Effect of short-chain fatty acids on cell volume and intracellular pH in rat distal colon

Essential Oils as an Intervention Strategy to Reduce Campylobacter in Poultry Production: A Review

Campylobacter is a major foodborne pathogen and can be acquired through consumption of poultry products. With 1.3 million United States cases a year, the high prevalence of Campylobacter within the poultry gastrointestinal tract is a public health concern and thus a target for the development of intervention strategies. Increasing demand for antibiotic-free products has led to the promotion of various alternative pathogen control measures both at the farm and processing level. One such measure includes utilizing essential oils in both pre- and post-harvest settings. Essential oils are derived from plant-based extracts, and there are currently over 300 commercially available compounds. They have been proposed to control Campylobacter in the gastrointestinal tract of broilers. When used in concentrations low enough to not influence sensory characteristics, essential oils have also been proposed to decrease bacterial contamination of the poultry product during processing. This review explores the use of essential oils, particularly thymol, carvacrol, and cinnamaldehyde, and their role in reducing Campylobacter concentrations both pre- and post-harvest. This review also details the suggested mechanisms of action of essential oils on Campylobacter.

Involvement of butyrate in electrogenic K+ secretion in rat rectal colon

Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, are synthesized from dietary carbohydrates by colonic bacterial fermentation. These SCFAs supply energy, suppress cancer, and affect ion transport. However, their roles in ion transport and regulation in the intracellular environment remain unknown. In order to elucidate the roles of SCFAs, we measured short-circuit currents (I_SC) and performed RT-PCR and immunohistochemical analyses of ion transporters in rat rectal colon. The application of 30 mM butyrate shifted I_SC in a negative direction, but did not attenuate the activity of epithelial Na⁺ channels (ENaC). The application of bumetanide, a Na⁺-K⁺-2Cl⁻ cotransporter inhibitor, to the basolateral side reduced the negative I_SC shift induced by butyrate. The application of XE991, a KCNQ-type K⁺ channel inhibitor, to the apical side decreased the I_SC shift induced by butyrate in a dose-dependent manner. The I_SC shift was independent of HCO₃⁻ and insensitive to ibuprofen, an SMCT1 inhibitor. The mucosa from rat rectal colon expressed mRNAs of H⁺-coupled monocarboxylate transporters (MCT1, MCT4, and MCT5, also referred to as SLC16A1, SLC16A3, and SLC16A4, respectively). RT-PCR and immunofluorescence analyses demonstrated that KCNQ2 and KCNQ4 localized to the apical membrane of surface cells in rat rectal colon. These results indicate that butyrate, which may be transported by H⁺-coupled monocarboxylate transporters, activates K⁺ secretion through KCNQ-type K⁺ channels on the apical membrane in rat rectal colon. KCNQ-type K⁺ channels may play a role in intestinal secretion and defense mechanisms in the gastrointestinal tract.

Organic Acids and Potential for Modifying the Avian Gastrointestinal Tract and Reducing Pathogens and Disease

Recently, antibiotics have been withdrawn from some poultry diets; leaving the birds at risk for increased incidence of dysbacteriosis and disease. Furthermore, mortalities occurring from disease contribute between 10 to 20% of production cost in developed countries. Currently, numerous feed supplements are being proposed as effective antibiotic alternatives in poultry diets, such as prebiotics, probiotics, acidic compounds, competitive exclusion products, herbs, essential oils, and bacteriophages. However, acidic compounds consisting of organic acids show promise as antibiotic alternatives. Organic acids have demonstrated the capability to enhance poultry performance by altering the pH of the gastrointestinal tract (GIT) and consequently changing the composition of the microbiome. In addition, organic acids, by altering the composition of the microbiome, protect poultry from pH-sensitive pathogens. Protection is further provided to poultry by the ability of organic acids to potentially enhance the morphology and physiology of the GIT and the immune system. Thus, the objective of the current review is to provide an understanding of the effects organic acids have on the microbiome of poultry and the effect those changes have on the prevalence of pathogens and diseases in poultry. From data reviewed, it can be concluded that the efficacy of organic acids on shifting microbiome composition is limited to the time of administration, the composition of the organic acid product, and the current health conditions of poultry.

Re-print of "Intestinal luminal nitrogen metabolism: role of the gut microbiota and consequences for the host"

Alimentary and endogenous proteins are mixed in the small intestinal lumen with the microbiota. Although experimental evidences suggest that the intestinal microbiota is able to incorporate and degrade some of the available amino acids, it appears that the microbiota is also able to synthesize amino acids raising the view that amino acid exchange between the microbiota and host can proceed in both directions. Although the net result of such exchanges remains to be determined, it is likely that a significant part of the amino acids recovered from the alimentary proteins are used by the microbiota. In the large intestine, where the density of bacteria is much higher than in the small intestine and the transit time much longer, the residual undigested luminal proteins and peptides can be degraded in amino acids by the microbiota. These amino acids cannot be absorbed to a significant extent by the colonic epithelium, but are precursors for the synthesis of numerous metabolic end products in reactions made by the microbiota. Among these products, some like short-chain fatty acids and organic acids are energy substrates for the colonic mucosa and several peripheral tissues while others like sulfide and ammonia can affect the energy metabolism of colonic epithelial cells. More work is needed to clarify the overall effects of the intestinal microbiota on nitrogenous compound metabolism and consequences on gut and more generally host health.

