Adenylate cyclase in human gastric mucosa: its activation by histamine in morphologically different biopsy specimens

Changes in acid secretion over the years. A 30-year longitudinal study

We studied the effect of aging on gastric acid secretion in 11 physicians who had augmented histamine tests while at medical school in 1962. One of them had a duodenal ulcer at the time. The augmented histamine test was repeated in 1991 and, in addition, upper gastrointestinal endoscopy was done to exclude peptic ulcer and to obtain biopsies for histologic analysis and assessment of Helicobacter pylori status. The mean basal acid output decreased from 7.3 to 1.9 mEq/hr during the 30-year period of follow-up (p < 0.001), and the mean maximum acid output decreased from 29.9 to 20.3 mEq/hr (p < 0.01). The maximum acid output data showed a profound decrease in 4 of the 11 participants, a lesser decrease in 4, and a minimal increase in the remaining 3. Histologic analysis suggested a greater likelihood of atrophic gastritis, H. pylori infection, or both in participants showing a pronounced decrease in acid secretion with aging.

Protein kinase C inhibits stimulation of adenylate cyclase by the histamine H2 receptor in rat parietal cells

The action of protein kinase C on the stimulation of adenylate cyclase activity by the histamine H2 receptor was investigated in rat parietal cells. Protein kinase C was activated by preincubating cells with 12-O-tetradecanoylphorbol 13-acetate (TPA), and adenylate cyclase activity was measured in sonicated extracts. TPA (100 nM) inhibited adenylate cyclase activity stimulated by histamine (100 nM-500 microM). This effect was related to the concentration of TPA. TPA (100 nM) enhanced the stimulation of adenylate cyclase activity by forskolin (100 microM) but had no effect on the stimulation by NaF (10 mM). In conclusion, protein kinase C inhibits stimulation of adenylate cyclase by the histamine H2 receptor. This action could be mediated by changes in the number of affinity of histamine H2 receptors or in the coupling of the receptor to the stimulatory guanine nucleotide regulatory subunit Gs.

Cyclic AMP and gastric secretion in man. An in vivo study in healthy volunteers, duodenal ulcer patients, and pernicious anaemia patients

The metabolism of cyclic AMP and HCl secretion has been studied in eight healthy volunteers, in eight duodenal ulcer (DU) patients, and in four pernicious anaemia patients. Pentagastrin showed a tendency to increase adenylate cyclase and cyclic nucleotide phosphodiesterase activities in the fundal mucosa and caused a significant increase in cyclic AMP output into the gastric juice in healthy volunteers and in DU patients. Cimetidine inhibited all these events but had no effect on basal cyclic AMP output. Vagotomy significantly inhibited basal cyclic AMP output. We conclude that cyclic AMP is involved both in basal and in pentagastrin-stimulated gastric secretion in man. Basal secretion is mainly controlled by vagal tone. The main pathway for this stimulus at the parietal cell may be via other than H2-receptors, probably through acetylcholinergic receptors. However, a significant part of the pentagastrin stimulation of the human parietal cell is via H2-receptors.

Effects of hormones (calcitonin, GIP) and pharmacological antagonists (ranitidine and famotidine) on isolated rat parietal cells

The rationale for the present study was to compare calcitonin and gastric inhibitory polypeptide (GIP) versus two histamine H2 receptor antagonists with respect to their potency of inhibiting parietal cell functions. Adenylate cyclase activity and acid production ([14C]aminopyrine uptake) of isolated rat parietal cells were stimulated by histamine. At 10(-7) and 10(-6) mol/l, calcitonin and GIP reduced the response to histamine by 10-20% following noncompetitive kinetics. Ranitidine and famotidine (MK 208) inhibited the response to histamine by about 50% at 10(-7)-10(-6) mol/l, and at 10(-5) mol/l abolished the histamine effect. On a molar basis famotidine turned out to be 6 times more potent than ranitidine. Both antagonists revealed competitive kinetics. Our data suggest direct inhibition of the parietal cells by the tested compounds which were shown to interfere at the adenylate cyclase cAMP system or at the histamine H2 receptor. However, compared to the histamine H2 receptor antagonists, hormonal inhibition is less pronounced and mediated by a different mechanism.

Adenylate cyclase and H+ production of isolated rat parietal cells in response to glucagon and histamine

The effect of glucagon and its interaction with histamine on adenylate cyclase (AC), cellular cAMP and [14C]aminopyrine ( [14C]AP) uptake, a reliable index of parietal cell H+ production, was studied in isolated rat gastric cells. AC activation in response to glucagon and histamine correlated with the number of parietal cells. Glucagon (10(-10)-10(-6) mol/l) increasingly stimulated AC (maximal effect: 92% by 10(-7) mol/l) and cellular cAMP (86% by 10(-9) mol/l) of fractions enriched with 80% parietal cells but did not cause a pronounced change of the histamine-stimulated enzyme. If there was any interaction, the effect of both hormones was additive. Glucagon neither changed basal [14C]AP uptake nor interfered with that in response to histamine. The data suggest that if glucagon activates a parietal cell AC this process is not followed by parietal cell H+ production. Furthermore, unlike other inhibitors such as somatostatin or PGE2, glucagon does not reduce acid secretion via the cAMP system of the parietal cell.

