Mechanism of action of the calcium-sensing receptor in human antral gastrin cells

Broad-spectrum L-amino acid sensing by class 3 G-protein-coupled receptors

The sensing of nutrients is essential to the control of growth and metabolism. Although the sensing mechanisms responsible for the detection and coordination of metabolic responses to some nutrients, most notably glucose, are well understood, the molecular basis of amino acid sensing by cells and tissues is only now emerging. In this article, we consider evidence that some members of G-protein-coupled receptor class 3 are broad-spectrum amino acid sensors that couple changes in extracellular amino acid levels to the activation of intracellular signaling pathways. In particular, we consider both the molecular basis of specific and broad-spectrum amino acid sensing by different members of class 3 and the physiological significance of broad spectrum amino acid sensing by the extracellular calcium-sensing receptor, heterodimeric taste receptors and the recently "deorphanized" receptor GPRC6A and its goldfish homolog, the 5.24 chemoreceptor.

Taste receptors in the gastrointestinal tract. II. L-amino acid sensing by calcium-sensing receptors: implications for GI physiology

The extracellular calcium-sensing receptor (CaR) is a multimodal sensor for several key nutrients, notably Ca2+ ions and L-amino acids, and is expressed abundantly throughout the gastrointestinal tract. While its role as a Ca2+ ion sensor is well recognized, its physiological significance as an L-amino acid sensor and thus, in the gastrointestinal tract, as a sensor of protein ingestion is only now coming to light. This review focuses on the CaR's amino acid sensing properties at both the molecular and cellular levels and considers new and putative physiological roles for the CaR in the amino acid-dependent regulation of gut hormone secretion, epithelial transport, and satiety.

Role of calcium and other trace elements in the gastrointestinal physiology

Calcium is an essential ion in both marine and terrestrial organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the regulation of neuronal function. The Ca²⁺ balance is maintained by three organ systems, including the gastrointestinal tract, bone and kidney.

Since first being cloned in 1993 the Ca²⁺-sensing receptor has been expressed along the entire gastrointestinal tract, until now the exact function is only partly elucidated. As of this date it still remains to be determined if the Ca²⁺-sensing receptor is involved in calcium handling by the gastrointestinal tract. However, there are few studies showing physiological effects of the Ca²⁺-sensing receptor on gastric acid secretion and fluid transport in the colon. In addition, polyamines and amino acids have been shown to activate the Ca²⁺-sensing receptor and also act as allosteric modifiers to signal nutrient availability to intestinal epithelial cells. Activation of the colonic Ca²⁺-sensing receptor can abrogate cyclic nucleotide-mediated fluid secretion suggesting a role of the receptor in modifying secretory diarrheas like cholera. For many cell types changes in extracellular Ca²⁺ concentration can switch the cellular behavior from proliferation to terminal differentiation or quiescence. As cancer remains predominantly a disease of disordered balance between proliferation, termination and apoptosis, disruption in the function of the Ca²⁺-sensing receptor may contribute to the progression of neoplastic disease. Loss of the growth suppressing effects of elevated extracellular Ca²⁺ have been demonstrated in colon carcinoma, and have been correlated with changes in the level of CaSR expression.

Dissimilar PTH, gastrin, and calcitonin responses to oral calcium and peptones in hypocalciuric hypercalcemia, primary hyperparathyroidism, and normal subjects: a useful tool for differential diagnosis

We analyzed gastrin, PTH, and calcitonin responses to oral calcium and peptones in hypocalciuric hypercalcemia, mild primary hyperparathyroidism, and normal controls. We observed diverse hormonal responses that may help in the differential diagnosis of these conditions.

INTRODUCTION

Hypocalciuric hypercalcemia (HH) is consequent to calcium-sensing receptor (CaSR) genetic mutations or anti-CaSR antibodies. CaSR is expressed in parathyroid tissue, thyroid C cells, and gastrin-secreting cells, where it has been suggested that on calcium and/or amino acid allosteric activation, promotes gastrin secretion.

MATERIALS AND METHODS

We evaluated gastrin, PTH, and calcitonin responses to oral calcium (1 g) and peptones (10 g) in 10 patients with HH (mean age, 58.5 +/- 10.3 years; F/M = 9/1), 15 patients with primary hyperparathyroidism (PH; mean age, 60.4 +/- 8.3 years; F/M = 11/4), and 30 healthy controls (mean age, 60.3 +/- 8.1 years). Statistical analyses for differences during oral loading tests were calculated with ANOVA for repeated measurements and comparisons between two groups were performed with Student's t-test.

