Subclasses of muscarinic receptors in isolated gastric mucosal cells: receptor characterization and parietal cell function

Carbachol-induced potentiation and inhibition of acid secretion by guinea pig gastric gland

The effects of muscarinic ligands on acid secretion were examined by estimating the accumulation of [14C]aminopyrine in gastric glands isolated from guinea pigs. The accumulation of [14C]aminopyrine in the presence of 0.1 mM histamine was potentiated by 1 microM carbachol but suppressed by 1 mM. These two effects of carbachol were abolished by atropine, pirenzepine and AF-DX 116. Assuming that the binding of carbachol to one site (Site 1) increases [14C]aminopyrine accumulation but its binding to the other site (Site 2) reduces [14C]aminopyrine accumulation, we analysed the dose-response curves for the carbachol effects in the absence and presence of different concentrations of atropine, pirenzepine and AF-DX 116. The dissociation constants determined for these ligands at Sites 1 and 2 were as follows: carbachol, 0.28 and 7.1 microM; atropine, 0.28 and 0.54 nM; pirenzepine, 45 and 560 nM; and AF-DX 116, 380 and 4400 nM, respectively. The binding of [3H]N-methylscopolamine to the gastric glands indicated the presence of two populations of binding sites with different affinities for the above ligands, other than atropine. The apparent dissociation constants, which were estimated by analysing the displacement curves for [3H]N-methylscopolamine binding, were as follows: carbachol, 0.18 microM (10%) and 31 microM (90%); atropine, 1.24 nM; pirenzepine, 15 nM (16%) and 220 nM (84%); and AF-DX 116, 370 nM (10%) and 2970 nM (90%). These results suggest that there are two kinds of muscarinic acetylcholine receptors in the guinea pig gastric gland, one potentiating and the other inhibiting the acid secretion induced by histamine.

The possible role of histamine-mediated cAMP formation as a link between H2-receptor stimulation and ultrastructural changes in guinea-pig oxyntic cells

Guinea-pig oxyntic cell tubulin has been isolated and in vitro aggregation has been studied. The spontaneous assembly of isolated tubulin was significantly accelerated and increased by 1 mmol/l GTP. Histamine and forskolin increased tubulin polymerization only when detergent dispersed oxyntic cells or crude membranes were added. The forskolin response occurred very rapidly with an EC50 of approximately 30 mumol/l and did not require GTP. Histamine promoted tubulin aggregation with an EC50 of about 5 mumol/l only in the presence of GTP. Ranitidine completely inhibited the effects of histamine. From these data it is suggested that, in the oxyntic cell, histamine H2-receptor activated adenylate cyclase and the corresponding increase in cAMP play a role in eliciting characteristic ultrastructural changes by initiating formation of microtubules as a first step in the cascade of events leading to an increase in the secretory surface area.

Histamine H2-receptor mediated stimulation of gastric acid secretion by mercaptomethylimidazole

Intraperitoneal administration of mercaptomethylimidazole (methimazole), a potent antithyroid drug belonging to the thionamide group, caused a significant increase in gastric secretion both in control and pylorus-ligated mice. The drug also induced significant stimulation of gastric acid and pepsinogen secretion in both the animal systems studied. The dose-response curve indicated a nearly 10-fold increase in acid output by injection of 0.55 mg mercaptomethylimidazole per 25 g body weight. The duration profile of the drug response at the dose mentioned showed acid secretion almost at a linear rate up to 2.5 hr, after which the response decreased to some extent. Of the other antithyroid drugs of the same family, only thiourea activated acid secretion but the response was much smaller than mercaptomethylimidazole. Histamine, one of the physiological secretagogues of gastric acid secretion, was found to be less active than mercaptomethylimidazole. Mercaptomethylimidazole-induced stimulation of acid secretion could be effectively blocked by prior administration of cimetidine and completely by omeprazole and not by atropine. Verapamil and nifedipine had also some inhibitory effect. These observations indicate that mercaptomethylimidazole stimulates HCl secretion through the involvement of H2-receptor and through the functioning of the H+-K+-ATPase of the parietal cells. The bulk movement of water during increased HCl secretion was partially sensitive to cimetidine and omeprazole and was also associated with an increased secretion of Na+ and K+ in the gastric juice. This indicates that mercaptomethylimidazole also induced water transport through a separate mechanism.

