Differential effects of clonidine, dopamine, dobutamine, and dopexamine on basal and acid-stimulated mucosal blood flow in the rat stomach

Pathophysiologic Role of Neurotransmitters in Digestive Diseases

Neurotransmitters are special molecules that serve as messengers in chemical synapses between neurons, cells, or receptors, including catecholamines, serotonin, dopamine, and other neurotransmitters, which play an important role in both human physiology and pathology. Compelling evidence has indicated that neurotransmitters have an important physiological role in various digestive diseases. They act as ligands in combination with central or peripheral receptors, and transmits signals through chemical synapses, which are involved in regulating the physiological and pathological processes of the digestive tract organs. For instance, neurotransmitters regulate blood circulation and affect intestinal movement, nutrient absorption, the gastrointestinal innate immune system, and the microbiome. In this review, we will focus on the role of neurotransmitters in the pathogenesis of digestive tract diseases to provide novel therapeutic targets for new drug development in digestive diseases.

Effects of dopamine, norepinephrine and dobutamine on gastric mucosal pH of septic shock patients

The effect of different vasoactive drugs on the pH [intracellular pH (pHi)] of gastric mucosa in patients with septic shock was evaluated in the present study. According to the vasoactive drugs applied, 48 patients with septic shock were divided into 3 groups: A, B and C, with 16 cases each. Cases of group A were treated with dopamine, those of group B with norepinephrine while those of group C were treated with norepinephrine plus dobutamine. The changes of pH of gastric mucosa were observed before treatment (baseline) and 6, 12, 24 and 48 h after treatment, and the hemodynamic indicators were observed before treatment (baseline) and 6 h after administration. The gastric mucosal pH was not significantly different between two of the three groups before treatment (each at P>0.05). The gastric mucosal pH of group A did not change 6, 12, 24 and 48 h after treatment with drugs compared with the baseline (all at P>0.05), while the gastric mucosal pH in groups B and C were each statistically higher at the time points of 6, 12, 24 and 48 h after treatment with drugs compared with the respective baselines (all at P<0.05). Following treatment with drugs, the gastric mucosal pH of group C at all the time points of 6, 12, 24 and 48 h after treatment were significantly higher than those of groups A and B at the same time points after treatment, while there were some statistical differences between groups A and B at these time points (6, 12, 24 and 48 h after treatment; P<0.05). The hemodynamic indicators of the patients before treatment were not significantly different between two of the three groups (all at P>0.05). Compared with the baseline values, the mean arterial pressure and the cardiac index of each group after treatment were significantly increased, the pulmonary capillary wedge pressure and the central venous pressure of groups B and C significantly increased (all at P<0.05) and the heart rate of group A was significantly increased (P<0.05). In conclusion, the gastric mucosal pH of the septic shock patients was increased when treated with norepinephrine or with dobutamine. Additionally, the gastric mucosal pH was significantly higher when the patients were treated with dobutamine and norepinephrine in combination than with norepinephrine or dopamine alone. Dopamine, norepinephrine and dobutamine can improve the systemic hemodynamic conditions in patients with septic shock.

Alosetron, cilansetron and tegaserod modify mesenteric but not colonic blood flow in rats

BACKGROUND AND PURPOSE

As the use of the 5-HT(3) receptor antagonist alosetron (GlaxoSmithKline) and the 5-HT(4) receptor agonist tegaserod (Novartis) in patients with irritable bowel syndrome has been associated with cases of ischaemic colitis, the effects of alosetron, cilansetron (Solvay) and tegaserod on the rat splanchnic circulation were evaluated.

EXPERIMENTAL APPROACH

Phenobarbital-anaesthetised rats were instrumented to record blood flow in the superior mesenteric artery and transverse colon and to calculate mesenteric and colonic vascular conductance.

