Hyperosmotic instillation of rat stomach

Gastroduodenal mucosal surface and luminal pH in gastric ulcer

Gastroduodenal mucosal surface pH was measured in situ by electrode in endoscopically normal and gastric ulcer patients. With the exception of the antrum, mucosal surface pH in the ulcer group (GU) resembled that of the endoscopically normal group. In contrast, the antral mucosal surface pH of the GU group of 7.06 +/- 0.11 (22) was significantly (P less than 0.001) higher than that of 5.46 +/- 0.36 (34) for the endoscopically normal group. This difference was also evident when comparison was restricted to subjects in both groups having a fundal luminal fluid pH of less than 3. When the fundal luminal pH was below pH 3, antral mucosal surface pH was 4.79 +/- 0.41 (24) in endoscopically normal subjects and significantly less (P less than 0.001) than the value of 6.98 +/- 0.18 (12) for gastric ulcer patients. Mucosal surface alkalinity of the GU antrum may be a response to damage but seems inappropriate in view of the likelihood of acid dependent inhibition of gastric acid secretion.

Intraluminal prostaglandin stimulation and bile salt inhibition of basal acid secretion in the instilled rat stomach

To determine the effect of prostaglandin on acid secretion and the effectiveness of cytoprotection afforded by prostaglandins against bile salt damage, the stomachs of anesthetized rats were exteriorized and cannulated for determination of net acid output (H+) and electrical potential difference (PD). The stomachs were successively instilled with acid buffer, 5 mM and 15 mM sodium taurocholate (NaTC) with or without 3.3 X 10-6M 16, 16-dimethyl prostaglandin E2 (dmPGE2). With dmPGE2 present in the instillate, H+ was twice as large in the control period. Exposure to 5mM NaTC reduced H+ by 63% and abolished the stimulatory effect of dmPGE2. Subsequent exposure to 15mM NaTC depressed H+ further. In the absence of dmPGE2, NaTC reduced H+ to equivalent rates. The PD was unaffected by NaTC in the presence of dmPGE2 and lowered by 45% in the absence of dmPGE2. It may be concluded that 1) prostaglandins stimulate rather than inhibit basal H+ in vivo, and 2) prostaglandins provide weak protection against the ionic actions of conjugated bile salts instilled in the stomach.

Effect of hyperosmotic xylitol on gastric secretion in the rat: lack of influence of cyclooxygenase inhibitors

The effects of hyperosmotic solutions of xylitol were tested on acid and bicarbonate outputs from the rat stomach as well as on acid back-diffusion. Hyperosmotic solutions were instilled into anesthetized rats by an esophageal cannula and drained through a duodenal catheter. Spontaneously secreting and histamine-stimulated rats were used in different experiments. Hyperosmotic xylitol solutions at concentrations of 18.4% or higher produced graded inhibitions of the histamine-induced acid secretion. A 34.5% xylitol solution also inhibited spontaneous acid secretion. The same solution also caused a certain degree of acid back-diffusion and increased output of bicarbonate. Neither the inhibition acid secretion nor stimulation of bicarbonate output were affected by cyclooxygenase inhibitors indomethacin or flufenamic acid. It is concluded that hyperosmotic xylitol reduces gastric acidity by three mechanisms namely inhibition of acid secretion, increased bicarbonate output and increased back-diffusion of acid. None of these mechanisms seem to depend on prostaglandin biosynthesis.

Hyperosmotic xylitol, prostaglandins and gastric mucosal barrier

We have previously reported that hyperosmotic solutions of sodium chloride or of xylitol possess potent anti-ulcer activity and reduce gastric acidity in the rat. They also stimulate gastric prostaglandin (PG) biosynthesis, which may bear a causal relationship to the above effects. In the present investigation we studied the effect of intragastric hyperosmolarity on the transmucosal potential difference (PD) and on the permeability to H+ ions in the rat stomach. We also studied the effect of the prostaglandin synthetase inhibitors, indomethacin and flufenamic acid, on these parameters. Rat stomach was perfused in vivo, under urethane anesthesia, by xylitol solutions made up in 0.01 N HCl. While moderately hyperosmotic (13%) xylitol was without effect, the perfusion of intensely hyperosmotic xylitol (34.5%) resulted in a long lasting reduction of the transmucosal PD from a mean (+/- SEM) of -63 +/- 4 mV to a trough value of -40 +/- 3 mV. This depression of transmucosal PD was inhibited in a dose-related fashion by prior treatment with the PG-synthetase inhibitors. Acid recovery in the effluent was significantly reduced by the 34.5% xylitol solution and indomethacin pretreatment did not modify the effect of hyperosmotic xylitol. It is concluded that, although intensely hyperosmotic xylitol produces some of the characteristic effects of a barrier breaker, i.e. depression of transmucosal PD and acid back diffusion, these two phenomena probably involve different mechanisms, as indicated by their differential response to indomethacin.