Membrane receptors in the gastrointestinal tract

This review focusses on the roles that membrane receptors and their transducers play in the physiology and pathology of the gastrointestinal tract. The multifactorial regulation of [correction] mucosal growth and function is discussed in relation to the heterogeneity of exocrine and endocrine populations that originate from progenitor cells in stomach and intestine.

The effects of nicotine on basal and stimulated gastric secretions in the conscious cat and in isolated guinea pig gastric mucosal cells

Intravenous (i.v.) administration of nicotine in conscious cats significantly stimulated basal gastric acid output. The effect was completely blocked by atropine and ranitidine. Submaximally stimulated gastric acid secretion was not further increased by nicotine. In isolated guinea pig parietal cells nicotine significantly increased basal acid secretion by about 20% and potentiated the response to maximally effective concentrations of histamine but had no influence on the carbachol response. In isolated parietal cells stimulated either by nicotine, histamine or both, atropine pretreatment increased or inhibited the acid response in concentration-dependent manner. From these data, it is concluded that nicotine had direct stimulatory effects on isolated parietal cells and potentiated the histamine mediated response in the isolated cell preparation but not in the intact animal model.

Nicotinic acetylcholine receptors in the rat stomach: I. (-)-[3H]nicotine binding

The nicotinic acetylcholine receptors in the rat stomach were characterized by means of a radioligand binding assay with (-)-[3H]nicotine as ligand. Saturation binding studies on the gastric fundus membranes revealed the presence of two binding sites with dissociation constant (KD) values of 3.1 and 289 nM, and maximum binding capacity (Bmax) values of 3.6 and 76 fmol/mg protein, respectively. The Bmax of the high affinity binding site was greatest in the cardia, followed by fundal mucosa, fundal muscle, and, finally antrum. The IC50 values of cholinergic drugs to inhibit (-)-[3H]nicotine binding in fundus membranes were as follows: (-)nicotine, 0.12 nM; cytosine, 9.3 nM; acetylcholine, 17.7 nM; carbachol, 700 nM; hexamethonium, 2270 nM. The IC50 values of alpha-bungarotoxin, d-tubocurarine and atropine were greater than 100 microM. The muscarinic acetylcholine receptors were also characterized with [3H]quinuclidinyl benzilate and the choline acetyltransferase activity was measured. These results suggest that nicotinic acetylcholine receptors as well as muscarinic acetylcholine receptors are present in the rat stomach and that the regional distribution of these receptors is uneven.

Multiple types of neurotransmitter binding sites in the rat neuroblastoma B 50 cell line. I. Characterization and binding affinity changes during various differentiation paradigms

In this study, it is shown that the neuroblastoma B 50 cell line of rat central nervous system origin possesses both adrenergic and cholinergic binding sites. The adrenergic binding site population comprises a major alpha 1-adrenergic component (using [3H]prazosin as ligand; KD = 0.55 nM; 19.3 fmol/mg protein), and a minor beta-adrenergic species (KD = 0.45 nM; 3.4 fmol/mg protein using [125I]cyanopindolol as ligand). The cholinergic binding site is of a muscarinic type. Binding studies with quinuclidinyl benzylate (QNB) indicate the presence of two binding sites with different affinities. This is supported by competition studies with agonists. However, in view of this unusual binding behavior of the antagonist, the possibility of a one-site model for QNB is also considered and affords the following parameters: KD = 0.247 nM, Bmax = 29.6 fmoles/mg protein. When the cells are induced to differentiate with dibutyryl cyclic AMP (db-cAMP) or high extracellular calcium, all of these binding sites undergo a variety of changes in their affinities for their respective ligands. Db-cAMP increases the affinity of the alpha 1-site for prazosin while high extracellular calcium lowers it. In both cases the number of binding sites remains unchanged. The affinity of the beta-adrenergic component is markedly enhanced upon induction of differentiation with either extracellular calcium or/and db-cAMP. Finally, the muscarinic cholinergic binding sites exhibit a decrease in receptor number upon db-cAMP treatment and an apparent loss of one of the binding sites upon calcium treatment. The significance of these affinity changes is discussed for each type of binding site.

