Role of nonesterified fatty acids in necrotizing pancreatitis: an in vivo experimental study in rats.

INTRODUCTION: In acute pancreatitis, nonesterified fatty acids (NEFA) might be released by lipase and cause tissue necrosis by their detergent properties, but this has not been established in vivo. AIMS: To measure the release of NEFA in the blood stream, pancreatic tissue, and peritoneal cavity during taurocholate-induced acute necrotizing pancreatitis in rats. METHODOLOGY: Ascites and blood were repeatedly sampled; after 24 hours, pancreatic lesions were scored, and NEFA were measured in the pancreas. The effects of a specific lipase inhibitor (Tetrahydrolipstatin [THL]) were also studied. RESULTS: A slight transient increase (22%) of NEFA concentration was observed in systemic circulation but did not parallel the time course of lipase activity, arguing against an intravascular production of NEFA by circulating lipase. Pancreatic NEFA did not differ between rats with pancreatitis and control rats. NEFA in ascites increased to threefold the basal value immediately after taurocholate and decreased rapidly thereafter, whereas lipase increased later in ascites and remained elevated during the 24-hour duration of the experiment. Lipase inhibition by THL neither modified the early increase of NEFA in ascites, nor altered the macroscopic, enzymatic, and histologic evolution of pancreatitis. CONCLUSION: This in vivo study does not confirm the hypothetical role of NEFA produced by pancreatic lipase in the necrotic process and its systemic complications, up to now mainly suggested on the basis of ex vivo experiments.

Neural mechanism of the antisecretory effect of peptide YY in the rat colon in vivo.

The purpose of this work was to determine the mechanism of the antisecretory effect of peptide YY in the rat colon and whether this effect is physiological. In this prospect, doses of exogenous peptide YY producing physiological and supraphysiological plasma levels were intravenously infused in rats provided with colonic and jejunal ligated loops in vivo, under secretory stimulation by vasoactive intestinal peptide. Peptide YY decreased the secretory effect of VIP in a dose-related fashion. The effect of peptide YY was blocked or strongly decreased by tetrodotoxin, hexamethonium, idazoxan, haloperidol, and the sigma antagonist BMY 14, 802 in both the colon and jejunum. We conclude that peptide YY decreases water and electrolyte secretion in the colonic mucosa by a complex neural mechanism involving at least two neurons connected through a nicotinic synapse, alpha-2 adrenoceptors and sigma receptors, and that this effect can occur with physiological doses of peptide YY.

Effect of sumatriptan on external pancreatic secretion and its interaction with endogenous norepinephrine in the rat.

We reported previously that blocking norepinephrine reuptake by nisoxetine could modulate external pancreatic secretion in the rat. We report in this study the interaction of serotonin (5-HT) with endogenous catecholamines by using sumatriptan, an agonist of 5-HT1 receptors, in combination with nisoxetine. Urethane-anesthetized male Wistar rats were fitted with an acute pancreatic fistula. Nisoxetine (0.3 mg/kg, i.v.) and sumatriptan (0.1-1 mg/kg, s.c.) were administered alone or in combination. Pancreatic secretion was measured under stimulation by 2-deoxy-D-glucose (2DG; 75 mg/kg, i.v.), by vagal electrical stimulation (4 V, 2 ms, 10 Hz), or by acetylcholine (60-1,800 microg/kg.h). (i) 2DG: Nisoxetine alone inhibited 2DG-induced pancreatic secretion (p < 0.01). Sumatriptan alone also produced a dose-related inhibition of 2DG-induced pancreatic secretion (p < 0.01). When sumatriptan and nisoxetine were combined, protein response to 2DG remained inhibited, whereas water and electrolyte secretion was restored. (ii) Vagal stimulation: Nisoxetine did not modify water and electrolyte output in response to vagal electrical stimulation (VES), whereas it inhibited protein response by 75%. Sumatriptan alone strongly inhibited pancreatic response to VES (p < 0.01). When nisoxetine and sumatriptan were combined, the protein response to VES remained inhibited, whereas water and electrolyte response to VES was restored. (iii) Acetylcholine: Nisoxetine and sumatriptan alone or combined did not modify pancreatic response to acetylcholine. These results indicate that noradrenergic and serotonergic agents can indirectly affect pancreatic secretion through a modulation of the vagal cholinergic pathway. Nisoxetine and sumatriptan interact negatively on hydroelectrolytic pancreatic secretion, whereas they inhibit the secretion of enzymes both alone and in combination.