KEY RESULTS

Intravenous alosetron (0.03-0.3 mg.kg(-1)) did not alter blood pressure or heart rate but reduced mesenteric blood flow and vascular conductance by 15-20%. This activity profile was also seen after intraduodenal alosetron and shared by the 5-HT(3) receptor antagonist cilansetron. In contrast, blood flow, vascular conductance and intraluminal pressure in the colon were not modified by alosetron and cilansetron. Intravenous or intraduodenal tegaserod (0.3-1.0 mg.kg(-1)) had no inhibitory effect on mesenteric and colonic blood flow. Peroral treatment of rats with alosetron or tegaserod for 7 days did not modify mesenteric haemodynamics at baseline and after blockade of nitric oxide synthesis. Mild inflammation induced by dextran sulphate sodium failed to provoke a vasoconstrictor effect of cilansetron in the colon.

CONCLUSIONS AND IMPLICATIONS

Alosetron and cilansetron, not tegaserod, caused a small and transient constriction of the rat mesenteric vascular bed, whereas blood flow in the colon remained unaltered. The relevance of these findings to the treatment-associated occurrence of ischaemic colitis in patients with irritable bowel syndrome remains open.

Effects of systemically applied clonidine on intestinal perfusion and oxygenation in healthy pigs during general anaesthesia and laparotomy 1

BACKGROUND AND OBJECTIVE

Clonidine, which is used for induction of sympatholysis and prevention or treatment of alcohol withdrawal in anaesthesia and intensive care medicine, may have deleterious effects on intestinal mucosal perfusion. This study was designed to investigate the effects of clonidine on intestinal perfusion and oxygenation.

METHODS

Following ethical approval 17 anaesthetized, and acutely instrumented pigs were randomly assigned to two groups: eight animals received intravenous clonidine (2 microg kg(-1) bolus and 2 microg kg(-1) h(-1)), nine animals served as a control group. Measurement points for systemic and regional haemodynamic and oxygenation parameters were 135 and 315 min after starting the clonidine application.

RESULTS

Clonidine elicited systemic haemodynamic changes (median [25-75th interquartile range]): heart rate (106 [91, 126] to 84 [71, 90] beats min(-1)) cardiac output (147 [123, 193] to 90 [87, 107] mL min(-1) kg(-1)) and mean arterial pressure (77 [72, 93] to 69 [61, 78] mmHg) decreased. Despite systemic haemodynamic changes, the superior mesenteric artery blood flow did not change in the clonidine group. The vascular resistance of the superior mesenteric artery decreased. The small intestinal oxygen supply, the mucosal and the serosal tissue oxygen partial pressure did not change.

CONCLUSIONS

Systemic sympatholysis induced by intravenously applied clonidine in addition to basic intravenous anaesthesia elicited a decrease in cardiac output and mean arterial pressure. However, regional macrohaemodynamic perfusion was maintained and intestinal oxygenation did not change. Clonidine does not impair intestinal mucosal and serosal oxygenation under physiological conditions.

Intragastric nitric oxide is abolished in intubated patients and restored by nitrite

OBJECTIVE

Nitrite in saliva is reduced to nitric oxide (NO) in the acidic stomach, and this NO may serve gastroprotective functions. We studied intragastric NO levels in healthy controls and in intubated intensive care unit patients before and after supplementation with nitrite.

DESIGN

Prospective observational study involving patients and controls.

SETTING

A mixed intensive care unit and a university laboratory.

PATIENTS AND SUBJECTS

Eight healthy volunteers and ten intubated, mechanically ventilated intensive care unit patients.

INTERVENTIONS

A tonometric catheter was first evaluated in vitro and then used for all NO measurements. In fasting controls, gastric NO levels were measured repeatedly during periods of saliva depletion and after an intragastric nitrite infusion. In patients, changes in levels of intragastric NO, nitrite in plasma and gastric juice, and S-nitrosothiols in gastric juice were measured in response to an intragastric nitrite infusion.

