Evidence for size and charge permselectivity of rat ascending colon. Effects of ricinoleate and bile salts on oxalic acid and neutral sugar transport

Oxalate Flux Across the Intestine: Contributions from Membrane Transporters

Epithelial oxalate transport is fundamental to the role occupied by the gastrointestinal (GI) tract in oxalate homeostasis. The absorption of dietary oxalate, together with its secretion into the intestine, and degradation by the gut microbiota, can all influence the excretion of this nonfunctional terminal metabolite in the urine. Knowledge of the transport mechanisms is relevant to understanding the pathophysiology of hyperoxaluria, a risk factor in kidney stone formation, for which the intestine also offers a potential means of treatment. The following discussion presents an expansive review of intestinal oxalate transport. We begin with an overview of the fate of oxalate, focusing on the sources, rates, and locations of absorption and secretion along the GI tract. We then consider the mechanisms and pathways of transport across the epithelial barrier, discussing the transcellular, and paracellular components. There is an emphasis on the membrane-bound anion transporters, in particular, those belonging to the large multifunctional Slc26 gene family, many of which are expressed throughout the GI tract, and we summarize what is currently known about their participation in oxalate transport. In the final section, we examine the physiological stimuli proposed to be involved in regulating some of these pathways, encompassing intestinal adaptations in response to chronic kidney disease, metabolic acid-base disorders, obesity, and following gastric bypass surgery. There is also an update on research into the probiotic, Oxalobacter formigenes, and the basis of its unique interaction with the gut epithelium. © 2021 American Physiological Society. Compr Physiol 11:1-41, 2021.

The mechanistic basis of hyperoxaluria following gastric bypass in obese rats

Roux-en-Y gastric bypass (RYGB) surgery is a popular and extremely effective procedure for sustained weight loss in the morbidly obese. However, hyperoxaluria and oxalate kidney stones frequently develop after RYGB and steatorrhea has been speculated to play a role. We examined the effects of RYGB and the role of dietary fat in an obese rat model by measuring fecal fat content and transmural oxalate fluxes across the distal colon compared to sham-operated controls (SHAM). Direct measurements of fecal fat content confirmed that RYGB on a 10 % fat diet excreted 40-fold more fecal fat than SHAM and, on a 40 % fat diet, RYGB excreted sevenfold more fecal fat than SHAM fed similarly. Results from the transport studies revealed a clear effect of high dietary fat (40 %) on colonic oxalate permeability and tissue conductance (G T) with comparable oxalate fluxes in RYGB and in SHAM. Administering a diet containing 10 % fat to both groups distinguished differences between RYGB and SHAM, revealing a 40 % increase in G T in RYGB and a reversal in the direction of net oxalate flux from absorption in SHAM to secretion in RYGB. These changes in colonic oxalate permeability were associated with a fourfold increase in urinary oxalate excretion in RYGB compared to SHAM. Therefore, oxalate solubility and permeability in the RYGB model are promoted by steatorrhea and result in enhanced passive oxalate absorption and hyperoxaluria. To our knowledge, these are the first measurements of intestinal oxalate transport in rats with RYGB.

Risk factors for nephrolithiasis in patients with ileal pouches

BACKGROUND AND AIMS

Restorative proctocolectomy with ileal pouch-anal anastomosis (IPAA) has become a standard of care in patients with ulcerative colitis (UC). Nephrolithiasis is common in patients with inflammatory bowel disease (IBD), but has never been studied as a complication of IPAA. We aimed to assess the risk factors for nephrolithiasis in patients with IPAA.

METHODS

Using an IRB-approved, prospectively maintained pouch registry, we identified 1221 patients between 2000 and 2010. Those with post-IPAA nephrolithiasis served as the study group whereas IPAA patients without nephrolithiasis served as the controls. Demographic and clinical variables were analyzed using multivariable logistic regression to identify risk factors.

