Roman Korolkiewicz

Role of nitric oxide in regulation of gastric acid secretion in rats: effects of NO donors and NO synthase inhibitor

1 The role of nitric oxide (NO) in the regulation of acid secretion was examined in the anaesthetized rat. 2 A rat stomach was mounted in an ex vivo chamber, instilled with 2 ml of saline every 15 min, and the recovered sample was titrated at pH 7.0 against 0.1 N NaOH by use of an automatic titrator for acid secretion. Gastric mucosal blood flow (GMBF) was measured simultaneously by laser Doppler flowmeter. 3 Intragastric application of NO donors such as FK409 (3 and 6 mg ml−1) and sodium nitroprusside (SNP; 6 and 12 mg  ml−1) as well as i.p. administration of cimetidine (60 mg kg−1), a histamine H2‐receptor antagonist, significantly inhibited the increase in acid secretion in response to pentagastrin (60 μg kg−1 h−1, i.v.), in doses that increased gastric mucosal blood flow (GMBF). 4 Intragastric application of FK409 (6 mg ml−1) increased both basal and stimulated acid secretion induced by YM‐14673 (0.3 mg kg−1, i.v.), an analogue of thyrotropin‐releasing hormone (TRH), but had no effect on the acid secretory response induced by histamine (4 mg kg−1 h−1, i.v.). 5 Pretreatment with NG‐nitro‐L‐arginine methyl ester (L‐NAME; 10 mg kg−1, i.v.) did not affect basal acid secretion, but significantly potentiated the increase in acid secretion induced by YM‐14673 and slightly augmented the acid secretory response to pentagastrin. 6 Both pentagastrin and YM‐14673 increased the release of nitrite plus nitrate (NOx), stable NO metabolites, into the gastric lumen, and these changes were completely inhibited by prior administration of L‐NAME (10 mg kg−1, i.v.). 7 Pentagastrin caused an increase in luminal release of histamine and this response was significantly suppressed by intragastric application of FK409 (6 mg ml−1). 8 These results suggest that either exogenous or endogenous NO has an inhibitory action on gastric acid secretion through suppression of histamine release from enterochromaffin‐like (ECL) cells.

Increased susceptibility of gastric mucosa to ulcerogenic stimulation in diabetic rats–role of capsaicin-sensitive sensory neurons

1 We examined the gastric mucosal blood flow (GMBF) and ulcerogenic responses following barrier disruption induced by sodium taurocholate (TC) in diabetic rats and investigated the role of capsaicin‐sensitive sensory neurons in these responses. 2 Animals were injected streptozotocin (STZ: 70 mg kg−1, i.p.) and used after 5, 10 and 15 weeks of diabetes with blood glucose levels of >350 mg dl−1. The stomach was mounted on an ex‐vivo chamber under urethane anaesthesia and exposed to 20 mm TC plus 50 mm HCl for 30 min in the presence of omeprazole. Gastric transmucosal potential difference (PD), GMBF, and luminal acid loss (H+ back‐diffusion) were measured before and after exposure to 20 mm TC, and the mucosa was examined for lesions 90 min after TC treatment. 3 Mucosal application of TC caused PD reduction in all groups; the degree of PD reduction was similar between normal and diabetic rats, although basal PD values were lower in diabetic rats. In normal rats, TC treatment caused luminal acid loss, followed by an increase of GMBF, resulting in minimal damage in the mucosa. 4 The increased GMBF responses associated with H+ back‐diffusion were mitigated in STZ‐treated rats, depending on the duration of diabetes, and severe haemorrhagic lesions occurred in the stomach after 10 weeks of diabetes. 5 Intragastric application of capsaicin increased GMBF in normal rats, but such responses were mitigated in STZ diabetic rats. The amount of CGRP released in the isolated stomach in response to capsaicin was significantly lower in diabetic rats when compared to controls. 6 The deleterious influences on GMBF and mucosal ulcerogenic responses in STZ‐diabetic rats were partially but significantly antagonized by daily insulin (4 units rat−1) treatment. 7 These results suggest that the gastric mucosa of diabetic rats is more vulnerable to acid injury following barrier disruption, and this change is insulin‐sensitive and may be partly accounted for by the impairment of GMBF response associated with acid back‐diffusion and mediated by capsaicin‐sensitive sensory neurons.

Effects of Diabetes mellitus on the Contractile Activity of Carbachol and Galanin in Isolated Gastric Fundus Strips of Rats

The role of the cholinergic and peptidergic pathways in the impairment of gastric motility associated with diabetic gastroparesis was assessed at the postsynaptic level using isolated fundus smooth muscle strips. Maximal contractile responses to carbachol and galanin were significantly decreased in fundus strips isolated from rats rendered diabetic by a single intraperitoneal injection of streptozotocin (STZ, 70 mg/kg) 1, 4 and 8 weeks before experiments. We also observed notable decrements in the slopes and Hill’s coefficients without conspicuous changes in the EC50 of the respective galanin concentration-response curves measured in strips obtained from STZ animals after 4 and 8 weeks. L-NAME reversed the above-mentioned alterations in an L-arginine-sensitive manner in STZ rats after 4 weeks but not in STZ rats after 8 weeks. The blood plasma nitrite/nitrate levels in STZ animals after 4 and 8 weeks were increased by 44.6 and 61.9%, respectively. Ca2+-independent nitric oxide synthase activity in gastric fundus strips and stomach corpus mucosa from STZ rats after 4 weeks was markedly enhanced by 37.4 and 31.9%, respectively, suggesting an enhanced nitric oxide production. In vivo insulin treatment prevented diabetes-induced alterations in smooth muscle contractility. We conclude that the smooth muscle dysfunction evoked by experimental diabetes causing diminished contractions of fundus strips to carbachol and galanin is at least partly due to the increased nitric oxide synthesis.