RESULTS

PTH response to peptones was markedly increased in patients with PH compared with flat responses in controls and HH patients (p < 0.05). Gastrin increase after oral calcium was absent in HH and PH subjects (p < 0.05 versus controls), and gastrin responses to peptones were blunted in HH and PH subjects compared with controls (p < 0.05). PTH drop and calcitonin increase after calcium load observed in controls were absent in HH and PH subjects (p < 0.05).

CONCLUSIONS

The marked difference in PTH response elicited by peptones observed in PH compared with subjects with HH may help in the differential diagnosis of these conditions without genetic studies. Peptones may stimulate CaSR-controlled hormones as an allosteric regulatory pathway. CaSR abnormalities may help to explain the different calcium- and peptones-induced hormonal responses observed in PH and HH compared with normal subjects.

Ciprofibrate stimulates the gastrin-producing cell by acting luminally on antral PPAR-alpha

The lipid-lowering drug ciprofibrate stimulates gastrin-producing cells in the rat stomach without lowering gastric acidity. Although suggested to be a luminal action on antral peroxisome proliferator-activated receptor-alpha (PPAR-alpha), the mechanism is still not fully elucidated. Gastric bypass was surgically prepared in male Sprague-Dawley rats. Gastric-bypassed and sham-operated rats were either given ciprofibrate (50 mg.kg(-1).day(-1) in methocel) or vehicle alone for 7 wk. PPAR-alpha knockout (KO) and wild-type (WT) mice were either given ciprofibrate (500 mg.kg(-1).day(-1) in methocel) or vehicle alone for 2 wk. The concentration of gastrin in blood was analyzed. Antral G cell density and gastrin mRNA abundance were determined by using immunostaining and Northern blot analysis. Ciprofibrate did not raise plasma gastrin or G cell density in gastric-bypassed rats, although the gastrin mRNA level was slightly increased. In contrast, ciprofibrate induced hypergastrinemia, a 50% increase in G cell density, and a threefold increase in gastrin mRNA in sham-operated rats. In PPAR-alpha KO mice, ciprofibrate did not raise G cell density or the gastrin mRNA level. The serum gastrin level was reduced by ciprofibrate. In WT mice, ciprofibrate induced hypergastrinemia, a doubling of G cell density, and a threefold increase in gastrin mRNA. Comparing animals dosed with vehicle only, PPAR-alpha KO mice had higher serum gastrin concentration than WT mice. We conclude that the main effects of ciprofibrate on G cells are mediated from the antrum lumen, and the mechanism is dependent on PPAR-alpha. The results indicate that PPAR-alpha may have a role in the physiological regulation of gastrin release.

L-type amino acids stimulate gastric acid secretion by activation of the calcium-sensing receptor in parietal cells

Parietal cells are the primary acid secretory cells of the stomach. We have previously shown that activation of the calcium-sensing receptor (CaSR) by divalent (Ca(2+)) or trivalent (Gd(3+)) ions stimulates acid production in the absence of secretagogues by increasing H(+),K(+)-ATPase activity. When overexpressed in HEK-293 cells, the CaSR can be allosterically activated by L-amino acids in the presence of physiological concentrations of extracellular Ca(2+) (Ca(o)(2+); 1.5-2.5 mM). To determine whether the endogenously expressed parietal cell CaSR is allosterically activated by L-amino acids, we examined the effect of the amino acids L-phenylalanine (L-Phe), L-tryptophan, and L-leucine on acid secretion. In ex vivo whole stomach preparations, exposure to L-Phe resulted in gastric luminal pH significantly lower than controls. Studies using D-Phe (inactive isomer) failed to elicit a response on gastric pH. H(+)-K(+)-ATPase activity was monitored by measuring the intracellular pH (pH(i)) of individual parietal cells in isolated rat gastric glands and calculating the rate of H(+) extrusion. We demonstrated that increasing Ca(o)(2+) in the absence of secretagogues caused a dose-dependent increase in H(+) extrusion. These effects were amplified by the addition of amino acids at various Ca(o)(2+) concentrations. Blocking the histamine-2 receptor with cimetidine or inhibiting system L-amino acid transport with 2-amino-2-norbornane-carboxylic acid did not affect the rate of H(+) extrusion in the presence of L-Phe. These data support the conclusion that amino acids, in conjunction with a physiological Ca(o)(2+) concentration, can induce acid secretion independent of hormonal stimulation via allosteric activation of the stomach CaSR.