RESULTS

There were a total of 218 IPAA patients: 81 with nephrolithiasis (37%) and 137 without (63%). Of the 81 patients in the study group, 17 were excluded due to limited clinical data. Three risk factors were found to be associated with nephrolithiasis: the presence of extra-intestinal manifestations (odd's ratio [OR]=2.9, 95% confidence interval [CI]: 1.4, 5.8, p=0.003), no use of antibiotics (OR=3.2, 95% CI: 1.5, 6.5, p=0.002) and low serum bicarbonate level (OR=0.87, 95% CI: 0.77, 0.99, p=0.038).

CONCLUSION

Nephrolithiasis was a common finding in our patients with IPAA. As pouch patients with nephrolithiasis can develop adverse clinical complications, those with at least one of the risk factors we identified may need to be monitored more closely and possibly receive prophylactic treatment with oral bicarbonate.

Net intestinal transport of oxalate reflects passive absorption and SLC26A6-mediated secretion

Mice lacking the oxalate transporter SLC26A6 develop hyperoxalemia, hyperoxaluria, and calcium-oxalate stones as a result of a defect in intestinal oxalate secretion, but what accounts for the absorptive oxalate flux remains unknown. We measured transepithelial absorption of [(14)C]oxalate simultaneously with the flux of [(3)H]mannitol, a marker of the paracellular pathway, across intestine from wild-type and Slc26a6-null mice. We used the anion transport inhibitor DIDS to investigate other members of the SLC26 family that may mediate transcellular oxalate absorption. Absorptive flux of oxalate in duodenum was similar to mannitol, insensitive to DIDS, and nonsaturable, indicating that it is predominantly passive and paracellular. In contrast, in wild-type mice, secretory flux of oxalate in duodenum exceeded that of mannitol, was sensitive to DIDS, and saturable, indicating transcellular secretion of oxalate. In Slc26a6-null mice, secretory flux of oxalate was similar to mannitol, and no net flux of oxalate occurred. Absorptive fluxes of both oxalate and mannitol varied in parallel in different segments of small and large intestine. In epithelial cell lines, modulation of the charge selectivity of the claudin-based pore pathway did not affect oxalate permeability, but knockdown of the tight-junction protein ZO-1 enhanced permeability to oxalate and mannitol in parallel. Moreover, formation of soluble complexes with cations did not affect oxalate absorption. In conclusion, absorptive oxalate flux occurs through the paracellular "leak" pathway, and net absorption of dietary oxalate depends on the relative balance between absorption and SLC26A6-dependent transcellular secretion.

Net intestinal transport of oxalate reflects passive absorption and SLC26A6-mediated secretion

Mice lacking the oxalate transporter SLC26A6 develop hyperoxalemia, hyperoxaluria, and calcium-oxalate stones as a result of a defect in intestinal oxalate secretion, but what accounts for the absorptive oxalate flux remains unknown. We measured transepithelial absorption of [(14)C]oxalate simultaneously with the flux of [(3)H]mannitol, a marker of the paracellular pathway, across intestine from wild-type and Slc26a6-null mice. We used the anion transport inhibitor DIDS to investigate other members of the SLC26 family that may mediate transcellular oxalate absorption. Absorptive flux of oxalate in duodenum was similar to mannitol, insensitive to DIDS, and nonsaturable, indicating that it is predominantly passive and paracellular. In contrast, in wild-type mice, secretory flux of oxalate in duodenum exceeded that of mannitol, was sensitive to DIDS, and saturable, indicating transcellular secretion of oxalate. In Slc26a6-null mice, secretory flux of oxalate was similar to mannitol, and no net flux of oxalate occurred. Absorptive fluxes of both oxalate and mannitol varied in parallel in different segments of small and large intestine. In epithelial cell lines, modulation of the charge selectivity of the claudin-based pore pathway did not affect oxalate permeability, but knockdown of the tight-junction protein ZO-1 enhanced permeability to oxalate and mannitol in parallel. Moreover, formation of soluble complexes with cations did not affect oxalate absorption. In conclusion, absorptive oxalate flux occurs through the paracellular "leak" pathway, and net absorption of dietary oxalate depends on the relative balance between absorption and SLC26A6-dependent transcellular secretion.

