Involvement of beta1- and beta2- but not beta3-adrenoceptor activation in adrenergic PYY secretion from the isolated colon.

The secretion of PYY by endocrine L cells of the terminal gut is under the control of nutrients, the autonomic nervous system and hormones. Catecholamines, and the non-specific beta-adrenergic agonist isoproterenol induce PYY secretion from rat isolated colon or ileum. Because beta3-adrenergic receptors now appear to mediate many of the effects of catecholamines in the gastrointestinal tract, we investigated the involvement of beta1-, beta2-, and beta3-adrenoceptor stimulation in PYY secretion from the isolated, vascularly perfused rat colon. Infusion of 10(-6) M isoproterenol induced a transient increase in PYY secretion (from 36+/-4 to 87+/-20 fmol/2 min; n=7, P<0.05), that was abolished by a previous infusion of the beta1- and beta2-adrenergic blocker (and partial beta3-agonist) alprenolol (10(-6) M). The beta1-adrenergic agonist dobutamine and the beta-2 agonist terbutaline also (both at 10(-5) M) significantly stimulated PYY secretion, from 29+/-1 to 79+/-12 fmol/2 min and from 19+/-1 to 73+/-13 fmol/2 min respectively (n=7, P<0.05). Neither of the beta3-adrenergic agonists tested (BRL 37 344 (10(-5), 10(-6) M) and SR 58 611A (10(-6) M)) significantly stimulated PYY secretion, thus confirming the exclusive involvement of beta1- and beta2-receptors in beta-adrenergic agonist induced hormone secretion.

Peptide YY, glucagon-like peptide-1, and neurotensin responses to luminal factors in the isolated vascularly perfused rat ileum.

Exposure of the ileum to nutrients markedly inhibits several upper gastrointestinal functions. Hormonal peptides of the ileal wall, i.e. peptide YY (PYY), glucagon-like peptide-1 (GLP-1), and neurotensin (NT), are thought to play a role in this negative feedback mechanism. The present study was conducted to comparatively assess the secretion of PYY, GLP-1, and NT upon luminal infusion of a variety of individual luminal factors in the isolated vascularly perfused rat ileum preparation. PYY, GLP-1, and NT were measured in the portal effluent with specific RIAs. Glucose (250 mM) induced a pronounced release of the three peptides, whereas a physiological concentration of 5 mM did not induce peptide secretion. Peptone (5%, wt/vol) evoked a sustained release of PYY, GLP-1, and NT. Only NT secretion was increased upon luminal administration of 100 mM sodium oleate. Short chain fatty acids (20 mM) evoked an early and transient release of the three peptides. In contrast, taurocholate (20 mM) induced a sustained release of PYY, GLP-1, and NT, but the threshold concentration for peptide release was lower for NT than for PYY or GLP-1. Cellulose or pectin (0.5%, wt/vol) did not modify peptide secretion. In conclusion, glucose and peptone are potent stimulants of PYY, GLP-1, and NT release. Only NT is released upon oleic acid stimulation. Finally, taurocholate is a potent stimulant of the release of the three peptides. Overall, PYY, GLP-1, and NT may participate cooperatively in the ileal brake. As relatively high concentrations of the various stimulants were required to elicit peptide release, it seems likely that this mechanism operates in cases of maldigestion or malabsorption.

Luminal peptide YY-releasing factors in the isolated vascularly perfused rat colon.

