Biphasic kinetics of inositol 1,4,5-trisphosphate accumulation in gastrin-stimulated parietal cells. Effects of pertussis toxin and extracellular calcium

PREDICTING EFFICACY OF PROTON PUMP INHIBITORS IN REGULATING GASTRIC ACID SECRETION

Developing drugs to treat gastric acid related illnesses such as ulcers and acid reflux disease is the leading focus of pharmaceutical companies. In fact, expenditure for treating these disorders is highest among all illnesses in the US. Over the last few decades, a class of drugs known as a proton pump inhibitors (PPIs) appeared on the market and are highly effective at abating gastric illnesses by raising stomach pH (reducing gastric acid levels). While much is known about the action of PPIs , there are still open questions regarding their efficacy, dosing and long-term effects. Here we extend a previous gastric acid secretion model developed by our group to incorporate a pharmacodynamic/pharmacokinetic model to study proton pump inhibitor (PPI) action. Model-relevant parameters for specific drugs such as omeprazole (OPZ) , lansoprazole (LPZ) and pantoprazole (PPZ) were used from published data, and we conducted simulations to study various aspects of PPI treatment. Clinical data suggests that duration of acid suppression is dependent on proton pump turnover rates and this is supported by our model. We found the order of efficacy of the different PPIs to be OPZ>PPZ>LPZ for clinically recommended dose values, and OPZ>PPZ=LPZ for equal doses. Our results indicate that a breakfast dose for once-daily dosing regimens and a breakfast-lunch dose for twice-daily dosing regimens is recommended. Simulation of other gastric disorders using our model provides atypical applications for the study of drug treatment on homeostatic systems and identification of potential side-effects.

The production and role of gastrin-17 and gastrin-17-gly in gastrointestinal cancers

The gastrointestinal peptide hormone gastrin is responsible for initiating the release of gastric acid in the stomach in response to the presence of food and/or humoral factors such as gastrin releasing peptide. However, it has a role in the growth and maintenance of the gastric epithelium, and has been implicated in the formation and growth of gastric cancers. Hypergastrinemia resulting from atrophic gastritis and pernicious anemia leads to hyperplasia and carcinoid formation in rats, and contributes to tumor formation in humans. Additionally, gastrin has been suspected to play a role in the formation and growth of cancers of the colon, but recent studies have instead implicated gastrin processing intermediates, such as gastrin-17-Gly, acting upon a putative, non-cholecystokinin receptor. This review summarizes the production and chemical structures of gastrin and of the processing intermediate gastrin-17-Gly, as well as their activities in the gastrointestinal tract, particularly the promotion of colon cancers.

A model for the study of Helicobacter pylori interaction with human gastric acid secretion

We present a comprehensive mathematical model describing Helicobacter pylori interaction with the human gastric acid secretion system. We use the model to explore host and bacterial conditions that allow persistent infection to develop and be maintained. Our results show that upon colonization, there is a transient period (day 1-20 post-infection) prior to the establishment of persistence. During this period, changes to host gastric physiology occur including elevations in positive effectors of acid secretion (such as gastrin and histamine). This is promoted by reduced somatostatin levels, an inhibitor of acid release. We suggest that these changes comprise compensatory mechanisms aimed at restoring acid to pre-infection levels. We also show that ammonia produced by bacteria sufficiently buffers acid promoting bacteria survival and growth.

A model for integrative study of human gastric acid secretion

We have developed a unique virtual human model of gastric acid secretion and its regulation in which food provides a driving force. Food stimulus triggers neural activity in central and enteric nervous systems and G cells to release gastrin, a critical stimulatory hormone. Gastrin stimulates enterochromaffin-like cells to release histamine, which, together with acetylcholine, stimulates acid secretion from parietal cells. Secretion of somatostatin from antral and corpus D cells comprises a negative-feedback loop. We demonstrate that although acid levels are most sensitive to food and nervous system inputs, somatostatin-associated interactions are also important in governing acidity. The importance of gastrin in acid secretion is greatest at the level of transport between the antral and corpus regions. Our model can be applied to study conditions that are not yet experimentally reproducible. For example, we are able to preferentially deplete antral or corpus somatostatin. Depletion of antral somatostatin exhibits a more significant elevation of acid release than depletion of corpus somatostatin. This increase in acid release is likely due to elevated gastrin levels. Prolonged hypergastrinemia has significant effects in the long term (5 days) by promoting enterochromaffin-like cell overgrowth. Our results may be useful in the design of therapeutic strategies for acid secretory dysfunctions such as hyper- and hypochlorhydria.

