Escherichia coli Heat Stable (STa) Enterotoxin and the Upper Small Intestine: Lack of Evidence in Vivo for Net Fluid Secretion

Linaclotide improves gastrointestinal transit in cystic fibrosis mice by inhibiting sodium/hydrogen exchanger 3

Gastrointestinal dysfunction in cystic fibrosis (CF) is a prominent source of pain among patients with CF. Linaclotide, a guanylate cyclase C (GCC) receptor agonist, is a US Food and Drug Administration-approved drug prescribed for chronic constipation but has not been widely used in CF, as the cystic fibrosis transmembrane conductance regulator (CFTR) is the main mechanism of action. However, anecdotal clinical evidence suggests that linaclotide may be effective for treating some gastrointestinal symptoms in CF. The goal of this study was to determine the effectiveness and mechanism of linaclotide in treating CF gastrointestinal disorders using CF mouse models. Intestinal transit, chloride secretion, and intestinal lumen fluidity were assessed in wild-type and CF mouse models in response to linaclotide. CFTR and sodium/hydrogen exchanger 3 (NHE3) response to linaclotide was also evaluated. Linaclotide treatment improved intestinal transit in mice carrying either F508del or null Cftr mutations but did not induce detectable Cl^- secretion. Linaclotide increased fluid retention and fluidity of CF intestinal contents, suggesting inhibition of fluid absorption. Targeted inhibition of sodium absorption by the NHE3 inhibitor tenapanor produced improvements in gastrointestinal transit similar to those produced by linaclotide treatment, suggesting that inhibition of fluid absorption by linaclotide contributes to improved gastrointestinal transit in CF. Our results demonstrate that linaclotide improves gastrointestinal transit in CF mouse models by increasing luminal fluidity through inhibiting NHE3-mediated sodium absorption. Further studies are necessary to assess whether linaclotide could improve CF intestinal pathologies in patients. GCC signaling and NHE3 inhibition may be therapeutic targets for CF intestinal manifestations. NEW & NOTEWORTHY Linaclotide's primary mechanism of action in alleviating chronic constipation is through cystic fibrosis transmembrane conductance regulator (CFTR), negating its use in patients with cystic fibrosis (CF). For the first time, our findings suggest that in the absence of CFTR, linaclotide can improve fluidity of the intestinal lumen through the inhibition of sodium/hydrogen exchanger 3. These findings suggest that linaclotide could improve CF intestinal pathologies in patients.

An investigation into the relationship between small intestinal fluid secretion and systemic arterial blood pressure in the anesthetized rat

The effects of changes in the steady level of diastolic blood pressure on fluid flux across the jejunum has been investigated in the anesthetized rat during perfusion with a nutrient-free and Na⁺-free solution. Diastolic blood pressure was manipulated by intravenous infusions, during the jejunal perfusions, of vasodilators (vasoactive intestinal polypeptide, acetyl-β-methylcholine, and phentolamine) and a vasoconstrictor (arginine vasopressin), each of which acts through a different cellular mechanism. The outcome was that fluid flux was related by a parabolic relationship with diastolic blood pressure in which net secretion occurred over the range 40–100 mmHg, whereas net absorption was recorded at diastolic pressures exceeding 100 mmHg and below 40 mmHg. Against a background of normal absorption promoted by perfusion with 145 mmol L⁻¹ Na⁺/5 mmol L⁻¹ glucose solution, reductions in diastolic blood pressure markedly reduced the mean rate of fluid absorption by 58% overall, whereas the rate of glucose absorption remained unchanged. Our results were explained on the basis that vasodilatation led to increased capillary pressure and then to net filtration of fluid from the mesenteric capillary bed. Experiments in which Escherichia coli heat-stable toxin was added to the jejunal perfusate confirmed the absence of a secretory response, which was consistent with the absence of effect of the toxin on diastolic blood pressure.

Recent advances in understanding enteric pathogenic Escherichia coli

Although Escherichia coli can be an innocuous resident of the gastrointestinal tract, it also has the pathogenic capacity to cause significant diarrheal and extraintestinal diseases. Pathogenic variants of E. coli (pathovars or pathotypes) cause much morbidity and mortality worldwide. Consequently, pathogenic E. coli is widely studied in humans, animals, food, and the environment. While there are many common features that these pathotypes employ to colonize the intestinal mucosa and cause disease, the course, onset, and complications vary significantly. Outbreaks are common in developed and developing countries, and they sometimes have fatal consequences. Many of these pathotypes are a major public health concern as they have low infectious doses and are transmitted through ubiquitous mediums, including food and water. The seriousness of pathogenic E. coli is exemplified by dedicated national and international surveillance programs that monitor and track outbreaks; unfortunately, this surveillance is often lacking in developing countries. While not all pathotypes carry the same public health profile, they all carry an enormous potential to cause disease and continue to present challenges to human health. This comprehensive review highlights recent advances in our understanding of the intestinal pathotypes of E. coli.