Peptide YY (PYY) is produced in endocrine L cells primarily localized in the distal bowel. These open-type L cells make contact with the intestinal chyme which may thus affect their secretory activity. The aim of the present study was to examine a large variety of luminal compounds found in colonic contents for their potential as PYY-releasing factors, using the isolated vascularly perfused rat colon. The release of PYY into the portal effluent was measured by a specific RIA. Luminal administration of 5 mM glucose or 0.5% (w/v) starch for 30 min did not induce significant release of PYY. Oleic acid (10 and 100 mM) also did not significantly increase PYY secretion. A pharmacological concentration of glucose (250 mM) and a mixture of amino acids (total concentration 250 mM) both induced PYY secretion (200% of basal). Pectin, a poly-galacturonic acid, evoked dose-dependent secretion of PYY-like immunoreactivity over the range 0.1-0.5% (w/v). The maximal response was observed after infusion of 0.5% pectin which induced a prompt and sustained release of PYY (300% of basal). Galacturonic acid itself (5%) produced marked PYY secretion. Gum arabic (0.5%) induced a gradual increase in portal PYY concentration (maximal response 250% of the basal value) whereas cellulose (0.5%) did not elicit PYY secretion. Luminal n-butyrate over the range 0.5-5 mM produced a dose-dependent release of PYY (maximal response 300% of the basal value with 5 mM n-butyrate). Increasing the concentration of n-butyrate to 100 mM provoked a gradual decrease in PYY secretion. Propionate was a less potent stimulant than n-butyrate, and acetate did not increase PYY secretion above the basal value. At a concentration of 2 or 20 mM, taurocholate, cholate and deoxycholate brought about PYY secretion while hyodeoxycholate was without effect. In conclusion, glucose and amino acids may mediate PYY release but only when they are present at high supraphysiological concentrations in the colon while oleic acid does not produce any PYY secretion. Physiological concentrations of fibers (pectin, gum arabic), short-chain fatty acids (n-butyrate, propionate) and bile salts (taurocholate, cholate, deoxycholate) are all potent stimulants of PYY release. Whether the release of PYY by luminal factors is coupled to water and electrolyte transfer via a local/paracrine pathway remains an open question which requires additional work with the isolated vascularly perfused colon preparation.

Regulation of glucagon-like peptide-1-(7-36) amide, peptide YY, and neurotensin secretion by neurotransmitters and gut hormones in the isolated vascularly perfused rat ileum.

Neurotensin (NT), peptide YY (PYY), and several peptides derived from proglucagon are promptly released from endocrine cells of the distal part of the gut after oral ingestion of a meal, thus suggesting that release of these peptides is partly under neural and/or hormonal control. Our previous studies conducted with a model of isolated vascularly perfused rat colon showed that colonic L cells are highly responsive to several transmitters of the gut and to the hormonal peptide GIP. To test the possibility that hormones produced by the proximal small intestine or transmitters of the enteric nervous system may also modulate the secretory activity of the ileal L cells, various intestinal regulatory peptides and neurotransmitters were administered intraarterially for 30 min in the isolated vascularly perfused rat ileum preparation. The secretory activity of the ileal N cells was comparatively assessed. The release of NT, PYY, and glucagon-like peptide-1 (GLP-1) in the portal effluent was measured with specific RIAs. The muscarinic cholinergic agonist bethanechol at a concentration of 10(-4) M provoked a biphasic release of PYY, GLP-1, and NT, consisting of an early peak followed by a sustained response. Similarly, bombesin (10(-7) M) induced a marked biphasic release of PYY and GLP-1. In contrast, the NT response was essentially monophasic, characterized by an early peak secretion. Tetrodotoxin did not modify the bombesin-induced release of PYY, GLP-1, and NT. The beta-adrenergic agonist isoproterenol at a concentration of 10(-6) M induced a transient rise in portal PYY and GLP-1 concentrations, whereas the effect on NT release was clearly biphasic. Calcitonin gene-related peptide (5 x 10(-8) M) induced a dramatic rise in PYY, GLP-1, and NT immunoreactivities in the portal effluent (peaks at 600%, 500%, and 550% of the basal values, respectively, 4 mi n after the start of infusion). Intraarterial infusion of GIP over the concentration range (0.5-3 nM) evoked a significant increase in portal concentration of the three peptides only at the threshold concentration of 3 nM. Secretin (50 pM) or cholecystokinin (50 pM) did not affect the release of ileal hormones. In conclusion, ileal L and N cells respond to a variety of transmitters of the gut. The pattern of peptide release depends on the cell type studied. The two cosynthesized peptides, PYY and GLP-1, appear to be cosecreted in the conditions of the present study.

Luminal glucagon-like peptide-1(7-36) amide-releasing factors in the isolated vascularly perfused rat colon.

