Na+-dependent bile acid transport in the ileum: the balance between diarrhea and constipation

Effects of chenodeoxycholate and a bile acid sequestrant, colesevelam, on intestinal transit and bowel function

BACKGROUND & AIMS

Di-alpha hydroxy bile salt, sodium chenodeoxycholate (CDC), and bile acid binding have unclear effects on colonic transit in health and disease.

METHODS

We performed 2 randomized, double-blind, placebo-controlled studies. In healthy volunteers (20 per group), we evaluated the effects of oral placebo, 500 mg, or 1000 mg of CDC (delayed-release, each given for 4 days) on gastrointestinal and colonic transit. A second trial compared the effects of colesevelam (1.875 g, twice daily) versus placebo in 24 patients (12 per group) with diarrhea-predominant irritable bowel syndrome (IBS-D) on transit, daily bowel frequency and consistency, and colonic mucosal permeability. Serum fasting 7alpha-hydroxy-4-cholesten-3-one (7alphaC4) was measured to screen for bile acid malabsorption. Effects of treatments on transit were compared using analysis of covariance with body mass index and 7alphaC4 as covariates.

RESULTS

In healthy volunteers, CDC significantly accelerated colonic transit (at 24 and 48 hours, P = .01 and P < .0001, respectively), increased stool frequency and ease of passage (both P < .001), and evacuation (P = .02), and decreased stool consistency (P < .001). Four of the 24 IBS-D patients had increased serum 7alphaC4 levels. In IBS-D, colesevelam modestly affected overall colonic transit (24 h; P = .22). Emptying of the ascending colon took an average of 4 hours longer in patients given colesevelam compared with placebo; treatment effect was associated with baseline serum 7alphaC4 levels (P = .0025). Colesevelam was associated with greater ease of stool passage (P = .048) and somewhat firmer stool consistency (P = .12). No effects on mucosal permeability or safety were identified.

CONCLUSIONS

Sodium chenodeoxycholate in health and colesevelam in IBS-D patients have opposite effects on colonic transit and fecal parameters.

Measurement of serum 7alpha-hydroxy-4-cholesten-3-one (or 7alphaC4), a surrogate test for bile acid malabsorption in health, ileal disease and irritable bowel syndrome using liquid chromatography-tandem mass spectrometry

Bile acid malabsorption (BAM) is reported in up to 50% of patients with functional diarrhoea and irritable bowel syndrome with diarrhoea (IBS-D). Serum 7alpha-hydroxy-4-cholesten-3-one (7alphaHCO or 7alphaC4), an indirect measurement of hepatic bile acid synthesis, has been validated as a measurement of BAM relative to the (75)SeHCAT retention test. Our aim was to develop a serum 7alphaC4 assay, normal values, and compare results from healthy controls, patients with ileal Crohn's disease or resection, and patients with IBS-D or IBS with constipation (IBS-C). Stored serum samples were used from adult men and women in the following groups: 111 normal healthy controls, 15 IBS-D, 15 IBS-C, 24 with distal ileal Crohn's disease and 20 with distal ileal resection for Crohn's disease. We adapted a published high pressure liquid chromatography, tandem mass spectrometry (HPLC-MS/MS) assay. The HPLC-MS/MS assay showed good linearity in concentration range 0-200 ng mL(-1), sensitivity (lowest limit of detection 0.04 ng mL(-1)), and high analytical recovery (average 99%, range 93-107%). The 5th to 95th percentile for 111 normal healthy controls was 6-60.7 ng mL(-1). There were significant overall group differences (anovaon ranks, P < 0.001), with significantly higher values for terminal ileal disease or resection. There were significant differences between health and IBS (anova, P = 0.043) with higher mean values in IBS-D relative to controls (rank sum test, P = 0.027). We have established a sensitive non-isotopic assay based on HPLC-MS/MS, determined normal 7alphaC4 values, and identified increased 7alphaC4 in IBS-D and in distal ileal resection and disease. This assay has potential as a non-invasive test for BAM in IBS.

In vitro determination of active bile acid absorption in small biopsy specimens obtained endoscopically or surgically from the human intestine

BACKGROUND

In the construction of a Kock reservoir for continent urinary diversion, 70 cm of the distal ileum are used. Impaired absorption of bile acids in these patients might cause diarrhoea. Data on the absorption of bile acids in different parts of the human intestine are limited.