Increased Susceptibility of Diabetic Rat Gastric Mucosa to Food Deprivation during Cold Stress

Background: We investigated the mechanisms responsible for the increased susceptibility of diabetic rat gastric mucosa to damage inflicted by overnight food deprivation (18 h) and its worsening by the cold restraint stress (4°C, 3 h). Methods: Gastric damage was measured in fasted animals, some of which were rendered diabetic by a single intraperitoneal injection of streptozotocin (STZ; 70 mg/kg) 5 weeks before experiments (STZ 5W). Results: STZ 5W rodents showed a number of hemorrhagic lesions in corpus mucosa (26.8 ± 5.2 mm2) which could be prevented by insulin or nitric oxide synthase (NOS) inhibitors: aminoguanidine or L-NAME (Nω-nitro-L-arginine methyl ester). Mucosal injury was further aggravated by low temperature (51.1 ± 7.8 mm2), the damage ameliorated by insulin, aminoguanidine, or L-NAME. The salutary actions of L-NAME were L-arginine sensitive. Low temperature and L-NAME did not significantly influence the gastric secretory parameters in normal rats. On the other hand, L-NAME and aminoguanidine counteracted the attenuation of gastric juice acidity and acid output in STZ 5W rodents. Blood plasma nitrite and nitrate levels and outputs in gastric juice were augmented in STZ 5W animals in comparison to controls. The total activities of NOS including inducible NOS but not constitutive NOS were markedly enhanced by fasting and cold restraint in gastric mucosa of STZ 5W animals (2.2- and 3.7- or 2.4- and 17.9-fold respectively). Conclusions: Stressful stimuli, such as food bereavement and cold challenge contribute to the elevated susceptibility of diabetic gastric mucosa to damage, even though the main aggressive factor, i.e., gastric acid secretion, is attenuated. The enhanced production of nitric oxide by inducible NOS during food deprivation and cold exposure seems to play an important role in gastric mucosal integrity disturbances during diabetes.

Inhibition of gastric acid secretion by galanin in rats: Relation to endogenous histamine release

Inhibitory effect of galanin on basal and secretagogs-stimulated gastric acid secretion was investigated in urethane-anesthetized rats. A rat stomach was mounted in an ex-vivo chamber, perfused with saline, and either gastric acid or alkaline secretion was determined by titrating the perfusate. Gastric mucosal blood flow (GMBF) was measured by a laser Doppler flowmeter. Intravenous infusion of galanin dose-dependently inhibited the increase of acid secretion induced by pentagastrin and carbachol but not by histamine, without any influence on the GMBF response. Galanin also reduced basal acid secretion while increasing GMBF, but did not evoke any change in basal gastric alkaline secretion. M15, which is a galanin receptor antagonist in the central nervous system but acts as a full agonist in the gastrointestinal smooth muscle, also suppressed pentagastrin-induced acid secretion, similar to galanin. In addition, pentagastrin increased the release of histamine into the gastric lumen, and this response was significantly inhibited by galanin. These results suggest that the inhibitory effect of galanin on acid secretion is mediated by suppression of endogenous histamine release from enterochromaffin-like cells and that the process may be related to the activation of the same subtype of galanin receptors as in the central nervous system and pancreatic beta-cells.

Impaired duodenal bicarbonate secretion in diabetic rats. Salutary effect of nitric oxide synthase inhibitor

We previously reported the impaired HCO3- secretion and the increased mucosal susceptibility to acid in the duodenum of streptozotocin (STZ)-induced diabetic rats. In this study, we investigated the salutary effect of the NO synthase inhibitor L-NAME (NG-nitro-L-arginine methyl ester) on these changes and compared it with those of insulin. Animals were injected streptozotocin (STZ: 70 mg/kg, ip) and used after 1, 3-4, and 5-6 weeks of diabetes with blood glucose levels of > 300 mg/dL. Under urethane anesthesia the HCO3- secretion was measured in the proximal duodenal loop using a pH-stat method and by adding 10 mM HCl. L-NAME (20 mg/kg x 2) or insulin (4 units/rat) was administered sc for 4-5 weeks, starting 1 week after STZ treatment. The duodenal HCO3- secretory responses to various stimuli such as mucosal acidification (10 mM HCl for 10 min), 16,16-dimethyl prostaglandin E2 (dmPGE2: 10 micrograms/kg, i.v.), and vagal stimulation (0.5 mA, 2 ms, 3 Hz) were significantly decreased in STZ-treated rats, depending on the duration of diabetes. Repeated administration of L-NAME, starting from 1 week after STZ treatment, significantly reduced blood glucose levels toward normal values and restored the HCO3- responses to various stimuli in STZ rats, the effects being similar to those observed after supplementation of insulin. Diabetic rats developed duodenal lesions after perfusion of the duodenum with 150 mM HCl for 4 h, but this ulcerogenic response was significantly inhibited by the repeated treatment with L-NAME as well as insulin. We conclude that L-NAME is effective in ameliorating hyperglycemic conditions in STZ-diabetic rats, similar to insulin, and restores the impaired HCO3- secretion and the increased mucosal susceptibility to acid in diabetic rat duodenums.