Gut microbial activity, implications for health and disease: the potential role of metabolite analysis

Microbial metabolism of proteins and amino acids by human gut bacteria generates a variety of compounds including phenol, indole, and sulfur compounds and branched chain fatty acids, many of which have been shown to elicit a toxic effect on the lumen. Bacterial fermentation of amino acids and proteins occurs mainly in the distal colon, a site that is often fraught with symptoms from disorders including ulcerative colitis (UC) and colorectal cancer (CRC). In contrast to carbohydrate metabolism by the gut microbiota, proteolysis is less extensively researched. Many metabolites are low molecular weight, volatile compounds. This review will summarize the use of analytical methods to detect and identify compounds in order to elucidate the relationship between specific dietary proteinaceous substrates, their corresponding metabolites, and implications for gastrointestinal health.

News from the end of the gut--how the highly segmental pattern of colonic HCO₃⁻ transport relates to absorptive function and mucosal integrity

A number of transport mechanisms in the colonic epithelium contribute to HCO₃⁻ movement across the apical and basolateral membranes, but this ion has been largely regarded as a by-product of the transport functions it is involved in, such as NaCl or short chain fatty acid (SCFA) absorption. However, emerging data points to several specific roles of HCO₃⁻ for colonic epithelial physiology, including pH control in the colonic surface microenvironment, which is important for transport and immune functions, as well as the secretion and the rheological properties of the mucus gel. Furthermore, recent studies have demonstrated that colonic HCO₃⁻ transporters are expressed in a highly segmental as well as species-specific manner. This review summarizes recently gathered information on the functional anatomy of the colon, the roles of HCO₃⁻ in the colonic epithelium, colonic mucosal integrity, and the expression and function of HCO₃⁻ transporting mechanisms in health and disease.

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.

Ischemic post-conditioning to counteract intestinal ischemia/reperfusion injury

Intestinal ischemia is a severe disorder with a variety of causes. Reperfusion is a common occurrence during treatment of acute intestinal ischemia but the injury resulting from ischemia/reperfusion (IR) may lead to even more serious complications from intestinal atrophy to multiple organ failure and death. The susceptibility of the intestine to IR-induced injury (IRI) appears from various experimental studies and clinical settings such as cardiac and major vascular surgery and organ transplantation. Whereas oxygen free radicals, activation of leukocytes, failure of microvascular perfusion, cellular acidosis and disturbance of intracellular homeostasis have been implicated as important factors in the pathogenesis of intestinal IRI, the mechanisms underlying this disorder are not well known. To date, increasing attention is being paid in animal studies to potential pre- and post-ischemia treatments that protect against intestinal IRI such as drug interference with IR-induced apoptosis and inflammation processes and ischemic pre-conditioning. However, better insight is needed into the molecular and cellular events associated with reperfusion-induced damage to develop effective clinical protection protocols to combat this disorder. In this respect, the use of ischemic post-conditioning in combination with experimentally prolonged acidosis blocking deleterious reperfusion actions may turn out to have particular clinical relevance.

Dietary pectin up-regulates monocaboxylate transporter 1 in the rat gastrointestinal tract

This work was undertaken to study the effect of pectin feeding on the expression level, cellular localization and functional activity of monocarboxylate transporter 1 (MCT1) in the gastrointestinal tract of rats. The results indicated that MCT1 protein level was significantly increased along the entire length of the gastrointestinal tract of pectin-fed rats in comparison with control animals. Immunohistochemical analysis revealed an increase in MCT1 in the stratified squamous epithelia of the forestomach as well as in the basolateral membranes of the cells lining the gastric pit of the glandular stomach of pectin-fed rats when compared with control animals. The parietal cells, which showed barely any or no detectable MCT1 in the control group, exhibited a strong intensity of MCT1 on the basolateral membranes in pectin-fed rats. In the small intestine of pectin-fed rats, strong immunopositivity for MCT1 was detected in the brush border and basolateral membranes of the absorptive enterocytes lining the entire villi, while in control rats, weak reactivity was detected on the brush border membrane in a few absorptive enterocytes in the villus tip. In the large intestine of control animals, MCT1 was detected on the basolateral membranes of the epithelia lining the caecum and colon. This staining intensity was markedly increased in pectin-fed rats, along with the appearance of strong reactivity for MCT1 on the apical membranes of the surface and crypt epithelia of caecum and colon. Our results also showed that MCT1 co-localizes with its chaperone, basigin (CD147), in the rat gastrointestinal tract, and that the pectin feeding increased the expression of CD147. In vivo functional studies revealed an enhanced acetate absorption in the colon of pectin-fed in comparison with control animals. We conclude that MCT1 is up-regulated along the gastrointestinal tract of pectin-fed rats, which might represent an adaptive response to the increased availability of its substrates.

Effects of amino acid-derived luminal metabolites on the colonic epithelium and physiopathological consequences

Depending on the amount of alimentary proteins, between 6 and 18 g nitrogenous material per day enter the large intestine lumen through the ileocaecal junction. This material is used as substrates by the flora resulting eventually in the presence of a complex mixture of metabolites including ammonia, hydrogen sulfide, short and branched-chain fatty acids, amines; phenolic, indolic and N-nitroso compounds. The beneficial versus deleterious effects of these compounds on the colonic epithelium depend on parameters such as their luminal concentrations, the duration of the colonic stasis, the detoxication capacity of epithelial cells in response to increase of metabolite concentrations, the cellular metabolic utilization of these metabolites as well as their effects on colonocyte intermediary and oxidative metabolism. Furthermore, the effects of metabolites on electrolyte movements through the colonic epithelium must as well be taken into consideration for such an evaluation. The situation is further complicated by the fact that other non-nitrogenous compounds are believed to interfere with these various phenomenons. Finally, the pathological consequences of the presence of excessive concentrations of these compounds are related to the short- and, most important, long-term effects of these compounds on the rapid colonic epithelium renewing and homeostasis.

