Abstract

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. In conclusion, some fibres and bile acids are capable of releasing colonic GLP-1 in rats and may contribute to the secretory activity of colonic L cells.

Journal of Endocrinology (1995) 145, 521–526

Abstract

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 polygalacturonic 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.

Journal of Endocrinology (1996) 151, 421–429

The neuropeptide galanin is widely distributed in the gastrointestinal tract and exerts several inhibitory effects, especially on intestinal motility and on insulin release from pancreatic beta-cells. The presence of galanin fibres not only in the myenteric and submucosal plexus but also in the mucosa, prompted us to investigate the regulatory role of galanin, and its mechanism of action, on the secretion of the insulinotropic hormone glucagon-like peptide-1 (GLP-1). Rat ileal cells were dispersed through mechanical vibration followed by moderate exposure to hyaluronidase, DNase I and EDTA, and enriched for L-cells by counterflow elutriation. A 6- to 7-fold enrichment in GLP-1 cell content was registered after elutriation, as compared with the crude cell preparation (929 +/- 81 vs 138 +/- 14 fmol/10(6) cells). L-cells then accounted for 4-5% of the total cell population. Bombesin induced a time-(15-240 min) and dose- (0.1 nM-1 microM) dependent release of GLP-1. Glucose-dependent insulinotropic peptide (GIP, 100 nM), forskolin (10 microM) and the phorbol ester 12-0-tetradecanoylphorbol-13-acetate (TPA, 1 microM) each stimulated GLP-1 secretion over a 1-h incubation period. Galanin (0.01-100 nM) induced a dose-dependent inhibition of bombesin- and of GIP-stimulated GLP-1 release (mean inhibition of 90% with 100 nM galanin). Galanin also dose-dependently inhibited forskolin-induced GLP-1 secretion (74% of inhibition with 100 nM galanin), but not TPA-stimulated hormone release. Pretreatment of cells with 200 ng/ml pertussis toxin for 3 h, or incubation with the ATP-sensitive K+ channel blocker disopyramide (200 microM), prevented the inhibition by galanin of bombesin- and GIP-stimulated GLP-1 secretion. These studies indicate that intestinal secretion of GLP-1 is negatively controlled by galanin, that acts through receptors coupled to pertussis toxin-sensitive G protein and involves ATP-dependent K+ channels.

Abstract

The effect of glucocorticoids on the expression of intestinal cholecystokinin (CCK) was investigated both in vivo and in cell culture systems. In vivo, 2-day administration of methylprednisolone to adult male rats induced a decrease in CCK-like immunoreactivity (CCK-LI) and CCK mRNA levels in mucosal extracts. In two CCK-producing cell lines, RIN 1056E and STC-1 of pancreatic and intestinal origin respectively, dexamethasone induced dose-dependent decreases in both CCK-LI and steady-state CCK mRNA levels. The decrease in CCK mRNA was totally prevented by incubation of cells with an excess of RU 38486, a competitive inhibitor for the binding of glucocorticoids to their receptor. Actinomycin D, used to prevent RNA synthesis, did not modify CCK mRNA stability in dexamethasone-pretreated cells as compared with cells not exposed to dexamethasone. When cells were first incubated with actinomycin D, subsequent addition of dexamethasone left the steady-state CCK mRNA levels unaltered in both cell lines. Nuclear run-on assays performed in RIN 1056E cells showed that glucocorticoids decreased the rate of transcription of the CCK gene. In addition, cycloheximide, used to prevent protein synthesis, abolished the inhibitory effects of dexamethasone on steady-state CCK mRNA levels. These results demonstrate that glucocorticoids down-regulate CCK gene expression in the rat intestinal mucosa and in two CCK-producing cell lines. The effect is blocked by a glucocorticoid receptor antagonist. Inhibition of CCK gene expression may result from a decrease in the transcription rate, and probably involves one or several steps that depend on protein synthesis.

Journal of Endocrinology (1996) 151, 137–145