Electrolyte absorption and secretion in the human colon

Bacterial growth, flow, and mixing shape human gut microbiota density and composition

The human gut microbiota is highly dynamic, and host physiology and diet exert major influences on its composition. In our recent study, we integrated new quantitative measurements on bacterial growth physiology with a reanalysis of published data on human physiology to build a comprehensive modeling framework. This can generate predictions of how changes in different host factors influence microbiota composition. For instance, hydrodynamic forces in the colon, along with colonic water absorption that manifests as transit time, exert a major impact on microbiota density and composition. This can be mechanistically explained by their effect on colonic pH which directly affects microbiota competition for food. In this addendum, we describe the underlying analysis in more detail. In particular, we discuss the mixing dynamics of luminal content by wall contractions and its implications for bacterial growth and density, as well as the broader implications of our insights for the field of gut microbiota research.

Effect of water flow and chemical environment on microbiota growth and composition in the human colon

The human gut harbors a dynamic microbial community whose composition bears great importance for the health of the host. Here, we investigate how colonic physiology impacts bacterial growth, which ultimately dictates microbiota composition. Combining measurements of bacterial physiology with analysis of published data on human physiology into a quantitative, comprehensive modeling framework, we show how water flow in the colon, in concert with other physiological factors, determine the abundances of the major bacterial phyla. Mechanistically, our model shows that local pH values in the lumen, which differentially affect the growth of different bacteria, drive changes in microbiota composition. It identifies key factors influencing the delicate regulation of colonic pH, including epithelial water absorption, nutrient inflow, and luminal buffering capacity, and generates testable predictions on their effects. Our findings show that a predictive and mechanistic understanding of microbial ecology in the gut is possible. Such predictive understanding is needed for the rational design of intervention strategies to actively control the microbiota.

Lactulose Differently Modulates the Composition of Luminal and Mucosal Microbiota in C57BL/6J Mice

In this study, C57BL/6J mice were fed diets supplemented with different proportions of lactulose (0%, 5%, and 15%) for 2 weeks to study its effects on the luminal and mucosal microbiota. The luminal and mucosal samples of cecum and colon were investigated. After high-lactulose treatment (15%), pH of the luminal contents decreased from 6.90-7.72 to 5.95-6.21 from the cecum to distal colon, and the amount of total short-chain fatty acids in the cecum was significantly increased. The luminal content was mostly dominated by Firmicutes, Actinobacteria, and Bacteroidetes, while the mucus was dominated by Firmicutes, Proteobacteria, and Bacteroidetes. The abundance of Actinobacteria was significantly increased in the content, and Proteobacteria was the most abundant phylum (∼50%) in the mucus after high-lactulose treatment. At the genus level, Bifidobacterium and Akkermansia were both significantly increased in the content, and Helicobacter was the most abundant in the mucus.

The physiology of continence and evacuation

Continence is maintained by the coordinated function of the pelvic floor, rectum and anal sphincters. Evacuation occurs through a relaxed pelvic floor. The rectum acts to either store or expel stool both of which require cortical sensory awareness acting in conjunction with intramural and spinal reflexes that ensure timely defecation. The anal sphincters act individually and in unison in response to rectal distension and the sensation of rectal filling. Reflex relaxation of the internal anal sphincter has an additional sensory function in allowing sampling of rectal contents in the upper anal canal. Voluntary control of the external anal sphincter is key in the voluntary deferring of evacuation until a socially opportune moment. This review describes the physiological roles of each of these continence organs in order to understand the complex process of defecation.

Development of colonic sodium transport in early childhood and its regulation by aldosterone

Aldosterone is important in the regulation of sodium conservation by both kidney and colon. In the very preterm neonate marked urinary salt wasting occurs because of immature renal tubular function, but little is known of the ontogeny of colonic transport processes. Using an in vivo rectal dialysis technique, we have shown that in the human infant the colon has well developed salt conserving mechanisms from early in the last trimester of gestation and that aldosterone is an important regulatory hormone. Sodium transport mechanisms in the colon appear to develop before those in the kidney and it is possible that the colon is the major organ of sodium conservation in the preterm neonate.

