VIP and PGE1 activate adenylate cyclase in rat intestinal epithelial cell membranes via different mechanisms

Enkephalin affects ion transport via the enteric nervous system in guinea-pig ileum

The endogenous opioid enkephalin drives ion transport towards absorption. To determine the site and mechanism of this effect, fractionated stripping of guinea-pig ileum was carried out. The muscularis propria, including myenteric plexus, was removed by partial stripping. The submucosa, including the submucosal plexus, plus the muscularis mucosae were removed by total stripping. For binding studies, epithelial cells were removed by the method of Weiser leaving the lamina propria mucosae with the mucosal plexus. Radio-receptor-assay with (3H)2-D-ala-5-D-leu-enkephalin revealed enkephalin binding sites in the submucosa plus muscularis mucosae (KD = 3.6 nmol l-1; Vmax = 7.3 fmol mg-1) and in the lamina propria mucosae (KD = 4.2 nmol l-1; Vmax = 5.1 fmol mg-1. The binding was stereospecific in both layers. No binding was detected on epithelial cells. In the Ussing chamber, partially stripped ileum exhibited spontaneous ISC which was abolished by addition of tetrodotoxin (TTX) or by total stripping indicating that this ISC was neuronally stimulated by the submucosal plexus. Electrogenic chloride secretion was identified as contributing to this ISC, since the TTX-sensitive part of ISC in the partially stripped ileum was lacking in Cl- and HCO3-free medium, reappeared after addition of Cl consistent with Michaelis-Menten kinetics (Km = 19 nmol l-1) and was reversed by serosal addition of bumetanide. In addition, enkephalin increased electroneutral NaCl-absorption as obtained by Na- and Cl-flux measurements. Enkephalin decreased this spontaneous neuronally stimulated electrogenic Cl-secretion in the partially stripped ileum, but had no effect in totally stripped ileum if ISC was stimulated at the cellular level by theophylline or PGE1. We conclude that ganglia located in the submucosal plexus regulate intestinal ion transport. Enkephalin acts by presynaptic inhibition via receptors on these neurons in the submucosa and/or via receptors on their neurites in the lamina propria mucosae.

Involvement of 5-hydroxytryptamine, prostaglandin E2, and cyclic adenosine monophosphate in cholera toxin-induced fluid secretion in the small intestine of the rat in vivo

The diarrhea of cholera is considered to rely solely on a cyclic adenosine monophosphate-mediated secretory mechanism. However, both 5-hydroxytryptamine and prostaglandin E2 have been proposed to be involved in the pathogenesis of cholera. In vivo experiments were performed, therefore, in the rat jejunum to investigate the influence of purified cholera toxin on fluid secretion, luminal release of 5-hydroxytryptamine and prostaglandin E2, and formation of mucosal cyclic adenosine monophosphate. Also the effects of ketanserin, indomethacin, verapamil, and nifedipine on the named parameters were studied. Cholera toxin dose-dependently (0.1-0.5 microgram/ml) and time-dependently (1-5 h) increased mean net fluid secretion with a maximum response at 4 h. It also caused a significant (p less than 0.01) rise in release of 5-hydroxytryptamine and prostaglandin E2, in addition to formation of cyclic adenosine monophosphate. The dose-response curve for cholera toxin-induced fluid secretion was shifted to the right by indomethacin (10 mg/kg s.c.) and ketanserin (200 micrograms/kg s.c.), none of which caused a change in cholera toxin-induced release of 5-hydroxytryptamine. However, both agents significantly decreased the release of prostaglandin E2. Verapamil (0.2-9.5 micrograms/min i.a.) and nifedipine (0.05-0.5 microgram/min i.a.) dose-dependently reduced cholera toxin-induced fluid secretion. The estimated local concentrations at half-maximal inhibition were 5 x 10(-7) M verapamil and 5 x 10(-8) M nifedipine, respectively. The cholera toxin-induced increase in release of 5-hydroxytryptamine and prostaglandin E2 and formation of cyclic adenosine monophosphate was unaffected by verapamil. These results support the concept that cholera toxin-induced fluid secretion in vivo is caused, in part, by release of 5-hydroxytryptamine, which in turn stimulates formation of prostaglandin E2.

Pathogenesis and pharmacology of diarrhea

The etiological factors involved in diarrhea are multiple. Also the mechanisms and mediators involved are multiple: intracellular mediators (Ca, cAMP, cGMP, calmodulin, phospholipids), extracellular mediators (hormones, neurotransmitters, prostaglandins, enterotoxins...), intramural blood flow and oxygen, intestinal motility (local- and peristaltic motility). Till now, antidiarrheals are not so versatile that they provide a solution to all types of diarrhea. The mechanisms of action of fluid replacement therapy, loperamide, alpha 2 agonist and some nonsteroidal anti-inflammatory substances are reviewed.

Tetrodotoxin inhibits directly acting stimulants of intestinal fluid secretion

Intravenous injection of tetrodotoxin (20 micrograms kg-1) to anaesthetized rats blocks PGE1- and inhibits VIP-induced fluid secretion from the mucosa of the jejunum without affecting the physiological rates of net water and glucose absorption. It is suggested that the toxin might release an endogenous inhibitor of secretion or that it might possess direct antisecretory activity on the epithelial cells of the jejunum. It is further suggested that results where tetrodotoxin has been used to investigate secretagogue action should be treated with caution.

