Meal-stimulated release of methionine-enkephalin into the canine jejunal lumen

Intestinal sensing of nutrients

Ingestion of a meal triggers a range of physiological responses both within and outside the gut, and results in the remote modulation of appetite and glucose homeostasis. Luminal contents are sensed by specialised chemosensitive cells scattered throughout the intestinal epithelium. These enteroendocrine and tuft cells make direct contact with the gut lumen and release a range of chemical mediators, which can either act in a paracrine fashion interacting with neighbouring cells and nerve endings or as classical circulating hormones. At the molecular level, the chemosensory machinery involves multiple and complex signalling pathways including activation of G-protein-coupled receptors and solute carrier transporters. This chapter will discuss our current knowledge of the molecular mechanisms underlying intestinal chemosensation with a particular focus on the relatively well-characterised nutrient-triggered secretion from the enteroendocrine system.

Release of endogenous opioids from duodenal enteroendocrine cells requires Trpm5

BACKGROUND & AIMS

Enteroendocrine cells, the largest and most diverse population of mammalian endocrine cells, comprise a number of different cell types in the gut mucosa that produce, store, and secrete small molecules, peptides, and/or larger proteins that regulate many aspects of gut physiology. Little is known about less typical endocrine cells in the intestinal mucosa that do not contain secretory granules, such as brush or caveolated cells. We studied a subset of these enteroendocrine cells in duodenum that produce several peptides, including endogenous opioids, and that also express the Trpm5 cation channel.

METHODS

We studied expression patterns of Trpm5 and other molecules by immunohistochemical and enzyme-linked immunosorbent assay analyses of intestinal tissues from transgenic mice that express green fluorescent protein from the Trpm5 promoter, as well as wild-type and Trpm5-null mice.

RESULTS

We describe a type of enteroendocrine cell in mouse duodenum that is defined by the presence of Trpm5 and that does not contain typical secretory granules yet expresses endogenous opioids (beta-endorphin and Met-enkephalin) and uroguanylin in apical compartments close to the lumen of the gut.

CONCLUSIONS

Solitary chemosensory cells that coexpress beta-endorphin, Met-enkephalin, uroguanylin, and Trpm5 exist in mouse duodenum. These cells are likely to secrete the bioactive peptides into the intestinal lumen in response to dietary factors; release of the opioid peptides requires the Trpm5 ion channel.

Racecadotril versus loperamide: antidiarrheal research revisited

Racecadotril is an enkephalinase inhibitor, presented as a purely antisecretory agent with advantages over the opiate-receptor agonist loperamide in the treatment of diarrhea. A critical review of the literature and the models used was performed. Although pretreatment with high doses of racecadotril reduced cholera toxin-induced secretion and although clinical efficacy was demonstrated in young infants--a population characterized by 10-fold higher plasma enkephalin concentrations compared with adults, the analysis calls into question the peripheral antisecretory selectivity and relative clinical efficacy. Conversely, loperamide can be proposed as an antisecretory agent at therapeutic concentrations. Its efficacy is well established in acute and chronic diarrhea. Current experimental and clinical comparative studies of both drugs have problems with regard to the selection of the doses, the validity of models, and/or the trial design. The conclusion is that more research is needed before reliable conclusions can be drawn on the place of racecadotril in diarrhea treatment.

Racecadotril demonstrates intestinal antisecretory activity in vivo

BACKGROUND

Racecadotril (acetorphan), a potent enkephalinase inhibitor, protects endogenous enkephalins from degradation. Racecadotril exhibits experimental and clinical antidiarrhoeal activity without any effect on intestinal motility, suggesting selective antisecretory activity. The antisecretory effect of racecadotril was directly assessed in the present study.

METHODS

A 1 m, jejunal, Thiry-Vella loop was created in six mongrel dogs, and water and ionic fluxes were evaluated during infusion (2 mL/min) of Tyrode solution labelled with 14C-polyethylene glycol. Fluxes were determined both in the basal state and 5-6 h after commencement of a 2-h infusion of cholera toxin (0.4 microgram/mL). Racecadotril (10 mg/kg) or vehicle was given orally with and without prior intravenous administration of naloxone (0.1 mg/kg) or phentolamine (0.2 mg/kg).

RESULTS

Basal absorption remained unchanged following racecadotril administration; however, racecadotril significantly decreased (P = 0.01) cholera toxin-induced water, sodium, and potassium hypersecretion, from 0.73 +/- 0.15 to 0.37 +/- 0.13 mL/min; from 125.0 +/- 16.1 to 14.7 +/- 9.5 microMol/min; and from 3.41 +/- 0.66 to 1.66 +/- 0.61 microMol/min, respectively. This antisecretory activity of racecadotril was suppressed by naloxone but not by phentolamine.

CONCLUSIONS

This study directly demonstrates the antisecretory activity of racecadotril in relation to the protection of endogenous enkephalins.

