Serotonin-induced chloride secretion in hen colon. Possible second messengers

Endocrine regulation of ion transport in the avian lower intestine

The lower intestine (colon and coprodeum) of the domestic fowl maintains a very active, transporting epithelium, with a microvillus brush border, columnar epithelial cells, and a variety of transport systems. The colon of normal or high salt-acclimated hens expresses sodium-linked glucose and amino acid cotransporters, while the coprodeum is relatively inactive. Following acclimation to low salt diets, however, both colon and coprodeum shift to a pattern of high expression of electrogenic sodium channels, and the colonic cotransporter activity is simultaneously downregulated. These changes in the transport patterns seem to be regulated, at least in part, by aldosterone. Our recent work with this tissue has focused on whether aldosterone alone can account for the low salt pattern of transport. Other work has looked at the changes in morphology and in proportions of cell types that occur during chronic acclimation to high or low salt diets, and on a cAMP-activated chloride secretion pathway. Recent findings suggesting effects of other hormones on lower intestinal transport are also presented.

Effect of 5-HT2 and 5-HT3 receptor antagonists on cholera toxin-induced fluid hypersecretion in the pig jejunum

5-Hydroxytryptamine is a mediator in cholera toxin-induced hypersecretion in the small intestine. The aim of this study was to determine the effect of the 5-hydroxytryptamine receptor antagonists ketanserin, granisetron, ondansetron and tropisetron on cholera toxin-induced hypersecretion in the pig jejunum. Hypersecretion was induced by cholera toxin in ligated jejunal loops. The antagonists were administered subcutaneously at a dose of 100 micrograms/kg. Furthermore, the effect of intraluminally instilled ondansetron was studied. None of the antagonists altered basal absorption or caused fluid hypersecretion. Cholera toxin caused a dose-dependent electrolyte and fluid hypersecretion. The apparent maximal effect, 6.8 +/- 0.4 mg fluid x mg dry loop-1, was reduced by ondansetron, granisetron and tropisetron by about 40%, 30%, and 20%, respectively, whereas ketanserin had no effect. Intraluminal ondansetron reduced the effect of cholera toxin by about 50%. These results demonstrate that 5-hydroxytryptamine3 antagonists administered subcutaneously reduce the cholera toxin-induced hypersecretion in the pig jejunum. Finally, the results support species differences with respect to the antagonistic effect of the tested drugs in cholera toxin-induced hypersecretion.

Experimental studies of intestinal ion and water transport

A major advance in transport physiology was H. H. Ussing's development of the voltage-clamp method, and later the Koefoed-Johnsen-Ussing model for Na+ transport. In the same decade, J. C. Skou identified the Na(+)-K(+)-ATPase, which maintains the Na+ and K+ gradients that drive most epithelial transport processes. With this foundation, Danish scientists have pursued the mechanism of ion transport and the resulting solute-linked water flow. Recent contributions have been on isosmotic transport, suggesting solute recycling, and KCl-water cotransport in the basolateral epithelial cell membrane. Efficient small intestinal nutrient absorption is dependent on coupling to the Na+ gradient. Cotransport of Na+ and glucose is quantitatively the most important absorptive mechanism in the small intestine, as illustrated by the success of oral rehydration solutions in diarrhoea. The majority of amino acids are likewise transported by Na+ dependent carriers, but recent experiments have identified a concomitant Cl- dependency for some. Regulation of intestinal secretion, both under normal digestive processes, and in response to enterotoxins, has turned out to be very complex. It involves local and central neuronal regulation through an array of neurotransmitters and local actions of gastrointestinal hormones. Major effectors are the submucosal neurons and the main transmitters serotonin, vasoactive intestinal peptide, acetylcholine, substance P, and neurotensin. Development of antisecretagogues is impeded by the existence of several receptor subtypes and significant species differences. The Na+ and water-conserving properties of the large intestine have been shown to be regulated by adrenocortical hormones, with aldosterone as a potent stimulator of colonic Na+ absorption. A major colonic function is the symbiosis with the anaerobic bacterial population. The fermentation of carbohydrate to short-chain fatty acids, which can be absorbed, supplements small intestinal digestive function.

