Wieslaw W. Pawlik

Role of endogenous nitric oxide in the control of canine pancreatic secretion and blood flow

BACKGROUND Endogenous nitric oxide has been implicated in the control of mesenteric circulation, but its role in the control of pancreatic blood flow and exocrine pancreatic secretion has not been studied. METHODS Secretory studies were performed on conscious dogs with chronic pancreatic fistulas, and changes in pancreatic blood flow were measured by laser Doppler flowmetry in anesthetized animals. RESULTS Infusion of NG-nitro-L-arginine did not affect basal pancreatic protein secretion but suppressed an increase of this secretion induced by L-arginine but not that induced by glyceryl trinitrate. Sham-feeding, meal feeding, and infusion of secretin plus cholecystokinin induced pancreatic protein outputs reaching, respectively, 30%, 74%, and 50% of cerulein maximum in these dogs. Infusion of NG-nitro-L-arginine caused a profound inhibition of these secretions, whereas the addition of L-arginine reversed this inhibition in part. NG-nitro-L-arginine or L-arginine added to the incubation medium of isolated canine pancreatic acini did not affect basal or cholecystokinin-induced amylase release. In anesthetized dogs, infusion of NG-nitro-L-arginine caused a significant reduction in the pancreatic blood flow both while resting and following stimulation with secretin plus cholecystokinin but did not affect this flow in animals treated with glyceryl trinitrate. Addition of L-arginine attenuated the decrease in pancreatic blood flow and the increase in systemic blood pressure caused by NG-L-nitro-arginine. CONCLUSIONS Endogenous NO affects pancreatic secretion probably through the changes in the vascular bed.

Prostaglandin/Cyclooxygenase Pathway in Ghrelin-Induced Gastroprotection against Ischemia-Reperfusion Injury

Ghrelin is involved in the control of food intake, but its role in gastroprotection against the formation of gastric mucosal injury has been little elucidated. We studied the effects of peripheral (i.p.) and central (i.c.v.) administration of ghrelin on gastric secretion and gastric mucosal lesions induced by 3 h of ischemia/reperfusion (I/R) with or without inhibition of ghrelin growth hormone secretagogue type 1a receptor (GHS-R1a) by using ghrelin antagonist, d-Lys3-GHRP-6; blockade of cyclooxygenase (COX)-1 (indomethacin, SC560 [5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole]) and COX-2 (rofecoxib); and bilateral vagotomy or capsaicin denervation. I/R produced typical gastric erosions, a significant fall in the gastric blood flow (GBF), an increase in gastric myeloperoxidase (MPO) activity and malonyldialdehyde (MDA) content, and the up-regulation of mucosal ghrelin mRNA. Ghrelin dose-dependently increased gastric acid secretion and significantly reduced I/R-induced gastric erosions, while producing a significant rise in the GBF and mucosal PGE2 generation and a significant fall in MPO activity and MDA content. The protective and hyperemic activities of ghrelin were significantly attenuated in rats pretreated with d-Lys3-GHRP-6 and capsaicin denervation and completely abolished by vagotomy. Indomethacin, SC560, and rofecoxib, selective COX-1 and COX-2 inhibitors, attenuated ghrelin-induced protection that was restored by supplying the methyl analog of prostaglandin (PG) E2. The expression of mRNA for COX-1 was unaffected by ghrelin, but COX-2 mRNA and COX-2 protein were detectable in I/R injured mucosa and further up-regulated by exogenous ghrelin. We conclude that ghrelin exhibits gastroprotective and hyperemic activities against I/R-induced erosions, the effects that are mediated by hormone activation of GHS-R1a receptors, COX-PG system, and vagal-sensory nerves.

Therapeutic Potential of 1-Methylnicotinamide against Acute Gastric Lesions Induced by Stress : Role of Endogenous Prostacyclin and Sensory Nerves