Effect of butyrate on membrane potential, ionic currents and intracellular Ca2+ concentration in cultured rat myenteric neurones

Myenteric neurones from 1-10-day-old rats were isolated from the small and large intestine by enzymatic digestion with collagenase. Single cells were collected and kept in culture for up to 1 week. After 1-5 days in culture, membrane potential and ionic currents were measured with the whole-cell patch-clamp technique. The intracellular Ca2+ concentration was measured with the fura-2 method. The short-chain fatty acid butyrate (50 mmol L-1) induced a reversible hyperpolarization of the myenteric neurones by about 10 mV. This hyperpolarization was concomitant with an inhibition of a TTX-sensitive Na+ current. The hyperpolarization could be suppressed by intracellular application of Cs+, a nonselective K+ channel blocker. Fura-2 experiments revealed that butyrate induced an increase of the intracellular Ca2+ concentration. The butyrate response was suppressed by thapsigargin, indicating that butyrate stimulates the release of intracellular Ca2+. This release is responsible for the voltage response, because intracellular chelation of Ca2+ inhibited the butyrate induced hyperpolarization. Consequently, butyrate acts on enteric neurones by releasing Ca2+ from intracellular stores with the consequence of the activation of K+ channels, followed by a hyperpolarization.

Intracellular pH regulation in colonocytes of rat proximal colon

The regulation of intracellular pH (pH(i)) in colonocytes of the rat proximal colon has been investigated using the pH-sensitive dye BCECF and compared with the regulation of pH(i) in the colonocytes of the distal colon. The proximal colonocytes in a HEPES-buffered solution had pH(i)=7.24+/-0.04 and removal of extracellular Na(+) lowered pH(i) by 0.24 pH units. Acid-loaded colonocytes by an NH(3)/NH(4)(+) prepulse exhibited a spontaneous recovery that was partially Na(+)-dependent and could be inhibited by ethylisopropylamiloride (EIPA). The Na(+)-dependent recovery rate was enhanced by increasing the extracellular Na(+) concentration and was further stimulated by aldosterone. In an Na(+)- and K(+)-free HEPES-buffered solution, the recovery rate from the acid load was significantly stimulated by addition of K(+) and this K(+)-dependent recovery was partially blocked by ouabain. The intrinsic buffer capacity of proximal colonocytes at physiological pH(i) exhibited a nearly 2-fold higher value than in distal colonocytes. Butyrate induced immediate colonocyte acidification that was smaller in proximal than in distal colonocytes. This acidification was followed by a recovery phase that was both EIPA-sensitive and -insensitive and was similar in both groups of colonocytes. In a HCO(3)(-)/CO(2)-containing solution, pH(i) of the proximal colonocytes was 7.20+/-0.04. Removal of external Cl(-) caused alkalinization that was inhibited by DIDS. The recovery from an alkaline load induced by removal of HCO(3)(-)/CO(2) from the medium was Cl(-)-dependent, Na(+)-independent and blocked by DIDS. Recovery from an acid load in EIPA-containing Na(+)-free HCO(3)(-)/CO(2)-containing solution was accelerated by addition of Na(+). Removal of Cl(-) inhibited the effect of Na(+). In summary, the freshly isolated proximal colonocytes of rats express Na(+)/H(+) exchanger, H(+)/K(+) exchanger ((H(+)-K(+))-ATPase) and Na(+)-dependent Cl(-)/HCO(3)(-) exchanger that contribute to acid extrusion and Na(+)-independent Cl(-)/HCO(3)(-) exchanger contributing to alkali extrusion. All of these are likely involved in the regulation of pH(i) in vivo. Proximal colonocytes are able to maintain a more stable pH(i) than distal cells, which seems to be facilitated by their higher intrinsic buffer capacity.

Na+-dependent recovery of intracellular pH from acid loading in mouse colonic crypt cells

The membrane transport mechanism for regulating the intracellular pH value (pHi) was investigated in mouse distal colon crypt cells. pHi was measured by microfluorometry in an isolated crypt fragment loaded with the pH-sensitive fluoroprobe, 2',7'-bis-(2-carboxyethyl)-5-(6) carboxyfluorescein. The pHi recovery process after acid loading induced by a 40 mM NH4Cl prepulse was almost totally dependent on Na+ in both the presence and absence of CO2/HCO3- in the perfusion solution. In the CO2/HCO3(-)-free, HEPES-buffered solution, amiloride partially inhibited the pHi recovery rate from acid loading with an ED50 value of 15 microM and maximum inhibition of 83%. In a CO2/HCO3- solution, amiloride inhibited the pHi recovery rate with an ED50 value of 18 microM, which was similar to that in the HEPES-buffered solution, while the rate of pHi recovery remaining in the presence of the maximum effective concentration of amiloride was significantly larger than that in the HEPES-buffered solution. The Na+-dependent pHi recovery from the acid loading was significantly less (by 18%) in the presence of forskolin. These results suggest that the pHi recovery from acid loading was mediated by 1) amiloride-sensitive Na+/H+ exchanger, 2) the amiloride-insensitive Na+/H+ exchanger, and 3) the Na+- and HCO3(-)-dependent acid extruder. The pHi recovery could be inhibited by cAMP.

pH heterogeneity at intracellular and extracellular plasma membrane sites in HT29-C1 cell monolayers

