Prevention and reversal of cholera enterotoxin-induced intestinal secretion by methylprednisolone induction of Na+-K+-ATPase.
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The relationship of the mucosal enzyme systems Na+-K+-activated adenosine triphophatase (Na-K-ATPase) and adenylate cyclase and their associated intestinal transport processes was studied in the rat ileum. Two ileal loops were constructed in each anesthetized rat; one loop was inoculated with saline, the other loop with choleragen. Net transport of water and electrolytes was measured in vivo after which enzyme activity was measured in the mucosa of the perfused loops. All doses of choleragen between 5 and 150 mug decreased water movement as early as 3 1/2 h after inoculation. A linear relationship between the dose of choleragen and the level of net water and electrolyte secretion was observed when choleragen doses between 5 and 150 mug were incubated in ileal loops for 4 h. Adenylate cyclase activity was always increased in secreting intestinal loops, whereas Na-K-ATPase was unaffected by choleragen. In animals pretreated with methylprednisolone acetate, 3 mg/100 g per day for 3 days before loop inoculation, saline loops had enhanced mucosal Na-K-ATPase activity had increased net water and electrolyte absorption; choleragen-exposed loops had increased adenylate cyclase and Na-K-ATPase activities, and net absorption of water and electrolytes 4 h after inoculation. These effects of methylprednisolone acetate were still present 19 1/2 h after inoculation. When a single injection of methylprednisolone acetate was given 3 1/2 h after choleragen inoculation, both adenylate cyclase and Na-K-ATPase were activated, and net intestinal absorption of water and electrolytes was observed 19 1/2 h after inoculation. These results suggest that methylprednisolone can prevent and reverse the secretory effects of choleragen by selectively stimulating a coexisting absorptive process. (+info)
Intestinal hypersecretion of the refed starved rat: a model for alimentary diarrhoea.
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Fluid transport was gravimetrically measured in vivo in the duodenum, jejunum, and ileum of anaesthetised fed, 72 hour starved and 72 hour starved rats refed for up to five days after starvation. Basal unstimulated fluid transport was monitored by instilling 0.9% NaCl into the lumen and measuring the gain or loss in weight of the closed intestinal loop. Fluid was absorbed in all the areas of the intestine in the fed rats. Increasing basal fluid absorption was observed in the duodenum over the three days of starvation but in the jejunum there was no significant change. In the ileum, the pattern was very different, on day 1 the fluid was absorbed but on days 2 and 3 there was an increasing secretion of fluid. Refeeding the rats with their normal diet restored the basal absorption of fluid in the duodenum within 24 hours, had no effect in the jejunum but in the case of the ileum the hypersecretion of fluid observed in the day 3 starved rat was maintained on day 1 of refeeding, increased further on day 2, decreased on day 3 but returned to absorption on day 4. The normal absorption was restored to the ileum on day 5 of refeeding. Fluid secretion was induced in all the rat groups by bethanechol (ip 60 micrograms/kg bw) a stable cholinergic agonist, PGE2 (ip 10 micrograms/kg (bw) and E coli STa (luminally instilled, 500 ng/ml) a secretory enterotoxin. All the secretagogues gave enhanced secretion compared with the fed by day 2 of starvation which increased considerably on day 3. Refeeding returned their secretion back to the fed level in the duodenum within 24 hours, in the jejunum within 48 hours but in the ileum their induced secretion on day 2 of refeeding was greater than that of the day 2 of refeeding was greater than that of day 3 starved and took until day 4 to return to the fed levels for behanechol and PGE2 and until day 5 for E. coli STa. This behaviour of rat small intestine showing even greater hypersecretion in the refed state than the starved mimics the human condition of alimentary induced diarrhoea where incautious feeding of starved humans induces severe, often lethal diarrhoea. The refed starved rat appears to be a possible model for this condition. (+info)
Liposomes targeted to deliver antisecretory agents to jejunal mucosa.
