Tamer Coşkun

Glucagon-like peptide-1 inhibits gastric emptying via vagal afferent-mediated central mechanisms

Exogenous administration of glucagon-like peptide-1-(7-36) amide (GLP-1), an insulinotropic hormone, inhibits gastric emptying and acid secretion in humans. The role of GLP-1 as a regulator of gastric function is elusive. In gastric fistula rats, vagal afferent denervation and peripheral administration of the GLP-1 receptor antagonist exendin(9-39) amide enhanced emptying of a glucose meal, whereas intracerebroventricular exendin was ineffective. The rate of saline emptying was attenuated by peripheral as well as by central administration of GLP-1, and pretreatment with exendin by the respective routes reversed the inhibition by GLP-1. Vagal afferent denervation abolished the central and peripheral action of GLP-1 on gastric emptying. Neither peripheral cholinergic nor adrenergic blockade altered the delay of methyl cellulose meal emptying by intracisternal GLP-1 injection. Acid secretion in conscious pylorus-ligated rats was inhibited by intracisternal GLP-1 administration, whereas systemic GLP-1 was ineffective. These results support the notion that GLP-1 receptors participate in the central and peripheral regulation of gastric function. Furthermore, vagal afferent nerves mediate the inhibitory action of GLP-1 on gastric motor function. GLP-1 may be a candidate brain-gut peptide that acts as a physiological modulator of gastric function.Exogenous administration of glucagon-like peptide-1-(7-36) amide (GLP-1), an insulinotropic hormone, inhibits gastric emptying and acid secretion in humans. The role of GLP-1 as a regulator of gastric function is elusive. In gastric fistula rats, vagal afferent denervation and peripheral administration of the GLP-1 receptor antagonist exendin-(9-39) amide enhanced emptying of a glucose meal, whereas intracerebroventricular exendin was ineffective. The rate of saline emptying was attenuated by peripheral as well as by central administration of GLP-1, and pretreatment with exendin by the respective routes reversed the inhibition by GLP-1. Vagal afferent denervation abolished the central and peripheral action of GLP-1 on gastric emptying. Neither peripheral cholinergic nor adrenergic blockade altered the delay of methyl cellulose meal emptying by intracisternal GLP-1 injection. Acid secretion in conscious pylorus-ligated rats was inhibited by intracisternal GLP-1 administration, whereas systemic GLP-1 was ineffective. These results support the notion that GLP-1 receptors participate in the central and peripheral regulation of gastric function. Furthermore, vagal afferent nerves mediate the inhibitory action of GLP-1 on gastric motor function. GLP-1 may be a candidate brain-gut peptide that acts as a physiological modulator of gastric function.

Pathways mediating CRF-induced inhibition of gastric emptying in rats

The corticotrophin-releasing factor (CRF) is shown to be released during stress suggesting that CRF has a physiological role in the mediation of central nervous system (CNS) response to stress, including an inhibitory effect on gastric emptying. In the present study, we have examined the pathways by which intracerebroventricularly (i.c.v.) administered CRF and central CRF activation during stress alter the gastric emptying rate of saline (0.14 M), acid (50 mM), peptone (4.5%) and peptone after preload. The emptying rates of all these test meals were significantly (p < 0.05-0.001) delayed with increasing doses of i.c.v. CRF (0.001, 0.003, 0.01, 0.1, 0.3 and 1 nmol/10 microl), when compared with their i.c.v. saline-treated controls. The 1-nmol dose of CRF inhibited the emptying of acid, peptone and peptone after a preload by 43.8%, 64.1% and 81.1%, respectively. Twenty-minute swim stress delayed gastric emptying rate of saline, acid and peptone solutions significantly (p < 0.001) and the CRF receptor antagonist, alpha-helical CRF (8 nmol/10 microl, i.c.v.), applied before the swim stress, abolished the inhibitory effect of stress on the emptying rate of these solutions. Acute intragastric administration of capsaicin (2 mg/rat) and systemic capsaicin (125 mg kg(-1)) treatment facilitated the gastric emptying rate of acid, peptone and peptone after preload significantly, almost abolishing the inhibitory effect of central CRF (p < 0.001). However, either capsaicin treatment had no effect on stress-induced inhibition of the gastric emptying of none of the solutions, except peptone after a preload. Our findings demonstrate that the gastric inhibitory response induced by swimming as a stress-producing stimulus is mediated by the endogenous release of CRF. They also suggest that CRF exerts its CNS actions on the gastrointestinal tract via vago-vagal, capsaicin-sensitive pathways, probably involving the central cholecystokinin (CCK) mechanisms.