Significance of calcium for the prostaglandin E2-mediated secretory response to 5-hydroxytryptamine in the small intestine of the rat in vivo

5-Hydroxytryptamine (5-HT) has been claimed to mediate intestinal secretion in morphine withdrawal diarrhea through stimulation of local prostaglandin formation without involving cyclic adenosine monophosphate. Therefore, experiments were performed to study (a) the effects of exogenous 5-HT and the cyclic adenosine monophosphate-dependent secretagogue, vasoactive intestinal polypeptide, on intestinal prostaglandin E2 (PGE2) formation and (b) the involvement of calcium in the secretory response to close intraarterial infusion of 5-HT, PGE2, or vasoactive intestinal polypeptide in tied-off loops of rat jejunum in vivo. 5-Hydroxytryptamine and vasoactive intestinal polypeptide reversed fluid absorption to net secretion (p less than 0.01), but only 5-HT caused an increase in luminal PGE2 output (p less than 0.01). Indomethacin and d,l-verapamil prevented only the secretory effect of 5-HT. Exogenous PGE2 (1.6-160 ng/min) reversed absorption to secretion (p less than 0.01) in a dose-dependent manner, irrespective of whether the rats were pretreated with indomethacin or not. Racemic and l-verapamil, but not d-verapamil, markedly reduced (p less than 0.01) the secretory effect of physiologically low doses of PGE2 (1.6 and 16 ng/min), whereas high doses of PGE2 (160 ng/min), which caused a significant increase in mucosal cyclic adenosine monophosphate (p less than 0.005), were not inhibited by verapamil. These data suggest that PGE2 may be an important intermediate in the transduction mechanism leading to 5-HT-induced intestinal secretion, and that physiologic doses of PGE2 may act by facilitating calcium entry, rather than by increasing intracellular calcium through activation of the adenylate cyclase.

Effects of some opiates and vasoactive intestinal peptide (VIP) on duodenal surface epithelial bicarbonate secretion in the rat

Bicarbonate secretion by 12 mm segments of duodenum just distal to the Brunner's glands area and devoid of pancreatic bicarbonate was titrated in situ in anaesthetised rats. The secretion increased significantly after intravenous injection of small amounts (20 ng/kg) of the endogenous opioid peptides beta-endorphin and methionine enkephalin and maximal (approximately twofold) stimulation occurred after 200-500 ng/kg. Morphine (50 micrograms/kg) caused a similar stimulation and the mu-opiate antagonist naloxone prevented stimulation by morphine. The synthetic analogue [D-Ala2, D-Leu5]-enkephalin (500 ng/kg) which is an agonist at delta-opiate receptors, did not affect the secretion, further suggesting that stimulation is mediated by mu-receptors. VIP (5-100 micrograms/kg) increased the secretion dose-dependently to levels considerably higher than those observed with opiates, and pretreatment with atropine or indomethacin did not affect the response to VIP. The results suggest a role of endogenous opioid peptides and VIP in the humoral and/or nervous control of duodenal surface epithelial bicarbonate secretion and mucosal protection.

Effect of opiates on human colonic adenylate cyclase activity

To elucidate a possible mechanism whereby opiates affect human intestinal fluid secretion, opiate effects on human basal and stimulated colonic adenylate cyclase activity were determined. Morphine (10 microM) and naloxone (1 microM) did not affect basal colonic adenylate cyclase activity: 32.6 +/- 2.6 (n = 23; X +/- S.E.) pmol cAMP/mg protein per 10 min or its stimulation induced by sodium fluoride or VIP. However, morphine inhibited the stimulation induced by prostaglandin E2, an effect which was blocked by naloxone. Synthetic enkephalins and metabolically stable retro inverso enkephalins also prevented PGE2-induced stimulation of colonic adenylate cyclase activity. The addition of calcium (0.1 mM) decreased the basal and PGE2-stimulated enzyme activities by 35% and 50%, respectively, regardless of the presence or absence of morphine. These results suggest that morphine and synthetic enkephalins do not affect basal human colonic adenylate cyclase activity but inhibit its stimulation induced by PGE2. This effect may be one of the mechanisms whereby opiates affect intestinal fluid transport.

Conversion of sodium channels to a form sensitive to cyclic AMP by component(s) from red cells

Sodium transport has been measured in the isolated epithelium from colons of male Sprague-Dawley rats. Sodium transport in colons was induced by pretreating the animals with dexamethasone (6 mg kg-1) which caused the appearance of an amiloride-sensitive short circuit current within a few hours. Forskolin, a diterpene, which activates adenylate cyclase, was found to increase the cyclic adenosine monophosphate (cyclic AMP) content of rat colons and also to increase short circuit current at the same time. However, measurements of chloride and sodium fluxes across the epithelium indicated that forskolin activates chloride secretion but has no effect on sodium transport. In confirmation of (3) it was found that the amiloride-sensitive short circuit current was unchanged after the short circuit current had been increased by forskolin under a variety of conditions. The behaviour of the mammalian colon as indicated in (3) and (4) is unlike that of amphibian sodium transporting epithelia. It is shown that in toad urinary bladder forskolin increases amiloride-sensitive short circuit current. Procedures were investigated which might make sodium transport in the mammalian colon sensitive to cyclic AMP. Exposing the apical surface to sonicated suspensions of nucleated red cells (frog, toad and duck), followed by washing, gave preparations with amiloride-sensitive short circuit currents which were increased by forskolin or dibutyryl cyclic AMP. It would appear that the sodium channel in the mammalian colon, unlike that of amphibian tissues, has lost the ability to have its properties modified by cyclic AMP. Incubation of colons with sonicated suspensions of nucleated red cells apparently modifies the tissues such that sodium transport across the tissue becomes sensitive to the nucleotide.