CCK-B receptor: chemistry, molecular biology, biochemistry and pharmacology

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract but also found in high density in the mammalian brain. The C-terminal sulphated octapeptide fragment of cholecystokinin (CCK8) constitutes one of the major neuropeptides in the brain; CCK8 has been shown to be involved in numerous physiological functions such as feeding behavior, central respiratory control and cardiovascular tonus, vigilance states, memory processes, nociception, emotional and motivational responses. CCK8 interacts with nanomolar affinities with two different receptors designated CCK-A and CCK-B. The functional role of CCK and its binding sites in the brain and periphery has been investigated thanks to the development of potent and selective CCK receptor antagonists and agonists. In this review, the strategies followed to design these probes, and their use to study the anatomy of CCK pathways, the neurochemical and pharmacological properties of this peptide and the clinical perspectives offered by manipulation of the CCK system will be reported. The physiological and pathological implication of CCK-B receptor will be confirmed in CCK-B receptor deficient mice obtained by gene targeting (Nagata el al., 1996. Proc. Natl. Acad. Sci. USA 93, 11825-11830). Moreover, CCK receptor gene structure, deletion and mutagenesis experiments, and signal transduction mechanisms will be discussed.

Modulation of Ca2+-activated K+ channels by Mg2+ and ATP in frog oxyntic cells

Ca2+-activated K+ channels in the basolateral plasma membrane of bullfrog oxynticopeptic cells are intimately involved in the regulation of acid secretion. Patch-clamp techniques were applied to study the regulating mechanism of these channels. In the excised inside-out configuration, intracellular Mg2+ decreased channel activity in a dose-dependent manner. In the absence of Mg2+, administration of adenosine 5'-trisphosphate (ATP) to the cytoplasmic side also inhibited channel activity. On the other hand, in the presence of Mg2+, addition of ATP markedly increased channel activity. At a fixed concentration of free Mg2+, the Mg-ATP complex caused channel activation and shifted the dose response relationship between channel activity and the intracellular Ca2+ concentration to the left. A nonhydrolysable ATP analogue, adenosine 5'-[beta,gamma-imido]triphosphate (AMP-PNP) adenylyl [beta,gamma-methylene]diphosphate (AMP-PCP), could not substitute for ATP in channel activation, but a hydrolysable ATP analogue, adenosine 5'-O-(3-thiotriphosphate) (ATP[gammaS]) could do so. Furthermore, application of alkaline phosphatase to the cytoplasmic side inhibited channel activity. These results demonstrate that Ca2+-activated K+ channels are regulated by Mg2+ and ATP, and suggest that a phosphorylation reaction may be involved in the regulation mechanism of these channels.

Characterization of cloned human cholecystokinin-B receptor as a gastrin receptor

The cholecystokinin (CCK)-B receptor cloned from human brain was characterized as a gastrin receptor by using heterologous expression systems of COS-7 cells and Xenopus oocytes. 125I-gastrin binding to human CCK-B receptor expressed in COS-7 was time-dependent, saturable and also specific, as well as 125-I-CCK-8. The binding of 125I-gastrin was inhibited by CCK-8 about 10-fold more potently than by gastrin. The rank order of potency of several antagonists to 125I-gastrin binding was YM022 > CI-988 > L-365,260 > L-364,718. Addition of GTP gamma S, a nonhydrolysable analog of GTP, dose-dependently inhibited 125I-gastrin binding, and lowered the gastrin binding affinity, Gastrin (10(-9)-10(-7) M) also evoked a Ca(2+)-dependent Cl- current in Xenopus oocytes expressing CCK-B receptors. These results suggest that the pharmacological profile of the cloned human CCK-B receptor using 125I-gastrin is closely parallel to that reported in gastric mucosa, and that the receptor transduces cellular signals of gastrin as well as those of CCK-8.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Effects of high extracellular calcium and strontium on inositol polyphosphates in bovine parathyroid cells