Metabolic, renal, and nutritional consequences of bariatric surgery: implications for the clinician

Management of obesity-associated comorbidities costs about $60 billion/year, about 5% of total US healthcare expenditure. Bariatric surgery is the only proven effective weight loss therapy for severely obese patients with a BMI > or =35 kg/m2. Bariatric surgery produces long-term weight loss, improves quality of life, and reduces the number of sick days and medication costs. Surgery has a profound effect on the metabolic milieu and nutritional status from the first few days after surgery, even before significant weight loss has been achieved. Metabolic effects of bariatric surgery reduce obesity-related comorbidities like type 2 diabetes, hypertension, metabolic syndrome, and cardiovascular disease risk. Improvement in renal function is seen, but adverse effects like oxalate nephropathy can lead to chronic kidney disease or end-stage renal disease (CKD/ESRD). Surgery can also lead to micronutrient deficiencies, making dietary supplementation necessary. Reduction in insulin resistance and hypertension after surgery makes medication adjustment imperative. Improvement in comorbidities and nutritional deficiencies after bariatric surgery has important clinical implications.

Obesity and urolithiasis

The current obesity epidemic in the United States has deleterious effects on the health of the population. Temporally related to the increase in obesity is an increase in the prevalence of urolithiasis. Epidemiologic studies have shown that the incident stone risk increases with body mass index. Obesity can increase stone risk in multiple ways. Excess nutritional intake increases traffic of lithogenic substances such as calcium, oxalate, and uric acid. Metabolic syndrome, commonly associated with obesity, alters renal acid-base metabolism, resulting in a lower urine pH and increased risk of uric acid stone disease. The low urine pH is caused by deficient ammonia production, which appears to be related to insulin resistance. Even weight-loss programs to combat obesity can influence stone risk. Contemporary bariatric surgery has been shown to frequently cause hyperoxaluria with associated stone formation and even oxalate nephropathy. Commonly used low-carbohydrate diets increase the risk of both calcium and uric acid stones. Certainly, the many health risks of obesity, including urolithiasis, necessitate weight loss, but recognition of the potential complications of such therapies is required to prevent induction of new and equally severe medical problems. The optimal approach to weight control that minimizes stone risk needs to be determined.

The roles and mechanisms of intestinal oxalate transport in oxalate homeostasis

The mammalian intestine has an important role in the dynamics of oxalate exchange and thereby is significant in the etiology of calcium oxalate nephrolithiasis. Here we review some of the phenomenologic observations that have led to the conclusion that anion exchangers (antiporters) are important mediators of secondarily active, net oxalate transport along the intestine (both absorptive and secretory). Understanding the mechanisms of transepithelial oxalate transport has been advanced radically in recent years by the identification of the solute-linked carrier (SLC)26 family of anion transporters, which has facilitated the identification of specific proteins mediating individual apical or basolateral oxalate transport pathways. Moreover, identification of specific exchangers has underscored their relative importance to oxalate homeostasis as revealed by using knockout mouse models and has facilitated studies of oxalate transport regulation in heterologous expression systems. Finally, the significance of oxalate degrading bacteria to oxalate homeostasis is considered from basic and applied perspectives.

The malabsorptive very, very long limb Roux-en-Y gastric bypass for super obesity: results in 257 patients

BACKGROUND

For the past 11 years, we have used a malabsorptive form of Roux-en-Y gastric bypass (RYGB), the "very, very long limb" RYGB, for selected patients with BMIs >50 kg/m(2) and in highly selected patients with BMI <50 kg/m(2). This modified distal gastric bypass establishes a 100-cm common channel (for digestion and absorption) and a "very, very" long Roux limb of 400 to 500 cm.

METHODS

To determine long-term efficacy and complications, we followed prospectively 257 consecutive patients; 188 (73%) participated in a postoperative survey.