Delayed gastric emptying in conscious male rats following chronic estrogen and progesterone treatment.

Several clinical observations and animal experiments have led to speculation concerning the possible effects of pregnancy and pregnancy-associated sex steroids on gastrointestinal function. It was reported that estrogen increases intestinal contractile activity, while progesterone or the combination of estrogen and progesterone decreases it. In order to measure gastric emptying, a methylcellulose test meal was given orally into the stomach of conscious rats. In progesteronetreated rats, at the dose of 0.2 mg/kg, gastric emptying was not significantly different from that of the control, but it was found to be significantly delayed at the dose of 10 mg/kg (P<0.05). Estrogen treatment at doses of 20 μg/kg and 600 μg/kg significantly delayed gastric emptying, when compared with controls (P<0.001). Combined therapy of estrogen and progesterone induced a significant delay in gastric emptying rate compared with the control group (P<0.001). In the animals with pseudopregnancy treatment (100 μg/kg estrogen+ 15 mg/kg progesterone; 7–12 days) the gastric empying rate was significantly different from that of the control (P<0.05). We conclude that both estrogen and progesterone exert inhibitory effects on gastric emptying, and this may account for the disturbances in gastrointestinal function that pregnant women frequently experience.

Capsaicin-sensitive vagal fibres and 5-HT3-, gastrin releasing peptide- and cholecystokinin A-receptors are involved in distension-induced inhibition of gastric emptying in the rat

The present study was undertaken to investigate how the activation of gastric mechanoreceptors by distension of the stomach in conscious gastric fistula rats influences gastric emptying; and the roles of capsaicin sensitive vagal afferent fibres and the 5-HT3, GRP and CCK-A receptors involved in mediating these responses. To activate mechanoreceptors by non-nutrient dependent pathways, methylcellulose in saline was used to distend the stomach (5 cm H2O) and the subsequent emptying of saline was examined immediately, and at 3, 5 and 10 min following distension. Prior distension delayed the subsequent emptying of saline instilled into the stomach compared with non-distended controls (2.28+/-0.09 ml/5 min; P < 0.001). Topical application of capsaicin, completely abolished the distension-induced inhibition of gastric emptying when compared with vehicle treated rats (2.82+/-0.09 vs. 2.38+/-0.04 ml/5 min; P < 0.001). Peripheral administration of a GRP antagonist (2258 U89UJ, 1 mg/kg), and a 5-HT3 antagonist (BRL4369UA, 50 microg/kg) significantly reversed (2.56+/-0.14 ml/5 min; P < 0.05 and 2.61+/-0.07 ml/5 min; P < 0.01; respectively) the delay in gastric emptying induced by distension. When the rats were treated with the CCK-A antagonist, gastric emptying of saline following distension was also significantly facilitated (2.56+/-0.07 ml/5 min; P < 0.001). In contrast, the CCK-B/gastrin receptor antagonist had no significant effect on the distension induced delay in gastric emptying (1.95+/-0.12 ml/5 min). The present results suggest that gastric distension in conscious gastric fistula rats delays gastric emptying by activating capsaicin-sensitive extrinsic afferent nerve fibres. Moreover, the results also indicate that distension-induced mechanisms involve GRP, 5-HT3 and CCK-A receptors, but not CCK-B receptors.