High-performance liquid chromatographic determination of intrathecally administered [D-Ala2-D-Leu5]-enkephalin concentrations in canine cerebrospinal fluid

A rapid and useful method for high-performance liquid chromatographic analysis of exogenous [D-Ala2-D-Leu5]-enkephalin (DADLE) in cerebrospinal fluid (CSF) is described. CSF (0.5 ml) samples were filtered using a 0.22-micron Co-Star filter. Chromatography was performed on a mu Bondapak C18 column using a mobile phase of A, 0.05 M sodium phosphate (monobasic, pH 6.0) and B, 60% acetonitrile in 0.05 M sodium phosphate (pH 6.0) with a flow-rate of 1 ml/min. Absorbance at 210 nm was measured. The procedure produced a linear curve for the concentration range 1-10 micrograms/ml. The development of the assay produced rapid, repeatable and accurate results for CSF analysis of DADLE at concentrations achieved with therapeutic administration of the peptide. This method could also be used in the future for analysis of compounds like DADLE.

Extrinsic neural contribution to ileal peptide YY (PYY) release

Peptide YY (PYY) release into the ileal lumen is stimulated by cholecystokinin (CCK) and glucose ingestion. Previous data have implicated vagal activity in the mediation of PYY release into both the systemic circulation and the ileal lumen. The present study was designed to evaluate extrinsic neural involvement in CCK and glucose-stimulated circulating and ileal intraluminal PYY release. Distal ileal Thiry-Vella loops (TVL) of 25 cm were created in seven mongrel dogs. On separate days fasted dogs were given continuous infusions of CCK at 500 ng/kg/hr during the first hour of the study or an oral glucose (1.5 g/kg) tolerance test (OGTT) was performed. Peripheral blood samples and ileal effusates were collected before tests and following either CCK or glucose stimulation for 120 min at 20-min intervals. Ileal PYY recoveries were measured by the instillation and collection of 20 cc of normal saline from the TVL for each 20-min period. The dogs were again tested after surgical denervation of the TVL. OGTT resulted in a significant rise of PYY recovery from the TVL (P less than 0.05), while not affecting circulating PYY. Intravenous CCK resulted in significant increases in both plasma and ileal PYY concentrations (P less than 0.05). Denervation of the TVL decreased PYY recovery from the TVL after both CCK and OGTT, whereas this procedure did not affect circulating PYY levels or basal luminal levels. These data demonstrate the inhibition of CCK- and glucose-stimulated ileal PYY recovery from denervated ileal loops. The extrinsic neural pathways are involved in the mediation of glucose- and CCK-stimulated mechanisms for ileal PYY release.

The postprandial circulatory and ileal intraluminal release of neuropeptide Y in conscious dogs

Neuropeptide Y (NPY), a 36-amino-acid peptide, was measured in the peripheral circulation and ileal lumen of conscious dogs using a sensitive radioimmunoassay. Fasting NPY concentrations averaged 448 +/- 15 pg/ml in the peripheral blood and 364 +/- 23 pg/ml in the ileal effluent. Following a mixed meal, circulating NPY levels rose to 499 +/- 37 pg/ml (P less than 0.05), whereas recoverable quantities of ileal intraluminal NPY fell to 257 +/- 19 pg/ml (P less than 0.05). Neither fat nor glucose meals significantly changed circulating or ileal intraluminal NPY recovery. These results demonstrate release of NPY into the blood and ileal lumen for the first time and support NPY as a candidate gut hormone.

Luminal adrenergic agents modulate intestinal transport

Intestinal transport is controlled by luminal solutes, neural pathways, and paracrine or humoral agents. The current study investigated the effect of luminally administered adrenergic agents on the intestinal transport of water and electrolytes. Dogs with 25-cm jejunal Thiry-Vella loops were studied. The loops were luminally perfused with an isotonic solution containing [14C]PEG, and the fluxes of H2O, Na+, and Cl- were calculated. Each experiment consisted of three 1-hr periods: basal, luminal agent infusion, and recovery. Luminal adrenergic agents did not alter heart rate. Norepinephrine (alpha 1 greater than alpha 2 and beta adrenergic agonist) and phenylephrine (alpha 1 adrenergic agonist) caused significant absorption of water and sodium. Clonidine (alpha 2 adrenergic agonist) and isoproterenol (beta adrenergic agonist) caused significant secretion of water, sodium, and chloride. Luminally administered adrenergic agents can influence small intestinal water and electrolyte transport. Alpha 1 agonists have a proabsorptive effect, while alpha 2 and beta agonists have a secretory effect. Luminally administered proabsorptive adrenergic agents may prove useful in pathologic secretory states such as diabetic diarrhea, small bowel transplantation, or diarrhea-associated endocrinopathies.

Endogenous opiates in the mediation of early meal-induced jejunal absorption of water and electrolytes

The purpose of this study was to investigate the role of endogenous opiates in mediating meal-stimulated jejunal absorption. Jejunal Thiry-Vella loops, 25 cm long, were studied in awake conditioned dogs, using luminal perfusion with carbon-14 polyethylene glycol. Fluxes of water, sodium, and chloride were calculated every 15 minutes over a 1-hour basal period, followed by a 3-hour experimental period. The animals were divided into four groups: control, naloxone, meal, and meal plus naloxone. In the control and naloxone groups, the fluxes did not change over the 4-hour observation period. Meal alone immediately stimulated the absorption of water and electrolytes in the Thiry-Vella loop (p less than 0.05). The addition of naloxone infusion to the meal stimulus resulted in significantly reduced absorption during the first hour after the meal (p less than 0.05). We concluded that endogenous opiates play a role in meal-stimulated jejunal absorption.