METHODS

Biopsies were taken during endoscopy from the duodenum, the terminal ileum or the right colon, and during surgery 10, 50, 100 and 150 cm proximally to the ileo-caecal valve using standard endoscopy biopsy forceps. The biopsy specimens were incubated in vitro with radio-labelled taurocholic acid at 37 degrees C for 22 or 45 min The radioactivity was determined using the liquid scintillation technique.

RESULTS

A linear increase in the uptake was observed, with increased concentrations of taurocholic acid between 100 and 500 microm in all specimens tested, that represented passive uptake or unspecific binding. The active uptake could be calculated from the intercept of the line representing passive uptake with the ordinate. The active uptake in the terminal ileum was 3-4 times greater than 100 cm proximal to the valve.

CONCLUSIONS

The active absorption of bile acids in humans can be determined in small biopsy specimens taken using standard biopsy forceps during endoscopy or surgery. This method is suitable for clinical studies of bile acid absorption. Active uptake of bile acids not only takes place in the very distal part of the ileum but also to a considerable degree 100 cm proximally to the ileo-colonic valve. This should be taken into account when selecting the ileal segment for continent urinary diversion.

Intestinal bile acid transport: biology, physiology, and pathophysiology

Intestinal reabsorption of bile salts plays a crucial role in human health and disease. This process is primarily localized to the terminal ileum and is mediated by a 48-kd sodium-dependent bile acid cotransporter (SLC10A2 = ASBT). ASBT is also expressed in renal tubule cells, cholangiocytes, and the gallbladder. Exon skipping leads to a truncated version of ASBT, which sorts to the basolateral surface and mediates efflux of bile salts. Inherited mutation of ASBT leads to congenital diarrhea secondary to bile acid malabsorption. Partial inhibition of ASBT may be useful in the treatment of hypercholesterolemia and intrahepatic cholestasis. During normal development in the rat ileum, ASBT undergoes a biphasic pattern of expression with a prenatal onset, postnatal repression, and reinduction at the time of weaning. The bile acid responsiveness of the ASBT gene is not clear and may be dependent on both the experimental model used and the species being investigated. Future studies of the transcriptional and posttranscriptional regulation of the ASBT gene and analysis of ASBT knockout mice will provide further insight into the biology, physiology, and pathophysiology of intestinal bile acid transport.

Association of bowel movement frequency and use of laxatives with the occurrence of symptomatic gallstone disease in a prospective study of women

OBJECTIVES

The authors prospectively examined the association between bowel movement frequency (used as a proxy for intestinal transit), laxative use, and the risk of symptomatic gallstone disease.

METHODS

A total of 79,829 women, aged 36-61 yr, without a history of symptomatic gallstone disease and free of cancer, responded to a mailed questionnaire in 1982 that assessed bowel movement frequency and use of laxatives. Between 1984 and 1996, 4,443 incident cases of symptomatic gallstone disease were documented. Relative risks (RRs) of symptomatic gallstone disease and 95% confidence intervals (CIs) were calculated using logistic regression.

RESULTS

After controlling for age and established risk factors, the multivariate RRs were, compared to women with daily bowel movements, 0.97 (95% CI 0.86-1.08) for women with bowel movements every third day or less, and 1.00 (95% CI 0.91-11.1) for women with bowel movement more than once daily. No trend was evident. As compared to women who never used laxatives in 1982, a significant modest inverse association was seen for monthly laxative use, with a multivariate RR of 0.84 (95% CI 0.72-0.98), and weekly to daily laxative use was associated with a RR of 0.88 (95% CI 0.78-1.02).

CONCLUSIONS

These findings do not support an association between infrequent bowel movements and risk of symptomatic gallstone disease in women, and indicate that simple questions directed at bowel movement frequency are unlikely to enhance our ability to predict risk of symptomatic gallstone disease. The slightly inverse association between use of laxatives and risk of symptomatic gallstone disease may be due to a mechanism that is not related to bowel movement frequency.