Enteroendocrine and neuronal mechanisms in pathophysiology of acute infectious diarrhea

BACKGROUND

While enterocyte secretion is the predominant mechanism considered responsible for secretory diarrhea in response to acute enteric infections, there are several lines of evidence that support alternative mechanisms controlling fluid and electrolyte secretion in diarrhea.

AIM

To review enteroendocrine and neuronal mechanisms that participate in the development of acute infectious diarrhea.

RECENT ADVANCES

Acute infectious diarrheas due to bacterial toxins (e.g., cholera, E. coli heat-stable enterotoxin, C. difficile) and rotavirus are all associated with secretion of transmitters from enteroendocrine cells (e.g., 5-HT) and activation of afferent neurons that stimulate submucosal secretomotor neurons. The latter secrete acetylcholine (which binds to muscarinic receptors on epithelial cells) and VIP. Involvement of nerves was demonstrated by inhibition of bacterial toxin-induced secretion by hexamethonium (nicotinic), tetrodotoxin (Na(+) channel blocker), and lidocaine (visceral/mucosal afferents). Nicotinic receptors are present on secretomotoneurons and these are activated by release of acetylcholine from enteric interneurons or extrinsic efferent fibers. Specific organisms also modify other mechanisms that may contribute to development of acute diarrhea. Thus, mucin secretion, activation of motor mechanisms, increased mucosal permeability and inhibition of bile acid absorption have been reported in specific types of acute infectious diarrhea.

CONCLUSION

New therapies targeting neural and transmitter mediation including 5-HT, VIP, NPY, as well as toxin receptors and channels activated during acute infectious diarrhea could usher in a novel approach to enhancing glucose-electrolyte solutions used in the treatment of acute diarrhea.

Lack of Restoration in Vivo by K-Channel Modulators of Jejunal Fluid Absorption after Heat Stable Escherichia coli Enterotoxin (STa) Challenge

Enhanced potassium ion permeability at the enterocyte basolateral membrane is assumed to facilitate sustained chloride ion and fluid secretion into the intestinal lumen during episodes of secretory diarrhoeal disease. To examine this concept in vivo, two potassium ion channel blockers and a channel opener were coperfused with E. coli heat stable STa enterotoxin to determine whether such compounds improved or worsened the inhibited fluid absorption. In the STa (80 ng/mL) challenged jejunal loop, the fluid absorption rate of 28.6 ± 5.8 (14) μL/cm/hr was significantly below (P < .001) the normal rate of 98.8 ± 6.2 (17) μL/cm/hr. Intraluminal (300 uM) glibenclamide added to STa perfused loops failed to improve the inhibited fluid absorption rate, which was 7.4 ± 3.2 (6) μL/cm/hr on coperfusion with STa. Similarly, on coperfusion with 30 uM clotrimazole, the fluid absorption rate with STa present remained inhibited at 11.4 ± 7.0 (4) μL/cm/hr. On coperfusion with intraluminal 1 uM cromakalim, STa reduced fluid absorption significantly (P < .02) to 24.7 ± 8.0 (10) μL/cm/hr, no different from STa challenge in the absence of cromakalim. Infusion i.v. with these agents also failed to restore fluid absorption after STa challenge. These observations do not support the proposed potassium ion permeability event as a necessary corollary of enterotoxin-mediated secretion. This makes it unlikely that modulators of such permeability prevent enterocyte secretion in diarrhoeal disease.

Cure and curse: E. coli heat-stable enterotoxin and its receptor guanylyl cyclase C

Enterotoxigenic Escherichia coli (ETEC) associated diarrhea is responsible for roughly half a million deaths per year, the majority taking place in developing countries. The main agent responsible for these diseases is the bacterial heat-stable enterotoxin STa. STa is secreted by ETEC and after secretion binds to the intestinal receptor guanylyl cyclase C (GC-C), thus triggering a signaling cascade that eventually leads to the release of electrolytes and water in the intestine. Additionally, GC-C is a specific marker for colorectal carcinoma and STa is suggested to have an inhibitory effect on intestinal carcinogenesis. To understand the conformational events involved in ligand binding to GC-C and to devise therapeutic strategies to treat both diarrheal diseases and colorectal cancer, it is paramount to obtain structural information on the receptor ligand system. Here we summarize the currently available structural data and report on physiological consequences of STa binding to GC-C in intestinal epithelia and colorectal carcinoma cells.

