Effect of acid inhibition on the growth of parietal cells

Effect of short- and long-term treatment with omeprazole on cell cycle distribution in the gastric mucosa. Results of a flow cytometric study

Omeprazole may exert an effect on gastric mucosal proliferation by inhibiting gastric acid secretion and increasing serum gastrin levels. It may also influence the kinetics of endocrine cells and the oxyntic mucosa. The aim of the present study was to evaluate the cell cycle in different gastric compartments following short- (1 month) and long-term (6 months) administration of two different dosages of omeprazole by means of a flow cytometric method. We also determined serum gastrin levels at the same time. No differences in cell cycle distribution of the antrum, body, and fundus were found in the two different dosage groups after 1 month of therapy, considering the synthetic phase (S-phase) of the cell cycle. A statistically significant increase in S-phase was reported after long-term therapy in the mucosa of the fundus and body of the stomach in both groups. Gastrin levels showed no clear correlation with cell cycle distribution variables. We postulate a proliferative adaptation of the oxyntic mucosa to long-term drug administration not mediated by gastrin influence.

Effects on the rat oxyntic mucosa of the histamine2-antagonist loxtidine and the H+, K(+)-ATPase inhibitor omeprazole

The present study examined whether histamine could affect the growth of the enterochromaffin-like (ECL) cell and the parietal cell. The effects of the unsurmountable histamine H2-receptor antagonist loxtidine (80 mg/kg) and the H+, K(+)-ATPase inhibitor omeprazole (100 mumol/kg) were compared in female Sprague-Dawley rats. Both drugs were given by gavage once daily for 3 months. Omeprazole induced a more pronounced and sustained hypergastrinaemia than loxtidine. In spite of marked hypergastrinaemia during most of the day, even in the loxtidine-treated rats, the weights of the stomach and oxyntic mucosa were elevated only in the omeprazole-treated rats. The ECL cell density was slightly higher in the loxtidine- than in the omeprazole-treated rats. Both treatments elevated the gastrin-stimulated histamine release from the vascularly perfused stomach. The parietal cell density was unaffected by omeprazole treatment, whereas it tended to be reduced in the loxtidine-treated rats. Simultaneous administration of loxtidine and omeprazole reduced the sustained hypergastrinaemia induced by omeprazole given alone. The present study may indicate that histamine inhibits the growth of the ECL cell, but further studies are needed to elucidate if histamine has any trophic effect on the parietal cells.

Acid secretory changes and early relapse following duodenal ulcer healing with ranitidine or sucralfate

Controversial aspects relating to acid secretory changes following duodenal ulcer healing, and the effect of histamine-2 (H2)-receptor blockers on such changes are discussed. Recent studies comparing the effects of ranitidine or sucralfate on changes in the acid secretory responses in the healing of duodenal ulcers are described, and the relevance of these changes to early relapse considered. The duration of therapy and timing of acid-secretory studies and endoscopies are standardized in all studies, but the acid-secretory parameters vary. These comprised acid secretory responses to graded doses of an intravenous infusion of pentagastrin or histamine, sham-feeding responses, and basal and nocturnal acid secretion. In all studies, healing with sucralfate was associated with a significant decrease in the various parameters of acid secretion. In the ranitidine-healed groups, no appreciable decrease was noted and there was, indeed, a significant increase in nocturnal acid secretion. Routine endoscopies performed 4-6 weeks after documented ulcer healing showed a recurrence of 3 of the 32 (9%) sucralfate-healed and in 14 of the 35 (40%) ranitidine-healed ulcers. Increased acid secretory responsiveness following duodenal ulcer healing was strongly correlated with subsequent early relapse.

Aspects of the role of prostaglandins in gastrin histamine regulation of gastric acid secretion

There is convincing evidence from in vitro studies that prostaglandins interfere with the gastrin histamine regulation of gastric acid secretion in an opposing manner. They can inhibit the action of histamine on the parietal cell when given as single-dose treatment. Conversely, when given at high doses, they liberate endogenous histamine, probably from nonparietal cells. In in vivo experiments prostaglandins are potent inhibitors of acid secretion when studied with single-dose treatment but are capable of stimulating basal acid secretion when chronically applied to rats. Studies performed in whole animal preparations on the effect of cyclooxygenase inhibitors have not fully clarified which of these two histamine-modulating effects of prostaglandins is of prime physiologic significance but favour suppression of histamine activity with subsequent acid inhibition. It appears that somatostatin release is the mechanism through which prostaglandin can simultaneously inhibit acid secretion and release plasma gastrin. At least in the rat, the pattern of plasma gastrin and somatostatin release is, similarly to acid secretion, partly reversed during prolonged prostaglandin treatment.

