Effect of insulin on gastric acid secretion, histamine formation, and on the incidence of gastric lesions

Glucose suppresses the activity of rat oxyntic histidine decarboxylase without affecting gastrin levels

Glucose suppressed the activity of oxyntic mucosal histidine decarboxylase within 2 h when given either intragastrically or intraperitoneally to rats fasted for 24 h. Serum levels of gastrin, secretin, glucagon, and somatostatin and oxyntic mucosal levels of gastrin, histamine, and somatostatin showed no significant changes after glucose. Glucose suppressed the aspirin-induced histidine decarboxylase activity without changing serum gastrin. It also suppressed the pentagastrin-induced histidine decarboxylase activity. Neither fructose nor mannitol had such an effect. These results suggest that glucose acts directly on the enterochromaffin-like cells in rat oxyntic mucosa to suppress histidine decarboxylase activation.

Relationship between the gastric myoelectric and mechanical activity in the genesis of ulcers in indomethacin-insulin-treated rats

We have proposed that gastric contractile activity mechanically induces ulcers in the nonsteroid antiinflammatory drug (NSAID)-treated rat. This study examines first the relationship between number (dose) of peristaltic contractions applied to the mucosa and the ulcer score. Second, it examines the relative roles of: altered gastric myoelectrical activity (MEA) resulting from indomethacin (Indo) pretreatment, insulin-induced gastric peristalsis, and a combination of the two in the generation of mucosal lesions. Third, it examines the effect of exogenous prostaglandin on the Indo-altered MEA and relates it to ulcerogenesis. Indo pretreatment increased gastric tone and MEA. In such animals, the dose of peristaltic contractions applied to the gastric wall was related to the ulcer score in a dose-dependent manner. Exogenous prostaglandin (PG) reversed the MEA effect of Indo and reduced ulceration. It is postulated that an altered smooth muscle state secondary to inhibition of prostaglandin synthesis (PG-S) renders the mucosa vulnerable to injury by peristaltic action.

Suppression of rat stomach histidine decarboxylase activity by histamine: H2-receptor-mediated feed-back

1. Gastrin activates rat stomach histidine decarboxylase. Exogenous histamine suppressed the basal enzyme activity in unoperated, in nephrectomized, in vagally denervated and in antrectomized rats, and counteracted the pentagastrin-induced enzyme activation in unoperated rats.2. Kinetic analysis of enzyme-catalysed histidine decarboxylation in extracts from untreated vagotomized and from histamine-treated vagotomized rats showed that the histamine-induced suppression of histidine decarboxylase activity probably reflects a reduced enzyme concentration. Moreover, the enzyme half-life in vagotomized rats after treatment with histamine was shorter than the half-life observed after inhibition of enzyme synthesis. These observations suggest that administration of histamine not only inhibits enzyme synthesis but also causes an accelerated rate of elimination of histidine decarboxylase.3. Intravenous infusion of histamine caused marked displacement of the pentagastrin dose-response curve, in a manner suggesting a reduced sensitivity to pentagastrin.4. After H(2)-receptor blockade, but not after H(1)-receptor blockade, histamine was less effective in suppressing the enzyme activity. Furthermore, H(2)-receptor blockade augmented the pentagastrin-induced enzyme activation.5. The results suggest that histamine (via H(2)-receptors) reduces the sensitivity of the histamine-storing cells to gastrin and that H(2)-receptor blockade induces the opposite effects.6. We propose that the histamine-storing cells in the rat stomach are endowed with H(2)-receptors and that exogenous histamine is capable of acting directly on the histamine cells. This may reflect a physiological control mechanism whereby mobilized endogenous histamine modifies its own synthesis and release.

The role of the antrum and the vagus nerve in the formation of gastric mucosal histamine

1. The effects of the vagus nerve and of antral gastrin on the rate of histamine formation (histamine forming capacity, HFC, i.e. histidine decarboxylase activity) in the parietal cell region of the gastric mucosa has been investigated in the following stomach preparations: gastric fistula, denervated Heidenhain pouch, antral resection with gastrojejunostomy, gastrojejunostomy with exclusion of the duodenum and in the intact stomach. The determinations of mucosal HFC were made on fasting rats and on re-fed animals when the effect of feeding was studied.2. In fasting rats with a gastrojejunostomy and the antrum intact the mucosal HFC of the innervated stomach was about 4 times higher than in the corresponding preparation with the antrum resected. In the innervated main stomach the mucosal HFC was about twice as high as in the denervated pouch, indicating that the vagus and endogenous antral gastrin each contribute to maintaining mucosal HFC in the fasting state.3. Acidifying the stomach caused a substantial lowering of the mucosal HFC presumably by inhibiting antral gastrin release, whereas acid in the stomach did not interfere with the elevation of mucosal HFC evoked by injection of gastrin.4. Injection of gastrin elevated mucosal HFC in the innervated main stomach and in the denervated pouch to approximately equal levels. With the dose of gastrin employed there was about a fourfold increase in the HFC of the pouch mucosa.5. In antrectomized rats enhanced vagal influence, evoked by insulin injection or by feeding, raised the mucosal HFC. In rats with the antrum intact and the stomach acidified, insulin injection produced an increased HFC. Thus, a vagal effect on mucosal HFC exists independent of participation of antral gastrin.6. The stable choline esters carbachol and methacholine act directly on the parietal cell without involving mucosal HFC. The vagus nerve and gastrin, however, are assumed to provide secretory stimulation by means of accelerated histamine formation.7. The interrelation between increased histamine formation and hydrochloric acid secretion is discussed.