Nalpha-methyl histamine and histamine stimulate gastrin release from rabbit G-cells via histamine H2-receptors

BACKGROUND

Gastrin release by Helicobacter pylori may be an important step in the pathway leading to duodenal ulceration. A histamine H3-receptor agonist was found to release gastrin from antral mucosal fragments; this was interpreted as being due to suppression of somatostatin release. H. pylori is reported to produce Nalpha-methyl histamine (NalphaMH), which is an agonist of H3 as well as other histamine receptors. H. pylori infection also recruits mast cells, which release histamine.

AIM

To determine the direct effects of histamine receptor agonists on isolated gastrin cells.

METHODS

Rabbit G-cells were prepared by countercurrent elutriation and cultured on 24-well plates.

RESULTS

NalphaMH (10-6-10-4 M) caused a dose-dependent increase in gastrin release from a basal level of 2.3 +/- 0.2% total cell content (TCC; mean +/- S.E.M.) to a maximum of 5.1 +/- 0.7%, an increase of 117% (P < 0. 005) above basal. This was abolished by the H2-antagonist ranitidine (10-5 M), but not by immunoblockade with anti-somatostatin antibody, the H1-antagonist chlorpheniramine (10-5 M) or the H3-antagonist thioperamide (10-4 M). The histamine H2-receptor agonist dimaprit (10-6-10-4 M) increased gastrin release from 2.4 +/- 0.2% to 3.6 +/- 0.2% TCC (P < 0.001). Gastrin release was also stimulated by histamine (10-7-10-4 M) from a basal value of 3.0 +/- 0.3% to 5.4 +/- 0.5% TCC (P < 0.001). This also was inhibited by ranitidine (10-5 M) (P < 0.01).

CONCLUSION

NalphaMH and histamine release gastrin from G-cells via H2-receptors; this might contribute to H. pylori-associated hypergastrinaemia.

Effect of PACAP-27 on 14C-aminopyrine accumulation in isolated rabbit parietal cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) is present in gastric mucosa, but its direct effect on parietal cells is unknown. We examined this using 14C-aminopyrine uptake in elutriated rabbit cells. PACAP-27 had no effect on basal cells but significantly increased the response to histamine (10(-4) M) at 10(-9) M and to carbachol (10(-4) M) in the presence of ranitidine (10(-4) M) at 10(-7) M and 10(-8) M. PACAP (6-38), an antagonist of PACAP, inhibited the effect of PACAP-27 on carbachol-stimulated cells. Vasoactive intestinal peptide had no significant effect. In conclusion, PACAP-27 has a direct additive effect on stimulated rabbit parietal cells in vitro.

How does Helicobacter pylori cause mucosal damage? Its effect on acid and gastrin physiology

Helicobacter pylori infection increases gastric acid secretion in patients with duodenal ulcers but diminishes acid output in patients with gastric cancer and their relatives. Investigation of the basic mechanisms may show how H. pylori causes different diseases in different persons. Infection of the gastric antrum increases gastrin release. Certain cytokines released in H. pylori gastritis, such as tumor necrosis factor alpha and specific products of H. pylori, such as ammonia, release gastrin from G cells and might be responsible. The infection also diminishes mucosal expression of somatostatin. Exposure of canine D cells to tumor necrosis factor alpha in vitro reproduces this effect. These changes in gastrin and somatostatin increase acid secretion and lead to duodenal ulceration. But the acid response depends on the state of the gastric corpus mucosa. The net effect of corpus gastritis is to decrease acid secretion. Specific products of H. pylori inhibit parietal cells. Also, interleukin 1 beta, which is overexpressed in H. pylori gastritis, inhibits both parietal cells and histamine release from enterochromaffin-like cells. H. pylori also promotes gastric atrophy, leading to loss of parietal cells. Factors such as a high-salt diet and a lack of dietary antioxidants, which also increase corpus gastritis and atrophy, may protect against duodenal ulcers by decreasing acid output. However, the resulting increase of intragastric pH may predispose to gastric cancer by allowing other bacteria to persist and produce carcinogens in the stomach.