Modulation of external pancreatic secretion by endogenous norepinephrine: study with a norepinephrine uptake blocker in the rat.

The effect of endogenous catecholamines on pancreatic secretion was analyzed with nisoxetine, a specific norepinephrine uptake blocker, and specific adrenoceptor antagonists in anesthetized acute fistula rats. Nisoxetine was administered alone or with alpha-1 (prazosin), alpha-2 (idazoxan or yohimbine), or beta (propranolol) adrenoceptor antagonists. Pancreatic secretion was measured in basal conditions or after stimulation by 2-deoxy-D-glucose (2DG), electrical vagal stimulation, or acetylcholine. (a) Basal. Nisoxetine alone had no effect. Associated with idazoxan or yohimbine, nisoxetine produced a dose-related stimulation (p < 0.01) of water and electrolyte without changing protein output. Addition of propranolol abolished this stimulation. (b) 2DG. Nisoxetine inhibited 2DG-induced secretion (p < 0.01). Idazoxan or yohimbine suppressed the nisoxetine inhibition of water and electrolyte output (p < 0.01) but had no effect on protein output, which was restored only by adding a mixture of idazoxan, prazosin, and propranolol. (c) Electrical stimulation. Nisoxetine did not modify water and electrolyte but inhibited protein response by 75%. Adding idazoxan to nisoxetine significantly increased (p < 0.01) water and bicarbonate response and partly restored protein response. Water and bicarbonate response was restored by propranolol, whereas protein response was only restored by adding a mixture of idazoxan, prazosin, and propranolol. (d) Nisoxetine did not modify pancreatic response to acetylcholine. In conclusion, endogenous norepinephrine affects the response to vagally mediated effects through several subtypes of adrenoceptors, without changing basal or acetylcholine stimulated secretion.

Neural modulation of the antisecretory effect of peptide YY in the rat jejunum.

The endocrine and neural peptide, peptide YY, inhibits intestinal secretion of water and electrolytes in several animal species and in man. Peptide YY receptors have been evidenced on isolated rat jejunal crypt cells, but neural receptors are also likely to participate in the antisecretory effect of peptide YY in vivo. The aim of the present study was to investigate the mechanisms of the peptide YY effect on vasoactive intestinal peptide (VIP)-stimulated jejunal net water flux in the rat. Antagonist experiments using several drugs affecting neurally mediated processes were done for the purpose. A small peptide YY dose (10 pmol/kg) inhibited significantly (P < 0.005) the jejunal net water flux produced by 30 microg/kg per h of VIP. The inhibitory effect of peptide YY was suppressed, or strongly and significantly reduced, by tetrodotoxin, hexamethonium, lidocaine, idazoxan and BMY14,802 (51-(4-fluorophenyl)-4-(-4-(5-fluoro-2pyrimidinyl)-1-piperazinyl)- 1-butanol), whereas devazepide and L-NAME (L-omega-N-arginine methyl ester) had no effect. These results suggest that peptide YY inhibits VIP-stimulated jejunal net water flux in vivo through a neural mechanism implicating the participation of nicotinic synapses, alpha2-adrenoceptors and sigma receptors.

Several receptors mediate the antisecretory effect of peptide YY, neuropeptide Y, and pancreatic polypeptide on VIP-induced fluid secretion in the rat jejunum in vivo.