Studies of osmotic diarrhea induced in normal subjects by ingestion of polyethylene glycol and lactulose

The purpose of these studies was to gain insight into the pathophysiology of pure osmotic diarrhea and the osmotic diarrhea caused by carbohydrate malabsorption. Diarrhea was induced in normal volunteers by ingestion of polyethylene glycol (PEG), which is nonabsorbable, not metabolized by colonic bacteria, and carries no electrical charge. In PEG-induced diarrhea, (a) stool weight was directly correlated with the total mass of PEG ingested; (b) PEG contributed 40-60% of the osmolality of the fecal fluid, the remainder being contributed by other solutes either of dietary, endogenous, or bacterial origin; and (c) fecal sodium, potassium, and chloride were avidly conserved by the intestine, in spite of stool water losses exceeding 1,200 g/d. Diarrhea was also induced in normal subjects by ingestion of lactulose, a disaccharide that is not absorbed by the small intestine but is metabolized by colonic bacteria. In lactulose-induced diarrhea, (a) a maximum of approximate 80 g/d of lactulose was metabolized by colonic bacteria to noncarbohydrate moieties such as organic acids; (b) the organic acids were partially absorbed in the colon; (c) unabsorbed organic acids obligated the accumulation of inorganic cations (Na greater than Ca greater than K greater than Mg) in the diarrheal fluid; (d) diarrhea associated with low doses of lactulose was mainly due to unabsorbed organic acids and associated cations, whereas with larger doses of lactulose unmetabolized carbohydrates also played a major role; and (e) the net effect of bacterial metabolism of lactulose and partial absorption of organic acids on stool water output was done dependent. With low or moderate doses of lactulose, stool water losses were reduced by as much as 600 g/d (compared with equimolar osmotic loads of PEG); with large dose, the increment in osmotically active solutes within the lumen exceeded the increment of the ingested osmotic load, and the severity of diarrhea was augmented.

The development of colonic transport mechanisms in early life: evidence for reduced anion exchange

An anion exchange mechanism exists in colonic mucosa whereby chloride is absorbed and bicarbonate secreted. Using an in vivo non-equilibrium dialysis method we investigated rectal electrolyte movement in preterm neonates and older children. Our results show that anion exchange is poorly developed in infancy and appears to be absent in premature neonates, and suggest that complete maturation of this mechanism is not present until the end of the first year of life. This may render the young infant, and more especially the preterm infant, more susceptible to chloride depletion.

Comparative modes of action of lactitol and lactulose in the treatment of hepatic encephalopathy

Lactitol, an unabsorbed sugar with defined laxative threshold and superior taste properties has been suggested as an alternative to lactulose in the treatment of hepatic encephalopathy. In the present study we have compared the colonic metabolism of the two sugars using an in vitro faecal incubation system. Both sugars were readily metabolised by faecal bacteria producing volatile fatty acids and the metabolism was inhibited by neomycin. The effect of lactitol and lactulose on terminal ileal and colonic pH was monitored in six normal subjects using a radiotelemetry technique. Both sugars significantly lowered right colonic pH (basal -6.51 +/- 0.48 vs lactitol -5.63 +/- 0.50; lactulose -5.18 +/- 0.82, p less than 0.05). The pH of rest of the colon and terminal ileum was unaffected. Neomycin given concurrently with lactulose abolished acidification of right colon. As lactitol and lactulose have similar effects within the colon, lactitol would appear to have a role in the treatment of hepatic encephalopathy. As neomycin antagonises the effect of lactulose in the colon, its concurrent use may be less effective in the treatment of hepatic encephalopathy.

Colonic dysfunction during cholera infection

To study the function of the colon in cholera, 12 patients with acute cholera diarrhea were subjected to measurements of ileocecal flow rates, fecal flow rates, and ionic compositions of stool and ileocecal fluid. Subtraction of fecal flow rates from ileocecal flow rates was taken as a measure of net fluid absorption by the colon. Additionally, these patients underwent colonoscopic perfusion of the colon that measured net colonic absorption rates of water and ions. The mean ileocecal flow rate was 7.9 ml/min compared with a mean fecal flow rate of 7.6 ml/min, indicating a small mean net fluid absorption by the colon of +0.30 ml/min. By colonoscopic perfusion, 6 patients showed net colonic absorption of water and 6 patients net secretion of water with a slight mean net fluid secretion of -0.03 ml/min. The handling of ions by the colon showed mean net absorption of sodium (100 mu Eq/min) and chloride (127 mu Eq/min), and net secretions of potassium (-42 mu Eq/min) and bicarbonate (-112 mu Eq/min). During convalescence, 5 patients who were studied again all showed net colonic absorption of water, and the handling of potassium changed significantly from net secretion in acute disease to net absorption (p less than 0.05). These results showed that the colon contributes to the clinical expression of cholera by failing to absorb water normally, and by secreting potassium at high rates.

Idiopathic chronic watery diarrhea from excluded rectosigmoid with goblet cell hyperplasia cured by restoration of large bowel continuity

A 73-year-old woman developed abnormal electrolyte and water loss from an excluded rectosigmoid segment after surgical treatment of a volvulus of the sigmoid colon. Rectal discharges lasted almost for a year, until it spontaneously resolved after restoration of large bowel continuity. Despite extensive investigation, including endoscopic, radiologic, microscopic, bacteriologic and parasitic examinations, no satisfactory explanation of the diarrhea could be found. The histologic pattern of the excluded segment showed a striking increase in mucosal thickness and in number and height of goblet cells. These abnormalities disappeared after closure of the colostomy. Electrolyte composition of the rectal fluid, which contained 134 mmol potassium and 22 mmol sodium per liter was remarkable and similar to that of normal stool water and anal discharges of patients with ulcerative proctitis.