RESULTS

Of the patients, 60% were female; overall age (x +/- SD) was 45 +/- 11 years, and BMI was 61 +/- 11 kg/m(2). Operative mortality was 1% with substantive postoperative morbidity occurring in 13%. Eighty-two percent of patients returning the survey an average of 48 months postoperatively (range, 12 to 148 months) lost >50% of excess body weight; BMI at follow-up was 37 +/- 9 kg/m(2). Resolution of comorbidities included diabetes mellitus (94%), hypertension (65%), sleep apnea (48%), and asthma (30%). Side effects included mild food intolerance (82%), occasional loose or watery stools (71%), nephrolithiasis (16%), and symptomatic steatorrhea (5%). Nine patients (4%) who developed or were developing impending protein/calorie malnutrition required proximal relocation of the enteroenterostomy with symptom resolution.

CONCLUSIONS

Overall, 90% were satisfied with the operation, and 93% would recommend it to a friend. The very, very long limb RYGB is relatively safe and effective and has acceptable side effects in the treatment of selected patients with super obesity (BMI >50). Because of the possibility of malabsorptive sequelae, patients should be selected based on degree of medical sophistication, insight, and compliance.

Gastrointestinal oxalic acid absorption in calcium-treated rats

We studied whether urinary oxalate excretion after an acute oral load of oxalic acid is influenced by concomitant administration of calcium in rats. Male Wistar rats weighing approximately 180 g were divided into six groups of five animals each. After inducing anesthesia, the animals were orally (via a gastrostomy) given 110 micromol of oxalic acid along with 0, 27.5, 55, 110, or 220 micromol of calcium (0, 27.5, 55, 110, or 220 micromol Ca group, respectively). Saline was given to the control group instead of oxalic acid. Urine specimens were collected before administration and then at hourly intervals up to 5 h afterward. Urinary oxalate and citrate levels were measured by capillary electrophoresis, while urinary calcium, magnesium, and phosphorus levels were measured by ICP spectrophotometry. Urinary oxalate excretion peaked at 1 h after administration and was higher in the 0, 27.5, and 55 micromol Ca groups than in the control group. The urinary recovery of oxalate in these groups was 10-15%, while the recovery rate was less than 3% in other groups. Urinary Ca excretion showed no significant changes, either over time or between groups. Free oxalic acid is absorbed more readily from the gastrointestinal tract than calcium oxalate, while simultaneous administration of calcium appears to block intestinal oxalic acid absorption in a dose-dependent manner.

Intestinal transport of an obdurate anion: oxalate

In this review, we focus on the role of gastrointestinal transport of oxalate primarily from a contemporary physiological standpoint with an emphasis on those aspects that we believe may be most important in efforts to mitigate the untoward effects of oxalate. Included in this review is a general discussion of intestinal solute transport as it relates to oxalate, considering cellular and paracellular avenues, the transport mechanisms, and the molecular identities of oxalate transporters. In addition, we review the role of the intestine in oxalate disease states and various factors affecting oxalate absorption.

Stones from bowel disease

Kidney stones are increased in patients with bowel disease, particularly those who have had resection of part of their gastrointestinal tract. These stones are usually CaOx, but there is a marked increase in the tendency to form uric acid stones, as well, particularly in patients with colon resection. These patients all share a tendency to chronic volume contraction due to loss of water and salt in diarrheal stool, which leads to decreased urine volumes. They also have decreased absorption, and therefore diminished urinary excretion, of citrate and magnesium, which normally act as inhibitors of CaOx crystallization. Patients with colon resection and ileostomy form uric acid stones, as loss of bicarbonate in the ileostomy effluent leads to formation of an acid urine. This, coupled with low urine volume, decreases the solubility of uric acid, causing crystallization and stone formation. Prevention of stones requires treatment with alkalinizing agents to raise urine pH to about 6.5, and attempts to increase urine volume, which increases the solubility of uric acid and prevents crystallization. Patients with small bowel resection may develop steatorrhea; if the colon is present, they are at risk of hyperoxaluria due to increased permeability of the colon to oxalate in the presence of fatty acids, and increased concentrations of free oxalate in the bowel lumen due to fatty acid binding of luminal calcium. EH leads to supersaturation of urine with respect to CaOx, in conjunction with low volume, hypocitraturia and hypomagnesuria. Therapy involves a low-fat, low-oxalate diet, attempts to increase urine volume, and agents such as calcium given to bind oxalate in the gut lumen. Correction of hypocitraturia and hypomagnesuria are also helpful.