Second messengers and ion channels in acetylcholine-induced chloride secretion in hen trachea

1. Hen tracheal epithelium can be stimulated by serosal application of acetylcholine (ACh) to secrete Cl- equal to approximately 60-90 microA/cm2. 2. Radio-ligand-displacement for IP3, cAMP and cGMP and ion channel selective drugs in voltage clamp set-ups were employed to characterize second messengers and Cl-, K+ and Ca2+ channels involved in the ACh response. 3. ACh induced a significant rise in IP3 in isolated tracheocytes, while ACh did not influence the production of cAMP in whole tissue, isolated tracheocytes or basolateral cell membrane vesicles. Further ACh desensitization did not effect cAMP level in tracheocytes. In addition neither ACh stimulation nor desensitization interfered with cAMP production in presence of 4.5 microM forskolin in tracheocytes, a level of forskolin rising base level cAMP by around five fold. 4. Around 35% of ACh Cl- secretion depends on Ca2+ mobilization from internal stores and about 65% on Ca2+ influx over basolateral membrane. The activated Ca2+ channel is insensitive to class I, II, III and IV Ca2+ antagonists. 5. A 23187 can mimic the ACh effect although 30% is indomethacin-sensitive demonstrating a prostaglandin activated adenylyl cyclase. 6. Two K+ channels are involved in ACh secretion, one sensitive to Ba2+ and quinine and both insensitive to 4-aminopyridine, apamin, charybdotoxin and TEA. 7. Flufenamate and triaminopyrimidine block a non-selective ion channel likely involved in the ACh response. An ACh activated apical Cl- channel is NPPB-sensitive.

Involvement of non-classical 5-HT receptor in serotonin and cisapride induced secretion in hen colon

1. In hen colon 5-HT induces a tetrodotoxin-resistant, bumetanide-sensitive, chloride secretion, positively coupled with adenylate cyclase activity. 2. The 5-HT receptor mediating this response seems non-classical since it cannot be blocked by 5-HT1-like, 5-HT2 or 5-HT3 antagonists. 3. Effects are presented of new putative 5-HT agonists and antagonists on short circuit current and cord conductance in the hen colon, using the Ussing chamber technique. 4. The substituted benzamides, cisapride and BRL 24924, induced a dose-dependent short circuit currents but both with less potency than 5-HT. 5. Cisapride mediated this dose-dependent bumetanide sensitive response mainly by release of acetylcholine, since atropine reduced cisapride response by 70%. 6. Neither BRL 24924, 5-HTP-DP, ketanserin, ICS 205-930, prazosin, yohimbine, atropine nor piroxicam, covering the 5-HT1P, 5-HT2P, 5-HT2, 5-HT3, 5-HT4, adrenergic and muscarinic receptor types and the prostaglandin synthesis, altered 5-HT induced increases in short circuit current and cord conductance. 7. Results suggest (a) cisapride mediates it's response mainly by releasing acetylcholine, which then stimulates muscarinic receptors to release 5-HT. (b) Involvement of a non-classical 5-HT receptor subtype in 5-HT induced chloride secretion in hen colon.

Serotonin-induced chloride secretion in hen colon. Possible second messengers

1. Serotonin, 100 microM, induces a peak increase in short circuit current of about 150 microA/cm2 and in cord conductance of about 7 mS/cm2 and a more prolonged increase of 30 microA/cm2 and 1.4 mS/cm2 which lasts more than 30 min in hen colon. 2. The peak increase in short circuit current and cord conductance is due to a concomitant Cl- secretion. 3. The second messenger, which mediates Cl- secretion, increases in short circuit current and cord conductance, is cyclic AMP as theophylline, 0.5 mM, increases the response in short circuit current to 1 microM serotonin from 38 +/- 5 to 78 +/- 8 microA/cm2 and in g from 1.1 +/- 0.4 to 2.0 +/- 0.3 mS/cm2. 4. Theophylline, 0.5 mM, also sensitizes the hen colon to cyclic AMP yielding an EC50 of 0.24 +/- 0.03 mM in the presence of theophylline compared with an EC50 of 2.3 +/- 0.2 mM in the absence of theophylline. 5. Manipulations of other putative second messenger systems, such as the prostaglandins/leucotrienes, the phosphoinositides and external Ca2+ or calmodulin-sensitive enzymes, did not influence the serotonin response in short circuit current and cord conductance, thus ruling out their importance as intracellular mediators.