Appearance of segmental discrepancy of anion transport in rat distal colon

Effects of stress-related peptides on chloride secretion in the mouse proximal colon

BACKGROUND

Stress increases intestinal secretion and exacerbates symptoms of irritable bowel syndrome (IBS). Peripherally derived corticotropin-releasing factor (CRF) is known to mediate stress-induced intestinal secretion, presumably by activation of CRF₁ receptors in the gut. The present study aimed to ascertain the role of CRF₂ activation in intestinal secretion by three other members of CRF peptide family, urocortin (UCN) 1-3, in wild type (WT) and CRF₂ knockout (Crhr2^-/- ) mice.

METHODS

Mucosal/submucosal preparations from proximal colon of WT and Crhr2^-/- mice of both sexes were mounted in Ussing chambers for measurement of short-circuit current (I_sc ) as an indicator of ion secretion.

KEY RESULTS

Male mice demonstrated a significantly higher baseline I_sc than female in both WT and Crhr2^-/- genotypes. CRF and UCN1-3 (1 μM) caused greater increases in colonic I_sc (ΔI_sc ) in male than female. Colonic I_sc response to the selective CRF₁ agonist, stressin1, was similar in both sexes. In male mice, the selective CRF₂ agonists (UCN2 and UCN3) caused significantly greater ΔI_sc than CRF and stressin1. UCN2- and UCN3-evoked ΔI_SC was significantly reduced in preparations pretreated with the selective CRF₂ antagonist antisauvagine-30 and in Crhr2^-/- mice. The prosecretory effects of urocortins were due to increases in Cl^- secretion and involved enteric neurons and mast cells.

CONCLUSIONS AND INFERENCE

The findings revealed sex differences in baseline colonic secretion and responses to stress-related peptides. CRF₂ receptors play a more prominent role in colonic secretion in male mice. The greater baseline secretion and responses to UCNs may contribute to the higher prevalence of diarrhea-predominant IBS in males.

Lactose-Induced Chronic Diarrhea Results From Abnormal Luminal Microbial Fermentation and Disorder of Ion Transport in the Colon

Diarrhea is one of the major abdominal symptoms in lactose-intolerant subjects. The changes in the large intestinal luminal environment and disorder of the epithelial ion transport in lactose-induced diarrhea remain unclear. The present study aimed to investigate the effect of an incremental high-lactose diet (IHLD, 30%/40%/50%) on luminal microbiota, microbiota-derived metabolite concentrations and colonic ion transport. Gut microbiota were analyzed by 16S rRNA amplicon sequencing and the concentration of SCFAs by gas chromatography, galactose, lactose and lactic acid through assay kit; Ussing chamber was performed to detect basal and stimulated ion transport; The expression and location of SCFA transporters, the Na-H exchanger 3(NHE3), cystic fibrosis transporter regulater (CFTR) and NKCC1 in the colon mucosa were analyzed by western and immunostaining. The concentrations of lactose, galactose and lactic acid of the cecal content were markedly increased (P < 0.01) and SCFA concentration was significantly decreased (P < 0.01). This was associated with depletion of the Lachnospiraceae NK4A136 group and Ruminococcaceae UCG-005 and increased relative abundance of Lactobacillus, escherichia-shigella and megamonas in the cecal microbiota. The expression of monocarboxylate transporter 1 was decreased in the colonic mucosa of the IHLD group. Low NHE3 expression and phosphorylation levels, and decreases in delta basal short circuit current after apical Na⁺ removal in the colonic mucosa of the IHLD group contributed to Na⁺ accumulation in the lumen and decrease stimulated Cl^– secretion with low CFTR and NKCC1 expression would compensate for water and electrolyte loss during the diarrhea process. These results indicated that the persistence of the diarrhea state was maintained by abnormal colonic microbiota fermentation leading to high concentrations of lactose, galactose and lactic acid and low SCFAs in the lumen, and decreased Na⁺ absorption with the low NHE3 expression and phosphorylation levels.

Effects of Fengliao-Changweikang in Diarrhea-predominant Irritable Bowel Syndrome Rats and Its Mechanism Involving Colonic Motility

Background/Aims

This study was designed to investigate the effect of Fengliao-Changweikang (FLCWK) in diarrhea-predominant irritable bowel syndrome (IBS-D) rats and explore its underlying mechanisms.