Diarrhoeal disease through enterocyte secretion: a doctrine untroubled by proof

For almost 40 years, one of the principal causes of diarrhoeal disease has been thought to be fluid secretion emanating from the epithelial cells of the small and large intestine. Given the extremely large fluid losses seen in cholera, where secretion can be up to several litres per day, this seems a plausible hypothesis. The enterocyte (epithelial cell) secretion hypothesis rapidly displaced all other alternatives, such as vasodilatation coupled with enhanced paracellular permeability. An essential mechanism underlying enterocyte secretion has always been assumed to be electrogenic chloride secretion, leading to a localized osmotic imbalance at the mucosal surface of the enterocytes that causes fluid entry into the lumen by osmosis. The chloride secretion basis for enterotoxin-deranged secretion is assumed to be measurable by changes in electrical currents and by altered transport of chloride ion. These can be detected after the small intestine is exposed to a heat-stable enterotoxin (STa) produced by Escherichia coli. However, in vivo, when the recovered volume technique is used, STa is found not to be secretory. The heat-stable enterotoxin is therefore a test case toxin, because the complex techniques used to demonstrate enterocyte secretion after STa exposure show apparent secretion, while the simplest technique based on fluid recovery and genuinely measuring the mass transport of fluid does not. This review scrutinizes the nature of the evidence put forward for enterocyte secretion and reaches the conclusion that there is no evidence for it. Debilitating secretion undoubtedly does take place in severe diarrhoeal disease, but secretion from enterocytes is unlikely to be the cause.

Pathophysiology of diarrhea in calves

Infectious diarrhea in calves is most commonly associated with enterotoxigenic Escherichia coli, Cryptosporidium parvum, rotavirus, coronavirus, or some combination of these pathogens. Each of these agents leads to diarrhea through either secretion or malabsorption/maldigestion, though the specific mechanisms and pathways may differ. Specific pharmacologic control and treatment are dependent on gaining a greater understanding of the pathophysiology of these organisms.

Enterocyte chloride and water secretion into the small intestine after enterotoxin challenge: unifying hypothesis or intellectual dead end?

Many forms of diarrhoeal disease, particularly so called "secretory" diarrhoeal disease are thought to arise by the active secretion of chloride ion from the enterocytes, creating an osmotic gradient for fluid movement into the small intestinal lumen. This model implies that normally occurring intestinal secretion is catastrophically enhanced by bacterial enterotoxins. This review advocates that neither normal nor abnormal intestinal secretion from the enterocytes occurs and that no competent proof for chloride secretion exists. Prior to 1970, the physiological evidence failed to support the concept of the formation of intestinal juice as a normal intestinal event. The concept was later revived to explain the high rate of fluid entry into the lumen after exposure to cholera toxin. Much evidence has been advanced for the chloride secretion hypothesis, the dominant secretory paradigm after 1974, but is the evidence sufficiently compelling for it to be regarded as proving the chloride secretory model? The evidence falls into four categories and a fifth conjectural argument that proposes that an abnormal chloride ion channel in cystic fibrotic sufferers confers a natural selective advantage by preventing diarrhoeal disease. Secretion is putatively demonstrated by 1) showing that mass transfer of fluid is into the lumen (secretion) and not merely a failure to transport out of the lumen (failed absorption). Support is offered by 2) chloride ion flux measurements in vitro in Ussing chambers and by 3) short-circuit current measurements that are consistent with and purport to show chloride ion movement into the lumen. In addition, 4) pharmacological agents are identified that affect short-circuit current and these are assumed to be anti-secretory, consistent with the biochemical mechanism for secretion, confirmed wherever possible by mouse knock-out models. Finally, the proxy methods used to study water movement such as elevated short-circuit current measurements show these to be absent in cystic fibrotic patients. The enterocyte secretion hypothesis is challenged here on the basis of an examination of the methods used to show secretion, particularly after exposing the small intestine to heat stable enterotoxin (STa) from E. coli. STa is thought to be secretory because fluid entry into the lumen is claimed, enhanced isotopic flux of chloride ion towards the lumen occurs, an increase in short-circuit current is found, preventable by various drugs that are deemed likely to be anti-secretory and also because the short-circuit current changes after STa are not seen in cystic fibrotic patients. Using volume recovery in vivo, STa is found not to be secretory but only anti-absorptive. Hence, other techniques used to show secretion are not fit for that purpose. If STa is identified as secretory and yet no secretion occurs, how reliable is the evidence for other toxins being secretory when these methods are used? This review concludes that chloride ion secretion is unproven. A review of the literature indicates that secretion occurs not because epithelial cells actively pump water but by interdiction of fluid absorption, increased conductivity through tight junctions and an increased hydrostatic driving force through elevated capillary pressure. The exclusive focus on chloride secretion may explain the failure to develop antisecretory drugs over the last three decades.

