Pentagastrin gastroprotection against acid is related to H2 receptor activation but not acid secretion

Gut homeostasis, injury, and healing: New therapeutic targets

The integrity of the gastrointestinal mucosa plays a crucial role in gut homeostasis, which depends upon the balance between mucosal injury by destructive factors and healing via protective factors. The persistence of noxious agents such as acid, pepsin, nonsteroidal anti-inflammatory drugs, or Helicobacter pylori breaks down the mucosal barrier and injury occurs. Depending upon the size and site of the wound, it is healed by complex and overlapping processes involving membrane resealing, cell spreading, purse-string contraction, restitution, differentiation, angiogenesis, and vasculogenesis, each modulated by extracellular regulators. Unfortunately, the gut does not always heal, leading to such pathology as peptic ulcers or inflammatory bowel disease. Currently available therapeutics such as proton pump inhibitors, histamine-2 receptor antagonists, sucralfate, 5-aminosalicylate, antibiotics, corticosteroids, and immunosuppressants all attempt to minimize or reduce injury to the gastrointestinal tract. More recent studies have focused on improving mucosal defense or directly promoting mucosal repair. Many investigations have sought to enhance mucosal defense by stimulating mucus secretion, mucosal blood flow, or tight junction function. Conversely, new attempts to directly promote mucosal repair target proteins that modulate cytoskeleton dynamics such as tubulin, talin, Ehm², filamin-a, gelsolin, and flightless I or that proteins regulate focal adhesions dynamics such as focal adhesion kinase. This article summarizes the pathobiology of gastrointestinal mucosal healing and reviews potential new therapeutic targets.

Dual age-dependent effect of ghrelin administration on serum level of insulin-like growth factor-1 and gastric growth in young rats

Ghrelin is a circulating growth hormone-releasing peptide primarily isolated from human and rat stomach. The aim of present study was to investigate the effect of ghrelin administration on gastric growth in suckling, and young peripubertal 7-week-old rats. Rats were treated for 7 days with saline or ghrelin (4, 8 or 16 nmol/kg/dose) intraperitoneally twice a day. Suckling rats were treated from the fist postnatal day.

RESULTS

Treatment with ghrelin did not affect animal weight in suckling rats; whereas in 7-week-old animals, administration of ghrelin caused a significant increase in body weight. In suckling rats, ghrelin decreased the gastric weight, DNA synthesis and DNA content. In young 7-week-old peripubertal rats, treatment with ghrelin increased food intake and animal body weight. This effect was accompanied with a significant increase in gastric mucosa weight, DNA synthesis and DNA content. Treatment with ghrelin increased serum level of growth hormone in all rats tested, but this result was much higher in 7-week-old peripubertal rats than in suckling rats. Serum level of insulin-like growth factor-1 was not affected by ghrelin administration in suckling rats. In contrast, ghrelin caused a significant increase in serum level of insulin-like growth factor-1 in 7-week-old peripubertal rats. We conclude that administration of ghrelin exhibits biphasic effect on gastric growth in young rats: in suckling rats, ghrelin reduces gastric growth, whereas in young 7-week-old animals, treatment with ghrelin stimulates gastric growth. The growth-promoting effect of ghrelin in the stomach seems to depend on the stimulation of food intake and the release of insulin-like growth factor-1.

Role of gastric mast cells in the regulation of central TRH analog-induced hyperemia in rats

RX 77368 (RX) increases gastric mucosal blood flow by a vagal cholinergic mechanism. The relative roles of mucosal and connective tissue mast cells (MMC and CTMC) were investigated in RX-injected rats. Blood flow and mast cell degranulation were measured after intracisternal RX. RX significantly increased gastric mucosal blood flow, and sequentially degranulated CTMC and MMC. Ketotifen or doxantrazole inhibited the hyperemic response. Ondansetron, RS-039604-90, or famotidine, but not ketanserin or pyrilamine, reduced hyperemia. Mast cells mediate RX-induced gastric hyperemia via 5-HT3, 5-HT4, and H2 receptors; initial increase depends upon CTMC whereas MMC contributes to the later response.