Release of transgenic human insulin from gastric g cells: a novel approach for the amelioration of diabetes

We explored the hypothesis that meal-regulated release of insulin from gastric G cells can be used for gene therapy for diabetes. We generated transgenic mice in which the coding sequence of human insulin has been knocked into the mouse gastrin gene. Insulin was localized specifically to antral G cells of G-InsKi mice by double immunofluorescence staining using antibodies against insulin and gastrin. Insulin extracted from antral stomach of G-InsKi mice decreased blood glucose upon injection into streptozotocin-diabetic mice. Intragastric administration of peptone, a known potent luminal stimulant of gastrin secretion, induced an increase in circulating levels of transgenic human insulin from 10.7 +/- 2 to 23.3 +/- 4 pm in G-InsKi mice. Although G cell-produced insulin decreased blood glucose in G-InsKi mice, it did not cause toxic hypoglycemia. Proton pump inhibitors, pharmacological agents that increase gastrin output, caused a further increase in the circulating levels of gastric insulin (41.5 +/- 2 pm). G cell-produced insulin was released into circulation in response to the same meal-associated stimuli that control release of gastrin. The most striking aspect of the results presented here is that in the presence of the G-InsKi allele, Ins2(Akita/+) mice exhibited a marked prolongation of life span. These results imply that G cell-derived transgenic insulin is beneficial in the amelioration of diabetes. We suggest that an efficient G cells-based insulin gene therapy can relieve diabetic patients from daily insulin injections and protect them from complications of insulin insufficiency while avoiding episodes of toxic hypoglycemia.

The extracellular calcium-sensing receptor increases the number of calcium steps and action currents in pituitary melanotrope cells

Secretion of alpha-melanophore-stimulating hormone (alpha-MSH) from the neuroendocrine melanotrope cells in the intermediate lobe of the pituitary gland of the clawed frog Xenopus laevis is regulated by various inhibitory, stimulatory and autocrine factors. The neuropeptide sauvagine stimulates alpha-MSH secretion by changing the pattern of intracellular Ca2+ oscillations and the electrical properties of the cell membrane. In the present study we investigated whether another secreto-stimulator, the extracellular Ca2+-sensing receptor (CaR), also affects the Ca2+ oscillatory pattern and electrical membrane properties. Using high-speed dynamic video-imaging we show that activation of the CaR with the specific agonist l-phenylalanine (l-Phe) changes the Ca2+ oscillatory pattern by increasing the number of Ca2+ steps, which are the "building blocks" of the oscillations. Moreover, using patch-clamp electrophysiology it is demonstrated that l-Phe affects membrane properties by increasing frequency and duration of action currents. Compared to sauvagine, the CaR has different effects on the action current parameters, suggesting that multiple mechanisms regulate the electrical properties of the melanotrope cell membrane and, thereby, the Ca2+ oscillation-dependent level of alpha-MSH secretion.

Gastrin: old hormone, new functions

It is exactly a century since the gastric hormone gastrin was first described as a blood-borne regulator of gastric acid secretion. The identities of the main active forms of the hormone (the "classical gastrins") and their cellular and molecular sites of action in regulating acid secretion have all attracted sustained attention. However, recent work on peptides derived from the gastrin precursor that do not stimulate acid secretion ("non-classical gastrins"), together with studies on mice over-expressing the gene, or in which the gastrin gene has been deleted, suggest hitherto unsuspected roles in regulating cell proliferation, migration, and differentiation. Moreover, microarray and proteomic studies have identified previously unsuspected target genes of the classical gastrins. Some of the newer actions have implications for our understanding of the progression to cancer in oesophagus, stomach, pancreas and colon, all of which have recently been linked in one way or another to dysfunctional signalling involving products of the gastrin gene. The present review focuses on recent progress in understanding the biology of both classical and non-classical gastrins.