Mechanism of gastric antisecretory effect of SCH 28080

The mechanism of the gastric antisecretory action of SCH 28080 has been studied utilizing two different in vitro test systems, isolated and enriched parietal cells from the guinea-pig and guinea-pig gastric membranes purified and enriched with K+/H+-ATPase. In guinea-pig isolated and enriched parietal cells SCH 28080 inhibited the acid response to histamine and high K+ concentrations with IC50 values not significantly different from each other. SCH 28080 inhibited the purified K+/H+-ATPase measured in the presence of 5 mM KCl with an IC50 value of 1.3 microM. Kinetic studies indicated a competitive inhibition of ATPase by SCH 28080 with respect to K+. Studies on Na+/K+-ATPase showed that this enzyme was only slightly depressed by SCH 28080. It is concluded that SCH 28080 acts with high selectivity on the parietal cell K%/H+-ATPase, establishing its antisecretory effect by a competitive interaction with the high affinity K+-site of the gastric ATPase.

Muscarinic receptors in isolated gastric fundic mucosal cells. Binding-activity relationships

Muscarinic receptors are involved in the control of gastric acid secretion. The characteristics of (3H)-N-methyl-scopolamine [(3H)-NMS] binding to isolated cells from rabbit fundic gastric mucosa and inhibition of this binding by muscarinic agonists, antagonists and other pharmacological agents known to regulate acid secretion are reported. Specific binding for (3H)-NMS was described: antagonists interact with high affinity sites (KD = 0.5 nM) whereas binding curves for agonists clearly deviated from the simple mass action isotherm with a flattening of the curve suggesting the presence of more than one class of sites. The low affinity sites for agonists are in the micromolar range. Pirenzine, a gastroselective antimuscarinic compound, known to differentiate between M1 and M2 sites, inhibited (3H)-NMS binding with an IC50 of 0.05 microM. On the same gastric cell population, muscarinic agonist carbachol stimulated (14C)-aminopyrine accumulation in a dose-dependent manner with an ED-50 of 10 microM, value close to that needed to 50% inhibit (3H)-NMS binding. This stimulation was competitively inhibited by muscarinic antagonists and pA2-values for atropine, QNB and pirenzepine, calculated from linear Schild plots, were in the following order: 9.2 for atropine, 8.6 for QNB and 7.0 for pirenzepine. In conclusion, fundic gastric mucosal cells from rabbit, isolated with collagenase and EDTA, contained specific muscarinic receptors coupled to the acid secretory mechanism and pirenzepine interact with these receptors with an intermediate affinity suggesting the presence of functional M2-sites.

The effects of antimitotic and microfilament disrupting agents on functions of isolated guinea-pig parietal cells

In isolated guinea-pig parietal cells pretreated for 60 min with the H2-antagonist ranitidine, the antimitotic agents colchicine and vinblastine, the microfilament-disrupting agent cytochalasin B resulted in a concentration-dependent inhibition of histamine-stimulated acid secretion up to 80%. Ranitidine reduced histamine binding to the membrane located H2-receptor. The anti-cytoskeletal agents inhibited the cellular histamine uptake but did not effect the histamine methyltransferase activity which was significantly reduced by ranitidine. The data suggest that cytoskeletal elements like microtubules and microfilaments are of very specific functional significance not only in the secretory process of the parietal cell but also for cellular transport mechanisms.