Absorption of electrolytes and volatile fatty acids in the hind-gut of the rabbit. Circadian rhythm of hind-gut electrolytes and plasma aldosterone

Interrelationships between the amount of water, solute concentrations in the hind-gut, plasma aldosterone concentrations, and the excretory cycle in the rabbit were studied as well as the effects of hormones generally implicated in electrolyte regulation. It appeared that the levels of water, electrolytes and volatile fatty acids (VFA) in the large intestine varied cyclicly with the excretion pattern: during the soft-phase, water, sodium, chloride and VFA were absorbed and potassium secreted as in other mammals; during the hard-phase two opposite phenomena occurred: secretion in the ascendant colon and absorption in the rest of the colon. Absorption of water, Na, K, chloride and VFA was increased and by this means only material without nutritional value was lost. Plasma levels of aldosterone showed a circadian rhythm parallel to hind-gut changes, i.e. the higher values were found during the hard-phase. Exogenous aldosterone enhanced secretive and absorptive processes, which occurred normally when hard faeces were elaborated.

A comparative study of bicarbonate secretion by ileum and colon of Oryctolagus cuniculus. Influence of caecal pH

The influence of caecal pH on movements of bicarbonate ion in rabbit colon and ileum has been studied. A net secretion of bicarbonate is noted in both intestinal segments (at higher rates in colon than in ileum) in our experimental conditions. The bicarbonate secretion rate is lowered when bicarbonate is added to caecum. The introduction of acetic acid at caecal level increased bicarbonate secretion rate both in ileum as in colon.

Evaluation of chloride/bicarbonate. Exchange in the human colon in vivo

During perfusion of a plasma-like solution, colonic absorption rate of chloride was much higher than the secretion rate of bicarbonate (34 vs. 3.5 meq/h, respectively). This might suggest that anion exchange (Cl/HCO3) accounts for only a small fraction of total chloride absorption. However, if the colon absorbs as well as secretes bicarbonate, this reasoning would underestimate the magnitude of the anion exchange. To see if the colon absorbs bicarbonate, we perfused a chloride-free solution (which would eliminate bicarbonate secretion via (Cl/HCO3 exchange) and found that the colon absorbed bicarbonate at a rate of 5.1 meq/h. Calculation of electrochemical gradients and measurement of luminal fluid PCO2 indicated that this bicarbonate absorption was mediated passively in response to electrical gradients, rather than via reversed Cl/HCO3 exchange or acid secretion. The combined results of the plasma-like and chloride-free perfusion experiments suggest Cl/HCO3 exchange at a rate of 8.6 meq/h (the sum of bicarbonate movements, 3.5 and 5.1 meq/h, observed in the two experiments). To obtain a second estimate under different experimental conditions, a choline chloride-choline bicarbonate (sodium-free) solution was perfused; with this solution, chloride and bicarbonate absorption dependent on active sodium transport should be eliminated or markedly reduced, and the magnitude of Cl/HCO3 exchange should be revealed. This experiment suggested a Cl/HCO3 exchange rate of 9.3 meq/h, similar to the first estimate. As chloride was absorbed at a rate of 34 meq/h during perfusion of the plasma-like solution, the Cl/HCO3 exchange provides for approximately one-fourth of total chloride absorption.

A comparative study of electrolyte absorption from the caecum and colon of Oryctolagus cuniculus-I. Chloride and bicarbonate

1. Chloride absorption by the rabbit colon occurs under all the experimental conditions tested, even when intraluminal concentrations are lower than the plasmatic ones. In contrast, at caecal levels, chloride absorption rate depends on the experimental conditions. 2. Bicarbonate is always secreted by the colon even for intraluminal concentrations twice the amount of plasmatic ones. At caecal level, it is absorbed only from concentrations similar to those of the plasma or greater. 3. There seems to be an exchange process affecting chloride and bicarbonate at colonic level but not in the caecum.