Cold restraint stress-induced gastric mucosal dysfunction role of nitric oxide

The objectives of this study were to determine the cold restraint stress-induced changes in gastric mucosal permeability and to assess whether nitric oxide synthesis inhibition affects gastric mucosal integrity after cold-restraint administration. Cold-restraint stress caused multiple gastric lesions in 90% of animals. The lesion index was found to be 3.87 ± 0.97 mm. Gastric mucosal permeability to the [51Cr]EDTA molecule was significantly elevated in the cold-restraint group compared to control. In order to evaluate the role of nitric oxide in cold restraint stress-induced gastropathy,l-arginine analogNG-nitro-l-arginie methyl ester (l-NAME) was given as a bolus (10 mg/kg, intravenously) and infused at a rate of 2 mg/ml/hr for 2 hr after cold-restraint administration.l-NAME greatly exacerbated gastric mucosal dysfunction associated with cold-restraint stress.d-NAME, the biologically inactive enantiomer, did not enhance mucosal dysfunction, whereasl-arginine, the substrate for nitric oxide, reversed the effect ofl-NAME. In an additional group of experiments, effects of cold-restraint stress andl-NAME on net transmucosal fluid flux as well as tissue myeloperoxidase activity (MPO) were assessed. Cold-restraint stress administration significantly reduced the absorptive capacity of stomach, whereasl-NAME treatment did not affect the stress-induced alterations on net fluid absorption. Furthermore,l-NAME treatment did not affect the cold restraint stress-induced changes in tissue MPO activity. Our results suggest that gastric barrier function is altered after cold-restraint stress and nitric oxide production is important in minimizing mucosal barrier dysfunction associated with cold-restraint stress administration. Our results also indicate thatl-NAME-induced alterations on mucosal permeability are not related to net transmucosal fluid flux and tissue neutrophils.

The Effect of Nitric Oxide Synthase Blockade and Indometacin on Gastric Emptying and Gastric Contractility

The aim of the present study was to investigate the effect of nitric oxide (NO) synthase inhibition on gastric emptying rate in conscious rats and on gastric muscle contractility. The involvement of NO was also investigated in indometacin-induced (25 mg/kg, s.c.) changes in gastric emptying rate and smooth muscle contractility. L-NAME (NG-nitro-L-arginine methyl ester; 10 mg/kg, i.v.) inhibited the gastric emptying rate compared to controls and this effect was abolished by L-arginine (300 mg/kg, i.v.). Similarly, indometacin treatment led to a significant delay of gastric emptying rate with respect to vehicle-treated rats. Gastric longitudinal and circular muscle strips of L-NAME or indometacin-treated rats showed a reduction in contractile responses to carbachol. The results demonstrate that NO synthase blockade and indometacin treatment delay gastric emptying in conscious rats, concomitant with reduced responsiveness to carbachol, in vitro.

Inhibitory effects of gastrin releasing peptide on gastric emptying in rats

Gastrin-releasing peptide (GRP) has a wide range of biological actions, including stimulation of the frequency of antral contractions and delaying gastric emptying. The present study was designed to evaluate the role of GRP in the control of gastric emptying of liquid test meals in the rat. The emptying of methyl cellulose given by gavage to fasted rats, or of saline given via the fistula to conscious gastric fistula rats was not influenced by the GRP antagonists, NC-8-89 (Leu13-psi-(CH2NH)-Leu14-bombesin) and 2258U89 ((de-NH2)Phe19, D-Ala24, D-Pro26 psi (CH2NH)Phe27(-GRP (19-27)), at 2 mg/kg, s.c. However, both antagonists (0.02, 0.2 and 2 mg/kg) reversed the inhibitory effect of HCI on gastric emptying in gastric fistula rats (P < 0.05-0.001). When peptone was administered after a preload, but not otherwise, the inhibition of emptying was also partly reversed by both antagonists at all doses used (P < 0.05-0.001). Interestingly, the delay in the emptying of hyperosmolal saline compared to saline, was enhanced at a dose of 0.2 mg/kg of both antagonists (P < 0.05 and P < 0.01). Food intake did not change significantly with the two lower doses of antagonists, but was decreased by the highest dose of NC 8-89. We conclude that GRP specifically inhibits gastric emptying of acid and peptone solutions in the conscious rat.

