Immunolocalization of gastrin-dependent histidine decarboxylase activity in rat gastric mucosa during feeding

Control of gastric acid secretion in somatostatin receptor 2 deficient mice: shift from endocrine/paracrine to neurocrine pathways

The gastrin-enterochromaffin-like (ECL) cell-parietal cell axis is known to play an important role in the regulation of gastric acid secretion. Somatostatin, acting on somatostatin receptor type 2 (SSTR(2)), interferes with this axis by suppressing the activity of the gastrin cells, ECL cells, and parietal cells. Surprisingly, however, freely fed SSTR(2) knockout mice seem to display normal circulating gastrin concentration and unchanged acid output. In the present study, we compared the control of acid secretion in these mutant mice with that in wild-type mice. In SSTR(2) knockout mice, the number of gastrin cells was unchanged; whereas the numbers of somatostatin cells were reduced in the antrum (-55%) and increased in the oxyntic mucosa (35%). The ECL cells displayed a reduced expression of histidine decarboxylase and vesicle monoamine transport type 2 (determined by immunohistochemistry), and an impaired transformation of the granules to secretory vesicles (determined by electron microscopic analysis), suggesting low activity of the ECL cells. These changes were accompanied by an increased expression of galanin receptor type 1 in the oxyntic mucosa. The parietal cells were found to respond to pentagastrin or to vagal stimulation (evoked by pylorus ligation) with increased acid production. In conclusion, the inhibitory galanin-galanin receptor type 1 pathway is up-regulated in the ECL cells, and the direct stimulatory action of gastrin and vagal excitation is enhanced on the parietal cells in SSTR(2) knockout mice. We suggest that there is a remodeling of the neuroendocrine mechanisms that regulate acid secretion in these mutant mice.

Fasting-induced changes in ECL cell gene expression

Gastric enterochromaffin-like (ECL) cells release histamine in response to food because of elevation of gastrin and neural release of pituitary adenylate cyclase-activating peptide (PACAP). Acid secretion is at a basal level in the absence of food but is rapidly stimulated with feeding. Rats fasted for 24 h showed a significant decrease of mucosal histamine despite steady-state expression of the histamine-synthesizing enzyme histidine decarboxylase (HDC). Comparative transcriptomal analysis using gene expression oligonucleotide microarrays of 95% pure ECL cells from fed and 24-h fasted rats, thereby eliminating mRNA contamination from other gastric mucosal cell types, identified significantly increased gene expression of the enzymes histidase and urocanase catabolizing the HDC substrate L-histidine but significantly decreased expression of the cellular L-histidine uptake transporter SN2 and of the vesicular monoamine transporter 2 (VMAT-2) responsible for histamine uptake into secretory vesicles. This was confirmed by reverse transcriptase-quantitative polymerase chain reaction of gastric fundic mucosal samples from fed and 24-h fasted rats. The decrease of VMAT-2 gene expression was also shown by a decrease in VMAT-2 protein content in protein extracts from fed and 24-h fasted rats compared with equal amounts of HDC protein and Na-K-ATPase alpha(1)-subunit protein content. These results indicate that rat gastric ECL cells regulate their histamine content during 24-h fasting not by a change in HDC gene or protein expression but by regulation of substrate concentration for HDC and a decreased histamine secretory pool.

Immunocytochemical localization of monoamine oxidase type B in enterochromaffin-like cells of rat oxyntic mucosa

We used immunohistochemistry to identify the localization of monoamine oxidase type B (MAOB) in the rat oxyntic mucosa. At light microscopic levels, MAOB-immunopositive cells were mostly located in the basal half of the oxyntic mucosa. By a double-labeling immunofluorescence method, it was shown that MAOB immunoreactivity was localized in almost all of histidine decarboxylase (HDC)-positive cells. Only a few MAOB-positive cells were negative for HDC. At electron microscopic levels, immunohistochemical reaction products of MAOB were detected on the mitochondrial outer membranes in cells that showed morphological characteristics of enterochromaffin-like (ECL) cells. These findings indicate that ECL cells contain MAOB in the rat. We provide a hypothesis that MAOB is involved in the inactivation mechanism of histamine that is released from ECL cells and activates parietal cells to secrete gastric acid.

The gastrins: their production and biological activities

Gastric epithelial organization and function are controlled and maintained by a variety of endocrine and paracrine mediators. Peptides encoded by the gastrin gene are an important part of this system because targeted deletion of the gene, or of the gastrin-CCKB receptor gene, leads to decreased numbers of parietal cells and decreased gastric acid secretion. Recent studies indicate that the gastrin precursor, preprogastrin, gives rise to a variety of products, each with a distinctive spectrum of biological activity. The conversion of progastrin to smaller peptides is regulated by multiple mechanisms including prohormone phosphorylation and secretory vesicle pH. Progastrin itself stimulates colonic epithelial proliferation; biosynthetic intermediates (Gly-gastrins) stimulate colonic epithelial proliferation and gastric epithelial differentiation; and C-terminally amidated gastrins stimulate colonic proliferation, gastric epithelial proliferation and differentiation, and acid secretion. The effects of progastrin-derived peptides on gastric epithelial function are mediated in part by release of paracrine factors that include histamine, epidermal growth factor (EGF)-receptor ligands, and Reg. The importance of the appropriate regulation of this system is shown by the observation that prolonged moderate hypergastrinemia in transgenic mice leads to remodelling of the gastric epithelium, and in the presence of Helicobacter, to gastric cancer.

Regulation of gastric acid secretion

This paper summarizes important developments, published over the past year, that improve our understanding of the regulation of gastric acid secretion at the central, peripheral, and intracellular levels and mechanisms by which various neurotransmitters, paracrine agents, and hormones regulate gastric secretion and are themselves regulated. The main stimulants of acid secretion from the parietal cell are histamine, gastrin, and acetylcholine. Histamine, released from fundic enterochromaffin-like cells, interacts with H(2) receptors on parietal cells that are coupled via separate G proteins to activation of adenylate cyclase and phospholipase C. The antral hormone gastrin, released by activation of cholinergic and bombesin/gastrin-releasing peptide neurons, acts mainly by release of histamine from enterochromaffin-like cells. Acetylcholine, released from gastric intramural neurons, interacts with muscarinic M(3) receptors on parietal cells and has little, if any, effect on histamine secretion. The main inhibitor of acid secretion is somatostatin, which, acting via sst(2) receptors, exerts a tonic restraint on parietal, enterochromaffin-like, and gastrin cells. In patients with duodenal ulcer, infection with Helicobacter pylori is associated with increased basal and stimulated plasma gastrin concentrations and acid outputs. The precise mechanisms mediating the effects are not known, but evidence suggests that both products of the bacteria and the inflammatory infiltrate are capable of stimulating gastrin and acid secretion.