Intestinal electrophysiology and transmural ion transport in freshwater prawns

A steady-state transmural potential difference of 1.63 +/- 0.26 mV (mean +/- SE, lumen negative) is recorded across the isolated, perfused intestine of the freshwater prawn, Macrobrachium rosenbergii, when saline approximating hemolymph in composition is present on both surfaces of the preparation. The magnitude of this potential is a hyperbolic function of bilateral K concentration. It is abolished by metabolic poinsons (NaN3 and iodoacetate) or ouabain and appears to result largely from the combined net transmural fluxes of Na, Cl, and K. Net fluxes of Na (1,49 +/- 0;30 mu mol . cm-2 . h-1) and Cl (0.72 +/- 0.22) are absorptive, whereas that of K (0.47 +/- 0.11) IS SECRETORY. Bilateral absence of Na abolishes net K secretion, whereas bilateral addition of ouabain (0.5 mM) eliminates net Na absorption. One-third of net K secretion appears to be transcellular and coupled to the net cellular transfer of Na in the opposite direction. The remaining component of K secretion can be attributed to paracellular cation flow responsive to transmural PD, Transmural diffusion potentials generated in the presence and absence of metabolic poisons provide the following passive permeability properties of the tissue: PK:PNa:PCl = 5.1:1.1:0.5. The symmetrical nature of these diffusion potentials implies the occurrence of a single rate-limiting barrier to ion flow with the above characteristics--probably the cell junction (septate desmosome) and paracellular channel. A model for transepithelial ion transport is presented where transmural potential difference is largely a result of an apparent 9 Na/1 K basolateral cationic exchange.

Absorption of inorganic ions and volatile fatty acids in the rabbit caecum

1. Rabbit caecal segments in situ were used to measure absorption rates of volatile fatty acids (VFA) and inorganic ions from a saline solution comparable in composition to normal caecal fluid. 2. Results confirm the importance of VFA absorption from caecal material found by other workers. 3. Like the mammalian colon, the rabbit caecum conserved large amounts of sodium, chloride and water. Bicarbonate was also absorbed. 4. VFA replacement studies showed that net water absorption was reduced, net electrolyte absorption was hardly influenced. 5. Na replacement completely inhibited net water absorption and decreased net VFA and Cl absorption, HCO3 was heavily secreted. 6. These findings indicate that VFA absorption in the rabbit caecum is partly dependent on Na absorption and that in the absence of Na an anion-exchange mechanism occurs.

Rectal potential difference and histology in Crohn's disease

The rectal potential difference (PD) was measured in 27 patients with Crohn's disease, and in 16 subjects without gastrointestinal disease to establish a normal range. Sigmoidoscopic assessment and rectal biopsy were performed in all patients with Crohn's disease, and the mean resting rectal PD was significantly reduced in patients with sigmoidoscopically active disease and in those with abnormalities of the superficial epithelium on rectal biopsy. Patients with diarrhoea had a significantly lower mean resting PD than those with normal bowel habit, suggesting that an abnormality of rectal sodium transport may be contributing to the diarrhoea in these patients. The response of rectal PD to mineralocorticoid stimulation with oral fludrocortisone was measured in 13 patients. The PD failed to rise only with patients with sigmoidoscopically active disease, and the test proved to be a less sensitive indication of minor mucosal abnormalities than sigmoidoscopy of biopsy.

Colonic electrolyte transport in health and in congenital chloride diarrhea

Congenital chloride diarrhea (CCD) is a rare autosomal recessive disorder, characterized by watery stools with C1- concentration around 150 meq/liter. We have perfused the colon of three patients and their three healthy siblings with different salt solutions containing 36C1- to determine the nature of the colonic defect in CCD. In the controls, net absorption of Na+ and C1- occurred against steep concentration gradients. The influx (lumen-to-plasms flux) of C1- was twice the effux. Omission of HCO3- from the perfusate caused a clear decrease in C1- efflux which suggests a coupling of C1- effux to HCO3- influx. In CCD, net Na+ absorption occurred normally when HCO3- was present in the lumen. However, Na+ absorption was always impaired when the luminal contents were acid, a situation that prevails in CCD. Net K+ secretion was clearly increased. Both influx and efflux of C1- were practically absent. Only slight net secretion occurred along a steep gradient. Net appearance of HCO3- was not observed, in contrast to controls. These findings and earlier studies of ileal function in CCD are best explained by a defect in the C1-/HCO3- exchange mechanism, which operates in both directions in the normal ileum and colon.

Effects of lactulose and other laxatives on ileal and colonic pH as measured by a radiotelemetry device

Using a pH-sensitive radiotelemetering device the effect of lactulose on luminal pH in the ileum, colon, and rectum has been compared with that of two other laxative agents. Lactulose produced marked acidification of proximal colonic contents but this effect was not consistently maintained into the distal colon. Sodium sulphate acidified distal rather than proximal colonic contents. However, for a similar degree of laxation it was not possible to produce a significantly more uniform reduction of pH along the length of the colon by combining these laxatives compared with lactulose alone. Magnesium sulphate had little effect upon luminal pH except in the rectum where a significant rise occurred. These results are discussed in relation to both normal colonic physiology and to their possible relevance to the treatment of chronic hepatic encephalopathy by colonic acidification.