Trophic effect of histamine on the stomach

On the basis of clinical observations and experimental animal studies it has been established that gastrin has a trophic effect on the oxyntic mucosa. On the other hand, histamine, being at least as efficient as gastrin as an acid secretagogue, has experimentally been reported not to have such trophic effect. However, during the last few years both endogenously and exogenously induced hypergastremia have been shown to have a specific trophic effect on the enterochromaffin-like (ECL) cell and a less pronounced and later detectable general trophic effect on the oxyntic mucosa. Moreover, in the rat (the species in which most of the trophic studies have been done) the acid-stimulatory effect of gastrin may be solely explained by stimulation of histamine release from ECL cells. Therefore, it seemed natural to evaluate whether the general trophic effect of gastrin could also be caused by histamine or another substance released from the ECL cells. In this review we challenge the concept that maximal pentagastrin-stimulated acid secretion only reflects the parietal cell mass, since the acid-stimulatory effect of gastrin is mediated by histamine release. Therefore, maximal pentagastrin-stimulated acid secretion reflects both the ECL cell mass and the parietal cell mass. With regard to the possible trophic effect of histamine, we show that the doses previously used have been inadequate. Furthermore, histamine has been reported to have a trophic effect on the parietal cell in the dog; some patients with hyperhistaminemia have an increased maximal histamine-stimulated acid secretion, suggesting an increase in the parietal cell mass; and there is parietal cell hyperplasia in the oxyntic mucosa surrounding the histamine-producing carcinoids in mastomys.(ABSTRACT TRUNCATED AT 250 WORDS)

Therapeutic achlorhydria and risk of gastric cancer

New, powerful gastric secretory inhibitors, such as omeprazole, produce gastric cancer in rats. The mechanism by which the drugs elicit gastric carcinogenesis is considered to depend on the production of therapeutic achlorhydria, with subsequent release in to the circulation of peptides (such as gastrin) which are trophic to the gastric mucosa. It has been argued that the drugs do not pose a carcinogenic risk to man because the neoplastic response to gastric inhibitors in rats is a reaction to a 'toxic' insult; or because rats and humans react differently to the drugs; or because the mechanisms of gastric carcinogenesis are different in the two species. In any case, since most of the powerful gastric secretory inhibitors produce carcinoid tumours in rats, and carcinoid tumours of the human stomach are rare and largely benign, there would be no risk even if the drugs did produce proliferative abnormalities of the human stomach. Not one of the above hypotheses has been confirmed or, indeed, even satisfactorily tested. The mechanisms of the drug-induced gastric carcinogenesis in rats has not been defined and consequently it is not even possible to attempt to guess the risk to man. Until information is available about the effects of the powerful gastric secretory inhibitors on the proliferative indices and patterns of the human gastric mucosa, the drugs must be categorized as too dangerous to use therapeutically, especially since the proposed therapeutic benefits are minimal.

Omeprazole: its influence on gastric acid secretion, gastrin and ECL cells

The H+,K+-ATPase inhibitor omeprazole is a highly effective gastric antisecretory agent, both in animals and man, with a long duration of action. These properties are shared by a number of recently described histamine H2-receptor antagonists. In life-long oncogenicity studies of these H2-receptor antagonists, as well as with the H+,K+-ATPase inhibitor omeprazole, gastric enterochromaffin-like cell (ECL cell) hyperplasia and carcinoids have been found. The purpose of this paper is to summarize available evidence for the "Gastrin Hypothesis" to explain the development of ECL-cell hyperplasia. The hypothesis may be outlined as follows: 1) Inhibition of gastric acid secretion leads to elevated antral pH and, secondarily, to release of gastrin from the antral gastrin cells into the blood stream. 2) Gastrin causes both general hypertrophy of the oxyntic mucosa and hyperplasia of the ECL cells in the oxyntic mucosa. That this sequence of events occurs not only with omeprazole but also with other effective gastric antisecretory agents has been verified in the rat by giving the H2-receptor antagonist ranitidine as a continuous infusion. Ranitidine caused a hypergastrinemia of a similar magnitude as that seen after omeprazole, provided that the acid secretion was inhibited to a similar degree. At similar gastrin levels, ECL-cell hyperplasia of the same magnitude developed during both ranitidine and omeprazole treatment. Antrectomy prevented the development of ECL-cell hyperplasia during omeprazole treatment, indicating that the hyperplasia was not due to the drug treatment per se, but rather to the hypergastrinemia. Both the hypergastrinemia and the ECL-cell hyperplasia were found to be reversible.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical relevance of mucosal protection

An inquiry was made among 70 of the 76 participants of an international symposium entitled 'Future Directions in Peptic Ulcer Research', held in Bellagio, Italy, in April 1986. There was almost unanimous agreement that research in the development of protective drugs should be intensified. The participants felt that in future studies the prevention of ulcer relapse, concomitant administration with potentially ulcerogenic drugs, prevention of stress ulceration, and treatment of ulcer hemorrhage should be emphasized rather than the healing of uncomplicated ulcers. They were not generally satisfied with the performance of available prostaglandin analogues in ulcer treatment. The moderate commercial success of sucralfate was attributed mainly to the complicated mode of intake, but also to the unknown mechanism of action of this substance. It was felt likely that antacids have a clinically relevant protective effect on gastroduodenal mucosa.