1-Methylnicotinamide (MNA) is one of the major derivatives of nicotinamide, which was recently shown to exhibit antithrombotic and antiinflammatory actions. However, it is not yet known whether MNA affects gastric mucosal defense. The effects of exogenous MNA were studied on gastric secretion and gastric lesions induced in rats by 3.5 h of water immersion and water restraint stress (WRS) or in rats administered 75% ethanol. MNA [6.25–100 mg/kg intragastrically (i.g.)] led to a dose-dependent rise in the plasma MNA level, inhibited gastric acid secretion, and attenuated these gastric lesions induced by WRS or ethanol. The gastroprotective effect of MNA was accompanied by an increase in the gastric mucosal blood flow and plasma calcitonin gene-related peptide (CGRP) levels, the preservation of prostacyclin (PGI2) generation (measured as 6-keto-PGF1α), and an overexpression of mRNAs for cyclooxygenase (COX)-2 and CGRP in the gastric mucosa. R-3-(4-Fluoro-phenyl)-2-[5-(4-fluoro-phenyl)-benzofuran-2-ylmethoxycarbonylamino]-propionic acid (RO 324479), which is the selective antagonist of IP/PGI2 receptors, reversed the effects of MNA on gastric lesions and GBF. MNA-induced gastroprotection was attenuated by suppression of COX-1 [5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazole; SC-560] and COX-2 [4-(4-methylsulfonylphenyl)-3-phenyl-5H-furan-2-one; rofecoxib] activity, capsaicin denervation, and by the pretreatment with CGRP8-37 or capsazepine. Addition of exogenous PGI2 or CGRP restored the MNA-induced gastroprotection in rats treated with COX-1 and COX-2 inhibitors or in those with capsaicin denervation. WRS enhanced MDA content while decreasing superoxide dismutase (SOD) activity in the gastric mucosa, but pretreatment with MNA reversed these changes. MNA exerts potent gastroprotection against WRS damage via mechanisms involving cooperative action of PGI2 and CGRP in preservation of microvascular flow, antioxidizing enzyme SOD activity, and reduction in lipid peroxidation.

Dopamine effects on the intestinal circulation

The effects of intra-arterial infusion of dopamine on superior mesenteric artery blood flow, intestinal flow, intestinal oxygen consumption, and capillary density were studied in anesthetized dogs before and after blockade of dopamine receptors with haloperidol and after beta-adrenergic receptor blockade with propranolol. Mesenteric blood flow to a distal segment of the small intestine was measured with an electromagnetic blood flow-meter and intestinal oxygen consumption was calculated from the measured arteriovenous oxygen difference across the intestine and total blood flow. Intestinal capillary density was estimated from the clearance of 86Rb. In normal animals prior to dopaminergic or beta-adrenergic blockade, dopamine caused a dose-related decrease in mesenteric blood flow, intestinal oxygen consumption, and 86Rb clearance. Only the lowest dose of the drug, 1 mug/Kg.-min., did not significantly change the intestinal capillary density. In dogs pretreated with the dopamine receptor, antogonist, haloperidol, dopamine (20 mug/Kg.-min.) caused a significant increase in blood flow and oxygen consumption and did not significantly alter the number of perfused intestinal capillaries. These increases in haloperidol-blocked animals administered dopamine were reversed by propranolol. Our results indicate that dopamine caused smooth muscle contraction in mesenteric arterioles and precapillary sphincters, thereby producing intestinal ischemia and hypoxia. These findings with haloperidol and propranolol indicate that dopamine stimulates at least two different receptors in the canine mesenteric vascular bed: a constrictor receptor blocked by haloperidol and a dilator receptor blocked by propranolol.

Melatonin and its precursor, L-tryptophan: influence on pancreatic amylase secretion in vivo and in vitro

Abstract:  Melatonin, considered as a main pineal product, may be also synthetized in the gastrointestinal tract from l‐tryptophan. Melatonin has been recently shown to affect insulin release and its receptors have been characterized in the pancreas however, the effects of melatonin on the pancreatic enzyme secretion have not been examined. The aim of this study was to investigate the effects of melatonin or l‐tryptophan on amylase secretion in vivo in anaesthetized rats with pancreato‐biliary fistulas, and in vitro using isolated pancreatic acini. Melatonin (1, 5 or 25 mg/kg) or l‐tryptophan (10, 50 or 250 mg/kg) given to the rats as a intraperitoneal (i.p.) bolus injection produced significant and dose‐dependent increases in pancreatic amylase secretion under basal conditions or following stimulation of enzyme secretion by diversion of bile‐pancreatic juice. This was accompanied by a dose‐dependent rise in melatonin plasma level. Stimulation of pancreatic enzyme secretion caused by melatonin or l‐tryptophan was completely abolished by vagotomy, deactivation of sensory nerves with capsaicin or pretreatment with CCK1 receptor antagonists (tarazepide or l‐364,718). Pretreatment with luzindole, an antagonist of melatonin MT2 receptor failed to affect melatonin‐ or l‐tryptophan‐induced amylase secretion. Administration of melatonin (1, 5 or 25 mg/kg i.p.) or l‐tryptophan (10, 50 or 250 mg/kg i.p.) to the rats resulted in the dose‐dependent increase of cholecystokinin (CCK) plasma immunoreactivity. Enzyme secretion from isolated pancreatic acini was not significantly affected by melatonin or l‐tryptophan used at doses of10−8–10−5 m. We conclude that exogenous melatonin, as well as that produced endogenously from l‐tryptophan, stimulates pancreatic enzyme secretion in vivo while increasing CCK release. Stimulatory effect of melatonin or l‐tryptophan on the exocrine pancreas involves vagal sensory nerves and the CCK release by these substances.