Renal stone disease in children

Renal stone disease has been regarded as an uncommon problem in children compared to adults. However, increased awareness of this problem in children may lead to early intervention preventing long-term consequences on the kidney and the urinary tract. This article reviews the epidemiology, pathogenesis, and the most common etiologies of renal stones in children. The clinical features and diagnostic and therapeutic modalities for the specific etiologies are also outlined. Using these guidelines may be helpful not only in the treatment but also in the prevention of renal stones.

Excessive urinary oxalate excretion occurs in long-term TPN patients both with and without ileostomies

OBJECTIVE

To determine if excessive oxalate and deficient citrate excretion were associated with TPN-associated nephropathy.

DESIGN

Crossectional cohort.

SETTING

Outpatient clinic.

SUBJECTS

Twenty-five patients (15 males, 10 females) aged 51 +/- 17 (mean +/- SD) years who had received home total parenteral nutrition (TPN) for 10 +/- 4 years. Fifteen subjects had ileostomies (Group A) and 10 had functional colons (Group B).

OUTCOME MEASURES

Glomerular filtration rate (GFR), tubular reabsorption of phosphate (TRP), urinary oxalate and citrate excretion.

RESULTS

The mean GFR was 68.1 +/- 34.5 ml/minute/1.73 m2 and did not differ between Groups A and B. The mean TRP was 65.0 +/- 32.2% for Group A and 80.5 +/- 16.0% for Group B. The difference was not statistically significant. Urinary oxalate and citrate excretion were 40.2 +/- 30.2 and 324.4 +/- 239.0 mg/day respectively for Group A, and 63.2 +/- 34.2 and 474.8 +/- 936.3 respectively for Group B. The differences were not statistically significant. Thirty-eight percent (38%) of patients with ileostomies and 78% of patients without ileostomies had excessive urinary oxalate excretion (> 40 mg/day). Fifteen percent (15%) of patients with ileostomies and 50% of patients without ileostomies had decreased urinary citrate excretion (< 140 mg/day).

CONCLUSIONS

Increased endogenous oxalate production may occur in patients receiving long-term TPN.

Nitric oxide involvement in sodium choleate-induced fluid secretion and diarrhoea in rats

Bile salt-induced diarrhoea, net water and electrolyte secretion, gastrointestinal transit and nitric oxide (NO) synthase activity were studied in rats. NG-Nitro-L-arginine methyl ester (2.5-25 mg/kg i.p.), an inhibitor of NO synthase, and dexamethasone (0.03-0.3 mg/kg i.p.), an inhibitor of the inducible isoform of NO synthase, antagonized the diarrhoeal response. The NO precursor, L-arginine and isosorbide-5-mononitrate (an NO donor), reversed the inhibitory effect of NG-nitro-L-arginine methyl ester. The bile salt-stimulated fluid secretion, transit through the gut and NO synthase all were inhibited by NG-nitro-L-arginine methyl ester (but not NG-nitro-D-arginine methyl ester). NO synthase activity also was inhibited by dexamethasone. The results are consistent with bile salt induction of epithelial cell injury and concomitant synthesis of NO, mainly through activation of the inducible form of the enzyme. We believe that in this study NO is a mediator of intestinal secretion and motility changes that enhance transit of luminal contents through the gut, resulting in diarrhoea.

Pathogenesis of nephrolithiasis post-partial ileal bypass surgery: case-control study. The POSCH Group