Involvement of protein kinase C-alpha and -epsilon in extracellular Ca2+ signalling mediated by the calcium sensing receptor

The sensing of extracellular Ca(2+) concentration ([Ca(2+)](o)) and modulation of cellular processes associated with acute or sustained changes in [Ca(2+)](o) are cell-type specific and mediated by the calcium sensing receptor (CaR). [Ca(2+)](o) signalling requires protein kinase C (PKC), but the identity and role of PKC isoforms in CaR-mediated responses remain unclear. Here we show that high [Ca(2+)](o) activated PKC-alpha and PKC- in parathyroid cells and in human embryonic kidney (HEK293) cells overexpressing the CaR (HEK-CaR) and that this response correlated with the CaR-dependent activation of mitogen-activated protein kinases ERK1/2. Activation of ERK1/2 by acute high [Ca(2+)](o) required influx of Ca(2+)through Ni(2+)-sensitive Ca(2+)channels and phosphatidylinositol-dependent phospholipase C-beta activity. Inhibition of PKC by co-expression of dominant-negative (DN) mutants of PKC-alpha or - with the CaR attenuated sustained ERK1/2 activation. Overexpression of a PKC phosphorylation site (T888A) mutant CaR in HEK293 cells showed that this site was important for ERK1/2 activation at high [Ca(2+)](o). Activation of ERK1/2 by high [Ca(2+)](o) was not necessary for the [Ca(2+)](o)-regulated secretion of parathyroid hormone (PTH) in dispersed bovine parathyroid cells. These data suggest that the CaR-mediated [Ca(2+)](o) signal leading to regulated PTH secretion that requires diacylglycerol-responsive PKC isoforms is not mediated via the ERK pathway.

Relationship between serum calcium and CA 19-9 levels in colorectal cancer

AIM: To examine the calcium metabolism of colorectal cancer (CRC) in patients with colorectal cancer and control patients.

METHODS: Seventy newly diagnosed CRC patients were included. The healthy control group was age and gender matched (n = 32). Particular attention was devoted to the relationship between serum calcium of patients, and levels of AFP, CEA, carbohydrate antigen 19-9 (CA 19-9) (that could be considered as prognostic factors). Furthermore, the Ca-sensing receptor (CaSR) gene A986S polymorphism was investigated in these patients, as well as the relationship between different CaSR genotypes and the data stated above.

RESULTS: A lower level of ionized calcium (also corrected for albumin) was found in the serum of CRC patients with normal 25 (OH) vitamin D levels. The ionized calcium concentration was inversely correlated with the serum level of CA 19-9. There was no difference in the distribution of CaSR genotypes, between CRC patients and general population. The genotypes did not correlate with other data examined.

CONCLUSION: Based on these results, lower levels of serum calcium might be a pathogenic and prognostic factor in colorectal cancer.

Functions and roles of the extracellular Ca2+-sensing receptor in the gastrointestinal tract

The gastrointestinal tract is vital to food digestion and nutrient absorption as well as normal salt and water homeostasis. Studies over the last several years have shown that the Ca2+-sensing receptor is expressed along the entire gastrointestinal tract. The potential roles for the receptor in gastrointestinal biology are now only beginning to be elucidated and much work remains. Well-studied physiological effects include regulation of gastric acid secretion and modulation of fluid transport in the colon. It remains to be determined if the Ca2+-sensing receptor is involved in calcium handling by the gastrointestinal tract. The ability of organic nutrient receptor agonists/allosteric modifiers, such as polyamines and L-amino acids, to activate the Ca2+-sensing receptor suggest potential roles in signalling nutrient availability to gastric and intestinal epithelial cells. In addition, polyamines are crucial for normal cell proliferation and differentiation required to sustain the rapid turnover of gastrointestinal epithelial cells and the Ca2+-sensing receptor may be involved in this function. Activation of the colonic Ca2+-sensing receptor can abrogate cyclic nucleotide-mediated fluid secretion suggesting a role for the receptor in modifying secretory diarrheas like cholera. Finally, the Ca2+-sensing receptor has been suggested to provide a mechanism for the effect of calcium intake in reducing the risk of colon cancer.

Calcium sensing by endocrine cells

The elucidation of the structure and function of the Ca2+(o)-sensing receptor (CaR) has provided important insights into the normal control of Ca2+(o) homeostasis, particularly the key role of the receptor in kidney and parathyroid. Further studies are needed to define more clearly the homeostatic role of the CaR in additional tissues, both those that are involved and those that are uninvolved in systemic Ca2+(o) homeostasis. The availability of the cloned CaR has also permitted documentation of the molecular basis of inherited disorders of Ca2+(o) sensing, including those in which the receptor is less and or more sensitive than normal to Ca2+(o). Antibodies to the CaR that either activate it or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the receptor is abnormally low in 1 degree and 2 degrees hyperparathyroidism, which could contribute to the defective Ca2+(o) sensing in these conditions. The recent discovery of calcimimetics, which sensitize the CaR to Ca2+(o), has provided what will likely be an effective medical therapy for the secondary/tertiary hyperparathyroidism of end stage renal failure as well as for 1 degree hyperparathyroidism.

