Pharmacological aspects of acid secretion.

The secretion of gastric acid is regulated both centrally and peripherally. The finding that H2-receptor antagonists are able to reduce or abolish acid secretion due to vagal, gastrinergic, and histaminergic stimulation shows that histamine plays a pivotal role in stimulation of the parietal cell. In the rat, the fundic histamine is released from the ECL cell, in response to gastrin, acetylcholine, or epinephrine, and histamine release is inhibited by somatostatin or by the H3-receptor ligand, R-alpha-methyl histamine.

The pharmacology of the gastric acid pump: the H+,K+ ATPase.

The gastric H+,K+ ATPase--the gastric acid pump--is the molecular target for the class of antisecretory drugs called the proton-pump inhibitors (PPIs). These compounds--omeprazole, lansoprazole, and pantoprazole--contain, as their core structure, 2-pyridyl methylsulfinyl benzimidazole. The H+,K+ ATPase is a heterodimer composed of a 1034-amino acid catalytic alpha peptide and a glycosylated 291-amino acid beta subunit.

Characterization of proton transport in bone-derived membrane vesicles.

ATP-dependent proton transport in membrane vesicles prepared from the medullary bone of egg-laying hens, a source rich in osteoclasts, was characterized. Proton transport was abolished by bafilomycin A1 (10 nM) and N-ethylmalemide (50 microM), but not by oligomycin (15 micrograms/ml), vanadate (100 microM) or SCH 28080 (100 microM), thereby differentiating this H(+)-ATPase from the F1F0- and phosphorylated-type of ATPases. Preincubation of the membrane vesicles at 0 degrees C for 1 h in the presence of KCl (0.

Characterization of proton transport in bone-derived membrane vesicles.

ATP-dependent proton transport was characterized in membrane vesicles, prepared by differential centrifugation from medullary bone of egg laying hens, a source rich in osteoclasts. The H(+)-ATPase present in this preparation showed the characteristics of a vacuolar H(+)-ATPase in its sensitivity to inhibitors, including bafilomycin A1, its sensitivity to cold treatment and its electrogenic mechanism. There was no evidence for a direct activation of the H(+)-ATPase by anions, including Cl-.

Omeprazole and bafilomycin, two proton pump inhibitors: differentiation of their effects on gastric, kidney and bone H(+)-translocating ATPases.

The effects of omeprazole and bafilomycin on processes dependent on two different types of H(+)-translocating ATPases were compared. A H(+)-ATPase of the E1E2-type, the H+,K(+)-ATPase, was purified from gastric mucosa. Vacuolar type H(+)-ATPases were prepared both from kidney medulla and from osteoclast-containing medullary bone.

Antiulcer agents. 5. Inhibition of gastric H+/K(+)-ATPase by substituted imidazo[1,2-a]pyridines and related analogues and its implication in modeling the high affinity potassium ion binding site of the gastric proton pump enzyme.

A number of substituted imidazo[1,2-a]pyridines and related analogues were selected for biochemical characterization in vitro against both the purified gastric proton pump enzyme, H+/K(+)-ATPase, and the intact gastric gland. The inhibitory activity in these two in vitro models was then examined for correlation with the gastric antisecretory potency determined for these compounds in vivo by using the histamine-stimulated Heidenhain pouch dog. Analysis of the biological data suggested that the inhibitory activity of the analogues determined in two in vitro models is predictive of their in vivo gastric antisecretory activity following intravenous, but not oral, administration.

Changes induced in growing rat bone by immobilization and remobilization.

We studied changes in bone mass and histology in growing rats after different relatively short periods of immobilization and during subsequent remobilization. Immobilization-induced loss of bone weight is mainly due to mineral losses as indicated by changes in wet weight, ash weight, and calcium content. 45Ca2+ incorporation was found to be decreased in immobilized bones and showed strong dependence upon the age of the rats.

Partial pronase digestion of rat gastric mucosa isolates cells undergoing replicative DNA synthesis.

A pronase digestion procedure for the isolation of gastric mucosal cells was evaluated for its usefulness in measuring unscheduled DNA synthesis (UDS). The method has been claimed to be suited for assessing the genotoxicity potential of compounds. Compounds were given orally to rats.

Evidence for the presence of a proton pump of the vacuolar H(+)-ATPase type in the ruffled borders of osteoclasts.

Microsomal membrane vesicles prepared either from chicken medullary bone or isolated osteoclasts were shown to have ATP-dependent H(+)-transport activity. This activity was N-ethylmaleimide-sensitive but resistant to oligomycin and orthovanadate, suggesting a vacuolar-type ATPase. Furthermore, immunological cross-reactivity of 60- and 70-kD osteoclast membrane antigens with Neurospora crassa vacuolar ATPase was observed when analyzed by immunoblotting.

Coupling of H(+)-K(+)-ATPase activity and glucose oxidation in gastric glands.

The production of 14CO2 from uniformly labeled glucose was shown to account for the entire increase in histamine-stimulated O2 consumption in rabbit gastric glands when no other substrate was added to the medium. The increased production of CO2 was correlated to the increase in O2 consumption and the accumulation of [14C]-aminopyrine (AP) after stimulation with several secretagogues. Inhibitors of H(+)-K(+)-ATPase reduced the secretagogue-induced increase in CO2 production by greater than 90%, showing that the activity of this enzyme was responsible for the greater part of gastric gland metabolism under stimulated conditions.

Inhibition of osteoclast proton transport by bafilomycin A1 abolishes bone resorption.

