Function and structure of parietal cells after H+-K+-ATPase blockade.

Omeprazole was administered to rabbits as a single dose or daily for 1 wk. H+-K+-ATPase and isolated gastric glands were prepared from the oxyntic corpus mucosa and used for functional and quantitative morphological studies. Both 10 and 100 mumol omeprazole/kg increased the pH of the gastric content when measured at death. The stimulated oxygen uptake and the rate of aminopyrine (AP) uptake were both inhibited in the isolated gastric gland preparations. Morphometric studies of biopsy specimens taken from the corpus mucosa and isolated gastric glands showed that omeprazole treatment increased the volume density of the acid compartments. This expansion provides an increased accumulation space for AP. Therefore, an increased AP uptake might be seen in glands isolated from omeprazole-treated animals. Blockade of the H2-receptor by ranitidine transformed the morphology of the cell into a more resting type and, furthermore, reduced the omeprazole-induced increase in the volume density of the acid compartments in the parietal cell. The H+-K+-ATPase activity measured in membrane fractions from the omeprazole-treated animals was decreased dose dependently and inhibited by 95% after 100 mumol omeprazole/kg. However, the concentration of the enzyme in these fractions did not change. These results indicate a specific inhibitory action of omeprazole on the H+-K+-ATPase.

Histamine H2-receptor of human and rabbit parietal cells.

The histamine H2-receptor on the human parietal cell has been characterized by using dose-response curves and the negative logarithm of the molar concentration of an antagonist (pA2) analyses of cimetidine antagonism of betazole, histamine, and impromidine stimulation in isolated human and rabbit gastric glands. To evaluate the in vitro results, betazole-stimulated gastric acid secretion with and without cimetidine was also studied in healthy subjects. In the in vivo model, individual dose-response curves were shifted to the right with increasing cimetidine concentrations, but this was counteracted by increasing betazole doses, indicating competitive, reversible antagonism. The pA2 values ranged from 6.1 to 6.3. In isolated human gastric glands, impromidine was shown to be eight times more potent than histamine, indicating higher receptor affinity, but the maximally stimulated aminopyrine accumulation was the same as for histamine, and the pA2 values for cimetidine antagonism did not differ significantly, i.e., 5.7 (histamine) and 6.1 (impromidine). In isolated rabbit gastric glands, cimetidine inhibited the histamine- and impromidine-stimulated response with pA2 values of 6.0 and 7.3, respectively. Impromidine was shown to be approximately 100 times more potent than in human gastric glands, whereas histamine had the same potency. This confirms the role of the histamine H2-receptor and suggests a difference between the species concerning receptor affinity.

Inhibitory action of omeprazole on acid formation in gastric glands and on H+,K+-ATPase isolated from human gastric mucosa.

The inhibitory effect of omeprazole on acid formation has been studied in vitro in gastric glands and partly purified H+,K+-ATPase, prepared from mucosa obtained either from healthy subjects by gastroscopic biopsy or from gastric ulcer patients during antrectomy. The effect of omeprazole was compared with the inhibitory pattern of the H2-antagonist cimetidine. Acid production in the glands was determined by measuring the accumulation of 14C-aminopyrine. In glands isolated from patients, omeprazole inhibited acid production maximally stimulated by histamine, db-cAMP, and potassium in a dose-dependent manner, with an IC50 value of about 50 nM irrespective of the agonist used. In contrast, cimetidine inhibited only histamine-induced aminopyrine accumulation, with an IC50 of about 30 micron. The inhibitory effect of omeprazole in db-cAMP-stimulated glands from healthy volunteers was of the same magnitude as seen in glands from gastric ulcer patients. Basal aminopyrine accumulation in glands from both patients and healthy volunteers was almost totally inhibited by omeprazole, whereas cimetidine was without effect. Omeprazole also concentration-dependently inhibited the H+,K+-ATPase activity in isolated gastric membrane vesicles. The estimated IC50 value was 4 micron.

Acid secretion in isolated gastric glands from healthy subjects and ulcer patients.

Isolated human gastric glands provide an in vitro model that can yield significant information about the mechanisms regulating gastric acid secretion at the parietal cell level. Aminopyrine, a weak base that accumulates in acid compartments, has been used as an indirect probe of H+ secretion. By means of a microscale technique it was possible to isolate oxyntic glands from gastroscopic biopsy specimens and thereby enable studies of healthy subjects and non-operated ulcer patients. Histamine (5.4 X 10(-5) M) and db-cAMP (10(-3) M) both induced a pronounced response, whereas the response to carbachol (4.5 X 10(-6) M), although still statistically significant, was less potent. The response to stimuli was twice as high in duodenal ulcer patients as in normal individuals. In contrast, the response in patients with a gastric ulcer located either in the prepyloric region or at the minor curvature on the antrum-corpus border was of the same magnitude as in healthy subjects. Pentagastrin did not induce any response in isolated gastric glands from normal individuals. Gastric acid secretion in vitro, measured as aminopyrine accumulation, did not decrease with increasing age of the individuals.

Effects of proximal gastric vagotomy and antrectomy on parietal cell function in humans.