G and D cells in rat antral mucosa: An immunoelectron microscopic study

AIM: To investigate the gastrin secreting cells (G cells) and the somatostatin secreting cells (D cells) of antral mucosa in rats at the ultrastructural level.

METHODS: Revised immunoelectron microscopic technique was used to detect the G cells and D cells in rat antral mucosa through gastrin and somatostatin antibodies labeled by colloidal gold. Also the relevant quantitative analysis regarding the granular number of colloidal gold in G cells and in D cells was conducted.

RESULTS: Immunological granules of colloidal gold were distributed in G cells and D cells. Gastrin labeled golden granules or somatostatin labeled ones presented mainly as lobation-like or island-like congeries. Most of the golden congeries were observed dissociated in cytoplasms of G cells or D cells, near the basement membrane. A few golden congeries were located in nuclei. The number of golden granules in one G cell was around 107.04 ± 19.68 and was 83.36 ± 17.58 in one D cell.

CONCLUSION: Gastrin secreting granules are located in cytoplasms and nuclei of G cells, and somatostatin secreting granules both in cytoplasms and in nuclei of D cells. The number of golden granules can be quantitatively analyzed to determine the relative amount of gastrin secreting granules or somatostatin secreting granules.

Chemoprevention of colon cancer by calcium, vitamin D and folate: molecular mechanisms

Recent findings have indicated that dietary calcium, vitamin D and folate can modulate and inhibit colon carcinogenesis. Supporting evidence has been obtained from a wide variety of preclinical experimental studies, epidemiological findings and a few human clinical trials. Important molecular events and cellular actions of these micronutrients that contribute to their tumour-modulating effects are discussed. They include a complex series of signalling events that affect the structural and functional organization of colon cells.

Ca2+-stimulated Ca2+ oscillations produced by the Ca2+-sensing receptor require negative feedback by protein kinase C

We examined the role of protein kinase C (PKC) in the mechanism and regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations elicited by an increase in the extracellular concentration of Ca(2+) ([Ca(2+)](e)) in human embryonic kidney 293 cells expressing the Ca(2+)-sensing receptor (CaR). Exposure to the PKC inhibitors bisindolylmaleimide I (GF I) or Ro-31-8220 converted oscillatory responses to transient, non-oscillatory responses, significantly reducing the percentage of cells that showed [Ca(2+)](i) oscillations but without decreasing the overall response to increase in [Ca(2+)](e). Exposure to 100 nm phorbol 12,13-dibutyrate, a direct activator of PKC, eliminated [Ca(2+)](i) oscillations. Addition of phorbol 12,13-dibutyrate at lower concentrations (3 and 10 nm) did not eliminate the oscillations but greatly reduced their frequency in a dose-dependent manner. Co-expression of CaR with constitutively active mutants of PKC (either epsilon or beta(1) isoforms) also reduced [Ca(2+)](i) oscillation frequency. Expression of a mutant CaR in which the major PKC phosphorylation site is altered by substitution of alanine for threonine (T888A) eliminated oscillatory behavior, producing [Ca(2+)](i) responses almost identical to those produced by the wild type CaR exposed to PKC inhibitors. These results support a model in which phosphorylation of the CaR at the inhibitory threonine 888 by PKC provides the negative feedback needed to cause [Ca(2+)](i) oscillations mediated by this receptor.

