relatively poor metabolized short-chain fatty acid (SCFA), induces HCO(3) secretion via a Cl-independent, DIDS-insensitive, carrier-mediated process as well as inhibiting both Cl-dependent and cAMP-induced HCO(3) secretion. The mechanism(s) responsible for these processes have not been well characterized. HCO(3) secretion was measured in isolated colonic mucosa mounted in Lucite chambers using pH stat technique and during microperfusion of isolated colonic crypts. (14)C-labeled butyrate, (14)C-labeled isobutyrate, and (36)Cl uptake were also determined by apical membrane vesicles (AMV) isolated from surface and/or crypt cells. Butyrate stimulation of Cl-independent, DIDS-insensitive 5-nitro-3-(3-phenylpropyl-amino)benzoic acid-insensitive HCO(3) secretion is greater than that by isobutyrate, suggesting that both SCFA transport and metabolism are critical for HCO(3) secretion. Both lumen and serosal 25 mM butyrate inhibit cAMP-induced HCO(3) secretion to a comparable degree (98 vs. 90%). In contrast, Cl-dependent HCO(3) secretion is downregulated by lumen 25 mM butyrate considerably more than by serosal butyrate (98 vs. 37%). Butyrate did not induce HCO(3) secretion in isolated microperfused crypts, whereas an outward-directed HCO(3) gradient-driven induced (14)C-butyrate uptake by surface but not crypt cell AMV. Both (36)Cl/HCO(3) exchange and potential-dependent (36)Cl movement in AMV were inhibited by 96-98% by 20 mM butyrate. We conclude that 1) SCFA-dependent HCO(3) secretion is the result of SCFA transport across the apical membrane via a SCFA/HCO(3) exchange more than intracellular SCFA metabolism; 2) SCFA-dependent HCO(3) secretion is most likely a result of an apical membrane SCFA/HCO(3) exchange in surface epithelial cells; 3) SCFA downregulates Cl-dependent and cAMP-induced HCO(3) secretion secondary to SCFA inhibition of apical membrane Cl/HCO(3) exchange and anion channel activity, respectively.

Bicarbonate secretion: a neglected aspect of colonic ion transport

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

SECRETION AND ABSORPTION BY COLONIC CRYPTS

The intestines play an important role in the absorption and secretion of nutrients. The colon is the final area for recapturing electrolytes and water prior to excretion, and in order to maintain this electrolyte homeostasis, a complex interaction between secretory and absorptive processes is necessary. Until recently it was thought that secretion and absorption were two distinct processes associated with either crypts or surface cells, respectively. Recently it was demonstrated that both the surface and crypt cells can perform secretory and absorptive functions and that, in fact, these functions can be going on simultaneously. This issue is important in the complexities associated with secretory diarrhea and also in attempting to develop treatment strategies for intestinal disorders. Here, we update the model of colonic secretion and absorption, discuss new issues of transporter activation, and identify some important new receptor pathways that are important modulators of the secretory and absorptive functions of the colon.

The basis for transport across epithelial cells

This essay looks at the historical significance of two APS classic papers that are freely available online:

Ingraham RC and Visscher MB. Further studies on intestinal absorption with the performance of osmotic work. Am J Physiol 121: 771–785, 1938 ( http://ajplegacy.physiology.org/cgi/reprint/121/3/771 ).

Visscher MB, Fetcher ES Jr, Carr CW, Gregor HP, Bushey MS, and Barker DE. Isotopic tracer studies on the movement of water and ions between intestinal lumen and blood. Am J Physiol 142: 550–575, 1944 ( http://ajplegacy.physiology.org/cgi/reprint/142/4/550 ).

Functions and roles of the extracellular Ca2+-sensing receptor in the gastrointestinal tract

The gastrointestinal tract is vital to food digestion and nutrient absorption as well as normal salt and water homeostasis. Studies over the last several years have shown that the Ca2+-sensing receptor is expressed along the entire gastrointestinal tract. The potential roles for the receptor in gastrointestinal biology are now only beginning to be elucidated and much work remains. Well-studied physiological effects include regulation of gastric acid secretion and modulation of fluid transport in the colon. It remains to be determined if the Ca2+-sensing receptor is involved in calcium handling by the gastrointestinal tract. The ability of organic nutrient receptor agonists/allosteric modifiers, such as polyamines and L-amino acids, to activate the Ca2+-sensing receptor suggest potential roles in signalling nutrient availability to gastric and intestinal epithelial cells. In addition, polyamines are crucial for normal cell proliferation and differentiation required to sustain the rapid turnover of gastrointestinal epithelial cells and the Ca2+-sensing receptor may be involved in this function. Activation of the colonic Ca2+-sensing receptor can abrogate cyclic nucleotide-mediated fluid secretion suggesting a role for the receptor in modifying secretory diarrheas like cholera. Finally, the Ca2+-sensing receptor has been suggested to provide a mechanism for the effect of calcium intake in reducing the risk of colon cancer.