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The B subunit of cholera toxin has been covalently attached to the surface of liposomes made from a mixture of phosphatidylethanolamine, phosphatidylcholine and cholesterol. Adenylate cyclase inhibitors and chloride conductance inhibitors were encapsulated within the liposomes. These "targeted" liposomes were used to study the combined effects of this novel delivery system, and a limited number of possible antisecretory agents, on net fluid flux into the pig jejunum. A state of net secretory fluid flux was induced in isolated jejunal loops in weanling pigs by adding theophylline or cholera toxin to the lumen of the isolated loops. There was no reduction in net fluid secretion when liposome suspensions without encapsulated secretory inhibitors were added to fluid in the lumen of loops treated with theophylline. There was also no reduction in net fluid secretion when miconazole, alpha-phenylcinnamate or 5 nitro-2-(3-phenethylamino)benzoate were encapsulated within targeted liposomes added to isolated jejunal loops. The net fluid flux induced by exposure of jejunal loops to theophylline was significantly reduced by adding targeted liposomes containing 2'-deoxy-3'-AMP. The reduction involved a reversal of net secretory fluid flux to an absorptive value. The net fluid secretory response to treatment of loops with cholera toxin was also inhibited by treating loops with targeted liposomes containing 2'-deoxy-3'-AMP. However, the reversal of secretion was less complete for secretion induced by cholera toxin than for secretion induced by theophylline. The reduced antisecretory efficacy versus cholera toxin was not improved by encapsulating higher concentrations of 2'-deoxy-3'-AMP.(ABSTRACT TRUNCATED AT 250 WORDS) (+info)
MUCUS IN INTESTINAL CONTENTS OF GERMFREE RATS.
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The fecal excretion of total nitrogen and of total hexosamines has been determined in germfree and conventional rats. Germfree rats excreted more hexosamines than the conventional rats, while no difference in the nitrogen excretion was found. Infection of the germfree rats with a normal flora resulted in a temporarily increased excretion of hexosamines and nitrogen over a period of 2 to 3 days after which they reached the level of the conventional animals. The contents of the germfree cecum contained 65 to 137 mg of hexosamines and 57 to 127 mg of nitrogen as compared to 1.2 to 5.3 and 7.4 to 23 mg in conventional animals. The high figures for hexosamines were due to an increase in the total amount of contents in the cecum and to a fivefold increase in the concentration of hexosamine-containing material. Studies on the distribution of hexosamine-containing cecal contents between sediment and supernatant after centrifugation at 20,000 g for 2 hours demonstrated that 5 to 10 per cent of the hexosamines occurred in the sediment in the germfree rats, while 75 to 85 per cent was found in this fraction in the conventional rats. The soluble part of the cecal contents in germfree as well as in the conventional rats contained 70 per cent of hexosamines in molecules with a molecular weight above approximatively 100,000 as found by gel filtration experiments on sephadex gels. The higher weight of the germfree cecal wall was reflected in a high total amount of nitrogen and hexosamines. Isolated strains of bacteria capable of reducing the cecal size in vivo did not show any capacity to degrade the mucus in vitro in a test system, where a full intestinal flora was highly active. (+info)
Intracellular potentiation between two second messenger systems may contribute to cholera toxin induced intestinal secretion in humans.
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BACKGROUND: Cholera toxin (CT) acts on intestinal epithelial cells both directly and indirectly via activation of a secretory neural reflex. The reflex may release acetylcholine as one of its final neurotransmitters. This opens up the possibility of a third mechanism of action for CT, namely a synergistic interaction between two secretagogues acting on different second messenger systems within the epithelial cell. AIMS: To establish evidence for cholinergic innervation to human ileal epithelial cells and to investigate whether CT potentiates the action of acetylcholine on human intestinal epithelial cells. METHODS: Transverse sections of human ileum were examined for mucosal cholinergic nerves and M3 muscarinic receptors using antibodies raised to choline acetyltransferase and M3 receptors. Short circuit current (Isc) responses and ion flux movements were elicited from T84 epithelial cell monolayers set up in Ussing chambers. RESULTS: Immunohistochemistry of native human ileal mucosa revealed the presence of both cholinergic nerves and muscarinic M3 receptors located to the basolateral domain of epithelial cells. Secretory responses of T84 cell monolayers to acetylcholine were greatly potentiated in the presence of CT. This effect, substituting forskolin for CT, was mirrored by increases in basolateral 86Rb and apical 125I efflux. Charybdotoxin plus apamin reduced both Isc and 86Rb efflux evoked by acetylcholine, in the presence of forskolin. CONCLUSIONS: Human ileal mucosa receives a direct cholinergic innervation to its epithelial cells. Secretory effects of acetylcholine on epithelial cells are augmented in the presence of CT. Such a synergistic response is dependent on optimum opening of basolateral potassium channels by acetylcholine and apical chloride channels by CT. The interaction may contribute to the mechanism of action of cholera toxin induced secretory diarrhoea. (+info)
Fluid secretory responses to enterotoxin STa and 8-bromo-cyclic GMP in fed and nutrionally-deprived gerbils: jejunum, ileum and colon in vivo.