Electrophysiological Effects of Burimamide and 16,16-Dimethyl Prostaglandin E2 on The Canine Gastric Mucosa

The electrophysiological effects of two potent inhibitors of gastric acid secretion, burimamide and 16,16-dimethyl prostaglandin E2 (dm-PGE2), were determined in an in vivo histamine-stimulated canine stomach preparation and an in vitro canine gastric mucosal preparation. In the in vivo stomach preparation, intravenous burimamide caused a decrease in acid secretion, an increase in transmucosal potential difference (PD) and the relative resistance (R) was essentially unchanged. Intravenous dm-PGE2 also inhibited acid secretion and increased PD but, in contrast to burimamide, increased R. In the in vitro preparation, the unidirectional flux of sodium from mucosa to serosa increased after dm-PGE2 but not after burimamide. Passive sodium fluxes and unidirectional chloride fluxes were not altered after either agent. These findings suggest that increased active transport of sodium from mucosa to serosa is at least partially responsible for the observed increase in transmural PD with dm-PGE2, an agent which also decreases hydrogen ion transport. With burimamide the increased PD was due primarily to inhibition of hydrogen ion secretion.

The circadian rhythm of melatonin modulates the severity of caerulein-induced pancreatitis in the rat

Abstract:  Melatonin, an antioxidant, protects the pancreas against acute inflammation but, although this indole is released mainly at night, no study has been undertaken to determine circadian changes of plasma melatonin levels and the severity of acute pancreatitis. The aims of this study were: (a) to compare the severity of caerulein‐induced pancreatitis (CIP) produced in the rat during the day and at the night, and (b) to assess the changes of plasma melatonin level and the activity of an antioxidative enzyme; superoxide dismutase (SOD), in the pancreas subjected to CIP during the day time and at night without or with administration of exogenous melatonin or its precursor; l‐tryptophan. Rats were kept in 12 hr light/dark cycle. CIP was induced by subcutaneous infusion of caerulein (5 μg/kg/hr for 5 hr). Melatonin (5 or 25 mg/kg) or l‐tryptophan (50 or 250 mg/kg) was given intraperitoneally 30 min prior to the start of CIP. CIP induced during the day time was confirmed by histological examination and manifested by pancreatic edema, and rises of amylase and lipase plasma activities (by 400 and 500%, respectively), whereas pancreatic SOD, pancreatic blood flow (PBF) and oxygen consumption by pancreatic tissue (VO2) were decreased by 70, 40 and 45%, respectively, as compared with the appropriate controls. All morphological and biochemical parameters of CIP induced at night were significantly less severe, compared with those recorded during the light phase. Plasma melatonin immunoreactivity was significantly higher during the night, than during the day, especially following administration of melatonin or its precursor, which reversed all manifestations of CIP. In conclusion, a circadian rhythm modulates the severity of CIP with a decrease of pancreatitis severity during the night compared with that at the day time and this may be due to the increased plasma level of melatonin and higher activity of SOD in the pancreas.

Involvement of Orexigenic Peptides in the Mechanism of Gastric Mucosal Integrity and Healing of Chronic Gastric Ulcers

Orexigenic peptides are group of endocrine hormones exerting a pleiotropic influence on many physiological functions including regulation of the feeding behaviour and energy expenditure, release of growth hormone (GH) and inotropic effects on the heart. Some of these peptides such as ghrelin, originally identified in the gastric mucosa, has been involved not only in control of food intake and growth hormone release but also exerts the immunomodulatory and anti-inflammatory properties. This review summarizes the recent attempts to prove the concept that orexigenic peptides such as ghrelin, orexin-A and obestatin besides playing an important role in the mechanism of food intake, exhibit a potent gastroprotective action against the formation of acute gastric mucosal injury induced by various ulcerogens. This protective effect depends upon vagal activity and hyperemia mediated by NOS/NO and COX/PG systems and CGRP released from sensory afferent nerves. In addition, the appetite peptides such as ghrelin and orexin-A are implicated in the mechanism of the healing of preexisting gastric ulcers due to an activation of specific GHS-R1a and OX-R1 receptors and PG/COX system.