Slow transit constipation: a model of human gut dysmotility. Review of possible aetiologies

Slow transit constipation is a severe condition of gut dysmotility that predominantly affects young women and may result in surgical intervention. Current medical treatments for STC are often ineffective, and the outcome of surgery is unpredictable. STC was first described almost a century ago. Since this time, progress in improving therapy for this condition has been complicated by a lack of understanding of the aetiology, and great variation in the methods and criteria used for the study of patients with this debilitating disorder. It is difficult to find unequivocal data, and harder still to give a definitive picture of the cause or causes of STC. Here we consider the evidence for various aetiologies of STC, in the light of the physiological and pathological findings.

Mechanism of inhibition of Na+-bile acid cotransport during chronic ileal inflammation in rabbits

In the chronically inflamed ileum, unique mechanisms of alteration of similar transport processes suggest regulation by different immune-inflammatory mediator pathways. In a rabbit model of chronic ileitis, we previously demonstrated that Na+-glucose cotransport was inhibited by a decrease in the cotransporter numbers, whereas Na+-amino acid cotransport was inhibited by a decrease in the affinity for the amino acid. In this study, we demonstrated that Na+-bile acid cotransport was reduced in villus cells from the chronically inflamed ileum. In villus cell brush-border membrane vesicles from the chronically inflamed ileum, Na+-bile acid cotransport was reduced as well, suggesting a direct effect at the cotransporter itself. Kinetic studies demonstrated that Na+-bile acid cotransport was inhibited by both a decrease in the affinity as well as a decrease in the maximal rate of uptake of the bile acid. Analysis of steady-state mRNA and immunoreactive protein levels of the Na+-bile acid cotransporter also demonstrated some reduction during chronic ileitis. Thus, in the chronically inflamed ileum, the mechanisms of inhibition of Na+-glucose, Na+-amino acid, and Na+-bile acid cotransport are different. These data suggest that different cotransporters are uniquely altered either secondary to their intrinsic differences or by different immune-inflammatory mediators during chronic ileitis.

Beneficial effect of a bile acid resin binder on enteral feeding induced diarrhea

OBJECTIVES

Diarrhea is a complication of enteral feeding, occurring in up to 68% of critically ill patients. We hypothesized that prolonged fasting results in abnormal bile acid homeostasis. Subsequent enteral feeding then causes a relative luminal excess of bile acids, which leads to choleretic diarrhea. Hence, diarrhea induced by enteral feeding should improve with the use of a bile acid binding agent, such as Colestid Granules.

METHODS

We evaluated the effect of Colestid on enteral feeding-induced diarrhea in a double-blind placebo-controlled study. Nineteen patients who were nil per os (NPO) for 5 days before initiation of enteral feeding were enrolled in the study and treatment continued for 7 days. The severity and frequency of diarrhea were quantified. Fecal bile acids were measured enzymatically. Stool nutrient loss was measured by fat extraction, microkjeldahl determination of nitrogen, and bomb calorimetry of dried fecal specimens.

RESULTS

Enteral feeding resulted in a high frequency of diarrhea (95%) at some time during the observation period. The majority of episodes of diarrhea in both groups were of low volume. Colestid significantly decreased the prevalence and severity of diarrhea. Colestid had no significant effect on fecal calorie or nutrient losses. The average bile acid concentration in the stool increased significantly after enteral feeding.

CONCLUSION

Enteral feeding-induced diarrhea is, at least in part, due to malabsorption of bile acids. The bile acid resin binding agent Colestid improves diarrhea induced by enteral feeding.

Primary bile acid malabsorption caused by mutations in the ileal sodium-dependent bile acid transporter gene (SLC10A2)

Primary bile acid malabsorption (PBAM) is an idiopathic intestinal disorder associated with congenital diarrhea, steatorrhea, interruption of the enterohepatic circulation of bile acids, and reduced plasma cholesterol levels. The molecular basis of PBAM is unknown, and several conflicting mechanisms have been postulated. In this study, we cloned the human ileal Na+/bile acid cotransporter gene (SLC10A2) and employed single-stranded conformation polymorphism analysis to screen for PBAM-associated mutations. Four polymorphisms were identified and sequenced in a family with congenital PBAM. One allele encoded an A171S missense mutation and a mutated donor splice site for exon 3. The other allele encoded two missense mutations at conserved amino acid positions, L243P and T262M. In transfected COS cells, the L243P, T262M, and double mutant (L243P/T262M) did not affect transporter protein expression or trafficking to the plasma membrane; however, transport of taurocholate and other bile acids was abolished. In contrast, the A171S mutation had no effect on taurocholate uptake. The dysfunctional mutations were not detected in 104 unaffected control subjects, whereas the A171S was present in 28% of that population. These findings establish that SLC10A2 mutations can cause PBAM and underscore the ileal Na+/bile acid cotransporter's role in intestinal reclamation of bile acids.