Effect of topical application of amino acids on gastric pepsin secretion in the rat

The effect of amino acids and other chemicals of intragastric perfusion on pepsin secretion was studied in anaesthetized rats. Irrigation of the stomach with glycine caused concentration-dependent increase in pepsin output, but not in acid output. Pepsin stimulatory effect was decreased by an increase of the carbon chain between the amino group and carboxyl group of glycine and by transposing the amino group from alpha- to gamma-position in amino-n-butyric acid. Acidification of perfusate, a local irrigation of lidocaine and an intravenous infusion of atropine reduced but did not abolish the pepsin response to chemical stimulation. Since serum gastrin level was not changed from basal levels during pepsin secretion induced by amino acids, the mechanism of chemical stimulation appears to be gastrin-independent. The comparison of the secretagogue activity of amino acids shows that glycine exhibited the strongest stimulation of pepsin output, reaching 208% of the response to tetragastrin at the dose of 8 microgram/kg/hour. All other amino acids tested were found to stimulate pepsin secretion, whereas bovine serum albumin and hydrochloric acid were inert in this respect. The result indicates that the chemical stimulation of the stomach by amino acids is capable of inducing pepsin secretion by a local, gastrin-independent mechanism sensitive to pH and related to the molecular configuration of amino acids.

Effect of intragastric acid on pepsinogen secretion in the rat

When the osmolarity of intragastric instillates was constant (200 mosmole/l) the changes in their hydrogen ion concentrations (maximally from 0.01 to 100 mM) did not significantly affect pepsinogen secretion in anaesthetised rats. Solutions of 100 mM HCl, 100 mM NaCl and 150 mM sucrose all produced a similar stimulation of pepsinogen secretion. The results do not support the view that the gastric mucosa contains receptors sensitive to hydrogen ions regulating pepsinogen secretion under physiological conditions.

Osmotic stimulation of pepsin secretion in the rat

The effect of the osmolarity of intragastric instillates on pepsin secretion was studied in rats anaesthetised with urethane. Irrigation of the stomach with solutions of sucrose and NaCl, resp. caused a concentration-dependent increase in pepsin output. A stimulation was observed already by hypotonic solutions and the maximal effect was obtained by 300 m-osmole/l of sucrose and by 600 m-osmole/l of NaCl (13- and 10-fold stimulation resp.). A similar time course in the increase of pepsin output was produced by hyperosmotic solutions (600 m-osmole/l) of sucrose, urea, NaCl and choline chloride. Pepsin output was stimulated maximally within 30 min and decreased thereafter, but remained at about 4--6-fold higher levels than during the previous irrigation with distilled water. Replacement of hyperosmotic instillates by distilled water reduced pepsin secretion to the initial level. Hypertonic ethanol (600 m-osmole/l) increased pepsin output only slightly. Vagotomy, pretreatment with atropine (1 mg/kg i.v.) or cimetidine (5 mg/kg i.v.), local anesthesia of the gastric mucosa with 4% lidocaine or intravenous infusion of PGE2 (2 microgram/kg X min) did not antagonise the stimulation of pepsin output induced by hyperosmotic NaCl (600 m-osmole/l). The results indicate that the increase of the osmolarity of intragastric instillates stimulates pepsin secretion in the rat without involvement of neural (vagal or local cholinergic reflexes) or hormonal mechanisms (release of gastrin) which are known to stimulate gastric secretion in the gastric phase.

Ethanol-induced changes in gastric mucosal content of cyclic AMP and ATP in the rat

The stomach of urethane-anaesthetised rats was perfused with 10% (v/v) ethanol. At 40 min, the secretion of acid was strongly inhibited and the contents of cyclic AMP and ATP were lowered in the superficial mucosa, but not in whole gastric mucosa. After discontinuation of the ethanol perfusion, the rate of gastric acid output as well as the cyclic AMP and ATP levels recovered. Most of the acid-secreting parietal cells were found in the superficial mucosa where the changes of cyclic AMP and ATP took place. The results suggest that lowering of the mucosal contents of cyclic AMP and ATP may be involved in the ethanol-induced inhibition of the gastric acid output.