PCO(2) in the large intestine of mice, rats, guinea pigs, and dogs and effects of the dietary substrate

A look at the smelly side of physiology: transport of short chain fatty acids

Fermentative organs such as the caecum, the colon, and the rumen have evolved to produce and absorb energy rich short chain fatty acids (SCFA) from otherwise indigestible substrates. Classical models postulate diffusional uptake of the undissociated acid (HSCFA). However, in net terms, a major part of SCFA absorption occurs with uptake of Na⁺ and resembles classical, coupled electroneutral NaCl transport. Considerable evidence suggests that the anion transporting proteins expressed by epithelia of fermentative organs are poorly selective and that their main function may be to transport acetate^-, propionate^-, butyrate^- and HCO₃^- as the physiologically relevant anions. Apical uptake of SCFA thus involves non-saturable diffusion of the undissociated acid (HSCFA), SCFA^-/HCO₃^- exchange via DRA (SLC26A3) and/or SCFA^--H⁺ symport (MCT1, SLC16A1). All mechanisms lead to cytosolic acidification with stimulation of Na⁺/H⁺ exchange via NHE (SLC9A2/3). Basolaterally, Na⁺ leaves via the Na⁺/K⁺-ATPase with recirculation of K⁺. Na⁺ efflux drives the transport of SCFA^- anions through volume-regulated anion channels, such as maxi-anion channels (possibly SLCO2A1), LRRC8, anoctamins, or uncoupled exchangers. When luminal buffering is inadequate, basolateral efflux will increasingly involve SCFA^-/ HCO₃^- exchange (AE1/2, SCL4A1/2), or efflux of SCFA^- with H⁺ (MCT1/4, SLC16A1/3). Furthermore, protons can be basolaterally removed by NHE1 (SCL9A1) or NBCe1 (SLC4A4). The purpose of these transport proteins is to maximize the amount of SCFA transported from the tightly buffered ingesta while minimizing acid transport through the epithelium. As known from the rumen for many decades, a disturbance of these processes is likely to cause severe colonic disease.

From IBS to DBS

Irritable bowel syndrome is a chronic gastrointestinal disorder characterized by abdominal pain and altered bowel habits in the absence of organic disease. We present 2 cases where diarrhea-predominant irritable bowel syndrome occurred in association with earlier intestinal infection or antibiotic treatment. Both were successfully treated with instillation of an anaerobic cultivated human intestinal microbiota. Thereafter, they were symptom free for at least 12 months. We now introduce the term dysbiotic bowel syndrome covering cases where a disturbed intestinal microbiota is assumed to be present. We recommend that restoration of the dysbiotic gut microbiota should be first-line treatment in these conditions.

Aquaporins and membrane diffusion of CO2 in living organisms

BACKGROUND

Determination of CO2 diffusion rates in living cells revealed inconsistencies with existing models about the mechanisms of membrane gas transport. Mainly, these discrepancies exist in the determined CO2 diffusion rates of bio-membranes, which were orders of magnitudes below those for pure lipid bilayers or theoretical considerations as well as in the observation that membrane insertion of specific aquaporins was rescuing high CO2 transport rates. This effect was confirmed by functional aquaporin protein analysis in heterologous expression systems as well as in bacteria, plants and partly in mammals.

SCOPE OF REVIEW

This review summarizes the arguments in favor of and against aquaporin facilitated membrane diffusion of CO2 and reports about its importance for the physiology of living organisms.

MAJOR CONCLUSIONS

Most likely, the aquaporin tetramer forming an additional fifth pore is required for CO2 diffusion facilitation. Aquaporin tetramer formation, membrane integration and disintegration could provide a mechanism for regulation of cellular CO2 exchange. The physiological importance of aquaporin mediated CO2 membrane diffusion could be shown for plants and cyanobacteria and partly for mammals.

GENERAL SIGNIFICANCE

Taking the mentioned results into account, consequences for our current picture of cell membrane transport emerge. It appears that in some or many instances, membranes might not be as permeable as it was suggested by current bio-membrane models, opening an additional way of controlling the cellular influx or efflux of volatile substances like CO2. This article is part of a Special Issue entitled Aquaporins.

Low carbon dioxide permeability of the apical epithelial membrane of guinea-pig colon

We have investigated the apical membrane permeability for CO2 of intact epithelia of proximal and distal colon of the guinea pig. The method used was the mass spectrometric 18O-exchange technique previously described. In a first step, we determined the intraepithelial carbonic anhydrase (CA) activity by studying vital isolated colonocytes before and after lysis with Triton X-100. Intraepithelial CA activity was found to be 41,000 and 900 for proximal and distal colon, respectively. Then 18O-exchange measurements were done with stripped intact epithelial layers, which on their apical side were exposed to the reaction solution containing 18O-labelled CO2 and HCO3-. The mass spectrometric signals in these measurements are determined by the intracellular epithelial CA activity, and by the apical membrane permeabilities for CO2 and HCO3-, P(CO2) and P(HCO3). From the signals, we calculated the two permeabilities while inserting the CA activities obtained from isolated colonocytes. From layers of intact colon epithelium, the apical P(CO2) was determined to be 1.5 x 10(-3) cm s(-1) for proximal and 0.77 x 10(-3) cm s(-1) for distal colon. These values are > or =200 times lower than the P(CO2) of the human red cell membrane as studied with the same technique (0.3 cm s(-1)). We conclude that the apical membrane offers a significant resistance towards CO2 diffusion, which implies that a major drop in CO2 partial pressure (pCO2) will occur across the apical membrane when luminal pCO2 is higher than basolateral or capillary pCO2. In view of the very high pCO2 that can occur in the colonic lumen, this property of the apical membrane constitutes a significant protection of the cell against the high acid load associated with high pCO2.