The addition of Ca2+ or a variety of divalent cations increases intracellular Ca2+ in parathyroid cells and suppresses secretion. Since 1,4,5-inositol trisphosphate (IP3) and 1,3,4,5-inositol tetrakisphosphate (IP4) mediate Ca2+ mobilization in other systems, we examined high Ca(2+)- and Sr(2+)-induced accumulation of IP3 and IP4 isomers by anion-exchange HPLC and measured 1,4,5-IP3 mass in parathyroid cells. Raising extracellular [Ca2+] from 0.5 to 3.0 mM increased 3H-1,4,5-IP3 within 5 s, which was confirmed by mass measurements. 3H-1,3,4-IP3 rose gradually by 10 s and increased for 60 s after the addition of Ca2+. Although we detected no change in 3H-1,3,4,5-IP4, the increase in 3H-1,3,4-IP3 suggests that 3H-1,3,4,5-IP4 was being formed. The addition of 4 mM SrCl2 produced similar changes in 1,4,5-IP3, which were confirmed by mass assay. 3H-1,3,4,5-IP4 did not change. However, Sr2+ induced a gradual increase in 3H-1,3,4-IP3, which remained above control levels for 5 minutes. Isotopic labeling studies in this system may underestimate changes in 1,4,5-IP3 mass, but both mass and radioisotopic analyses indicate that high extracellular Ca2+ and Sr2+ stimulate substantial increases in 1,4,5-IP3 without significant accumulation of 1,3,4,5-IP4. These studies suggest a role for 1,4,5-IP3 in intracellular Ca2+ mobilization by divalent cations in parathyroid cells.

Receptor-operated Ca2+ channels in gastric parietal cells: gastrin and carbachol induce Ca2+ influx in depleting intracellular Ca2+ stores

The mechanism whereby gastrin-type receptor and muscarinic M3-type receptor regulate free intracellular Ca2+ concentration ([Ca2+]i) was studied in rabbit gastric parietal cells stimulated by either gastrin or carbachol. Both agonists induced a biphasic [Ca2+]i response: a transient [Ca2+]i rise, followed by a sustained steady state depending on extracellular Ca2+. Gastrin and carbachol also caused a rapid and transient increase in Mn2+ influx (a tracer for bivalent-cation entry). Pre-stimulation of cells with one agonist drastically decreased both [Ca2+]i increase and Mn2+ influx induced by the other. Neither diltiazem nor pertussistoxin treatment had any effect on agonist-stimulated Mn2+ entry. Thapsigargin, a Ca(2+)-pump inhibitor, induced a biphasic [Ca2+]i increase, and enhanced the rate of Mn2+ entry. Preincubation of cells with thapsigargin inhibits the [Ca2+]i increase as well as Mn2+ entry stimulated by gastrin or by carbachol. Thapsigargin induced a weak but significant increase in Ins(1,4,5)P3 content, but this agent had no effect on the agonist-evoked Ins(1,4,5)P3 response. In permeabilized parietal cells, Ins(1,4,5)P3 and caffeine caused an immediate Ca2+ release from intracellular pools, followed by a reloading of Ca2+ pools which can be prevented in the presence of thapsigargin. We conclude that (i) gastrin and carbachol mobilize common Ca2+ intracellular stores, (ii) Ca2+ permeability secondary to receptor activation involves neither a voltage-sensitive Ca2+ channel nor a GTP-binding protein from the G1 family, and (iii) agonists regulate common Ca2+ channels in depleting intracellular Ca2+ stores.