Actions of Nicotine on Renal Function in Dogs

Abstract Renal excretory and circulatory responses to nicotine were investigated in anesthetized dogs under three sets of conditions: (a) infusion of nicotine into the left renal artery (ia) at a dose of 0.5 μg·min−1 ·kg body wt−1 × 15 min; (b) ia nicotine after 1.0 mg/kg ia propranolol; and (c) ia nicotine after bilateral adrenalectomy. Measured and calculated left and right renal excretory variables included sodium, potassium, and chloride excretion rates (UNaV, UKV, and UClV, respectively), total solute excretion (UOsV), glomerular filtration rate (GFR), fractional sodium excretion (FENa), and urine flow rate. Systemic arterial pressure and left renal artery blood flow (RBF) were also measured. In seven intact dogs administered nicotine alone, there were significant increases in UNaV, UClV, UOsV, GFR, and urine flow rates from both kidneys. However, nicotine did not significantly affect UKV, FENa, arterial pressure, or RBF. The lack of circulatory effects of nicotine was also observed after either propranolol or adrenalectomy. However, when nicotine was administered after propranolol, the drug evoked significant decreases in UOsV, UNaV, UClV, and GFR, compared with prenicotine values. When nicotine was administered after bilateral adrenalectomy, the drug evoked decreases in the excretory parameters similar to those observed after propranolol. These findings seem to support several inferences: (a) nicotine stimulates renal excretory functions—the alkaloid is saluretic and diuretic; (b) the action of nicotine on the kidney is mediated mainly by the release of catecholamines from the adrenal medulla; (c) catecholamines released by nicotine act mainly on β-adrenergic receptors; and (d) the saluresis prompted by the release of catecholamines in response to nicotine is due to a subsequent increase in GFR.

Lipoxins, The Novel Mediators of Gastroprotection and Gastric Adaptation to Ulcerogenic action of Aspirin

Previous studies revealed that prostaglandins contribute to the mechanism of maintenance of gastrointestinal integrity and mediate various physiological aspects of mucosal defense. The suppression of prostaglandin synthesis in the stomach is a critical event in terms of the development of mucosal injury after administration of various NSAID including aspirin (ASA). A worldwide use of ASA is now accepted due to its remarkable analgesic, antipyretic and anti-thrombotic prophylactics against myocardial infarct and coronary disorders despite the fact that the use of NSAIDs is associated with the risk of gastrointestinal bleedings, haemorrhagic lesions and ulcerations. It has become clear that other mediators besides prostaglandins can similarly act to protect the gastrointestinal mucosa of experimental animals and humans from injury induced by ASA. For instance, nitric oxide (NO) released from vascular epithelium, epithelial cells of gastrointestinal tract and sensory nerves can influence many of the same components of mucosal defense as do prostaglandins. This review was designed to provide an updated overview based on the experimental and clinical evidence on the involvement COX-2 derived products, lipoxins in the mechanism of gastric defense, gastroprotection and gastric adaptation to ASA. Lipoxins were recently considered as another group of lipid mediators that can protect the stomach similarly as NO-donors known to exert protective influence on the stomach from the injury under condition where the mucosal prostaglandin levels are suppressed. The new class of NO-releasing NSAIDs, including NO-aspirin or NO-naproxen, represent a very promising approach to reducing the toxicity of their parent NSAIDs. Aspirin-triggered lipoxin (ATL) synthesis, via COX-2, acts to reduce the severity of damage induced by this NSAID. Lipoxin analogues may prove to be useful for preventing mucosal injury and for modulating mucosal inflammation. Evidence presented in this review documents that ATL also play in important role in gastric adaptation during chronic ASA administration. Suppression of COX-2 activity by selective COX-2 inhibitors such as rofecoxib or celecoxib was shown to abolish the production of ATL and to diminish the gastric tolerability of ASA and gastric adaptation developed in response to repetitive administration of this NSAID. Synthetic analogues of lipoxins as well as newer class of NSAIDs releasing NO may be used in the future as the therapeutic approach to counteract adverse effects in the stomach associated with NSAIDs ingestion.