In the colonic mucosa, short-chain fatty acids change intracellular pH (pH(i)) and extracellular pH (pH(e)). In this report, confocal microscopy and dual-emission ratio imaging of carboxyseminaphthorhodofluor-1 were used for direct evaluation of pH(i) and pH(e) in a simple model epithelium, HT29-C1 cells. Live cell imaging along the apical-to-basal axis of filter-grown cells allowed simultaneous measurement of pH in the aqueous environment near the apical membrane, the lateral membrane, and the basal membrane. Subapical cytoplasm reported the largest changes in pH(i) after isosmotic addition of 130 mM propionate or 30 mM NH(4)Cl. In resting cells and cells with an imposed acid load, lateral membranes had pH(i) values intermediate between the relatively acidic subapical region (pH 6.3-6.9) and the relatively alkaline basal pole of the cells (pH 7.4-7.1). Transcellular pH(i) gradients were diminished or eliminated during an induced alkaline load. Propionate differentially altered pH(e) near the apical membrane, in lateral intracellular spaces between adjacent cells, and near the basal membrane. Luminal or serosal propionate caused alkalinization of the cis compartment (where propionate was added) but acidification of the trans compartment only in response to luminal propionate. Addition of NH(4)Cl produced qualitatively opposite pH(e) excursions. The microscopic values of pH(i) and pH(e) can explain a portion of the selective activation of polarized Na/H exchangers observed in HT29-C1 cells in the presence of transepithelial propionate gradients.

Effect of butyrate on paracellular permeability in rat distal colonic mucosa ex vivo

BACKGROUND AND AIMS

The effects of butyrate on colonic epithelial barrier function are poorly understood. The aim of this study was to examine the short-term effects of butyrate on paracellular permeability of rat distal colonic epithelium.

METHODS

Mucosa mounted in Ussing chambers was treated with butyrate (1-10 mmol/L) for 4 h. Transepithelial conductance, [51Cr]-EDTA flux, mucosal brush border hydrolase activity and epithelial kinetics, using proliferating cell nuclear antigen (PCNA) staining, were measured.

RESULTS

On exposure to butyrate (10 mmol/L, but not 1 or 5 mmol/L), transepithelial conductance was 65 +/- 2% higher (mean +/- SEM; n = 8, P < 0.05, paired t-test) and the rate coefficient for [51Cr]-EDTA flux was 65 +/- 25% higher (P = 0.03) than those of control tissue. Histologically, the epithelium exhibited no signs of injury, but butyrate-treated tissue exhibited interstitial oedema consistent with water uptake in association with butyrate absorption. Butyrate caused a reduction in crypt column height to 30.6 +/- 1.6 cells from 33.4 +/- 1.8 cells in controls (n = 10, P < 0.03), but the number of cells per crypt column staining with PCNA was unchanged. Butyrate significantly reduced the mucosal activities of alkaline phosphatase by 40 +/- 16%, maltase by 54 +/- 12% and dipeptidyl peptidase IV by 41 +/- 14%.

CONCLUSIONS

Acute exposure to butyrate increased paracellular permeability in rat distal colon. The mechanism involved may relate to the loss of differentiated surface epithelial cells, or as a physiological response to Na+-coupled butyrate uptake.

K+ and Cl- conductances in the distal colon of the rat

1. K+ and Cl- conductances and their putative regulation have been characterized in the rat colonic epithelium by Ussing-chamber experiments, whole-cell and single-channel patch-clamp recordings. 2. The apical Cl- conductance is under the control of intracellular cAMP. An increase in the concentration of this second messenger induces transepithelial Cl- secretion due to the activation of an apical 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB)- and glibenclamide-sensitive Cl- conductance. 3. In addition to the apical Cl- conductance, the basolateral membrane is equipped with Cl- channels. They are stimulated by cell swelling and play a role in cell volume regulation and transepithelial Cl- absorption. 4. The basolateral K+ conductance is under the dominant control of intracellular Ca2+. An increase in the cytosolic Ca2+ concentration leads to the opening of basolateral K+ channels, which causes a hyperpolarization of the cell membrane, indirectly supporting Cl- secretion owing to an increase in the driving force for Cl- exit. The predominant effect of cAMP on the basolateral K+ conductance is an inhibitory one, probably due to a decrease in the intracellular Ca2+ concentration. 5. The apical K+ conductance, which is involved in transepithelial K+ secretion, is stimulated by an increase in the intracellular Ca2+ concentration. 6. The differential regulation of apical and basolateral ion conductances in the epithelium of the rat distal colon provides an interesting example for the mechanisms underlying vectorial transport of ions across polarized cells.

Nonionic diffusion of short-chain fatty acids across rat colon

Short-chain fatty acid (SCFA) transport across the colon may occur by nonionic diffusion and/or via apical membrane SCFA-/HCO3- exchange. To examine the relative importance of these processes, stripped segments of rat (Ratus ratus) proximal and distal colon were studied in Ussing chambers, and the unidirectional fluxes of radiolabeled SCFA butyrate, propionate, or weakly metabolized isobutyrate were measured. In N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) or 1 or 5 mM HCO3- Ringer, decreases in mucosal pH stimulated mucosal-to-serosal flux (Jm-->s) of all SCFA, decreases in serosal pH stimulated serosal-to-mucosal flux (Js-->m), and bilateral pH decreases stimulated both fluxes equally. These effects were observed whether the SCFA was present on one or both sides of the tissue, in both proximal and distal colon, in the absence of luminal Na+, and in the presence of either luminal or serosal ouabain. Changes in intracellular pH or intracellular [HCO3-] did not account for the effects of extracellular pH. Luminal Cl- removal, to evaluate the role of apical membrane Cl-/SCFA- exchange, had no effect on Jm-->s but decreased Js-->m 32% at pH 6.5 and 22% at 7.2. Increasing SCFA concentration from 1 to 100 mM, at pH 6.4 or 7.4, caused a linear increase in Jm-->s. We conclude that SCFA are mainly transported across the rat colon by nonionic diffusion.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Effects of short-chain fatty acids on cell volume regulation and chloride transport in the rat distal colon