Role of capsaicin-sensitive nerves and histamine H1, H2, and H3 receptors in the gastroprotective effect of histamine against stress ulcers in rats

It is assumed that an overproduction of gastric acid is the most important factor in the development of peptic ulcer. However, it has been also demonstrated that gastric defense mechanisms, which prevent mucosal injury, are enhanced by the same factors that increase acid secretion. The aim of this study was to examine the role of capsaicin-sensitive sensory nerves and histamine H1, H2, and H3 receptors in histamine-induced gastroprotection against stress ulcers. Studies were performed on rats with intact or ablated sensory nerves. Ablation of sensory nerves was induced by neurotoxic doses of capsaicin. Gastric ulcers were induced by water immersion and restrain stress. Before exposure to stress, rats were pretreated with saline (control), histamine (10 micromol/kg), histamine H1 receptor antagonist pyrilamine (100 micromol/kg), histamine H2 receptor antagonist ranitidine (100 micromol/kg), histamine H3 receptor antagonist thioperamide (100 micromol/kg), or a combination of histamine with these histamine receptor antagonists.

RESULTS

Histamine alone reduced ulcer area evoked by stress and this effect was accompanied by an increase in gastric mucosal blood flow and mucosal DNA synthesis, as well as a decrease in serum pro-inflammatory interleukin-1beta concentration. Treatment with combination of pyrilamine plus histamine caused an increase in gastric ulcer area and serum interleukin-1beta above the value observed in animals treated with saline, and this effect was accompanied by a decrease in gastric mucosal DNA synthesis. Ranitidine, in combination with histamine, reduced the ulcer area and serum interleukin-1beta to a minimal value, whereas gastric mucosal blood flow and DNA synthesis reached a maximal value. Pretreatment with thioperamide before histamine administration abolished the histamine-evoked reduction in gastric ulcer area. Ablation of sensory nerves increased the ulcer area in animals treated with saline or histamine, or histamine in combination with pyrilamine or ranitidine. In animals with sensory nerves ablation combined with administration of thioperamide plus histamine, the ulcer area was similar to that in saline-treated animals with intact sensory nerves. We conclude that: (1) histamine exhibits protective effect against stress-induced gastric ulcer and that this gastroprotection is related to stimulation of histamine H1 and H3 receptors; (2) blockade of histamine H2 receptors exhibited beneficial effect on gastric mucosa against stress-induced gastric ulcers; and (3) ablation of sensory nerves aggravates stress-induced gastric ulcer and reduces histamine-evoked gastroprotection related to stimulation of histamine H3 receptors.

13C urea breath test (UBT) in the diagnosis of Helicobacter pylori: why does it work better with acid test meals?

BACKGROUND

Acid test meals may improve the accuracy of the (13)C urea breath test (UBT). This has been attributed to changes in gastric emptying rather than to the effects of gastric pH on Helicobacter pylori urease.

AIMS

To determine whether enhancement of (13)CO(2) excretion in the UBT in H pylori infected volunteers by acidification of a test meal is due to a delay in gastric emptying.

METHODS

Urease activity in vitro was measured in intact bacteria and in bacterial homogenates. Urease activity in vivo was assessed by means of the UBT. Eleven H pylori infected subjects underwent UBTs with neutral Ensure (pH 7.0), acidified Ensure (pH 3.0), and apple juice (pH 3.0). Gastric emptying was assessed by (13)C sodium acetate breath test.

RESULTS

From pH 7 to pH 3, the in vitro urease activity of intact bacteria increased sixfold. In contrast, urease activity of bacterial homogenates was inactivated by low pH. In vivo, urease activity, as measured by the UBT 20 minutes after meal ingestion, was higher with apple juice (delta (13)CO(2)=21.1; p=0.03) and acidified Ensure (delta (13)CO(2)=25.5; p=0.01) than with neutral Ensure (delta (13)CO(2)=12.5). Gastric emptying was faster with apple juice (T(max)=36.7 (8) minutes) but not with acidified Ensure (T(max)=63.3 (5) minutes; p=0.06) than with neutral Ensure (T(max)=65.0 (3) minutes; p=0.04).

CONCLUSIONS

The higher UBT found with acidified compared with neutral test meals was independent of the emptying rates of the test meals but may have been due to medium acidity dependent activation of intra-bacterial urease in intact H pylori.