Lack of evidence in vivo for a remote effect of Escherichia coli heat stable enterotoxin on jejunal fluid absorption

On contact with the mucosa, heat stable (STa) enterotoxin from Escherichia coli reduces fluid absorption in vivo in the perfused jejunum of the anaesthetized rat. The question of whether it also has a vagally mediated remote action on jejunal absorption, when instilled into the ileum, was re-examined, given contradictory findings in the literature. A standard perfused loop preparation was used to measure luminal uptake of fluid in vivo by means of volume recovery. STa in the ileum was found to have no effect on jejunal absorption, regardless of cervical or sub-diaphragmatic vagotomy and also regardless of the nature of the perfusate anion. The batches of toxin were shown in parallel experiments to reduce fluid absorption directly in the jejunum and also in the ileum. Similarly, vagal nerves prior to section had demonstrable in vivo physiological function. There was therefore no evidence for an indirect, vagally mediated ileal effect of STa on proximal fluid absorption.

Lack of evidence in vivo for nitrergic inhibition by Escherichia coli (STa) enterotoxin of fluid absorption from rat proximal jejunum

Fluid absorption from the proximal jejunum of the anaesthetised rat was measured in vivo by fluid recovery. As expected, heat stable (STa) enterotoxin from E. coli reduced fluid absorption. Neither intraperitoneal L-NAME, thought to inhibit a putative neurally mediated action of STa, nor similar doses of D-NAME, ameliorated the inhibitory effect on jejunal fluid absorption of STa. Luminally perfused 10 mM sodium nitroprusside (SNP) had no effect on fluid absorption when expressed per gram dry weight per hour but reduced fluid absorption when expressed per cm length per hour. Similarly, 80 but not 40 mg/Kg of L-NAME reduced fluid absorption when expressed per cm length per hour, while the same dose of D-NAME did not. L-NAME and SNP significantly increased the wet weight to dry weight and the length to dry weight ratio of perfused loops. We conjecture that smooth muscle relaxation caused by these compounds increases interstitial fluid volumes that can be misconstrued as changes in absorption when this is expressed per cm length or per tissue wet weight. When fluid absorption is expressed per gram dry weight of tissue, there is no evidence for a role of nitric oxide in normal or STa inhibited fluid absorption.

Thermodynamic Investigation of Staphylococcus epidermidis interactions with protein-coated substrata

We evaluated self-assembled monolayers (SAMs) as potential coatings to prevent bacterial adhesion to biomaterials. Bacterial retention experiments were conducted on SAMs, some of which were coated with the model proteins fetal bovine serum (FBS) and fibronectin (FN). A thermodynamic approach was applied to calculate the Gibbs free energy changes of adhesion (DeltaG(adh)) of Staphylococcus epidermidis interacting with the substrates. When only nonspecific interactions controlled bacterial attachment, such as for the non-protein-coated substrates or the FBS substrates, the correlation between the thermodynamic predictions and measured values of bacterial retention was strong. However, when FN was adsorbed to the surfaces, the thermodynamic modeling underestimated bacterial adhesion, presumably since specific interactions between proteins of S. epidermidis and FN led to stronger attachment. Bacterial viability on the substrates was correlated with thermodynamic properties. For example, although bacteria attached more to surfaces having negative DeltaG(adh) values, these cells experienced the greatest loss of viability, presumably since strongly attached bacteria were unable to divide and grow. When the DeltaG(adh) values were decoupled into their components, we saw that acid-base interactions due to hydrogen bonding dominated the interactions of bacteria and proteins with each other and with the substrates in aqueous media. Finally, we discuss concerns regarding the use of the thermodynamic model to predict bacterial adhesion behavior in biomaterials systems.

The hormonal control of uterine luminal fluid secretion and absorption

The secretion of uterine luminal fluid initially provides a transport and support medium for spermatozoa and unimplanted embryos, while the absorption of uterine luminal fluid in early pregnancy results in the closure of the lumen and allows blastocysts to establish intimate contact with the uterine epithelium. We have established an in vivo perfusion technique of the lumen to study the hormonal control of the events in the peri-implantation period. Fluorescein-labelled dextran was included in the perfusion medium to monitor fluid movements and the concentrations of Na(+) and CI(-) ions in the effluent were monitored. Using an established regimen of steroid treatment of ovariectomized rats mimicking early pregnancy, oestradiol caused fluid secretion, while progesterone resulted in an amiloride-sensitive fluid absorption. Fluid absorption peaked at about the expected time of implantation. The effect of progesterone could be inhibited by treatment with a high dose of oestradiol, by the anti-progestin RU486, and by the presence of an intra-uterine contraceptive device. Studies of expression of Na(+) and CI(-) channels (ENaC, CFTR) indicated that these channels were subject to tissue-specific regulation within the uterus, but more work is required to determine their role and the factors controlling their abundance and localization in early pregnancy.