Superfusion of isolated crypts from rat colon with sodium butyrate-containing solutions induced an amiloride-sensitive swelling of the cells at the upper one-third of the crypt. In HCO3(-)-containing buffer, swelling was followed by regulatory volume decrease, which was inhibited by K+ and C- channel blockers and inhibitors of leukotriene synthesis. Whole-cell patch-clamp recordings revealed that butyrate induced a membrane depolarization, which was dependent on Cl- and was accompanied by an increase in membrane inward current, indicating an increase in Cl- conductance. Membrane outward (K+) current, however, behaved inconsistently, suggesting an activation of swelling-induced K+ currents, but an inhibition of pH-sensitive K+ currents due to the cellular acidification. Cell-attached patch-clamp recordings showed an activation of basolateral Cl- channels by butyrate. The lipoxygenase inhibitor, NDGA (nordihydroguaiaretic acid), inhibited the butyrate response and even reversed it into a slight hyperpolarization indicating that the butyrate-induced Cl- channels, but not the K+ channels, are stimulated by a leukotriene. Short-chain fatty acids concentration-dependently decreased short-circuit current (Isc). The decrease in Isc was diminished by a Cl- channel blocker, NPPB (5-nitro-2-[3-phenylpropylamino]-benzoate), and a lipoxygenase inhibitor, NDGA. Butyrate stimulated the mucosa to serosa fluxes (Jms) of Na+ and Cl-. The effect on J(ms)Cl was blocked by NPPB or NDGA. The stimulation of J(ms)Cl correlated with the degree of metabolism of the short-chain fatty acid. Consequently, two factors seem to be responsible for the stimulation of Cl- absorption by short-chain fatty acids: (a) the intracellular production of HCO3- during the oxidation of short-chain fatty acids as substrate for the apical Cl-/HCO3- exchanger, and (b) the activation of volume-sensitive basolateral Cl- channels.

Transepithelial SCFA gradients regulate polarized Na/H exchangers and pH microdomains in colonic epithelia

Short chain fatty acids (SCFAs) stimulate electroneutral sodium absorption by activation of apical Na/H exchange in colonocytes. It is often assumed that activation of Na/H exchange is via an intracellular acidification caused by SCFA uptake. These lecture notes review shortcomings in this model of SCFA-stimulated sodium absorption, revealed by recent reports in the literature. This is supplemented by information generated in our laboratory using both a tissue culture model of colonocytes (HT29-C1 cells) and a native tissue preparation (mouse distal colonic mucosa). In both preparations, evidence suggests that physiologic SCFA gradients may generate pH heterogeneity in aqueous microdomains near the plasma membrane of colonocytes. Finally, direct observation of such extracellular microdomains with confocal microscopy is used to support a new model, in which pH microdomains play an important role in regulating both SCFA fluxes and sodium absorption.

Diet and carcinogen alter luminal butyrate concentration and intracellular pH in isolated rat colonocytes

A 2 x 2 factorial experiment was conducted to examine the effects of two different dietary fibers and carcinogen treatment on colonic luminal short-chain fatty acid (SCFA) concentrations and intracellular pH (pHi) in rats. Twenty-four male Sprague-Dawley rats were divided into four groups, injected with a carcinogen [azoxymethane (AOM)] or normal saline (Sal), and fed one of two diets differing only in the type of dietary fiber [cellulose (Cell) or pectin (Pect)]. After 38 weeks of consuming these diets, the rats were euthanized, luminal contents were collected for analysis of SCFA concentrations, and colonocytes were isolated from the proximal and distal colon for subsequent determination of pHi. Changes in pHi after the addition of exogenous sodium butyrate to the culture medium were also tested. The highest concentrations of SCFAs were produced by the control rats (saline injected) consuming the pectin diet. Luminal butyrate concentrations were reduced in three of four colonic segments of carcinogen-injected groups [proximal and distal cellulose (Prox Cell and Dist Cell) and distal pectin (Dist Pect)] compared with saline controls. The pHi was consistently higher in colonocytes isolated from carcinogen-injected rats (Prox Cell/AOM = 6.95 vs. Prox Cell/Sal = 6.65, Prox Pect/AOM = 6.75 vs. Prox Pect/Sal = 6.65, Dist Cell/AOM = 6.94 vs. Dist Cell/AOM = 6.85, Dist Pect/AOM = 6.92 vs. Dist Pect/Sal = 6.79) than in cells from saline-injected rats. Furthermore, in the majority of rats, pHi was lower in the proximal than in the distal colon. Addition of butyrate to cultured colonocytes consistently lowered pHi, but the effect was more pronounced in the carcinogen-injected animals. These data identify changes that occur intraluminally and intracellularly in colons of rats injected with AOM and suggest that, during tumorigenesis, alterations in butyrate production and basic colonocyte physiology may play an important role in the process.