Chloride secretion in a morphologically differentiated human colonic cell line that expresses the epithelial Na+ channel

Cell line models of colonic electrolyte transport have been extensively used despite lacking some of the characteristics of native tissue. While native colonic crypts absorb or secrete NaCl, immortalized cell lines only retain the secretory phenotype. In the present study we have characterized functionally and molecularly, vectorial fluid and electrolyte transport in the morphologically differentiated human colonic cell line LIM1863. LIM1863 cells form morphologically differentiated organoids resembling native human colonic crypts, which secrete fluid and electrolytes across the apical membrane into a centrally located lumen. Net fluid secretion was evaluated by means of morphometric measurement of lumens formed in LIM organoids in response to known secretagogues. Pharmacological profiling of the channels and transporters involved in fluid and electrolyte transport showed that net fluid transport requires Cl- uptake across the basolateral membrane through a Na(+)-K(+)-2Cl- cotransporter (NKCC1) and its subsequent exit across an apical cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel. Similar to the native colon, net Cl- secretion in the LIM1863 cell line is activated by cAMP-mediated agonists. Carbachol, a Ca(2+)-mediated agonist, does not induce net Cl- secretion but modulates the cAMP-activated response. Expression of chloride channels (CFTR and the Ca(2+)-dependent Cl- channel, ClCa1), potassium channels (KCNN4 and KCNQ1), epithelial Na+ channel (ENaC) alpha, beta and gamma subunits and ion transporters (NKCC1; anion exchanger, AE2; Na+/H+ exchangers, NHE1-3) was detected by RT-PCR and Western blot in the case of ENaC. Based on this evidence we propose that LIM1863 cells provide a unique model for studying CFTR-dependent Cl- secretion in a morphologically differentiated human colonic crypt cell line that also expresses ENaC.

Extracellular polyamines regulate fluid secretion in rat colonic crypts via the extracellular calcium-sensing receptor

BACKGROUND AND AIMS

Polyamines are essential for the normal postnatal development, maintenance, and function of gastrointestinal epithelia. The extracellular Ca(2+) (Ca(2+)(o)/nutrient)-sensing receptor is expressed on both luminal and basolateral membranes of colonocytes, and, in other cell systems, this receptor has been shown to respond to polyamines. Thus, the Ca(2+)-sensing receptor could provide a mechanism for modulation of colonocyte function by dietary and systemic extracellular polyamines. In the present study, we investigated the interaction of polyamines, particularly spermine, and extracellular Ca(2+) on second messenger generation by, and on function of, rat distal colonic crypts.

METHODS

Calcium-sensing receptor activation was assessed in colonic epithelial cells and intact crypts freshly isolated from distal colon by monitoring intracellular IP(3) and Ca(2+) accumulation using radioimmunoassay and Fluo-3 fluorometry, respectively. Interactions of extracellular Ca(2+) and spermine on regulation of both basal and forskolin-stimulated fluid transport were measured in crypts microperfused in vitro.

RESULTS

Polyamine (spermine > spermidine > putrescine)-mediated enhancement of intracellular D-myo-inositol 1,4,5-trisphosphate (IP(3)) and Ca(2+) accumulation required extracellular Ca(2+), and the EC(50) for extracellular Ca(2+)-mediated activation of the calcium-sensing receptor was reduced by polyamines. Extracellular spermine modulated both basal and forskolin-stimulated fluid secretion in perfused colonic crypts, and the EC(50) for spermine-induced reduction in forskolin-stimulated fluid secretion was inversely dependent on extracellular Ca(2+) (Ca(2+)(o)).

CONCLUSIONS

The interactions of extracellular Ca(2+) and polyamines on second messenger accumulation and fluid secretion support a role for the luminal and basolateral calcium-sensing receptors in mediating some of the effects of polyamines on distal colonic epithelial cells.