Pediatric constipation

The purpose of this article is to present an overview of pediatric constipation. Discussion of the definition from a medical and nursing point of view is included. Intestinal pathophysiology as well as etiological theories of pediatric constipation are reviewed. Current research to date and clinical treatment and experience in the area are presented. A rationale for further nursing research in this area is described.

Intestinal transport of taurocholate in the cat

In the present study, the transport of taurocholate by brush border membrane vesicles (BBMV), prepared from the proximal and distal half of the small intestine of cats was investigated. Uptake of taurocholate (0.01 mmol/l) into BBMV from the distal small intestine was clearly enhanced in the presence of an inwardly directed initial Na+ gradient compared to choline+ gradient conditions, whereas uptake by BBMV from the proximal half of the small intestine was not substantially different between the two incubation conditions. Calculated uptake of taurocholate at a 10s incubation period was mainly due to transfer of taurocholate into the vesicular lumen rather than to extensive binding of taurocholate to the membranes. Evaluation of the kinetics of taurocholate transport across the brush border membrane of the distal half of the small intestine under Na+ gradient conditions revealed a saturable Na(+)-dependent component and a diffusive component with the subsequent apparent parameters: Vmax (maximal transport velocity) = 0.59 nmol/mg protein.10s, KM (affinity constant) = 0.12 mmol/l, D (diffusion constant) = 2.0 microliters/mg protein.10s. The results indicate Na+/taurocholate co-transport across the brush border membrane of the distal half of the small intestine in cats. Thus, the carrier-mediated intestinal bile salt absorption in the cat appears to be similar as described in other species.

Negative feedback regulation of the ileal bile acid transport system in rodents

BACKGROUND

Active transport of conjugated bile acids by ileal enterocytes is a key mechanism for conservation of the bile acid pool. Experiments were performed to determine whether such transport is regulated by substrate load.

METHODS

Using anesthetized biliary fistula guinea pigs or rats, the ileum was perfused with ursodeoxycholyltaurine at a concentration causing maximal ileal transport of this bile acid; absorption was assessed by biliary recovery. Before ileal perfusion, animals ingested one of three diets: chow, chow with added conjugated bile acid, or chow with added cholestyramine.

RESULTS

In the guinea pig, ingestion of a taurocholate-enriched diet resulted in a 75% decrease in the absorption rate of ursodeoxycholyltaurine. Similar results were obtained with cholylsarcosine (a deconjugation-dehydroxylation resistant analogue) or with chenodeoxycholylglycine, the endogenous bile acid of the guinea pig. In contrast, cholestyramine ingestion caused an increase in ursodeoxycholyltaurine absorption. In the rat, cholyltaurine or cholylsarcosine ingestion also caused decreased ileal transport. In the guinea pig, maximal down-regulation of active ileal bile acid transport occurred after 2-3 days of bile acid feeding; up-regulation required 3-4 days.

CONCLUSIONS

Bile acid metabolism is regulated by feedback inhibition of active ileal transport in addition to the well-established feedback inhibition of bile acid biosynthesis in the liver. Together, these two regulatory mechanisms ensure constancy of bile acid secretion.

Ileal mucosal absorption of bile acid in man: validation of a miniature flux chamber technique

A method that allows the quantitative assessment of ileal mucosal cell uptake and transport of bile acids in mucosal biopsy specimens has been validated. Viability of the tissue was confirmed by maintenance of normal cell morphology, wet weight, extracellular space, porosity to polyethylene glycol-900, lactate dehydrogenase release, and transmucosal potential difference. Using 14C-taurocholic acid, absorption was shown to be directional, capable of working against a concentration gradient, reduced by metabolic inhibitors, and sodium dependent. The system showed saturation kinetics with an estimated Km of 10 mumol/l. At a standard substrate concentration of 10 mumol/l ileal mucosal bile acid absorption was compared in patients with colorectal cancer (n = 6), ulcerative colitis (n = 10), and slow transit constipation (n = 8). There was no significant difference in tissue uptake or transport between the three groups.