Both proximal gastric vagotomy and antrectomy reduce maximal gastric acid secretion in vivo by about 60%. The combination of vagotomy and antrectomy reduces the maximal acid secretion by about 80%. This additive effect indicates that these surgical procedures differ in their mode of action. The function of isolated human oxyntic glands was studied before and after vagotomy and antrectomy, respectively, using radioactively labeled aminopyrine as a marker of parietal cell response. The basal accumulation increased after vagotomy, suggesting a vagally controlled inhibitory component. The carbachol response disappeared and the maximal response induced by histamine or dibutyryl-cyclic adenosine monophosphate was reduced by 60% (p less than 0.01) after vagotomy. This reduction could not be overcome by increasing the dose of dibutyryl-cyclic adenosine monophosphate. This indicates an intracellular effect of vagotomy peripheral to dibutyryl-cyclic adenosine monophosphate point of action. Antrectomy did not induce any statistically significant change at the glandular level, indicating that the reduced gastric acid secretion in vivo may be caused by a reduction in the number of oxyntic glands due to a removal of a trophic effect of antral gastrin.

Inhibition of acid secretion in isolated gastric glands by substituted benzimidazoles.

A new class of gastric acid inhibitors, substituted benzimidazoles (H 83/69 and H 149/94), have been tested in an isolated rabbit gastric gland preparation. Acid formation in the glands was stimulated by histamine, dibutyryl cAMP (DBcAMP), and high extracellular K+ concentrations, and the glandular secretory response was measured by changes in oxygen consumption and in accumulation of the weak base [14C]aminopyrine (AP). The substituted benzimidazoles inhibited AP accumulation induced by all stimulants in a dose-dependent noncompetitive manner. In contrast, cimetidine only inhibited histamine-induced AP accumulation. Basal AP accumulation, not affected by cimetidine, was also inhibited by the substituted benzimidazoles, as was the increase in glandular oxygen consumption produced by the addition of histamine and DBcAMP. Basal oxygen consumption was inhibited by about 15%. The substituted benzimidazoles, like AP, are weak bases and were also found to accumulate in the glands. Semiquantitative morphological studies of glands stimulated by histamine plus theophylline did not show any change in the enlarged secretory surface area after stimulation in the presence of inhibitory concentrations of H 149/94 (10(-4) M). The results suggest that substituted benzimidazoles have a mechanism of action different from that of H2-receptor antagonists and indicate a very distal site of action in the events leading to acid formation.

The effects of secretagogues of isolated human gastric glands.

The function of isolated human gastric glands has been studied in vitro by measuring the 14C-aminopyrine accumulation (RAP) in basal, unstimulated, conditions and after stimulation with different secretagogues. A microscale technique was used which enabled determinations of RAP in tissue obtained as gastroscopic biopsies. In addition, oxyntic-gland-containing mucosa was obtained at gastric resections for gastric or prepyloric ulcer disease. Histamine and cAMP derivative both induced maximal stimulation; RAP was approximately three times larger than in basal states. Carbachol induced a smaller but still significant stimulation. Pentagastrin did not increase RAP above the unstimulated level. Combinations of histamine and carbachol or pentagastrin did not induce a larger response than carbachol alone. The peak acid response to pentagastrin or betazole in vivo did not correlate with the maximum RAP in vitro.

Acid secreting gastric heterotopia in the duodenum.

Heterotopic gastric mucosa was found in the duodenum of a female patient with duodenitis and acid hyposecretion. The heterotopic isolated fundic glands were shown to accumulate 14C-aminopyrine in basal state and on stimulation with histamine suggesting that the heterotopic mucosa secreted acid. The ectopic acid secretion in the duodenal bulb suppressed gastric acid secretion, and might have caused the duodenitis and at least partly the clinical symptoms. The heterotopic gastric mucosa could be removed surgically by local excision. This procedure, combined with a Nissen fundoplication for suspected reflux oesophagitis, was followed by total symptomatic relief and normalization of gastric acid secretion.

Substituted benzimidazoles inhibit gastric acid secretion by blocking (H+ + K+)ATPase.

Studies both in vivo and in vitro have shown that substituted benzimidazoles inhibit the stimulation of acid secretion produced by dibutyryl cyclic AMP and histamine. Furthermore, the results differ from those produced by H2 antagonists and anticholinergic agents in that the inhibition is not competitive, and the site of action is intracellular and peripheral to that of dibutyryl cyclic AMP. To investigate the biochemical mechanism of action of substituted benzimidazoles, one such compound, H 149/94 (2-([2-(3-methyl)pyridyl-methyl]-sulphinyl)-5-methoxycarbonyl-6-methylbenzimidazol), has been tested either directly on an (H+ + K+)ATPase isolated from pig and human gastric mucosa or on the function of this enzyme in gastric glands isolated from rabbit and human gastric mucosa. (H+ + K+)ATPase, which has only been found at the secretory surface of the parietal cell, catalyses a one-to-one exchange of protons and potassium ions. It is possibly the proton pump within the gastric mucosa, and may thus be the terminal or one of the terminal steps of the acid secretory process. We show here that H 149/94 inhibits (H+ + K+)ATPase, which may explain its inhibitory action on acid secretion in vitro and in vivo. Because of the unique distribution and properties of the (H+ + K+)ATPase, the inhibitory action of H 149/94 on this enzyme may be a highly selective clinical means of suppressing the acid secretory process.