pH gradients and a mirco-pore filter at the luminal surface affect fluxes of propionic acid across guinea pig large intestine

Potential fat-lowering and prebiotic effects of enzymatically treated okara in high-cholesterol-fed Wistar rats

This study evaluates the effect of the lipid profile on serum, liver and faeces, and the potential prebiotic effect of diets supplemented with enzymatically treated okara (okara(ET)) in high-cholesterol fed Wistar rats. Triglyceride levels were significantly reduced in the serum (p < 0.01) and liver (p < 0.01) of okara(ET) treated rats. Total lipids, triglycerides and bile acids were significantly higher (p < 0.001) in the faeces of rats fed the okara(ET) diet. The pH of faecal contents from treated okara(ET) rats was lower (p < 0.001), probably due to the significantly higher (p < 0.001) production of short-chain fatty acids (SCFA). Okara(ET), therefore, reduced triglycerides in serum and liver, and increased the excretion of total lipids, triglycerides and bile acids, improving the lipid profile in rats fed with high-cholesterol diets. Okara(ET) fibre can improve intestinal transit by increasing faecal bulk. The decreased pH and increased SCFA production indicated that okara(ET) fibre fermentation occurred, suggesting a potential prebiotic effect.

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.

Transepithelial transport and intraepithelial metabolism of short-chain fatty acids (SCFA) in the porcine proximal colon are influenced by SCFA concentration and luminal pH

Short-chain fatty acids (SCFA) are end products of bacterial fermentation in the colon and cecum of monogastric animals. As SCFA serve as relevant energy suppliers for colonocytes and various tissues, it is important to reveal fundamental mechanistic characteristics of their transepithelial transport subjected to transient variations of fermentations rates. We performed Ussing chamber studies with porcine (Sus scrofa) colon epithelium under physiological conditions and examined individual mucosal disappearance, metabolized loss, tissue concentrations and serosal release of acetate, propionate and butyrate by gas chromatography. Reduction of initial SCFA concentrations from 80 to 40 mmol/L resulted in diminished absolute flux rates, but the relative proportions of mucosal disappearance and intracellular metabolization of individual SCFA were slightly enhanced. Simulation of high fermentation rates by lowering the mucosal pH induced an increase in mucosal disappearance and serosal release of all SCFA, while their tissue contents trended to lower levels. With respect to the metabolization at lowered pH we found increased acetate concentrations and a decrease of propionate and butyrate. Our data indicate that the colon epithelium possesses a high adaptive capacity to ensure its energetic maintenance under various intraluminal fermentation rates by utilizing the unique features of individual SCFA as energy sources.

Segregation of Na/H exchanger-3 and Cl/HCO3 exchanger SLC26A3 (DRA) in rodent cecum and colon

The colon is believed to absorb NaCl via the coupled operation of apical Na/H exchanger-3 (NHE3) and Cl/HCO(3) exchanger SLC26A3 (DRA). Efficient coupling requires that NHE3 and DRA operate in close proximity within common luminal and cytosolic microenvironments. Thus we examined whether these proteins coexist along the apical margin of surface enterocytes by quantitative immunofluorescence microscopy in consecutive colon segments from nonfasted mice and rats. The cecocolonic profiles of NHE3 and DRA expression were roughly inverse; NHE3 was highest in proximal colon (PC) and negligible in distal colon, whereas DRA was absent in early PC and highest in the late midcolon, and DRA was prominent in the cecum whereas NHE3 was not. NHE3 and DRA coexisted only in the middle third of the colon. The consequences of unpaired NHE3/DRA expression on mucosal surface (subscript MS) pH and Na(+) concentration ([Na(+)]) were assessed in nonfasted rats in situ using miniature electrodes. In the cecum, where only DRA is expressed, pH(MS) was approximately 7.5, markedly higher than underlaying stool (6.3), consistent with net HCO(3)(-) secretion. In the early PC, where NHE3 is not expressed with DRA, pH(MS) was acidic (6.2), consistent with unopposed H(+) secretion. [Na(+)](MS) was approximately 60 mM in the cecum, decreased along the PC to approximately 20 mM, and declined further to approximately 10 mM distally. Cl(-) was secreted into the PC, then reabsorbed distally. Our results suggest a model in which 1) unpaired DRA activity in the cecum maintains an alkaline mucosal surface that could neutralize fermentative H(+); 2) unpaired NHE3 activity in the early PC preserves an acidic mucosal surface that could energize short-chain fatty acid absorption; and 3) coupled NHE3/DRA activities in the midcolon allow for vigorous NaCl absorption at a neutral pH(MS).