Role of protein kinase C in the regulation of histamine and bradykinin stimulated inositol polyphosphate turnover in adrenal chromaffin cells

1. The possibility that bradykinin- or histamine-stimulated inositol polyphosphate accumulation may be regulated by protein kinase C (PKC) in bovine adrenal chromaffin cells has been addressed. 2. Initial experiments confirmed that the phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) dramatically inhibited agonist-stimulated [3H]-inositol phosphate accumulations in [3H]-inositol prelabelled cells. In contrast, the PKC inhibitor, Ro 31-8220, did not affect this response. 3. Histamine (100 microM) or bradykinin (100 nM) evoked rapid increases in inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) mass accumulations (maximal accumulations within 10 s and 30 s, respectively) which declined towards basal values over a 10 min incubation period. TPA (1 microM) significantly attenuated the peak Ins(1,4,5)P3 response to bradykinin and histamine by 30% and 70% respectively. In contrast, TPA did not significantly affect agonist-stimulated Ins(1,3,4,5)P4 responses. 4. Ro 31-8220 (10 microM) significantly enhanced the maximal Ins(1,4,5)P3 accumulations elicited by both bradykinin and histamine. 5. The results indicate that the initial Ins(1,4,5)P3 response to either bradykinin or histamine in bovine adrenal chromaffin cells can be attenuated by PKC activation by phorbol ester and enhanced by PKC inhibition by Ro 31-8220. In contrast, agonist-stimulated Ins(1,3,4,5)P4 accumulation does not appear to be affected by these manipulations of PKC activity. Possible bases for differential modulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 are discussed.

Inositol 1,4,5-trisphosphate receptor immunoreactivity in SH-SY5Y human neuroblastoma cells is reduced by chronic muscarinic receptor activation

Inositol 1,4,5-trisphosphate (InsP3) receptor immunoreactivity in SH-SY5Y human neuroblastoma cells was monitored with a monoclonal antibody raised against the mouse cerebellar InsP3 receptor. Recognition of a protein corresponding to the InsP3 receptor (molecular mass, approximately 275 kDa) was inhibited markedly following exposure of cells to 0.1 mM carbachol. This effect was half-maximal and maximal at approximately 2 and approximately 6 h, respectively; was blocked by atropine; but was not mimicked by thapsigargin, K+, or phorbol 12-myristate 13-acetate. However, the decrease in immunoreactivity following exposure of cells to carbachol for 5 h was blocked if the extracellular Ca2+ concentration was reduced from 1.3 mM to 200 nM. This manipulation also reduced markedly carbachol-induced increases in InsP3 concentration at 5 h. These data indicate that chronic muscarinic stimulation of phosphoinositide hydrolysis reduces InsP3 receptor concentration in SH-SY5Y cells, perhaps via a mechanism that involves prolonged elevation of InsP3 levels.

Expression cloning and characterization of the canine parietal cell gastrin receptor

Gastrin is an important stimulant of acid secretion by gastric parietal cells and is structurally related to the peptide hormone cholecystokinin (CCK). The pharmacologic properties of the parietal cell gastrin receptor are very similar to the predominant CCK receptor in the brain, CCK-B. Neither the gastrin nor the CCK-B receptor have been cloned thus far, making it difficult to resolve whether these two receptors are distinct. We have isolated a clone encoding the canine gastrin receptor by screening a parietal cell cDNA expression library using a radioligand-binding strategy. Nucleotide sequence analysis revealed an open reading frame encoding a 453-amino acid protein with seven putative hydrophobic transmembrane domains and significant homology with members of the beta-adrenergic family of G protein-coupled receptors. The expressed recombinant receptor shows the same binding specificity for gastrin/CCK agonists and antagonists as the canine parietal cell receptor. Gastrin-stimulated phosphatidylinositol hydrolysis and intracellular Ca2+ mobilization in COS-7 cells expressing the cloned receptor suggest second-messenger signaling through phospholipase C. Affinity labeling of the expressed receptor in COS-7 cells revealed a protein identical in size to the native parietal cell receptor. Gastrin receptor transcripts were identified by high-stringency RNA blot analysis in both parietal cells and cerebral cortex, suggesting that the gastrin and CCK-B receptors are either highly homologous or identical.