Methods

IBS-D model rats were induced by neonatal maternal separation (NMS) combined with restraint stress (RS). In in vivo experiments, the model rats were randomly divided into 5 groups: NMS + RS, FLCWK (low dose, middle dose, and high dose), and pinaverium bromide. The normal control (no handling) rats were classified as the NH group. The therapeutic effect of FLCWK was evaluated by fecal characteristics, electromyographic response and abdominal withdrawal reflex scores. In in vitro experiments, the model rats were randomly divided into 2 groups: NMS + RS, FLCWK (middle dose), and no handling rats were used as the NH group. The differences in basic tension and ACh-induced tension of isolated colonic longitudinal smooth muscle strips (CLSMs) among the 3 groups were observed. In addition, different inhibitors (nifedipine, TMB-8, L-NAME, methylene blue, and 4-AP) were pretreated to explore the underlying mechanisms.

Results

In in vivo experiments, fecal characteristics, electromyographic response, and abdominal withdrawal reflex scores significantly improved in the FLCWK group, compared with the NMS + RS group. In in vitro experiments, the basic tension and ACh-induced tension of CLSMs in IBS-D rats were significantly inhibited by FLCWK. After pre-treatment with different inhibitors, the ACh-induced tension of CLSMs in each group showed no significant difference.

Conclusions

FLCWK manifested curative effect in IBS-D rats by inhibiting colonic contraction. The underlying mechanisms may be related to regulatory pathway of nitric oxide/cGMP/Ca²⁺ and specific potassium channels.

Effect of vasopressin on Na(+)-K(+)-2Cl(-) cotransporter (NKCC) and the signaling mechanisms on the murine late distal colon

It has been demonstrated that the antidiuretic hormone vasopressin is able to regulate the expression of Na-K-Cl cotransporters (NKCC1 and NKCC2) in the kidney. The present study investigated the effects of long- and short-term administration of vasopressin on NKCC and the possible signaling mechanism of vasopressin in the mouse distal colon using the siRNA, real-time PCR, western blotting and Ussing chambers method. The results showed the presence of NKCC2 expression in the colon, which was verified with a siRNA technique. The mRNA and protein expression level of NKCC2 significantly increased by about 40% and 90% respectively in response to restricting water intake to 1ml/day/20g for 7 days. In contrast, the NKCC1 expression level was unchanged in the colon. To determine the short-term activation of NKCC2 by vasopressin in vitro, we found that the administration of vasopressin caused a 3-fold increase in mouse colon NKCC2 phosphorylation, which was detected with phosphospecific antibody R5. In addition, the Ussing chamber results showed that NKCC2, cAMP and Ca(2+) signaling pathway may be involved in the vasopressin-induced response. Further, adenylate cyclase inhibitor MDL-12330A and PKA inhibitor H89 and Ca(2+) chelator BAPTA-AM reversed the vasopressin induced NKCC2 phosphorylation level increase by about 35%, 28% and 42% respectively suggesting vasopressin stimulate NKCC2 phosphorylation increase mediated by cAMP-PKA and Ca(2+) signaling in the colon. Collectively, these data suggest that the expression and phosphorylation of NKCC2 are increased in the colon by vasopressin stimulation, in association with enhanced activity of the vasopressin/cAMP and Ca(2+) pathways.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

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.

Cellular localization of NKCC2 and its possible role in the Cl- absorption in the rat and human distal colonic epithelia