Comparison of the effect of PGE2 and somatostatin on histamine stimulated 14C-aminopyrine uptake and cyclic AMP formation in isolated rat gastric mucosal cells

In isolated rat gastric cells somatostatin and PGE2 were compared in respect to their effects on the cAMP system and on the histamine-stimulated H+-production, measured by 14C-aminopyrine (14C-AP) uptake. Like PGE2 somatostatin activated adenylate cyclase (AC) for all in non-parietal cells. This effect on AC declined in cell fractions with increasing number of parietal cells. Activation of AC or elevation of cellular cAMP and uptake of 14C-AP in response to histamine were inhibited by 10(-9) to 10(-5) mol/1 PGE2 and somatostatin. The results indicate remarkable similarity between somatostatin and PGE2: both activate a non-parietal cell AC and both inhibit H+-production, likely by interfering at the histamine sensitive AC of the parietal cell.

Adrenergic stimulation of isolated rat gastric mucosal cells. Effect on adenylate cyclase and 14C-aminopyrine uptake

Adrenergic stimulation of the adenylate cyclase (AC)-cAMP-system and 14C-aminopyrine accumulation, an indirect measure of parietal cell H+-production, was studied in a different preparations of gastric mucosal cells. The beta 2-adrenoceptor agonist hexoprenaline activated AC of crude homogenates from the gastric corpus of mouse, rat, guinea-pig, hog, dog and man. In isolated rat gastric cells (20% parietal cells), treated by low power sonication, 10(-8) to 10(-3) mol/l adrenaline and hexoprenaline activated AC equally potently and efficaciously by maximally 170%. Isoprenaline proved to be less effective activating up to 80%. 5.10(-5) mol/l GMP-PNP augmented basal activity 8.5 times and reduced the maximal efficacy. Adrenaline and hexoprenaline activated AC by maximally 120%, isoprenaline by 40%. The potency of adrenaline was 4 times lower, that of hexoprenaline 2 and that of isoprenaline 4 times higher in the presence of GMP-PNP. Adrenergic stimulation was inhibited by the beta-adrenoceptor antagonist propranolol, the effect of alpha-adrenoceptor-blockade by phenoxybenzamine was less pronounced. In fractions with 7-80% of parietal cells, prepared by isopycnic centrifugation with Percoll, adrenaline and hexoprenaline activated AC or hexoprenaline enhanced the cellular levels of cAMP in parietal cell poor and rich fractions. The degree of activation in response to histamine correlated with the number of parietal cells. 14C-Aminopyrine uptake was increasingly stimulated through 10(-8) to 10(-5) mol/l hexoprenaline, maximally by doubling the basal accumulation. 10(-4) mol/l histamine was 8 times more effective. 3.10(-7) mol/l propranolol inhibited the effect of 10(-5) mol/l hexoprenaline by 80%. The data suggest the localization of beta-adrenoceptors (likely beta 2-adrenoceptors) on parietal and other nonidentified gastric cells. At the parietal cell, adrenaline and hexoprenaline initiate activation of AC and hexoprenaline leads to H+-production. The responses are small compared to the effect of histamine. Thus, beta-adrenoceptor agonists exert intrinsic activity in relation to H+-production. Their influence on stimulated secretion of isolated cells remains to be elucidated.

Adenylate cyclase activity in gastric mucosal biopsies and cAMP in gastric juice before and after cimetidine treatment in healthy subjects

Previous studies have revealed augmented parietal call sensitivity to histamine stimulation after cessation of cimetidine treatment in healthy subjects. To elucidate one possible mechanism for this effect, we measured cAMP in gastric mucosa and gastric juice during histamine stimulation in vivo and histamine-stimulated adenylate cyclase activity in vitro in biopsy material obtained by gastroscopy. In eight healthy subjects the cAMP content of gastric corpus mucosal biopsies was measured before, during, and after infusion of 2.5 and 25.6 microgram . kg-1 . h-1 of histamine dihydrochloride. No increase in cAMP content was found. In 10 healthy subjects cAMP secretion into gastric juice was studied before and 36 to 84 h after cessation of 4 weeks' treatment with cimetidine, 1 g/day. No significant increase of cAMP output was found in response to infusion of 2.5 and 25.6 microgram . kg-1 . h-1 of histamine dihydrochloride either before or after the treatment period. Thus histamine does not stimulate cAMP secretion into gastric juice in man. In the same 10 subjects histamine-stimulated adenylate cyclase activity was studied in vitro in biopsy material obtained before and 60 h after cessation of cimetidine treatment. The dose-response curve of the adenylate cyclase activity was not shifted to the left. The results indicate that the augmented parietal cell sensitivity to histamine, seen after stopping cimetidine treatment in healthy subjects, is not due to increased sensitivity of the adenylate cyclase-cAMP system to histamine stimulation.