Short-chain fatty acids inhibit cAMP-mediated chloride secretion in rat colon

Butyrate stimulates salt absorption in mammalian colon. We examined whether butyrate also affects Cl- secretion. Mucosal segments of distal colon of male Sprague-Dawley rats and T84 cells were studied in Ussing chambers. In control colon, 1 mM dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) increased short-circuit current (Isc) and serosal-to-mucosal Cl- flux (JsmCl) by 3.2 +/- 0.8 and 2.9 +/- 0.8 mueq.cm-2.h-1, respectively. Mucosal or serosal 25 mM butyrate prevented DBcAMP-induced increases in Isc and JsmCl. Four and eight millimolar butyrate caused half-maximal inhibition of the increases in JsmCl and Isc, respectively. Butyrate also inhibited basal JsmCl (by 2.0 +/- 0.4 mueq.cm-2.h-1) but not carbachol-mediated Cl- secretion. The relative inhibitory potency at 25 mM of other short-chain fatty acids (SCFA) paralleled their degree of cellular metabolism: butyrate > acetate = propionate > isobutyrate. At 25 mM, all SCFA reduced mucosal intracellular pH (pHi) transiently by 0.1 pH unit. In intact T84 cells, 50 mM butyrate inhibited the DBcAMP-induced rise in Isc by 55%. In T84 cells with nystatin-permeabilized basolateral membranes, butyrate inhibited the increase in Isc by 82%. We conclude that butyrate inhibits basal and cAMP-mediated Cl- secretion by a mechanism independent of pHi, possibly located at the apical membrane.

Effects of extracellular pH on intracellular pH-regulation and growth in a human colon carcinoma cell-line

Mechanisms of intracellular pH (pHi) regulation seem to be involved in cellular growth and cell division. Little is known about how extracellular acidosis, known to occur in central regions of solid tumors, or alkaline conditions affect pHi regulation in colonic tumors. pHi changes in the colonic adenocarcinoma cell-line SW-620 were recorded by spectrofluorimetric monitoring of the pH-sensitive, fluorescent dye BCECF, and proliferative activity was assessed by [3H]thymidine uptake. Resting pHi in Hepes-buffered solution was 7.53 +/- 0.01 (n = 36). Both 1 mM amiloride and Na(+)-free solution inhibited pHi recovery from acidification and decreased pHi in resting cells. In HCO3-/CO2-buffered media resting pH1 was 7.42 +/- 0.01 (n = 36). Recovery from acidification was Na(+)-dependent, CI(-)-independent, and only partially blocked by 1 mM amiloride. In the presence of amiloride and 200 microM H2DIDS pHi recovery was completely inhibited. In Na(+)-free solution pHi decreased from 7.44 +/- 0.04 to 7.29 +/- 0.03 (n = 6) and no alkalinization was observed in CI(-)-free medium. Addition of 5 microM tributyltin bromide (an anion/OH-exchange ionophore) caused pHi to decrease from 7.43 +/- 0.05 to 7.17 +/- 0.08 (n = 5). The effects of pH0 on steady-state pHi, pHi recovery from acidification and proliferative activity after 48 h were investigated by changing buffer [CO2] and [HCO3-]. In general, increases in pH0 between 6.7 and 7.4 increased pHi recovery, steady-state pHi and growth rates. In summary, SW-620 cells have a resting pHi > 7.4 at 25 degrees C, which is higher than other intestinal cells. Acid extrusion in physiological bicarbonate media is accomplished by a pHi-sensitive Na+/H+ exchanger and a pHi-insensitive Na(+)-HCO3-cotransporter, both of which are operational in control cells at the resting pHi. No evidence for activity of a CI-/HCO3- exchanger was found in these cells, which could account for the high pHi observed and may explain why the cells continue to grow in acidic tumor environments.

Dissociation of colonic apical Na/H exchange activity from bulk cytoplasmic pH

Intracellular acidification by stimuli rather than CO2 fails to stimulate colonic apical Na/H ex-change and Na absorption. We examined whether Na absorption could be stimulated in the absence of changes in cytoplasmic pH (pHi). Distal colon of male Sprague-Dawley rats was used for pHi measurements with 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and for flux measurements in Ussing chambers. In 21 mM HCO3-Ringer, increasing PCO2 from 20 to 70 mmHg decreased pHi from 7.51 to 7.03 and increased net Na flux (JnetNa) from 4.2 +/- 0.4 to 6.8 +/- 0.6 mu eq.cm-2.h-1. Similar increases in JnetNa occurred in the absence of mucosal CI and in the presence of phalloidin to inhibit microfilaments or penzolamide to inhibit membrane-bound carbonic anhydrase. sohydric increases in Pco2 did not alter pHi but stimulated JnetNa from 5.1 +/- 0.6 to 7.2 +/- 0.8 mu eq.cm-2.h-1. Carbonyl cyanide m-chlorophenylhydrazone (CCCP) decreased pHi from 7.45 to 7.35 but did not stimulate JnetNa. Butyrate (25 mM) decreased pHi from 7.15 to 7.02 with recovery to baseline within 6 min; however, JnetNa increased by 2.2 mu eq.cm-2.h-1 for 60 min. We conclude that apical Na/H exchange activity is unresponsive to changes in bulk pHi and is independent of Cl/HCO3 exchange, microfilaments, and membrane-bound carbonic anhydrase. The presence of an H-tight, CO2, and butyrate-permeable subapical domain is postulated.