Between 1975 and 1983, 838 patients were randomized into the Program on the Surgical Control of Hyperlipidemias (POSCH) trial: 417 to standard medical care and 421 to partial ileal bypass (PIB) surgery. During the course of the trial, an increased incidence of kidney stone formation was found in the surgery group (4%/year) as compared to the control group (0.4%/year). A matched triplet case-control study was conducted to assess the possible causes for the increased incidence of kidney stones. Three groups were studied: PIB stone-formers (S); PIB non-stone formers (N); and non-PIB, non-stone formers in the control group (C). Initially, 162 patients (54 triplets) were selected. Ten percent of the patients declined to participate which resulted in a sample size of 146 patients. The PIB patients had statistically significant (P less than 0.05) lower levels of serum vitamin D metabolites; lower urine volume, pH, citrate, magnesium, carbon dioxide, and sulfate, and higher urinary oxalate, ammonia and relative supersaturation for calcium oxalate and uric acid than the control patients. Although S and N had similar results, those S with no prior history of stones had a higher calcium oxalate supersaturation than similar N with a negative prior history of stones (P less than 0.025). Based on these results, all PIB patients appear to be at risk for kidney stone formation. The combination of reduced urinary volume and calcium oxalate precipitation inhibitor substance with increased calcium oxalate relative supersaturation produced an increase in nephrolithiasis risk in the PIB groups.

Intestinal permeability changes in rodents: a possible mechanism for degraded carrageenan-induced colitis

Rats and guinea-pigs were treated with degraded carrageenan (50 g/litre in the drinking-water) and their intestinal permeability was studied at weekly intervals over the last 4 wk of the test period by determining the recovery of orally administered tracer doses of [3H]polyethylene glycol (PEG-900) or D-[3H]mannitol in 16-hr urine collections. A freely diffusible dye, Azure A, was administered simultaneously to compensate for non-intestinal factors that could modify renal excretion. Animals were killed after a total treatment period of 5 months for rats and 6 wk for guinea-pigs. After 3 wk of carrageenan treatment, excretion of PEG-900 (expressed as a ratio of the Azure A excretion) in guinea-pigs showed a statistically significant increase over that in the control group. At autopsy, the caeca showed numerous macroscopically visible erosions of the entire mucosal surface and histological examination showed ulcerations largely in the mucosa with abscesses in the crypts. Although no such histological changes were seen in the intestines of the treated rats, even after 5 months, a statistically significant increase in PEG-900 excretion was again found compared with the control group. This increase did not occur when deoxycholate was administered with the carrageenan solution. No effect of carrageenan treatment on mucosal permeability to D-[3H]mannitol was demonstrated in either species. The results suggest that degraded carrageenan-induced colitis could be a result of increased intestinal permeability, since ingestion of this polysaccharide by rats increased PEG-900 absorption without causing mucosal damage.

Oxalate transport by anion exchange across rabbit ileal brush border

This study demonstrates the presence of oxalate transporters on the brush border membrane of rabbit ileum. We found that an inside alkaline (pH = 8.5 inside, 6.5 outside) pH gradient stimulated [14C]oxalate uptake 10-fold at 1 min with a fourfold accumulation above equilibrated uptake at 5 min. 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonate (disodium salt; DIDS) profoundly inhibited the pH-gradient stimulated oxalate uptake. Using an inwardly directed K+ gradient and valinomycin, we found no evidence for potential sensitive oxalate uptake. In contrast to Cl:HCO3 exchange, HCO3 did not stimulate oxalate uptake more than was seen with a pH gradient in the absence of HCO3. An outwardly directed Cl gradient (50 mM inside, 5 mM outside) stimulated oxalate uptake 10-fold at 1 min with a fivefold accumulation above equilibrated uptake. Cl-stimulated oxalate uptake was largely inhibited by DIDS. Addition of K+ and nigericin only slightly decreased the Cl gradient-stimulated oxalate uptake, which indicates that this stimulation was not primarily due to the Cl gradient generating an inside alkaline pH gradient via Cl:OH exchange. Further, an outwardly directed oxalate gradient stimulated 36Cl uptake. These results suggested that both oxalate:OH and oxalate:Cl exchange occur on the brush border membrane. To determine if one or both of these exchanges were on contaminating basolateral membrane, the vesicle preparation was further fractionated into a brush border and basolateral component using sucrose density gradient centrifugation. Both exchangers localized to the brush border component. A number of organic anions were examined (outwardly directed gradient) to determine if they could stimulate oxalate and Cl uptake. Only formate and oxaloacetate were found to stimulate oxalate and Cl uptake. An inwardly directed Na gradient only slightly stimulated oxalate uptake, which was inhibited by DIDS.