Glucagon-like peptide-1 (GLP-1) is released from endocrine cells of the distal part of the gut after ingestion of a meal. GLP-1 secretion is, in part, under the control of hormonal and/or neural mechanisms. However, stimulation of the colonic L cells may also occur directly by the luminal contents. This was examined in the present study, using an isolated vascularly perfused rat colon. GLP-1 immunoreactivity was measured in the portal effluent after luminal infusion of a variety of compounds which are found in colonic contents (nutrients, fibers, bile acids, short-chain fatty acids (SCFAs)). Oleic acid (100 mM) or a mixture of amino acids (total concentration 250 mM), or starch (0.5%, w/v) did not increase GLP-1 secretion over basal value. A pharmacological concentration of glucose (250 mM) elicited a marked release of GLP-1 which was maximal at the end of infusion (400% of basal), while 5 mM glucose was without effect on secretion. Pectin evoked a dose-dependent release of GLP-1 over the range 0.1-0.5% (w/v) with a maximal response at 360% of basal when 0.5% pectin was infused. Cellulose or gum arabic (0.5%) did not modify GLP-1 secretion. The SCFAs acetate, propionate or butyrate (5, 20 and 100 mM) did not induce a significant release of GLP-1. Among the four bile acids tested, namely taurocholate, cholate, deoxycholate and hyodeoxycholate, the last one was the most potent at eliciting a GLP-1 response with a maximal release at 300% and 400% of the basal value when 2 and 20 mM bile acid were administered respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholyltaurine absorption by the isolated vascularly perfused rat small bowel.

Isolated, vascularly perfused, rat duodenojejunum and ileum preparations were used to study the transport parameters of cholyltaurine (C-tau; taurocholate). After a bolus administration of C-tau containing trace amounts of [3H]C-tau into the lumen of the isolated vascularly perfused duodenojejunum, the rate of appearance of C-tau in the portal effluent was dose-dependent over the range 5-20 mM. At a concentration of 20 mM, the rate of absorption was constant over time and amounted to 1.25 nmol.min-1.cm-1; 2.6% of the luminal load of C-tau was recovered in the portal effluent over the 40-min experimental period. Intra-arterial infusion of ouabain (10(-3)M) did not modify significantly the absorption rate of C-tau. The C-tau absorption from the lumen of the isolated vascularly perfused ileum was 7-fold higher than that from the duodenojejunum. Total absorption of C-tau was dose-dependent over the range 1-20 mM and the estimated Km and Tmax of the absorptive area of the rat ileum were 11.5 mM and 17.7 nmol.min-1.cm-1, respectively. Intraarterial infusion of ouabain reduced by 84% the recovery of C-tau in the portal effluent. In conclusion, the absorption parameters of bile acids in the duodenojejunum and in the ileum of the ex vivo rat intestinal preparations are consistent with a passive diffusion process in the proximal small intestine and an active transport in the ileum. The isolated vascularly perfused duodenojejunum and ileum preparations are therefore appropriate models for studying bile acid absorption process and the coupling with the associated local metabolic, motor, secretory and vascular events.

Luminal bile salts and neurotensin release in the isolated vascularly perfused rat jejuno-ileum.

Bile salts in the distal small intestine are strong stimulants of neurotensin (NT) release, but the underlying mechanisms are not known. They were investigated using an isolated vascularly perfused rat jejuno-ileum preparation. Luminal administration of crude ox bile extract (0.25-1.5%, wt/vol) produced a dose-dependent release of NT-like immunoreactivity (NT-LI), with a maximal effect after infusion of 1% bile extract (500% of basal). Pretreatment of the 1% bile extract with the bile salt-sequestering resin cholestyramine (2%, wt/vol) abolished NT-LI release. Taurocholate (TC), the major bile salt in rats, dose dependently increased the release of NT. The maximal secretion of NT-LI was observed after infusion of 20 mM TC (400% of basal). Taurodeoxycholate (20 mM) was as potent as TC in stimulating NT-LI release, but the threshold concentration of taurodeoxycholate for NT-LI secretion was lower than that of TC. Glycocholate and cholate were 2- to 3-fold less potent than TC in releasing NT-LI over the concentration range 5-20 mM. Luminal infusion of oleic acid (sodium salt; 100 mM) increased by 100% the level of NT-LI in the portal effluent, whereas 20 mM oleate had no effect. In contrast, the micellar form of oleic acid (20 and 100 mM) in bile extract (1%) or TC (20 mM) dose dependently reduced the release of NT-LI induced by bile extract or TC alone. Neither intraarterial tetrodotoxin (10(-6) M), EGTA (2 mM), verapamil (5 x 10(-5) M), nor atropine (10(-5) M) had any effect on TC-induced NT-LI release. These results show that the tauro-conjugated forms of cholic and deoxycholic acid are strong stimulants of NT-LI release. The N-cell response is blunted when bile salts are complexed in the lumen by oleic acid. Finally, bile salt-induced NT-LI release is not mediated by intramural nerves and is not dependent on the activation of calcium channels.