Acute adaptive cellular base uptake in rat duodenal epithelium

We studied the role of duodenal cellular ion transport in epithelial defense mechanisms in response to rapid shifts of luminal pH. We used in vivo microscopy to measure duodenal epithelial cell intracellular pH (pH(i)), mucus gel thickness, blood flow, and HCO secretion in anesthetized rats with or without the Na(+)/H(+) exchange inhibitor 5-(N,N-dimethyl)-amiloride (DMA) or the anion transport inhibitor DIDS. During acid perfusion pH(i) decreased, whereas mucus gel thickness and blood flow increased, with pH(i) increasing to over baseline (overshoot) and blood flow and gel thickness returning to basal levels during subsequent neutral solution perfusion. During a second brief acid challenge, pH(i) decrease was lessened (adaptation). These are best explained by augmented cellular HCO uptake in response to perfused acid. DIDS, but not DMA, abolished the overshoot and pH(i) adaptation and decreased acid-enhanced HCO secretion. In perfused duodenum, effluent total CO(2) output was not increased by acid perfusion, despite a massive increase of titratable alkalinity, consistent with substantial acid back diffusion and modest CO(2) back diffusion during acid perfusions. Rapid shifts of luminal pH increased duodenal epithelial buffering power, which protected the cells from perfused acid, presumably by activation of Na(+)-HCO cotransport. This adaptation may be a novel, important, and early duodenal protective mechanism against rapid physiological shifts of luminal acidity.

Intraluminal acid and gastric mucosal integrity: the importance of blood-borne bicarbonate

The acid-secreting gastric mucosa resists intraluminal acid better than the nonsecreting. Here we investigated pH at the epithelial cell surface, mucosal permeability, and blood flow during intraluminal administration of acid (100 mM) in acid-stimulated and nonstimulated gastric corpus mucosae. Surface pH (H(+)-selective microelectrodes), permeability (clearance of (51)Cr-EDTA), and mucosal blood flow (laser-Doppler flowmetry) were studied in Inactin-anesthetized rats. Acid secretion was stimulated with pentagastrin (40 microg. kg(-1). h(-1)) or impromidine (500 microg. kg(-1). h(-1)), or HCO(3)(-) (5 mmol. kg(-1). h(-1)) given intravenously. Surface pH was only slightly reduced by intraluminal acid in acid secretion-stimulated or HCO(3)(-)-treated rats but was substantially lowered in nonstimulated rats. Clearance increased threefold and blood flow increased by approximately 75% in nonstimulated rats. During stimulated acid secretion or intravenous infusion of HCO(3)(-), clearance was unchanged and blood flow increased by only approximately 30% during intraluminal acid. Increased epithelial transport of HCO(3)(-) buffering the mucus gel is most probably the explanation for the acid-secreting mucosa being less vulnerable to intraluminal acid than the nonsecreting.

Dynamic regulation of mucus gel thickness in rat duodenum

We examined the dynamic regulation of mucus gel thickness (MGT) in vivo in rat duodenum in response to luminal acid, cyclooxygenase (COX) inhibition, and exogenous PGE(2). An in vivo microscopic technique was used to measure MGT with fluorescent microspheres in urethan-anesthetized rats. Duodenal mucosa was topically superfused with pH 7.0 or pH 2.2 solutions with or without PGE(2) and indomethacin treatments. Glycoprotein concentration of duodenal loop perfusates was measured with periodic acid/Schiff (PAS) or Alcian blue (AB) staining. MGT and perfusate glycoprotein concentration were stable during a 35-min perfusion with pH 7.0 solution. Acid exposure increased MGT and PAS- and AB-positive perfusate glycoprotein concentrations. Indomethacin pretreatment increased both PAS- and AB-positive perfusate glycoprotein at baseline; subsequent acid superfusion decreased perfusate glycoproteins and gel thickness. PGE(2) (1 mg/kg iv) simultaneously increased MGT and PAS-positive perfusate glycoprotein concentrations followed by a transient increase in AB-positive glycoprotein concentration, suggesting contributions from goblet cells and Brunner's glands. Parallel changes in MGT and perfusate glycoprotein concentration in response to luminal acid and PGE(2) suggest that rapid MGT variations reflect alterations in the balance between mucus secretion and exudation, which in turn are regulated by a COX-related pathway. Luminal acid and PGE(2) augment mucus secretion from goblet cells and Brunner's glands.