Possible relation of gastrin and histamine receptors in gastric hydrochloric acid secretion

Gastrin-histamine as a normal sequence in gastric acid stimulation in the rabbit

In a suspension of isolated gastric glands the effect of secretagogues on the oxyntic cells can only be detected by direct stimulation. An indirect stimulus like gastrin inducing one type of cell to liberate histamine which then acts on the oxyntic cells will not be detectable because of the very high dilution of the liberated substance. Thus the isolated gland preparation presents a means by which two steps in a sequential stimulation can be separated. There is no evidence that gastrin acts directly on the oxyntic cells but it does liberate histamine in a dose-effect relationship, which would in an intact stomach give histamine concentrations sufficient to effectively stimulate the acid secretion. Thus in the rabbit histamine seems to be a normal physiological mediator for gastrin stimulation.

The metabolism of [14C]histamine during pentagastrin-stimulated acid secretion in the cat

1. The metabolism of exogenous [14C]histamine has been examined in cats during gastric secretion. The appearance of 14C-labelled metabolites has been measured chromatographically in urine and gastric juice under non-secreting and pentagastrin-stimulated conditions, and in gastric mucosa after pentagastrin stimulation. 2. The main metabolites detected were (i) an acidic product, presumed to be methylimidazoleacetic acid, (ii) Ntau-methylhistamine, and (iii) an unidentified metabolite, moving rapidly in the chromatographic systems used, and designated 'preacetylhistamine'. No evidence was found for the presence of side chain (i.e. Nalpha)-methylated histamines, either in the non-stimulated or pentagastric-stimulated state. 3. During a 3 hr non-stimulated period, followed by 3 hr of pentagastrin administration, the relative proportions of 14C-labelled metabolites excreted in urine remained unchanged. During acid secretion there was an increase in the proportion of 'pre-acetylhistamine' in gastric juice, which occurred at the expense of the acidic metabolites. It is suggested however that this effect is more likely to be the result of a slow adaptation to the administration of exogenous [14C]histamine than the result of acid secretion. 4. The results of the present work do not appear to support the hypothesis of Code (1970), that a local control mechanism for acid secretion might involve the formation Nalpha-methylated histamine metabolites in gastric mucosa.

Gastric acid secretion

Advances in the understanding of physiologic control of gastric secretion raise the hope that an effective nonsurgical therapy for peptic ulcer disease will be developed soon. This paper reviews these advances with special emphasis upon those aspects of hormonal control, cyclic nucleotide function, and histamine activity which may have therapeutic applications.

Classification and biological distribution of histamine receptor sub-types

The distribution and classification of histamine receptors in mammalian and avian tissues have been summarized in Tables 1-4. It is evident that histamine receptors are present on a number of morphologically distinct cell types and the proportion of cells bearing H1- and H2-receptors varies not only with the species but also with the cell source. The pharmacological receptors mediating mepyramine-sensitive histamine responses have been defined as H1-receptors. Receptors mediating mepyramine-resistant, but burimamide or metiamide-sensitive histamine responses have been classified as H2-receptors. Histamine responses mediated via H2-receptors seem to involve the adenylcyclase system resulting in elevation of intracellular cyclic-AMP level, which is susceptible to burimamide blockade but insensitive to beta-adrenergic blocking agents. This mode of action of histamine involving H2-receptors and the adenyl cyclase system has been shown to stimulate the mammalian heart; promote gastric acid secretion; inhibit antigen-induced histamine release from leucocytes and inhibit lymphocyte-mediated cytotoxicity. It can further be concluded that both H1- and H2-receptors are widely distributed throughout the animal body in the gastro-intestinal, reproductive, respiratory and cardiovascular systems, nervous system and on mast cells and blood leucocytes. In these tissues, histamine receptors play an important role in physiological, immunological and immunopathological processes. Interaction of histamine with both H1- and H2-receptors in varying proportions modulates the overall manifestation of cardiovascular and respiratory syndromes during certain immunopathological conditions (e.g. inflammation, allergy and anaphylaxis). Histamine receptors also appear to play and important role in the development of immuno-competence and immunity.