Oxygen consumption and chloride secretion in rat distal colon isolated mucosa

The aerobic metabolic cost of chloride secretion was studied in rat distal colon isolated mucosa under several conditions by simultaneous measurement of short-circuit current and oxygen consumption under conditions that preserve vectorial ion transport. A low-chloride solution and the presence of bumetanide plus diphenylamine-2-carboxylate reduced short-circuit current by 75% and oxygen consumption by 25%. Ouabain decreased short-circuit current by 93% and oxygen consumption by 32%. Serotonin increased both variables by 59% and 33%, respectively. Bumetanide and diphenylamine-2-carboxylate reduced but did not abolish the effect of serotonin on short-circuit current and oxygen consumption. Changes in short-circuit current and oxygen consumption were linearly correlated under all conditions tested. It is concluded that, in the unstimulated rat distal colon epithelium, chloride secretion accounts for about 75% of ouabain-sensitive short-circuit current and oxygen consumption. Stimulated chloride secretion may demand over 40% of total oxygen consumption.

The future of GI and liver research: editorial perspectives: I. Visions of epithelial research

Epithelial cells are gatekeepers that sit at the interface between two compartments. By controlling the flow of molecules and information between two compartments, epithelial cells provide unique benefit to the body. This article provides a brief appraisal of our current knowledge about the functions of gastrointestinal epithelial cells as a functionally diverse set of cells mediating transepithelial transport and as a continually renewing layer of cells. The convergence of new methodologies in laser capture microdissection, microarray analyses, microscopic analyses, and generation of mutant animals provides an exciting template for future research.

New light shed on fluid formation in the seminiferous tubules of the rat

In this study the effects of perfusing isolated seminiferous tubules of the testes are reported for the first time. Initial perfusion studies (fast rate perfusion) resulted in gross morphological damage to the seminiferous tubules. The recorded transepithelial potential (V(t)) was close to 0 mV. Slow perfusion rates eliminated morphological damage to the perfused tubules. These tubules exhibited a V(t) of -5.4 +/- 1.8 mV which was significantly different (P < 0.0001) from tubules that were perfused at a fast rate. Additional non-perfusion electrophysiological experiments (oil-gap and agar probe techniques) provided the confirmation that tubules not morphologically compromised produced a higher V(t) which was not statistically different (P < 0.0001) from slowly perfused tubules. A revised hypothesis on fluid secretion is postulated. In brief, that the seminiferous tubule is solely responsible for the production of its luminal fluid. This hypothesis is contrary to the long standing 'Tuck' hypothesis which suggested that the source of luminal fluid in the seminiferous tubule originated from secretions of Sertoli cells as well as from distal testicular structures, e.g. the rete testis.

Apical Na+/H+ exchange near the base of mouse colonic crypts

Colonic crypts can absorb fluid, but the identity of the absorptive transporters remains speculative. Near the crypt base, the epithelial cells responsible for vectorial transport are relatively undifferentiated and often presumed to mediate only Cl- secretion. We have applied confocal microscopy in combination with an extracellular fluid marker [Lucifer yellow (LY)] or a pH-sensitive dye (2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein) to study mouse colonic crypt epithelial cells directly adjacent to the crypt base within an intact mucosal sheet. Measurements of intracellular pH report activation of colonocyte Na+/H+ exchange in response to luminal or serosal Na+. Studies with LY demonstrate the presence of a paracellular fluid flux, but luminal Na+ does not activate Na+/H+ exchange in the nonepithelial cells of the lamina propria, and studies with LY suggest that the fluid bathing colonocyte basolateral membranes is rapidly refreshed by serosal perfusates. The apical Na+/H+ exchange in crypt colonocytes is inhibited equivalently by luminal 20 microM ethylisopropylamiloride and 20 microM HOE-694 but is not inhibited by luminal 20 microM S-1611. Immunostaining reveals the presence of epitopes from NHE1 and NHE2, but not NHE3, in epithelial cells near the base of colonic crypts. Comparison of apical Na+/H+ exchange activity in the presence of Cl- with that in the absence of Cl- (substitution by gluconate or nitrate) revealed no evidence of the Cl--dependent Na+/H+ exchange that had been previously reported as the sole apical Na+/H+ exchange activity in the colonic crypt. Results suggest the presence of an apical Na+/H+ exchanger near the base of crypts with functional attributes similar to those of the cloned NHE2 isoform.

Channel-mediated water movement across enclosed or perfused mouse intrahepatic bile duct units

We previously reported the development of reproducible techniques for isolating and perfusing intact intrahepatic bile duct units (IBDUs) from rats. Given the advantages of transgenic and knockout mice for exploring ductal bile formation, we report here the adaptation of those techniques to mice and their initial application to the study of water transport across mouse intrahepatic biliary epithelia. IBDUs were isolated from livers of normal mice by microdissection combined with enzymatic digestion. After culture, isolated IBDUs sealed to form intact, polarized compartments, and a microperfusion system employing those isolated IBDUs developed. A quantitative image analysis technique was used to observe a rapid increase of luminal area when sealed IBDUs were exposed to a series of inward osmotic gradients reflecting net water secretion; the choleretic agonists secretin and forskolin also induced water secretion into IBDUs. The increase of IBDU luminal area induced by inward osmotic gradients and choleretic agonists was reversibly inhibited by HgCl2, a water channel inhibitor. With the use of a quantitative epifluorescence technique in perfused mouse IBDUs, a high osmotic water permeability (P(f) = 2.5-5.6 x 10(-2) cm/s) was found in response to osmotic gradients, further supporting the presence of water channels. These findings suggest that, as in the rat, water transport across intrahepatic biliary epithelia in mice is water channel mediated.

