Effects of heat-stable Escherichia coli enterotoxin on intestinal alkaline secretion and transepithelial potential difference in the rat intestines in vivo

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.

Effects of guanylin and uroguanylin on rat jejunal fluid and electrolyte transport: comparison with heat-stable enterotoxin

The effects of rat guanylin, human guanylin, human uroguanylin and STa on net fluid and electrolyte transport in the closed jejunal loop were compared in anesthetized rats. STa administered into the lumen caused a concentration-dependent (10(-8) to 10(-6) M) inhibition of net fluid and NaCl absorption in the jejunal loop. Uroguanylin had a similar but weaker effect than STa. Both rat and human guanylin inhibited fluid and NaCl absorption only at 10(-6) M. Their order of potency was STa > human uroguanylin > rat guanylin = human guanylin. Changing the luminal pH from 5 to 8 failed to affect the action of guanylin on fluid absorption. Both STa and uroguanylin, but not guanylin, increased the luminal pH by stimulating bicarbonate secretion. Pretreatment of the jejunal loop with guanylin (10(-6) M) 5 min before the instillation of STa (10(-7) M) significantly reduced the inhibitory effect of STa on fluid absorption. It is concluded that guanylin and uroguanylin administered into the rat jejunal lumen have an STa-like action on fluid and electrolyte transport. Guanylin may act as an endogenous antagonist of STa in the rat jejunum and prevent excessive fluid loss by STa.

Nervous control of alkaline secretion in the duodenum as studied by the use of cholera toxin in the anaesthetized rat

There is experimental evidence for an axon reflex control of alkaline secretion in the rat duodenum. We have investigated if there is also an intramural reflex control of alkaline secretion similar to that demonstrated with regard to the control of the fluid transport in the rat jejunum. Alkaline secretion in the duodenum of an anesthetized rat was continuously monitored using an in situ titration technique. The segment was extrinsically denervated. Exposing the duodenal segment to 80 microg cholera toxin markedly increased alkaline secretion. This response was abolished by hexamethonium (28 micromol (10 mg) kg(-1) body wt), a nicotinic receptor blocker, lidocaine (0.5 mL of a 1% solution on the serosal surface), a local anaesthetic, and nifedipine (5.75 micromol (2 mg) kg(-1) body wt i.v.), a calcium channel blocker. The response to cholera toxin was partially abolished by granisetron (0.11 micromol (40 microg) kg(-1) body wt i.v.), a 5-HT3 receptor blocker. Atropine (1.7 micromol (0.5 mg) kg(-1) body wt i.v.), a muscarinic receptor blocker, had no effect. We therefore conclude that the alkaline secretion in the rat jejunum evoked by cholera toxin exhibits the same pharmacological properties as the fluid secretion caused by the toxin in the jejunum. This suggests that the alkaline secretion in the rat duodenum is controlled not only by an axon reflex but also by an intramural secretory reflex similar to that controlling fluid transport in the rat jejunum.

Model of bicarbonate secretion by resting frog stomach fundus mucosa. I. Transepithelial measurements

In the present in vitro experiments on gastric fundus mucosa of Rana esculenta we try to define the mechanism of alkaline secretion that is observed in summer frogs in the resting stomach (blockage of HCl secretion by ranitidine, 10(-5) mol/l). The transepithelial voltage and the rate of alkalinization (ASR) of an unbuffered gastric lumen perfusate was measured as a function of serosal (and mucosal) fluid composition. ASR was high (0.88 +/- S.E. 0.09 microEq.cm-2.h-1, n = 11) during serosal bath perfusion with HCO(3-)-Ringer solution, decreased slightly to 0.50 +/- 0.07 microEq.cm-2.h-1 (n = 6) in HCO(3-)-free HEPES-buffered Ringer solution of the same pH, and decreased to approximately 20% when carbonic anhydrase was inhibited by acetazolamide. While replacement of mucosal or serosal Cl- did not--within 1 h--significantly alter ASR, replacement of serosal Na+ in the presence or absence of HCO3- strongly reduced ASR, and a similar reduction was observed after serosal application of the anion transport inhibitor DIDS (4,4-diisothiocyanatostilbene-2,2-disulphonate, 2.10(-4) mol/l), the metabolic poison rotenone (10(-5) mol/l), the uncoupler dinitrophenol (10(-4) mol/l), and the Na+ pump inhibitor ouabain (10(-4) mol/l), while serosal amiloride (10(-4) mol/l) had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Segmental differences in the effects of guanylin and Escherichia coli heat-stable enterotoxin on Cl- secretion in human gut