Recently, we demonstrated the expression of NKCC2, an absorptive isoform of NKCC specifically expressed in the kidney, in the rat gastrointestinal tract including the distal colonic mucosa. This study aims to investigate its localization in colonic epithelia and possible role in the colonic ion transport. Reverse transcription polymerase chain reaction (RT-PCR), Western blotting, and immunohistochemistry were used to investigate the expression and localization of NKCC2. The role of NKCC2 on the colonic ion transport was examined by mean of short-circuit current (I(SC)) monitoring. The results indicated that NKCC2 was expressed in the apical region of the epithelia in rat distal colon and human sigmoid colon. NKCC1, which is a secretive NKCC isoform, was localized predominantly in the basolateral membrane, which has been well documented. Serosal (basolateral) administration of bumetanide, an inhibitor of both NKCC1 and NKCC2, inhibited serosal forskolin-induced I(SC) increase by 66% but enhanced the luminal (apical) forskolin-induced I(SC) response by 63%. Furthermore, the blocking of epithelial Na(+) channels by apical addition of amiloride (10 μmol/L), K(+) channels by tetraethylammoniumion (TEA) (5 mmol/L), or glibenclimide (0.1 mmol/L) did not affect apical forskolin-induced I(SC) increase, excluding the involvement of cations, Na(+) and K(+), in the I(SC) response. The luminal forskolin-induced I(SC) increase was enhanced markedly by the apical pretreatment with bumetanide or the reduction of apical Cl(-) concentration by 114% and 198%, respectively, which were inhibited by apical addition of glibenclimide (1 mmol/L) by more than 60%. This finding suggests the involvement of an anion. Furthermore, the removal of basolateral HCO(3)(-) reduced apical forskolin-induced I(SC) by more than 75% indicated that the apical forskolin-induced I(SC) increase in rat distal colon was mediated by Cl(-) absorption and HCO(3)(-) secretion. In conclusion, NKCC2 is expressed widely in the colonic epithelium in rat distal colon and human sigmoid colon, especially in the apical membrane. It involves the process of colonic Cl(-) absorption coupled with HCO(3)(-) secretion.

Janice Drew’s work on diet and cancer

Obesity and associated reduced consumption of plant derived foods are linked to increased risk of colon cancer as well as a number of other organ specific cancers. Inflammatory processes are a contributing factor but the precise mechanisms remain elusive. Obesity and cancer incidence are increasing worldwide, presenting bleak prospects for reducing, or preventing, obesity related cancers. The incidence of these preventable cancers can be achieved with greater understanding of the molecular mechanisms linking diet and carcinogenesis. Janice Drew has developed a research program over recent years to investigate molecular mechanisms related to consumption of anti-inflammatory metabolites generated from consumption of plant based diets, the impact of high fat diets and associated altered metabolism and obesity on regulation of colon inflammatory responses and processes regulating the colon epithelium. Comprehensive strategies have been developed incorporating transcriptomics, including the novel gene expression technology, the GenomeLab System and proteomics, together with biochemical analyses of plasma and tissue samples to assess correlated changes in oxidative stress, inflammation and pathology. The approaches developed have achieved success in establishing antioxidant and anti-inflammatory activity of dietary antioxidants and associated genes and pathways that interact to modulate redox status in the colon. Cellular processes and genes altered in response to obesity and high fat diets have provided evidence of molecular mechanisms that are implicated in obesity related cancer.

Expression and activation of β-adrenoceptors in the colorectal mucosa of rat and human

BACKGROUND

The functions of the distal colon are regulated by local and extrinsic neural pathways. In previous studies, we have found that dopamine (DA) and norepinephrine (NE) could evoke colonic ion transport by activating β-adrenoceptors. The present study aims to investigate the segmental differences in expression and activation of β-adrenoceptors in the distal colon in physiological and pathophysiological conditions.

METHODS

Real-time PCR, immunofluorescence, and Western blotting were used to detect the expression of β-adrenoceptors in the rat and human distal colon. Short-circuit current measurements (Isc) were used to assess the role of β-adrenoceptors in ion transport.

KEY RESULTS

DA and NE caused greater suppression of baseline Isc in distal colon adjacent to the rectum than in segments further away from the anus. These responses were inhibited by selective antagonists of β₁- and β₂-adrenoceptors, but not β₃-adrenoceptor. The expression levels of β₁- and β₂-adrenoceptors in colonic mucosa were higher in colorectum than the regions away from the anus of rats and humans. In wrap-restraint stress (2 h), DA-, NE-induced ΔIsc and the expression of β-adrenoceptors in the colorectum were significantly reduced. However, when endogenous catecholamines were depleted by 6-hydroxydopamine (75 mg kg(-1), i.p., 3 days), DA-, NE-induced ΔIsc as well as the expression of β-adrenoceptors were significantly enhanced in the rat colorectum but not in more proximal regions of the distal colon.

CONCLUSIONS & INFERENCES

β₁- and β₂-adrenoceptors are predominantly expressed in the colorectal mucosa. Perturbation of endogenous catecholamine levels influences the expression and activation of β-adrenoceptors in the colorectal region.