Expression of calcium-sensing receptor in rat colonic epithelium: evidence for modulation of fluid secretion

The calcium-sensing receptor (CaSR) is activated by extracellular calcium (Ca2+(o)) and mediates many of the known effects of extracellular divalent minerals on body cells. Both surface and crypt cells express CaSR transcripts and protein on both apical and basolateral surfaces. Raising Ca2+(o) elicited increases in intracellular calcium (Ca2+(o)) in both surface and crypt cells with an EC50 of 2 mM. The Ca-induced increase in Ca2+(i) was associated with increases in inositol 1,4,5-trisphosphate and eliminated by U-73129, an inhibitor of phosphatidylinositol-phospholipase C, as well as by thapsigargin. Other CaSR agonists, Gd3+ and neomycin, mimicked these Ca2+(o)-induced responses. Both luminal and bath Ca2+(o), Gd3+, and neomycin induced increases in Ca2+(i) in isolated perfused crypts. The stimulatory effect of forskolin on net fluid secretion in perfused crypts was abolished by increasing Ca2+(o) in either luminal or bath perfusates. Thus both apical and basolateral CaSR on crypt cells are functional and provide pathways modulating net intestinal fluid transport that may have important implications for the prevention and treatment of certain diarrheal diseases associated with elevated cAMP.

Alterations of the VIP-stimulated cAMP pathway in rat distal colon after abdominal irradiation

Ionizing radiation induces hyporesponsiveness of rat colonic mucosa to vasoactive intestinal peptide (VIP). Possible mechanisms responsible for this hyporesponsiveness of the cAMP communication pathway in rat colon were investigated. VIP- and forskolin-stimulated short-circuit current (I(sc)) responses were studied after a 10-Gy abdominal irradiation in Ussing chambers as well as in single, isolated crypts. Adenylyl cyclase (AC) activity and VIP receptor characteristics were determined in mucosal membrane preparations. In addition, alterations in crypt morphology were studied. Impaired secretory responses to VIP and forskolin were observed 4 days after irradiation (decrease of 80%). cAMP analog-stimulated I(sc) responses were unchanged. In isolated crypts, VIP- and forskolin-stimulated cAMP accumulation was markedly reduced by 80 and 50%, respectively. VIP-stimulated AC activity and VIP receptor number were decreased in membrane preparations. No major change of cellularity was associated with these functional alterations. In conclusion, the decreased secretory responses to VIP of rat colon are associated with reduced cAMP accumulation, decreased AC activity, and diminution of VIP receptor numbers without a marked decrease of crypt cell number.

Cloning and expression of a chloride-dependent Na+-H+ exchanger

Electroneutral Na(+)-H(+) exchange is present in virtually all cells, mediating the exchange of extracellular Na(+) for intracellular H(+) and, thus, plays an important role in the regulation of intracellular pH, cell volume, and transepithelial Na(+) absorption. Recent transport studies demonstrated the presence of a novel chloride-dependent Na(+)-H(+) exchange in the apical membrane of crypt cells of rat distal colon. We describe the cloning of a 2.5-kb full-length cDNA from rat distal colon that encodes 438 amino acids and has six putative transmembrane spanning domains. Of the 438 amino acids 375 amino acids at the N-terminal region are identical to Na(+)-H(+) exchange (NHE)-1 isoform with the remaining 63 amino acids comprising a completely novel C terminus. In situ hybridization revealed that this transcript is expressed in colonic crypt cells, whereas Northern blot analysis established the presence of its 2.5-kb mRNA in multiple tissues. Despite its much smaller size compared with all other known Na(+)-H(+) exchange isoforms, NHE-deficient PS120 fibroblasts stably transfected with this cDNA exhibited Na(+)-dependent intracellular pH recovery to an acid load that was chloride-dependent and inhibited both by 5-ethylisopropylamiloride, an amiloride analogue, and by 5'-nitro-2-(3-phenylproplyamino)benzoic acid, a Cl(-) channel blocker, but only minimally affected by 25 microm 3-methylsulfonyl-4piperidonbenzoylguanidine, an NHE-1 and NHE-2 isoform inhibitor. In contrast to other Na(+)-H(+) exchange isoforms in colonic epithelial cells, chloride-dependent Na(+)-H(+) exchange mRNA abundance was increased by dietary sodium depletion. Based on these results we predict that chloride-dependent Na(+)-H(+) exchange represents a new class of Na(+)-H(+) exchangers that may regulate ion transport in several organs.