Gastric secretion

Overlapping neural, hormonal, and paracrine pathways finely regulate gastric acid secretion. In rats and guinea pigs, most of the intrinsic neural innervation to the gastric mucosa originates in the myenteric plexus. In contrast, human stomachs have a clearly defined submucosal plexus that contains a variety of transmitters including nitric oxide, vasoactive intestinal peptide (VIP), gastrin-releasing peptide (GRP), substance P, and calcitonin gene-related peptide (CGRP). Although GRP is known to participate in meal-stimulated acid secretion by releasing gastrin in a variety of laboratory animals, recent studies were unable to demonstrate a role for endogenous GRP in meal-stimulated gastrin secretion in humans. Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the secretin-glucagon-VIP family, has been localized to gastric mucosal neurons and may participate in vagally mediated acid secretion. Two novel peptides, ghrelin and leptin, have been localized to the stomach. Peripheral administration of ghrelin stimulates and of leptin inhibits acid secretion. The binding of secretagogues to parietal cells generates changes in second messengers that regulate the translocation and activation of the proton pump, HK-ATPase. In resting cells, HK-ATPase is contained within cytoplasmic tubulovesicles in an inactive form. At stimulation, the tubulovesicles fuse with the apical canaliculi and the HK-ATPase is incorporated into the apical membrane where it actively pumps H ions in exchange for K. Acute infection with Helicobacter pylori results in hypochlorhydria, whereas chronic infection can cause either hypo- or hyperchlorhydria, depending on the distribution of the infection and the degree of corpus gastritis. Recent studies suggest that inflammatory cytokines, produced in response to the organism, can play a role in the perturbations in acid and gastrin secretion induced by H. pylori.

Amino acids and Ca2+ stimulate different patterns of Ca2+ oscillations through the Ca2+-sensing receptor

We determined the effect of aromatic amino acid stimulation of the human extracellular Ca2+-sensing receptor (CaR) on intracellular Ca2+ concentration ([Ca2+]i) in single HEK-293 cells. Addition of L-phenylalanine or L-tryptophan (at 5 mM) induced [Ca2+]i oscillations from a resting state that was quiescent at 1.8 mM extracellular Ca2+ concentration ([Ca2+]e). Each [Ca2+]i peak returned to baseline values, and the average oscillation frequency was approximately 1 min(-1) at 37 degrees C. Oscillations were not induced or sustained if the [Ca2+]e was reduced to 0.5 mM, even in the continued presence of amino acid. Average oscillation frequency in response to an increase in [Ca2+]e (from 1.8 to 2.5-5 mM) was much higher (approximately 4 min(-1)) than that induced by aromatic amino acids. Oscillations in response to [Ca2+]e were sinusoidal whereas those induced by amino acids were transient. Thus both amino acids and Ca2+, acting through the same CaR, produce oscillatory increases in [Ca2+]i, but the resultant oscillation pattern and frequency allow the cell to discriminate which agonist is bound to the receptor.

Cellular mechanisms of calcium and vitamin D in the inhibition of colorectal carcinogenesis

Convincing evidence is available showing that dietary calcium and vitamin D impede the development of colonic carcinogenesis. The major cellular modes of action of calcium and vitamin D which can contribute to the inhibition of colonic neoplasia are reviewed in this article. These consist of complex series of signaling events induced by the chemopreventive agents acting at various tiers of colonic cell organization.

Gastric secretion

The influence of central and peripheral stimuli on gastric acid secretion is mediated via activation of histaminergic, gastrinergic, and cholinergic pathways coupled to intracellular second-messenger systems that determine the trafficking and activity of H+ K+-ATPase, the proton pump of the parietal cell. Histamine, released from enterochromaffin-like cells stimulates the parietal cell directly via H-2 receptors coupled to generation of cAMP. Gastrin, acting via cholecystokinin-2 receptors on enterochromaffin-like cells coupled to an increase in intracellular calcium, stimulates the parietal cell indirectly by activating histidine decarboxylase, releasing histamine, and inducing enterochromaffin-like cell hypertrophy and hyperplasia. Acetylcholine, released from gastric postganglionic intramural neurons, stimulates the parietal cell directly via M-3 receptors coupled to intracellular calcium release and calcium entry. The second-messenger systems activated in the parietal cell converge on H+ K+-ATPase that catalyzes the exchange of luminal K+ for cytoplasmic H+ and is responsible for gastric luminal acidification. The main inhibitor of acid secretion is somatostatin which, acting via sst2 receptors, exerts a tonic inhibitory influence on parietal, enterochromaffin-like, and gastrin cells. Acute infection with Helicobacter pylori results in hypochlorhydria, whereas chronic infection may be associated with either hypo- or hyperchlorhydria. Although prostaglandins are thought to play a physiologic role in the regulation of acid secretion and maintenance of gastric mucosal integrity, the precise roles of cyclooxygenase-1 and cyclooxygenase-2 in these processes still eludes us.