Osteoclasts are the main bone resorbing cells with capacity to acidify their intimate contact area with bone. Recent studies have suggested that osteoclast acid secretion is carried out by an H(+)-ATPase. We demonstrate here, that specific inhibitor of vacuolar type H(+)-ATPases, bafilomycin A1, inhibits bone resorption in osteoclast cultures as well as blocks proton transport in isolated medullary bone derived microsomes containing a vacuolar type H(+)-ATPase.

Effect of 20 weeks' ranitidine treatment on plasma gastrin levels and gastric enterochromaffin-like cell density in the rat.

In this study, the effect of ranitidine treatment on the activation and proliferation of rat gastric enterochromaffin-like (ECL) cells was investigated. The drug was given in a high dose in the food (1.7-1.

The gastric H+,K(+)-ATPase.

Mammalian extramitochondrial pumps can be divided into two different classes: the vacuolar H(+)-ATPases, which are responsible for acidification of intracellular compartments, and the E1E2-type of ATPases, which are represented by the Na+,K(+)-ATPase, the Ca2(+)-ATPase and the gastric H+,K(+)-ATPase. The latter enzyme is confined to the tubulovesicles and to the secretory membranes of the parietal cell and has been shown to be the proton pump of the gastric mucosa. The H+,K(+)-ATPase carries out the electroneutral exchange of H+ and K+ and thereby generates a pH of less than 1 in the secretory canaliculus.

Tritiated thymidine incorporation into DNA in rat gastric mucosal cells.

The parietal cell acid pump inhibitor, omeprazole, has undergone numerous genotoxicity studies, the conclusions of which have all been negative. A recent report by Burlinson described a method which is claimed to measure unscheduled DNA synthesis (UDS) in gastric mucosa, based on the incorporation of tritiated thymidine (3H-TdR) into DNA. In a subsequent letter it was concluded that omeprazole induces a significant increase in UDS, and it was suggested that 'for omeprazole, a genotoxic action cannot be discounted'.

Omeprazole: mode of action and effect on acid secretion in animals.

Stomach acid is generated by the parietal cell of the gastric mucosa. During the last two decades an enzyme called H+,K+-ATPase, has been discovered and shown to be the agent responsible for acid transport in the parietal cell. H+,K+-ATPase is thus the hydrogen ion pump of the stomach.

Biological basis of omeprazole therapy.

There are two means of reducing acid secretion. The best studied is inhibition of stimulation of the parietal cell. There are three major types of receptors that activate secretion by this cell and two classes of receptor antagonists, as well as at least two intracellular messenger pathways.

The gastric H+,K+-ATPase: the site of action of omeprazole.

The mammalian parietal cell is dedicated to the secretion of HCl in response to various stimuli and second messengers. Oxidative metabolism in the cell increases about 10-fold in order to supply ATP to the gastric proton pump, the H+,K+-ATPase. This pump appears to be present only in the parietal cell.

Omeprazole: mode of action and effect on acid secretion in animals.

H+,K+-ATPase constitutes the final step in the acid secretory process that takes place in the parietal cell, and this enzyme has recently been recognized as a target for inhibitors of acid secretion. The gastric H+,K+-ATPase is located in the uniquely acidic environment of the parietal cell. Omeprazole, a weak base, is concentrated within the acid canaliculus of the parietal cell and is rapidly converted to an inhibitor of the H+,K+-ATPase in the acid compartments of the parietal cell.

Inhibition of gastric H+/K+-ATPase by substituted imidazo[1,2-a]pyridines.

A hydrophobic imidazopyridine, SCH 28080 (3-cyanomethyl-2-methyl-8-phenylmethoxy)imidazo[1,2-a]pyridine) has previously been shown to inhibit gastric acid secretion in vivo and in vitro. Studies of isolated gastric H+/K+-ATPase have demonstrated that SCH 28080 reversibly inhibited the enzyme and competitively interacted with the K+-stimulated ATPase and p-nitrophenylphosphatase activities of the H+/K+-ATPase. To elucidate the mechanism of inhibition further, for example to establish whether the inhibitor interaction occurs on the luminal or the cytosolic side of the enzyme or if compound pKa influences inhibition, SCH 28080 and three analogues have been studied.

Time-course of development and reversal of gastric endocrine cell hyperplasia after inhibition of acid secretion. Studies with omeprazole and ranitidine in intact and antrectomized rats.

In intact rats plasma gastrin levels were increased during a 20-wk treatment course with either omeprazole or ranitidine. Although plasma gastrin levels were the same during treatment, the enterochromaffinlike (ECL) cell density increased approximately linearly with time at a rate correlated to the plasma gastrin level. Antrectomy prevented the ECL cell hyperplasia seen in omeprazole-treated rats, suggesting that it was not caused by omeprazole per se.

Effects of some antisecretory drugs on acid production, intracellular free Ca2+, and cyclic AMP production in isolated pig parietal cells.

The effects of some inhibitors of acid secretion were tested on isolated, purified pig parietal cells. The cells were stimulated with 10(-4) M histamine, 10(-5) M carbachol, or 10(-7) M pentagastrin. The H,K-ATPase inhibitors SCH 28080 and omeprazole inhibited both the basal and secretagogue-stimulated acid production, as measured by aminopyrine accumulation, irrespective of the type of stimulator used.

Inhibitory effects of cations on the gastric H+, K+ -ATPase. A potential-sensitive step in the K+ limb of the pump cycle.

The presence of a cation inhibitory site on the dephosphoform of the H+, K+ -ATPase was confirmed by comparing the effects of K+ and NH4+ on overall activity and on phosphorylation and dephosphorylation. Inhibition of ATPase activity was pronounced at high cation/ATP ratios, but NH4+ was much less effective. At 60 mM cation, although the ATPase activity was greater in the presence of NH4+ (17.