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Fluid transport was measured gravimetrically in vivo in the jejunum, ileum and colon of fed, fasting (four days) and undernourished (50 % of control food intake for 21 days) gerbils (Gerbillus cheesmani). The effects of luminal enterotoxin Escherichia coli STa (50 ng/ml) and luminal 8-bromo-cyclic GMP (cGMP 1 mM) on fluid transport across jejunum, ileum and colon were also assessed. Fasting and undernourishment reversed the normal basal fluid absorption measured in fed ileum and colon into secretion. Neither fasting nor undernourishment had any effect on jejunal basal fluid absorption. In jejuna, ilea and colons of fed animals as well as in jejuna from fasting and undernourished gerbils STa (50 ng) reversed the normal absorptive "tone" to secretion but it had no significant effects on fluid secretion in either the ileum or colon from fasted gerbils. STa increased significantly the fluid secretion in ileum from undernourished gerbils. Luminal cGMP had no effect on basal absorptive tone in the jejunum of fed and fasted gerbils, but reversed absorption into secretion in the jejuna from undernourished gerbils. In the ilea taken from fed animals the small basal absorption was reversed to secretion by luminal cGMP. Although cGMP produced no significant changes in fluid secretion in the ilea taken from fasted gerbils, yet it caused a significant increase in those from undernourished gerbils. In the colon taken from fed animals cGMP decreased the basal fluid absorption significantly, but it had no significant effect on fluid secretion in the colon of fasted or undernourished gerbils. We conclude that fasting and undernourishment have no significant effects on fluid transport across the gerbil jejunum but reversed basal absorption in the fed ileum and colon into secretion. cGMP mimic the effects of STa in the jejunum taken from undernourished gerbils, in the ileum obtained under the three nutritional states and in the colon taken from fasting animals. (+info)
Recurrent networks of submucous neurons controlling intestinal secretion: a modeling study.
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Secretomotor neurons, immunoreactive for vasoactive intestinal peptide (VIP), are important in controlling chloride secretion in the small intestine. These neurons form functional synapses with other submucosal VIP neurons and transmit via slow excitatory postsynaptic potentials (EPSPs). Thus they form a recurrent network with positive feedback. Intrinsic sensory neurons within the submucosa are also likely to form recurrent networks with positive feedback, provide substantial output to VIP neurons, and receive input from VIP neurons. If positive feedback within recurrent networks is sufficiently large, then neurons in the network respond to even small stimuli by firing at their maximum possible rate, even after the stimulus is removed. However, it is not clear whether such a mechanism operates within the recurrent networks of submucous neurons. We investigated this question by performing computer simulations of realistic models of VIP and intrinsic sensory neuron networks. In the expected range of electrophysiological properties, we found that activity in the VIP neuron network decayed slowly after cessation of a stimulus, indicating that positive feedback is not strong enough to support the uncontrolled firing state. The addition of intrinsic sensory neurons produced a low stable firing rate consistent with the common finding that basal secretory activity is, in part, neurogenic. Changing electrophysiological properties enables these recurrent networks to support the uncontrolled firing state, which may have implications with hypersecretion in the presence of enterotoxins such as cholera-toxin. (+info)
Indomethacin decreases jejunal fluid secretion in addition to luminal release of prostaglandin E2 in patients with acute cholera.
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Human cholera is associated with an increased luminal release of prostaglandin E2 (PGE2), but whether inhibition of increased PGE2 synthesis will reduce or control intestinal secretion is uncertain. 'Steady state' perfusions (10 ml/minute) in 12 patients with acute cholera, and repeat perfusions in nine of these patients during the convalescent phase were therefore performed using the triple lumen technique. The proximal jejunum was perfused with isotonic saline containing sodium-sulphobromophthalein as a non-absorbable marker. After intravenous administration of indomethacin (1.0 mg/kg) the jejunal net transfer of fluid and the jejunal flow rate of PGE2 were determined in 30 minute periods for 120 minutes after a 120 minute control period. Indomethacin decreased net fluid secretion (2.1 (0.3-4.2) v 4.5 (2.5-8.4) ml/hour x cm; medians, Q50 ranges, p less than 0.01) and the jejunal flow rate of PGE2 (1.5 (1.2-2.7) v 2.2 (1.4-4.9) ng/minute, p less than 0.05). The results of similar perfusion studies in 22 patients with acute cholera, used to establish the spontaneous time related change in fluid secretion, showed no significant change in net fluid transfer (3.5 (2.2-6.2) to 3.5 (2.6-11.6) ml/hour x cm, p greater than 0.25) over 240 minutes. These data provide further evidence in favour of the hypothesis that prostaglandins have a role in the cholera toxin induced intestinal fluid secretion in man. (+info)