Fluid absorption in isolated perfused colonic crypts

A spatial segregation of ion transport processes between crypt and surface epithelial cells is well-accepted and integrated into physiological and pathophysiological paradigms of small and large intestinal function: Absorptive processes are believed to be located in surface (and villous) cells, whereas secretory processes are believed to be present in crypt cells. Validation of this model requires direct determination of fluid movement in intestinal crypts. This study describes the adaptation of techniques from renal tubule microperfusion to hand-dissect and perfuse single, isolated crypts from rat distal colon to measure directly fluid movement. Morphologic analyses of the isolated crypt preparation revealed no extraepithelial cellular elements derived from the lamina propria, including myofibroblasts. In the basal state, crypts exhibited net fluid absorption (mean net fluid movement = 0.34 +/- 0.01 nl.mm-1.min-1), which was Na+ and partially HCO3- dependent. Addition of 1 mM dibutyryl-cyclic AMP, 60 nM vasoactive intestinal peptide, or 0.1 mM acetylcholine to the bath (serosal) solution reversibly induced net fluid secretion (net fluid movement approximately -0.35 +/- 0.01 nl.mm-1.min-1). These observations permit speculation that absorption is a constitutive transport function in crypt cells and that secretion by crypt cells is regulated by one or more neurohumoral agonists that are released in situ from lamina propria cells. The functional, intact polarized crypt described here that both absorbs and secretes will permit future studies that dissect the mechanisms that govern fluid and electrolyte movement in the colonic crypt.

An Na(+)-independent short-chain fatty acid transporter contributes to intracellular pH regulation in murine colonocytes

Short-chain fatty acids (SCFAs) are the major anions in the colonic lumen. Experiments studied how intracellular pH (pHi) of isolated colonocytes was affected by exposure to SCFAs normally found in the colon. Isolated crypt fragments were loaded with SNARF-1 (a fluorescent dye with pH-sensitive excitation and emission spectra) and studied in a digital imaging microscope. Intracellular pH was measured in individual colonocytes as the ratio of fluorescence intensity in response to alternating excitation wavelengths (575/505 nm). After exposure to 65 mM acetate, propionate, n-butyrate, or iso-butyrate in isosmotic Na(+)-free media (substituted with tetramethylammonia), all colonocytes acidified rapidly and then > 90% demonstrated a pHi alkalinization (Na(+)-independent pHi recovery). Upon subsequent removal of the SCFA, pHi alkalinized beyond the starting pHi (a pHi overshoot). Using propionate as a test SCFA, experiments demonstrate that the acidification and pHi overshoot are explained by transmembrane influx and efflux of nonionized SCFA, respectively. The basis for the pHi overshoot is shown to be accumulation of propionate during pHi alkalinization. The Na(+)-independent pHi recovery (a) demonstrates saturable propionate activation kinetics; (b) demonstrates substrate specificity for unmodified aliphatic carbon chains; (c) occurs after exposure to SCFAs of widely different metabolic activity, (d) is electroneutral; and (e) is not inhibited by changes in the K+ gradient, Cl- gradient or addition of the anion transport inhibitors DIDS (1 mM), SITS (1 mM), alpha-cyano-4-hydroxycinnamate (4 mM), or probenicid (1 mM). Results suggest that most mouse colonocytes have a previously unreported SCFA transporter which mediates Na(+)-independent pHi recovery.

Extracellular pH regulation in microdomains of colonic crypts: effects of short-chain fatty acids

It has been suggested that transepithelial gradients of short-chain fatty acids (SCFAs; the major anions in the colonic lumen) generate pH gradients across the colonic epithelium. Quantitative confocal microscopy was used to study extracellular pH in mouse distal colon with intact epithelial architecture, by superfusing tissue with carboxy SNARF-1 (a pH-sensitive fluorescent dye). Results demonstrate extracellular pH regulation in two separate microdomains surrounding colonic crypts: the crypt lumen and the subepithelial tissue adjacent to crypt colonocytes. Apical superfusion with (i) a poorly metabolized SCFA (isobutyrate), (ii) an avidly metabolized SCFA (n-butyrate), or (iii) a physiologic mixture of acetate/propionate/n-butyrate produced similar results: alkalinization of the crypt lumen and acidification of subepithelial tissue. Effects were (i) dependent on the presence and orientation of a transepithelial SCFA gradient, (ii) not observed with gluconate substitution, and (iii) required activation of sustained vectorial acid/base transport by SCFAs. Results suggest that the crypt lumen functions as a pH microdomain due to slow mixing with bulk superfusates and that crypts contribute significant buffering capacity to the lumen. In conclusion, physiologic SCFA gradients cause polarized extracellular pH regulation because epithelial architecture and vectorial transport synergize to establish regulated microenvironments.

Colonic fermentation: metabolic and clinical implications

Colonic SCFA formation from fermentable carbohydrate is important for the maintenance of morphologic and functional integrity of the colonic epithelium. Carbohydrate-induced diarrhea occurs when the amount of carbohydrate entering the colon exceeds its fermentation capacity. Deficient availability or utilization of SCFA, mainly of n-butyrate, is the cause of diversion colitis and may play important roles in colonic carcinogenesis, in starvation and enterotoxigenic diarrhea, and in idiopathic UC.

HCO3- secretion by rat distal colon: effects of inhibitors and extracellular Na+

BACKGROUND/AIMS

The large intestine secretes HCO3- via a Cl-/HCO3- exchange mechanism located in the apical membrane of colonocytes. However, an additional transport system(s) must facilitate HCO3- (OH-) entry or H+ exit across the basolateral cell surface. The aim of this study was to determine that mechanism(s).

METHODS

A modified Ussing apparatus was used to measure net HCO3- secretion in segments of rat distal colon.

RESULTS

When added to the serosal solution, 10 mmol/L 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (SITS), 1 mmol/L SITS and 0.1 mmol/L diisothiocyanostilbene-2,2'-disulfonic acid, inhibited HCO3- secretion by 88%, 51%, and 30%, respectively. However, the Na+/H+ exchange inhibitors, amiloride (1 mmol/L), dimethylamiloride (0.1 mmol/L), ethylisopropylamiloride (0.1 mmol/L), failed to affect HCO3- secretion. Acetazolamide (1 mmol/L) blocked HCO3- secretion by approximately 60% when in the serosal solution but had little effect when in the mucosal solution. Ion substitution studies showed that HCO3- secretion required Na+ in the serosal solution (K0.5 approximately 12 mmol/L). HCO3- secretion was unaffected by depolarizing the basolateral membrane potential with K(+)-rich medium.