Effects of radiation damage on intestinal morphology

The current flow of papers on intestinal structure, radiation science, and intestinal radiation response is reflected in the contents of this review. Multiparameter findings and changes in compartments, cells, or subcellular structure all contribute to the overall profile of the response. The well-recognized changes in proliferation, vessels, and fibrogenesis are accompanied by alterations in other compartments, such as neuroendocrine or immune components of the intestinal wall. The responses at the molecular level, such as in levels of hormones, cytokines, or neurotransmitters, are of fundamental importance. The intestine responds to localized radiation, or to changes in other organs that influence its structure or function: some structural parameters respond differently to different radiation schedules. Apart from radiation conditions, factors affecting the outcome include the pathophysiology of the irradiated subject and accompanying treatment or intervention. More progress in understanding the overall responses is expected in the next few years.

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.

Evidence of amiloride-sensitive fluid absorption in rat descending colonic crypts from fluorescence recovery of FITC-labelled dextran after photobleaching

1. Fluorescence recovery after photobleaching (FRAP) of fluorescein isothiocyanate (FITC)-labelled 10 and 250 kDa dextran (FITC dextran) in isolated rat descending colonic crypts was measured at 35 degrees C using laser scanning confocal microscopy. 2. FRAP of either 10 or 250 kDa FITC dextran in crypt lumens was almost complete within 2-3 min. 3. In the presence of amiloride (0.1 mM), or in the absence of Na+, the rate of FITC dextran uptake into the crypt lumens was reduced by 70-80 %. 4. The rate of fluid uptake into the crypt lumen, as estimated from the rate of total FITC dextran uptake into the crypt lumen and its adjacent pericryptal region after FRAP, was between 1.3 x 10(-3) and 1.7 x 10(-3) cm x s(-1). 5. Convective flow during FRAP was also determined from the initial rate of FITC dextran advance along the crypt lumen. This effect was almost completely blocked by amiloride (0.1 mM). 6. The permeability of 10 kDa FITC dextran across the descending colonic crypt wall was found to be higher than that of 250 kDa FITC dextran (3.7 (+/- 0.6) x 10(-5) and 1.8 (+/- 0.3) x 10(-6) cm x s(-1), respectively; n = 3 for both, P < 0.01). The permeability of the caecal crypt wall to 10 kDa dextran was higher than that of the descending crypt wall (2.03 (+/- 0.21) x 10(-5) cm x s(-1); n = 3, P < 0.025). 7. Simulation of the flow of Na+, water and FITC dextran into the crypt lumen and across the crypt wall and pericryptal sheath corroborates the observed parameters of water and Na+ flows.

Characterization of apical membrane Cl-dependent Na/H exchange in crypt cells of rat distal colon

A novel Cl-dependent Na/H exchange (Cl-NHE) has been identified in apical membranes of crypt cells of rat distal colon. The presence of Cl is required for both outward proton gradient-driven Na uptake in apical membrane vesicles (AMV) and Na-dependent intracellular pH recovery from an acid load in the crypt gland. The present study establishes that Cl-dependent outward proton gradient-driven (22)Na uptake 1) is saturated with increasing extravesicular Na concentration with a Michaelis constant (K(m)) for Na of approximately 24.2 mM; 2) is saturated with increasing outward H concentration gradient with a hyperbolic curve and a K(m) for H of approximately 1.5 microM; 3) is inhibited by the Na/H exchange (NHE) inhibitors amiloride, ethylisopropylamiloride, and HOE-694 with an inhibitory constant (K(i)) of approximately 480.2, 1.1, and 9.5 microM, respectively; 4) is inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, an anion exchange inhibitor at low concentration and a Cl channel blocker at high dose, and by 5-nitro-2(3-phenylpropylamino)benzoic acid, a Cl channel blocker, with a K(i) of approximately 280.6 and 18.3 microM, respectively; and 5) substantially stimulated Cl-NHE activity by dietary Na depletion, which increases plasma aldosterone and inhibits NHE in surface cell AMV. These properties of Cl-NHE are distinct from those of NHE1, NHE2, and NHE3 isoforms that are present in colonic epithelial cells; thus these results suggest that the colonic crypt cell Cl-NHE is a novel NHE isoform.