1. Mucosally added synthetic guanylin and Escherichia coli heat-stable enterotoxin (STa) increased short-circuit current (ISC) across isolated muscle-stripped human intestine in vitro. 2. Serosal bumetanide inhibited ISC responses indicating that guanylin and STa stimulate electrogenic chloride secretion. 3. ISC responses were markedly greater in the colon than in the jejunum. 4. Pretreatment with indomethacin did not significantly alter the effects of guanylin and STa. 5. Both peptides induced concentration-dependent increases in the cyclic GMP content of human intestinal mucosa in vitro; cyclic AMP levels remained unchanged. 6. In contrast to ISC responses, increases in cyclic GMP content induced by guanylin and STa were markedly greater in the jejunum than in the colon. 7. Sodium nitroprusside (SNP) but not human alpha-atrial natriuratic peptide (CDD/ANP(99-126)) increased chloride secretion in human intestine; both agents induced small increases in intestinal cyclic GMP content. 8. Guanylin, STa and the nitric oxide (NO) donor SNP increased electrogenic chloride secretion across human intestinal mucosa in vitro by stimulation of cyclic GMP. The discrepancy between the effects on chloride secretion and intracellular cyclic GMP content suggest different cellular action sites of guanylin and STa in human small and large intestine.

Effects of luminal stimuli on polyamine metabolism in the small intestine of the rat: the role of enteric nerves

The aim of this study was to investigate to what extent polyamine metabolism in the small intestine of the rat is controlled by the enteric nervous system. Polyamine metabolism was followed by measuring the activity of ornithine decarboxylase (ODC) and in some instances also the content of polyamines (putrescine, spermidine and spermine). ODC activity in the intestine was increased when intraluminal pressure was increased and 3 h after placing cholera toxin in the intestinal lumen. Cholera toxin also increased the tissue putrescine content. Atropine or hexamethonium given i.v. did not influence the evoked changes of ODC activity. The pressure induced changes were not decreased by placing lidocaine on the serosal surface. On the other hand, the ODC activity of control segments were decreased by hexamethonium or atropine. The presence of glucose in the intestinal perfusate did not augment tissue ODC activity, neither did the heat stable enterotoxin from Escherichia coli (STa). It is concluded that the effect on polyamine metabolism evoked by luminal pressure or cholera toxin seems not to be mediated via nerves, while nerves seem to influence ODC activity during control conditions. The experiments with enterotoxins suggest that cAMP is the intracellular second messenger controlling intestinal ODC activity.

5-HT receptor antagonists and heat-stable Escherichia coli enterotoxin-induced effects in the rat

The effect of heat-stable E. coli enterotoxin on intestinal fluid secretion is commonly considered to be mediated by stimulation of mucosal cyclic guanosine monophosphate (cGMP). It was demonstrated recently that 5-hydroxytryptamine (5-HT) acts as an important mediator in cholera toxin-induced fluid secretion. To elucidate the possible involvement of 5-HT in the secretory response to heat-stable E. coli enterotoxin, in vivo experiments were performed in the rat jejunum. The inhibitory effects of the 5-HT2 receptor antagonist ketanserin, the 5-HT3 receptor antagonist tropisetron and indomethacin were studied in heat-stable E. coli enterotoxin-induced fluid secretion. Tropisetron and ketanserin (100 micrograms/kg each) alone only partially reduced the secretory effect of the toxin. However, in combination, the two blockers (100 plus 100 micrograms/kg) significantly reduced and at 200 plus 200 micrograms/kg totally abolished heat-stable E. coli enterotoxin-induced secretion without influencing the enterotoxin-induced increase in cGMP. Pretreatment with indomethacin (10 mg/kg) reduced the secretory response to the enterotoxin by about 50%. These results support the concept that 5-HT is an important mediator in intestinal fluid secretion induced by heat-stable E. coli enterotoxin. The enterotoxin may use 5-HT to stimulate prostaglandin formation via 5-HT2 receptors and to activate neuronal structures via 5-HT3 receptors.