CONCLUSIONS

These data are consistent with Na+ linked HCO3- transport across the colonocyte basolateral membrane, which appears to be electroneutral.

The role of volume-sensitive Cl- channels in the stimulation of chloride absorption by short-chain fatty acids in the rat colon

The short-chain fatty acids acetate, propionate and butyrate induced a concentration-dependent decrease in short-circuit current (Isc) of the rat colon in vitro. The decrease in Isc, being more pronounced in the distal than in the proximal colon, was dependent on the presence of Cl- ions and partly on the presence of HCO3-. In the distal colon, the fall in Isc could be inhibited by amiloride, indicating that the activity of the Na+/H+ exchanger is necessary for the induction of this response. The decrease in Isc was diminished by the Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoate, and the lipoxygenase inhibitor, nordihydroguaiaretic acid. In contrast, inhibitors of the leukotriene pathway or a Cl- channel blocker did not affect the Isc response in the proximal colon. Measurements of unidirectional fluxes revealed that butyrate caused a stimulation of the mucosa to serosa fluxes (Fms) of Na+ and Cl- in the distal, but only of FNams in the proximal colon. Unidirectional Rb+ fluxes were not altered. The stimulation of Fclms correlated with the degree of metabolism of the short-chain fatty acid. The increase in FClms was most pronounced for butyrate, smaller for acetate and not observed with the poorly metabolizable short-chain fatty acid, isobutyrate. Consequently, two factors seem to be responsible for the stimulation of Cl- absorption by short-chain fatty acids in the distal colon: (1) the intracellular production of HCO3- during the oxidation of short-chain fatty acids as substrate for the apical Cl-/HCO3- exchanger, and (2) the activation of volume-sensitive basolateral Cl- channels.

The effect of short-chain fatty acids on Cl- and K+ conductance in rat colonic crypts

The effect of butyrate on membrane potential and membrane currents of colonic enterocytes was studied with the whole-cell patch-clamp method. Superfusion of crypts from the rat distal colon with butyrate-containing solutions induced a membrane depolarization of 16.5 +/- 2.3 mV. This response was only observed in the upper third of the crypt. The depolarization was dependent on the presence of Cl- and was accompanied by an increase in membrane inward current, indicating that it is caused by an increase in Cl- conductance. Membrane outward current, however, behaved inconsistently. Whereas in most cells an increase was observed, about 25% of the cells responded with a decrease. This unexpected inhibition of the outward current probably represents a decrease of K+ conductance caused by the cellular acidification in the presence of butyrate. Manoeuvres carried out to acidify the cell interior, like perfusion with acid buffer solutions or inhibition of the Na+/H+ exchanger by amiloride, mimicked this inhibition of the K+ conductance. Orientating cell-attached patch-clamp recordings performed in parallel revealed an activation of previously silent basolateral Cl- channels by butyrate. They had a linear current/voltage relationship and a single-channel conductance of 20-30 pS. The butyrate-induced depolarization was not dependent on intracellular adenosine 5'-triphosphate (ATP) and was also observed when the buffer capacity of the pipette for Ca2+ was increased. It was also not inhibited by guanosine-5'-O(2-thiodiphosphate) (GDP[beta S]).(ABSTRACT TRUNCATED AT 250 WORDS)

Segmental differences along the crypt axis in the response of cell volume to secretagogues or hypotonic medium in the rat colon

VIP caused a decrease in the diameter of rat colonic crypts. This decrease was followed by a volume increase at the middle and the upper third of the crypt, which finally led to an increase of crypt diameter above the initial control values. At the fundus only a cell shrinkage was observed. The volume increase at the upper parts of the crypt was suppressed by the inhibitor of the Na(+)-K(+)-Cl(-)-cotransporter, furosemide. When crypts were exposed to a hypertonic medium, cell shrinkage was followed by a regulatory volume increase at the middle and the upper third of the crypt but not at the fundus region. These results suggest a gradient in the distribution of the Na(+)-K(+)-Cl(-)-cotransporter along the crypt axis.

The leukotriene D4 receptor blocker, SK&F 104353, inhibits volume regulation in isolated crypts from the rat distal colon

The effect of SK&F 104353 (2-hydroxy-3-carboxyethylthio-3-[2-(8-phenyloctyl)phenyl] propanoic acid), a specific leukotriene D4 receptor blocker, on volume regulation in isolated rat colonic crypts was studied. SK&F 104353 (10(-7)-5 x 10(-6) mol.l-1) concentration dependently inhibited the regulatory decrease in volume following cell swelling induced either by uptake of a short-chain fatty acid, butyrate, or by exposure to hypotonic medium. Whole-cell patch-clamp experiments revealed that SK&F 104353 suppressed the depolarization of crypt cells during the regulatory decrease in volume. Also the effect of leukotriene D4 (5 x 10(-7) mol.l-1), which has an action on the membrane potential similar to that induced by cell swelling, was suppressed nearly completely by the receptor blocker. In contrast, the precursor of leukotriene D4, leukotriene C4, had no effect on the membrane potential. These results are in accordance with the hypothesis that leukotriene D4 acts as mediator for the activation of basolateral Cl- channels in the rat colonic epithelium during regulatory decrease in volume.