Several Y receptor subtypes have been cloned and/or pharmacologically characterized that mediate the effects of the regulatory peptides peptide YY (PYY), neuropeptide Y (NPY), and pancreatic polypeptide (PP). These peptides possess antisecretory properties on the intestine. This effect can be blocked in vivo by neural antagonists, suggesting the intervention of neural receptors, although epithelial PYY-preferring receptors have been evidenced on jejunal crypt cells. The purpose of the present experiments was to compare the antisecretory properties in vivo of a series of PYY and NPY derivatives with various affinities for different Y receptor subtypes, in order to determine which subtypes were involved. A model of VIP-stimulated secretion by rat jejunal loops was used. The results were compared with the binding affinities for PYY-preferring receptors determined on rat jejunal crypt cell membranes. Full-length PYY(1-36) was about three times more potent than NPY(1-36), and 10 times more potent than PP in the low dose range. PP, however, had a low efficacy limited to about 50% inhibition of VIP effect. Both Y1 agonists ([Leu31, Pro34]PYY and [Leu31,Pro34]NPY), and Y2 agonists [C-terminal fragments ranging from PYY (3-36) and NPY(3-36) to PYY(22-36) to NPY(22-36)] displayed potent antisecretory properties. PYY derivatives and fragments were always more potent than their respective NPY counterparts. In contrast, Y1 derivatives and PP had very low affinity for the epithelial PYY receptor as measured in vitro by radioreceptor assay. These data suggest that the antisecretory effect of PYY/NPY/PP peptides in vivo involves the effects of several receptors: a Y2-like, PYY-preferring receptor identical to the epithelial receptor, a Y1-like receptor, and a third receptor with high affinity for PP.

A new biological contribution of cyclo(His-Pro) to the peripheral inhibition of pancreatic secretion.

The tripeptide pyro-Glu-His-Pro-NH2[thyrotropin-releasing hormone (TRH)] was isolated from the hypothalamus as a thyrotropin-releasing factor. It has a broad spectrum of central nervous system-mediated actions, including the stimulation of exocrine pancreatic secretion. TRH is also synthesized in the endocrine pancreas and found in the systemic circulation. Enzymatic degradation of TRH in vivo produces other bioactive peptides such as cyclo(His-Pro). Because of the short half-life of TRH and the stability of cyclo(His-Pro) in vivo, we postulated that at least part of the peripheral TRH effects on the exocrine pancreatic secretion may be attributed to cyclo(His-Pro), which has been shown to have other biological activities. This study determines in parallel the peripheral effects of TRH and cyclo(His-Pro) as well as the putative contribution of other TRH-related peptides on exocrine pancreatic secretion in rats. TRH and its metabolite cyclo(His-Pro) dose dependently inhibited 2-deoxy-D-glucose (2-DG)-stimulated pancreatic secretion. TRH and all the related peptides tested had no effect on the basal and cholecystokinin-stimulated amylase release from pancreatic acinar cells in vitro. These data indicate that cyclo(His-Pro) mimics the peripheral inhibitory effect of TRH on 2-DG-stimulated exocrine pancreatic secretion. This effect is not detected on isolated pancreatic acini. Our findings provide a new biological contribution for cyclo(His-Pro) with potential experimental and clinical applications.

Modulation by alcohol and methadone of 2-deoxyglucose-stimulated pancreatic secretion in the rat.

Alcohol intake is a major problem in drug addicts, and it is not clear whether the effects of alcohol and opiates are additive or potentiating. Vagally stimulated pancreatic secretion in rats is potently inhibited by opiates acting centrally at mu-receptors. In the present experiments, we determined the effects of methadone on 2-deoxyglucose (2DG)-stimulated pancreatic secretion in rats treated with acute (1.9 g/kg.3 h, intravenously) or chronic (1 or 3 month drinking) ethanol. In both acute and 1 month chronic alcoholic rats, methadone administered at its 50% inhibitory dose (ID50) reduced by about 50% 2DG-stimulated pancreatic secretion of sodium, bicarbonate and protein, and ethanol had only faint, nonsignificant inhibitory effects. In 3 month chronic alcoholic rats, similar results were obtained, but methadone inhibited 2DG-stimulated pancreatic secretion by 60 to 90% in these older rats. No significant interaction was found in any condition between ethanol and methadone, suggesting that they had only additive, but not potentiating effects in this method.