Escherichia coli enterotoxin (STa) binds to receptors, stimulates guanyl cyclase, and impairs absorption in rat colon

To determine the contribution of the colon in Escherichia coli heat-stable enterotoxin-mediated diarrheal disease, toxin binding, guanyl cyclase activation, and toxin-induced water flux in the rat colon and ileum were compared. Scatchard analysis suggested a single class of heat-stable enterotoxin receptors with an affinity constant of binding of 10(9) L/mol in both colonocytes and ileocytes; however, the number of toxin receptors per cell was 3.5-fold greater in coloncytes than ileocytes (8.32 +/- 1.33 x 10(5) vs. 2.33 +/- 0.28 x 10(5) receptors per cell; P = 0.02). Heat-stable enterotoxin stimulated guanyl cyclase activation in an identical dose-dependent manner in proximal colonic and ileal membranes, with similar sensitivity and maximum response. Heat-stable enterotoxin also inhibited net water flux to a similar degree in both colon and ileum (-47.8 vs. -48.4 microL.cm-1.h-1, respectively) at a dose of 8 nmol/L. At this dose in the colon, because of a higher baseline of absorption, absorption continued, but at a diminished level. At this dose in the ileum, heat-stable enterotoxin induced net secretion. These data are consistent with the concept that heat-stable enterotoxin-induced diarrheal disease results from a decreased absorptive capacity in the colon in the face of increased small intestinal fluid secretion.

On the mechanisms of the basal alkaline secretion in the rat ileum in vivo

Basal alkaline secretion of the denervated rat ileum was monitored by a pH-stat method. Changes of transepithelial electrical potential difference (PD) were also continuously registered. In other experiments net fluid transport was measured with a gravimetric method. The importance of the enteric nervous system for the recorded variables was investigated by giving i.v. hexamethonium, neuropeptide Y (NPY) or methionine-enkephalin or by stimulating electrically the mesenteric nerves surrounding the superior mesenteric artery. Alkaline secretion was inhibited by about 20% by mesenteric nerve stimulation or by neuropeptide Y (NPY) or met-enkephalin i.v. A somewhat greater inhibition (approximately 30%) of transepithelial electrical potential difference was elicited by the mesenteric nerve stimulation and NPY whereas met-enkephalin did not cause any transepithelial electrical potential difference change. Net fluid absorption was markedly diminished (by approximately 65-75%) by met-enkephalin but not by NPY. The cellular mechanisms underlying alkaline secretion were investigated by means of amiloride, SITS and acetazolamide. The basal alkaline secretion and transepithelial electrical potential difference were not influenced by 10(-3)M or 10(-4) SITS. In contrast 10(-3) M amiloride caused a significant increase of alkaline secretion but not of transepithelial electrical potential difference. A 35% reduction in the alkaline secretion but not transepithelial electrical potential difference was observed after acetazolamide had been given intravenously. A similar decrease was observed after giving hexamethonium. We conclude: (1) Enteric nerves are of comparatively small importance in controlling the ileal alkaline secretion recorded during basal conditions; (2) About 35% of the basal ileal alkaline secretion is carbonic anhydrase dependent. This mechanism is not influenced by nicotinic receptor blockade; (3) Under the present experimental conditions there may be an alkaline secretion which is concealed by a simultaneously operating Na+/H+ exchanger and; (4) No consistent quantitative correlation exists between alkaline secretion, transepithelial electrical potential difference and net field transport in the denervated rat ileum.