The effect of aqueous garlic extract on the levels of arachidonic acid metabolites (leukotriene C4 and prostaglandin E2) in rat forebrain after ischemia-reperfusion injury

Leukotriene C4 (LTC4) and prostaglandin E2 (PGE2) are known to be highly potent cerebral vasoconstrictors which are formed from arachidonic acid (AA). They enhance vascular permeability, inducing vasogenic edema that may damage the ischemic penumbra after ischemia and reperfusion. The inhibitory effect of aqueous garlic extract (AGE) on AA metabolism in human platelets is known. In this study, following the global ischemic model application to the rats, all underwent 10 min ischemia and were reperfused for different periods. The levels of LTC4 and PGE2 in rat forebrain were then measured. One rat group consisted of 8 rats. In the combined reperfused groups both metabolites increased significantly when compared with the 10 min ischemia alone, no reperfusion group (p < 0.05). In the 8 min reperfused group, PGE2 and LTC4 levels increased significantly at 60 min of reperfusion compared with each corresponding control group (P < 0.005). PGE2 and LTC4 levels were reduced significantly at 60 min of reperfusion compared with the 8 min reperfused group (P < 0.005). AGE (1 ml/kg) reduced both LTC4 and PGE2 levels significantly in the 8 min and 60 min reperfused group (P < 0.001, P < 0.001, P < 0.05, P < 0.01). In conclusion, AGE reduced LTC4 and PGE2 levels at a dosage of 1 ml/kg following 8 and 60 min reperfusion. It may be helpful in reducing AA metabolite levels and preventing injury after ischemic phenomena.

ROLE OF NEUTROPHILS IN INDOMETHACIN-INDUCED GASTRIC MUCOSAL LESIONS IN RATS

Nonsteroidal anti-inflammatory drugs (NSAIDs) cause clinically important gastric damage by several mechanisms. In order to evaluate the role of neutrophil infiltration in lesion formation, tissue myeloperoxidase activities were assessed in different gastric layers of the stomach both in rats with normal neutrophil levels and in neutropenic rats. Sprague-Dawley rats were treated either with indomethacin (Indo; 25 mg/kg, s.c.) or the vehicle. A group of rats were made neutropenic by administration of methotrexate (MTX; 2.5 mg/kg i.p.) once a day for 3 days. The stomachs were removed for the determination of lesion index, glutathione, lipid peroxide levels, protein oxidation and tissue myeloperoxidase activities. MTX treatment appeared to reduce neutrophil infiltration significantly while producing insignificant effects on eosinophils and macrophages. Indo administration caused multiple gastric lesions and treatment with MTX significantly reduced lesion index. In rats treated with Indo, neither glutathione nor LP levels showed any significant changes but the protein oxidation was significantly higher than that of other groups. The MPO level of gastric mucosa was increased in Indo-treated rats and reversed by MTX pretreatment. The results of the present study indicate that neutrophil infiltration in the gastric mucosa of rats may be involved in the pathogenesis of NSAID-induced gastric mucosal injury, but no correlation was found between lesion formation and protein oxidation in the gastric mucosa.

Inflammatory response to cold injury in remote organs is reduced by corticotropin-releasing factor.

Current experimental evidence concerning the potential activity of corticotropin releasing factor (CRF) in inflammatory processes still remains controversial. To determine whether CRF has protective effects on three remote organs (liver, lung and stomach) affected by cold injury and to characterize the role of neutrophils in cold-induced inflammation, dorsums of anesthetized rats were exposed for 5 min to a 22% NaCl solution maintained at -20+/-0.5 degrees C and the rats were sacrificed at 24 h after the cold injury. The results indicate that cold-exposure-induced edema in the liver, lung and stomach was blocked by subcutaneous (s.c.; 1.2 and 12 nmol/kg; 30 min before cold trauma) CRF pretreatment, while the central administration of CRF (intracisternally (i.c.); 0.30 and 1.5 nmol/rat; 15 min before cold) had the similar effect at the higher dose. Histological assessment and the tissue myeloperoxidase activities also revealed that CRF given peripherally has a protective role in damage generation. Moreover, CRF had a facilitatory effect in the recovery of the body temperature following cold exposure. In conclusion, CRF is likely to act on its peripheral receptors in the inflamed remote organs, suppressing the edematogenic effects of inflammatory mediators, some of which are neutrophil-derived.