Elevated plasma levels of noradrenaline in duodenal ulcer

The effect of the human gut-signalling hormone, norepinephrine, on the growth of the gastric pathogen Helicobacter pylori

BACKGROUND AND AIMS

Helicobacter pylori is an important human pathogen, infecting around half the population of the world. It has developed a number of refinements to allow it to persist in the human stomach. Catecholamine hormones have been shown to enhance growth of other bacterial species and are found in the gastric niche. We aimed to study growth enhancement of H. pylori by the human catecholamine hormones epinephrine and norepinephrine.

METHODS

Growth studies were carried out in complex and defined media containing the hormones epinephrine, norepinephrine, and normetanephrine, the main host metabolite of norepinephrine. Bacterial density was measured by viable count or optical density. Intracellular ATP was measured using a bioluminescence assay technique.

RESULTS

Both epinephrine and norepinephrine enhanced H. pylori growth in a dose-dependent strain-independent fashion, with norepinephrine being more effective than epinephrine. We showed a rapid (4 hours) dose-dependent effect on metabolic activity, as measured by intracellular ATP levels. We used a chemically defined medium to study mechanisms: chelation of ferric iron blocked H. pylori growth, which could be overcome by addition of norepinephrine. Disruption of the catechol group of norepinephrine abrogated its H. pylori-growth-promoting activity.

CONCLUSIONS

Norepinephrine stimulates growth of H. pylori under otherwise growth-restricted conditions, and this effect is related to the ability of norepinephrine to bind ferric iron. This supports the notion that norepinephrine may aid H. pylori persistence in the stomach.

Gastroduodenal mucus bicarbonate barrier: protection against acid and pepsin

Secretion of bicarbonate into the adherent layer of mucus gel creates a pH gradient with a near-neutral pH at the epithelial surfaces in stomach and duodenum, providing the first line of mucosal protection against luminal acid. The continuous adherent mucus layer is also a barrier to luminal pepsin, thereby protecting the underlying mucosa from proteolytic digestion. In this article we review the present state of the gastroduodenal mucus bicarbonate barrier two decades after the first supporting experimental evidence appeared. The primary function of the adherent mucus gel layer is a structural one to create a stable, unstirred layer to support surface neutralization of acid and act as a protective physical barrier against luminal pepsin. Therefore, the emphasis on mucus in this review is on the form and role of the adherent mucus gel layer. The primary function of the mucosal bicarbonate secretion is to neutralize acid diffusing into the mucus gel layer and to be quantitatively sufficient to maintain a near-neutral pH at the mucus-mucosal surface interface. The emphasis on mucosal bicarbonate in this review is on the mechanisms and control of its secretion and the establishment of a surface pH gradient. Evidence suggests that under normal physiological conditions, the mucus bicarbonate barrier is sufficient for protection of the gastric mucosa against acid and pepsin and is even more so for the duodenum.

Does nicotine administration influence intragastric acidity?

Smokers have an increased incidence of duodenal ulcer with a high relapse rate whether they receive maintenance therapy with H2-receptor antagonists or not. They also tend to be slow healers. The etiology behind this is still unknown, and there is general disagreement as to whether smoking affects gastric secretion. In an earlier study we found a small but significant decrease in intragastric pH a short time after smoking a cigarette. The aim of the present investigation was to study whether intragastric pH changed during nicotine administration per se. Nicotine was given as a nasal spray to eight healthy smokers. Nicotine did not induce any acute detectable changes in gastric acidity when the 5-min period before spraying was compared with the 35-min period after spraying (median pH, 1.47 (25-75 percentiles, 1.40-2.32) and 1.55 (25-75 percentiles, 1.42-2.06), respectively). When different time periods during a day with hourly nicotine administration were analyzed, and the results compared with those of a similar day when placebo was given, nicotine was found to impair postprandial gastric neutralization. Median pH during the lunch hour was 1.93 (25-75 percentiles, 1.80-2.37) after nicotine and 2.86 (25-75 percentiles, 2.37-3.70) after placebo; p less than 0.025. Possible explanations for this might be nicotine-mediated effects on gastric motility or gastrin release.

Effects of physiological increases of plasma noradrenaline on gastric acid secretion and gastrointestinal hormones

It is not known if the increased plasma concentration of noradrenaline in patients with chronic duodenal ulcer disease is a pathogenetic factor or not. The aim of the present study was to investigate if physiologic changes of noradrenaline would evoke any alterations in gastric acid secretion or in the plasma concentration of some gastrointestinal hormones (gastrin, secretin, PP, PYY, and GIP) known to affect gastric physiology. The results show that basal plasma noradrenaline concentration was 1.8 nM and after infusion with noradrenaline at 0.04 or 0.2 nmol/kg/min plasma levels of 2.5 and 4.4 nM were obtained. No appreciable changes could be found in basal or pentagastrin stimulated acid secretion or in any of the gastrointestinal peptides studied. If the elevated plasma noradrenaline concentration observed in duodenal ulcer patients is a pathogenetic factor; it is probable that it interferes with other variables such as blood flow, bicarbonate secretion, or prostaglandin synthesis.

Effects of alpha-adrenoceptor agonists and antagonists on duodenal surface epithelial HCO3-secretion in the rat in vivo

In anesthetized rats a 12 mm segment of duodenum, distal to the Brunner's gland area and devoid of pancreatic and bile secretions, was cannulated in situ. Secretion of HCO3- by the surface epithelium was measured by continuous titration at luminal pH 7.40. Noradrenaline at doses of 25-200 micrograms kg-1 h-1 had no (net) effect on duodenal HCO3- secretion while the non-selective alpha-adrenoceptor antagonist phentolamine (20-1000 micrograms kg-1 intravenously) dose-dependently increased secretion. The phentolamine-induced rise in alkaline secretion was partially inhibited by noradrenaline but this effect was transient and was followed by an increase in secretion in spite of continuous infusion of noradrenaline. The alpha 1-adrenoceptor agonist, phenylephrine (100 and 500 micrograms kg-1 h-1) stimulated HCO3- secretion in a dose-dependent manner and this response was abolished by the alpha 1-adrenoceptor antagonist prazosin (0.5 mg kg-1) while the beta-adrenoceptor antagonist propranolol (1 mg kg-1) was without effect. Basal secretion, as well as secretion stimulated by phentolamine and/or phenylephrine, was inhibited by the alpha 2-adrenoceptor agonist clonidine (0.75-15.0 micrograms kg-1). The results thus strongly suggest that alpha 1-adrenoceptor stimulation increases while alpha 2-adrenoceptor stimulation decreases duodenal surface epithelial HCO3(-)-secretion. This might explain the absence of a net effect of noradrenaline.