Primary bile acid malabsorption: a pathophysiologic and clinical entity?

Primary bile acid malabsorption is defined as chronic diarrhoea with bile acid malabsorption of unknown cause and a symptomatic response to cholestyramine. Convincing evidence of the proposed pathophysiology--a defect of the active bile acid absorption in the distal ileum--has never been substantiated. We found no evidence of a bile acid transport defect across the ileal brush border membrane in 10 patients with primary bile acid malabsorption; moreover, transport was significantly higher than in a control group. In the patients with primary bile acid malabsorption the estimated bile acid pool was significantly larger than in a control group and in a group of patients with ileal disease. In addition, the oro-anal transit time of radiopaque markers was shorter in the primary bile acid malabsorption group than in both other groups. This suggests that the bile acid pool size as well as intestinal motility could play a role in the pathophysiology of primary bile acid malabsorption.

Primary bile acid diarrhoea without an ileal carrier defect: quantification of active bile acid transport across the ileal brush border membrane

Unexplained bile acid malabsorption associated with diarrhoea that responds to cholestyramine was first described in 1973 but convincing evidence of the proposed mechanism--a defective active ileal bile acid transport--has never been substantiated. Active bile acid transport was quantified in vitro using brush border membrane vesicles prepared from terminal ileal biopsy specimens from 10 patients who fulfilled the criteria of idiopathic bile acid diarrhoea. They were recruited from 181 patients with bile acid malabsorption of various causes. Transport was quantified as in vitro Na+ dependent bile acid transport (INBAT), expressed as pmol taurocholate/mg brush border membrane protein/15 seconds, and in vitro Na+ dependent bile acid local transport capacity (INBALTC), expressed as pmol taurocholate/g ileal biopsy tissue/15 seconds. The lowest INBAT and INBALTC values in the 10 patients with idiopathic bile acid diarrhoea were well above the 10th centile values of a control group of 132 patients. Both INBAT (mean (range) 88 (30-136)) and INBALTC (158 (85-268)) values were significantly higher in the 10 patients than in the control group (INBAT: mean (range) 63 (1-244), INBALTC: mean (range) 98 (1-408)). Quantification of active ileal bile acid transport in these 10 patients with idiopathic bile acid malabsorption suggests that a genetic (carrier) defect is rare in adults.

Upper gastrointestinal manifestations in rheumatoid arthritis patients: intrinsic or extrinsic pathogenesis?

Apart from the complication of gastrointestinal vasculitis it is not known whether the upper gastrointestinal (UGI) tract has any special disease characteristics in rheumatoid arthritis (RA). However, oesophageal motility disorders have been reported in 30% of RA patients. Hypergastrinaemia has been found in 23-43% of RA patients, usually in combination with a decreased gastric acid output. Another finding suggestive of a decreased secretory state, namely a decreased level of pepsinogen A, was found in RA patients with the sicca syndrome and in patients with active disease. The risk for peptic ulcer disease with regard to nonsteroidal anti-inflammatory drugs (NSAID) is possibly higher in RA patients than in patients with other rheumatic diseases. These findings suggest that in RA patients intrinsic factors play a role in the pathogenesis of the oesophageal motility disorders, in hypergastrinaemia and hypopepsinogenaemia and the related decreased gastric secretory state, and possibly in the increased susceptibility to NSAID-related ulcers. However, there are also indications that oesophageal motility disorders, hypergastrinaemia, and NSAID-related ulcers in RA are the result of extrinsic factors, mainly the use of NSAIDs. The effects of NSAIDs and gold compounds on infection with Helicobacter pylori, a possible pathogenetic factor in ulcer disease, is discussed. It is clear that the discussion about intrinsic and extrinsic factors as a cause of the UGI manifestations in RA remains an intriguing but difficult subject for further studies.