Carbonic anhydrase in the gastrointestinal mucus of mammals—possible protective role against carbon dioxide

We show here that luminal mucus from the colon and the stomach of guinea pigs, mice and humans exhibits substantial carbonic anhydrase (CA) activity, by which the velocity of the CO(2) hydration reaction is accelerated 1000-2000-fold, approximately 1/10 of what is found in the red cell. Although this CA shares several properties with CA II, studies with CA II-deficient mice show that gastrointestinal mucus CA is not affected in these animals and thus does not appear to be CA II. We speculate that the mucus layer covering the luminal surface of gastrointestinal epithelium can, due to the presence of CA, maintain a normal tissue pCO(2) in the epithelium, even when the pCO(2) values in the lumen are much higher, as is known for stomach and colon. To test this hypothesis, we have developed a mathematical model which describes (a) diffusion of CO(2) and HCO(3)(-) across the mucus layer and (b) H(+) transport mediated by continuous secretion of mucus, which due to its high H(+) buffer capacity transports H(+) by convection towards the lumen. The model predicts that continuous transport of the reaction products of CO(2) towards the lumen, by diffusion and convection, protects the epithelium against high CO(2) partial pressures in the lumen.

Etiology of cecal and hepatic lesions in mice after administration of gas-carrier contrast agents used in ultrasound imaging

The aim of the study was to investigate the etiology of cecal and hepatic lesions in mice and rats after intravenous administration of gas-carrier contrast agents (GCAs). A modified fluorescein flowmetry technique and 24 h necropsy were used in mice (conventional and germ free), rats, and guinea pigs after GCA administration. Different diets and oral nonabsorbable antibiotics were used. Nonfluorescence, edema, congestion, hemorrhage, and mucosal erosion in cecum and colon and nonfluorescent areas in the liver were observed from 16 min after GCA administration in conventional mice on standard diet. Numerous gas bubbles (>50 microm) were observed in the vasculature around the nonfluorescent areas of cecum and colon and in mesenteric vessels draining to the portal vein. Acute inflammation, edema, hemorrhage, and ulceration of the cecum and colon and liver necrosis were seen 24 h after GCA administration in conventional mice on standard diet. When mice were maintained on either a diet with glucose as the only carbohydrate source or on a standard diet supplemented with antibiotics, uniform fluorescence and no organ lesions were observed after GCA administration. Uniform fluorescence and no organ lesions were observed in germ-free mice, rats, and guinea pigs dosed with GCAs and in control animals (mice, rats, and guinea pigs) dosed with sucrose. The results indicate that intravascular growth of GCA microbubbles occurs in the cecal and colonic wall of mice, leading to occlusive ischemia and necrosis in these intestinal segments and secondary gas embolisation in the liver. Transmural gas supersaturation in the cecal wall may explain the intravascular bubble growth in mice.

Intestinal and hepatic lesions in mice, rats, and other laboratory animals after intravenous administration of gas-carrier contrast agents used in ultrasound imaging

Single intravenous administration of three different gas-carrier contrast agents used in ultrasound imaging to mice caused inflammation, necrosis, and ulceration of cecum and proximal colon (cecocolonic area) and focal necrosis in the liver. Similar intestinal lesions were also found in rats after treatment with a single iv administration of a gas-carrier contrast agent. Strain differences in the incidences of these lesions were found in both rats and mice. HsdHan:NMRI mice were among the most sensitive of the strains of mice studied. Even at the lowest dose of Sonazoid technically possible to inject in HsdHan:NMRI mice, lesions were found and a no-effect dose could not be determined. In a time-course experiment in HsdHan:NMRI mice, it was found that the lesions began to develop in the cecum and colon within 15 to 30 min after dosing. Lesions in the liver were first observed 120-240 min after dosing. Diet played a role in the etiology of the lesions, as HsdHan:NMRI mice given a diet with reduced amounts of cellulose and starch had reduced incidences of lesions, and when glucose was the only carbohydrate source, no lesions were observed. No intestinal or hepatic lesions were found in guinea pigs or rabbits after repeated intravenous administrations of Sonazoid. In dogs, minimal to mild granulocytic inflammation of the cecum and/or colon was found after daily repeated intravenous injections for 28 days, but not after daily repeated administration for 14 days nor after a single administration. It is proposed that the intestinal and hepatic lesions in rats and mice after a single intravenous injection of gas-carrier contrast agents are caused by a common mechanism: intravascular growth of gas-carrier agents in tissues with gas supersaturation, as occurs in the cecal wall of rats and mice. In this particular environment the growing gas bubbles cause ischemia and necrosis in the cecal and colonic wall and liver. This proposed mechanism of action is consistent with the absence of clinical reports indicative of intestinal and/or hepatic lesions in humans after administration of gas-carrier contrast agents.