MEASUREMENTS AND MAIN RESULTS

The tonometric catheter had a recovery of 80% with a high intraclass and interclass correlation. Median baseline NO levels in healthy volunteers were 21.6 ppm (interquartile range, 11.4-22.3 ppm) and decreased by 90% to 3.3 ppm (2.0-5.2 ppm) during 25-50 mins of saliva depletion. The NO level was restored by an intragastric nitrite infusion. Baseline NO levels in patients were almost abolished (0.1 ppm [0.07-0.4 ppm]) but increased rapidly to 124 ppm (range, 65-180 ppm) during intragastric nitrite infusion. Nitrite levels in plasma increased from 0.18 +/- 0.03 to 1.3 +/- 0.2 microM (p < .01), and levels of S-nitrosothiols in gastric juice increased from 0.12 +/- 0.09 to 6.7 +/- 1.8 microM.

CONCLUSIONS

Intragastric generation of NO requires continuous delivery of nitrite-containing saliva and is almost abolished in critically ill, intubated patients. Enteral supplementation with nitrite could however fully restore gastric NO levels. Future studies will reveal if low NO levels contribute to stress ulcers and gastric overgrowth of bacteria often seen in these patients and in turn if restoring gastric NO with nitrite could be a useful therapeutic approach.

Effects of sleep deprivation on gastric mucosal blood flow and gastric mucosal lesions in rats

AIM: To observe the changes of gastric mucosal blood flow and gastric mucosal lesions in rats after sleep deprivation.

METHODS: Sleep deprivation was induced in male Sprague-Dawley rats housed on small platform over water. Control mice were housed either in normal cages or on large platform over water. Laser Doppler blood flow meter was used to measure the gastric mucosal blood flow, and index of gastric mucosal lesions was quantified. The arterial plasma levels of endotoxin (ET) and calcitonin gene related peptide (CGRP) were measured by radioimmunoassay.

RESULTS: Compared with control mice in normal cage, on big platform, or the mice on the 1st day of sleep deprivation, the gastric mucosal blood flow of mice on the 3rd, 5th and 7th day of sleep deprivation (54.2±2.5, 53.7±3.0, 48.3±2.5 mv, respectively) were significantly lower. The gastric mucosal injury became more and more serious: the injury indices were 29.8±3.2, 3.7±3.6 and 34.8±3.5 on the 3rd, 5th and 7th day of sleep deprivation, respectively. The serum levels of ET were 123±28, 139±36 and 149±38 ng/L in mice on the 3rd, 5th and 7th day of sleep deprivation, and the levels of CGRP were 193±32 and 221±40 ng/L in the mice on the 5th and 7th day of sleep deprivation. All were significantly higher than those in control mice.

CONCLUSION: Sleep deprivation can cause the decrease of gastric mucosal blood flow and the gastric mucosal injury, which are accompanied by the elevation of serum ET and CGRP.

Role of N-methyl-D-aspartate receptors in gastric mucosal blood flow induced by histamine

Ionotropic N-methyl-D-aspartate (NMDA) receptor agonists, L-aspartic acid (L-Asp) and NMDA, have been shown to inhibit histamine-stimulated acid secretion, but their effect on gastric mucosal blood flow (GMBF) is largely unknown. The aim of this study was to investigate whether L-Asp and NMDA inhibit histamine-stimulated GMBF and to examine the expression patterns of NMDA receptor subunits NR1, NR2A, and NR2B in rat stomach. Laser Doppler flowmetry was used to measure gastric blood flow in anesthetized rats. The GMBF was assessed during an intravenous infusion of histamine in the presence of tripelennamine. The effects of L-Asp and NMDA on histamine-induced gastric blood flow were examined. In addition, the distribution patterns of NR1-, NR2A-, and NR2B-contaning NMDA receptors in rat stomach were determined immunohistochemically by using specific antibodies against NR1, NR2A, and NR2B. Histamine-induced enhancement of GMBF depended on acid secretion and the activation of H(2)-receptors. Neither L-Asp nor NMDA had an effect on the spontaneous GMBF. However, L-Asp and NMDA reduced the histamine-induced increase in GMBF. DL-2-amino-5-phosphonopentanoic acid (AP-5), an NMDA receptor antagonist; and prazosin, an alpha(1)-receptor antagonist; but not propanolol, a beta(2)-receptor antagonist; or yohimbine, a alpha(2)-receptor antagonist; reversed the inhibitory effect of L-Asp and NMDA on the histamine-induced increase in GMBF. Therefore, L-Asp and NMDA inhibit histamine-induced GMBF via a mechanism involving the activation of NMDA receptors and alpha(1)- adrenoceptors. The fact that NMDA receptor subunits NR1, NR2A, and NR2B were found to be localized in the rat stomach as visualized immunohistochemically with specific antibodies against NR1, NR2A, and NR2B is consistent with this hypothesis.