Further studies of the changes in alkaline secretion, transepithelial potential difference and net fluid transport induced by the heat-stable enterotoxin of Escherichia coli (STa) in the rat jejunum in vivo

A pH-stat technique was used to study the mechanisms underlying the intestinal alkalinization evoked by the heat-stable enterotoxin of Escherichia coli (STa) in the rat denervated jejunum in vivo. In addition, concomitant changes of transepithelial potential difference and fluid transport were also investigated. To test the possible involvement of the enteric nervous system in the STa-stimulated alkaline secretion and potential difference, the mesenteric nerves were electrically stimulated or neuropeptide Y or methionine-enkephalin was infused intravenously. None of these interventions inhibited to any large extent the STa-stimulated alkaline secretion, whereas a greater suppression was noted on the concomitantly increased potential difference. Furthermore, neuropeptide Y but not methionine-enkephalin significantly inhibited STa-induced jejunal fluid secretion although neuropeptide Y was without effect on basal fluid transport. It is concluded that the enteric nervous reflex(es) which are of significant importance in explaining STa-evoked fluid secretion plays a minor role in controlling alkaline secretion. Furthermore, alkaline secretion may not contribute to the increase in potential difference caused by STa Amiloride (10(-4) or 10(-3) M) had no effect on the STa-stimulated alkaline secretion, implying that some cellular mechanism other than an inhibition of Na+/H+ exchanger explains the observed response. Similarly, acetazolamide had no effect on the STa-stimulated alkaline secretion or potential difference, suggesting that the secreted alkaline is of extracellular origin rather than from the cellular metabolism in the enterocytes.

Neuronal influence on intestinal transport

Reflex activation of the enteric nervous system (ENS) from the intestinal lumen and also from the serosa induces intestinal secretion. Thus mechanical distention, cholera toxin, heat-stable enterotoxin from E. coli, bile acids, mucosal inflammation and chemical peritonitis all induce an intestinal secretion that is inhibited by 60-100% by nerve-blocking agents. As a result of a large number of in vitro and in vivo studies, a picture of the organization of the secretory enteric nervous reflexes is now emerging. In secretory states with preserved intact intestinal epithelium, it is proposed that the reflex activation occurs via stimulation of receptor cells, i.e. epithelial endocrine cells such as EC and N-cells, which release peptides/amines into the interstitial space and thereby activate nerves close to the epithelium. The afferent neurones appear to transfer the reflex to the myenteric plexus, probably by using tachykinins as transmitters. This is in agreement with a superior and co-ordinating role for the myenteric plexus in the control of intestinal function by the ENS. Interneurones in turn mediate the transmission of the nerve signal to the submucosal plexus and the efferent neurones via cholinergic, nicotinic postganglionic receptors. The transmitters at the effector cells are acetylcholine and probably VIP.

Effect of antisecretory factor on Escherichia coli STa enterotoxin-induced alkalinisation of pig jejunal acid microclimate

The effect of challenge by Escherichia coli STa enterotoxin on pig jejunal mucosal surface pH was investigated in vivo. Exposure to STa resulted in a rapid and reversible alkalinisation (P less than 0.001) of the jejunal mucosa from 6.27 +/- 0.11 (5) to 6.89 +/- 0.03 (5). This action of STa is probably mediated through cyclic 3'5'-guanosine monophosphate (cGMP) since the 8-bromo analogue of cGMP induced the same effect as that observed after STa challenge. The action of STa on mucosal pH was partially inhibited by pre-administration of an antisecretory factor (ASF) preparation. The action of 8-bromo cGMP was unchanged by the presence of ASF. This implies that ASF inhibition occurs during the early stages of STa action prior to stimulation of guanylate cyclase. This effect of STa on the pig jejunal mucosal surface pH, or acid microclimate, may explain why weak acid supplementation of oral rehydration solutions can be ineffective in certain types of diarrhoeal disease.