Dopexamine but not dopamine increases gastric mucosal oxygenation during mechanical ventilation in dogs

OBJECTIVE

To compare the effects of dopamine and dopexamine on gastric mucosal oxygenation during mechanical ventilation without and with positive end-expiratory airway pressure (PEEP) and after compensation of the PEEP-induced hemodynamic suppression.

DESIGN

Randomized controlled animal study.

SETTING

University research department of experimental anesthesiology.

SUBJECTS

Ten anesthetized dogs with chronically implanted ultrasound flow probes around the pulmonary artery for continuous measurement of cardiac output.

INTERVENTIONS

On different days, the dogs randomly received dopamine (2.5 and 5.0 microg x kg(-1) x min(-1), n = 10), dopexamine (0.5 and 1.0 microg x kg(-1) x min(-1)) without (n = 8) or with pretreatment with a selective beta2-adrenoceptor antagonist (ICI 118,551, n = 7), or saline (control, n = 7). To simulate common clinical situations, these interventions were performed during different ventilation modes: during mechanical ventilation without and with high levels of PEEP, and after compensation of the PEEP-induced systemic hemodynamic suppression by titrated volume resuscitation with hydroxyethyl starch.

MEASUREMENTS AND MAIN RESULTS

We continuously measured microvascular hemoglobin saturation (mu-Hbo2) by light-guide spectrophotometry in the gastric mucosa. Dopexamine, but not dopamine, significantly increased gastric mucosal mu-Hbo2 by about 20%, regardless of the dose and the ventilation mode. Both catecholamines dose-dependently increased cardiac output and oxygen delivery by up to 75% without effects on systemic oxygen saturation. The effects of dopexamine on mu-Hbo2 as well as on cardiac output and oxygen delivery were prevented by selective beta2-adrenoceptor-blockade.

CONCLUSIONS

Dopexamine but not dopamine improved gastric mucosal oxygenation in dogs. This effect was independent of the dosage and the ventilation mode. Thus, dopexamine may reverse a decrease in splanchnic oxygenation induced by ventilation with PEEP. The dopexamine-induced increase in gastric mucosal oxygenation was mediated by beta2-adrenoceptors, which explains the superior effects of dopexamine to dopamine on mu-Hbo2. The regional effects of both catecholamines were not mirrored by systemic hemodynamics.

Gastroduodenal mucosal defense: coordination by a network of messengers and mediators

Gastroduodenal mucosal defense is composed of structural features of the mucosa, cellular monitors of pending or actual injury, and a web of effector cells that protect the mucosa from damage and govern its recovery from injury. By virtue of these systems, the gastroduodenal mucosa can cope with the harmful ingredients of ingested food and the potentially deleterious effects of gastric acid and pepsin. It is increasingly appreciated that a network of chemical messengers coordinates the alarm, defensive, and healing mechanisms. This article highlights some of the advances from the past year that have furthered our understanding of the regulatory systems that govern gastroduodenal mucosal homeostasis. Particular emphasis is given to control of the mucous and epithelial phospholipid barriers, the mucosal microcirculation, and the epithelial, neural, immune, and inflammatory mediators of the mucosal repair mechanisms.