Pharmacological analysis of the muscarinic receptors involved when McN-A 343 stimulates acid secretion in the mouse isolated stomach

M1 receptor-mediated nitric oxide-dependent relaxation unmasked in stomach fundus from M3 receptor knockout mice

Muscarinic receptors can mediate both contractile and relaxant responses in smooth muscle. The stomach fundus from wild-type mice possesses a neuronal M(1) receptor that mediates relaxation to carbamylcholine and (4-hydroxy-2-butynyl)-1-trimethylammonium-3-chlorocarbanilate chloride (McN-A-343) but is masked by M(3) receptor-mediated contraction to both agonists. When the M(3) receptor was deleted, cholinergic-induced relaxation was unmasked. M(1) receptor antagonism with pirenzepine, nitric oxide (NO) synthase inhibition with N(omega)-nitro-L-arginine methyl ester hydrochloride, and inhibition of neuronal activation with tetrodotoxin abolished relaxation to McN-A-343 in tissues from M(3) receptor knockout mice, supporting the neuronal localization of an M(1) receptor that activated NO release to effect relaxation. However, the cyclooxygenase inhibitor indomethacin did not affect contraction or relaxation to carbamylcholine in stomach fundus from wild-type or M(3) receptor knockout mice, indicating that cyclooxygenase products played no role in these responses. The neuronal M(1) receptor modulated relaxation induced by carbamylcholine and McN-A-343 but not relaxation induced by electric field stimulation of the stomach fundus. These data support the presence of M(1) receptor-mediated relaxation in the stomach and suggest that when the M(3) receptor is eliminated or blocked, M(1) receptor-mediated gastric relaxation may be enhanced, possibly leading to alterations in gastric emptying and subsequent effects on body weight.

Antisecretory effect of somatostatin on gastric acid via inhibition of histamine release in isolated mouse stomach

Somatostatin is known to inhibit gastric acid secretion via both inhibition of histamine release from gastric enterochromaffin-like cells and direct inhibition of parietal cell function. We tried to clarify which of these two mechanisms plays a more important role in the inhibition of gastric acid section by somatostatin using isolated mouse stomach preparations. The gastric acid secretion stimulated by histamine was not inhibited by pretreatment with somatostatin (1 micro M), but somatostatin abolished acid secretion induced by 4-[[[(3-chlorophenyl)amino]carbonyl]oxy]-N,N,N,-trimethyl-2-butynyl-1-aminium chloride (McN-A-343), a muscarinic M(1) receptor agonist. In addition, the histamine-H(2) receptor antagonist famotidine also completely inhibited the secretion stimulated by McN-A-343. Similarly, pretreatment with both somatostatin and famotidine completely abolished pentagastrin-induced acid secretion. Somatostatin partially inhibited the acid secretion stimulated by bethanechol. The late sustained acid secretion induced by bethanechol was reduced more strongly by somatostatin than the initial peak secretion. In addition, somatostatin had no effect on the transient increase in bethanechol-induced acid secretion in famotidine-pretreated preparations. Somatostatin had no effect on basal histamine secretion in isolated mouse stomach preparations, but markedly reduced histamine release induced by McN-A-343 and bethanechol. The present study showed that the acid secretory response via the endogenous histamine-mediated pathway was inhibited by somatostatin, but the response to a direct activation of gastric parietal cells was not. These results suggest that the inhibition of histamine release from enterochromaffin-like cells plays a more important role in the inhibition of gastric acid secretion by somatostatin than the direct inhibition of parietal cells. In addition, somatostatin inhibited the sustained acid secretion more strongly than the initial peak secretion after the cholinergic stimulation.

Inhibitory effect of N(omega)-nitro-L-arginine on gastric secretion induced by secretagogues and vagal stimulation in the isolated stomach

The involvement of endogenous nitric oxide (NO) in the control of gastric acid secretion induced by some secretagogues was studied in the mouse isolated whole stomach. The gastric acid secretion induced by McNeil A-343 [4-[[[(3-chlorophenyl)amino]carbonyl]oxy]-N,N,N-trimethyl-2-butyn- 1-aminium chloride], a muscarinic M1 receptor agonist, pentagastrin or electrical vagus nerve stimulation was markedly inhibited by pretreatment with the NO synthase inhibitor N(omega)-nitro-L-arginine (L-NNA). This inhibitory effect of L-NNA was reversed by L-arginine, but not by D-arginine. Histamine-induced gastric acid secretion was not influenced by treatment with L-NNA. Famotidine completely inhibited the gastric acid secretion induced by McNeil A-343, pentagastrin or electrical vagus nerve stimulation, showing that these stimulations induced gastric acid secretion mainly through histamine release from histamine-containing cells in the gastric mucosa. Moreover, the pentagastrin- and bethanechol-induced histamine release from gastric mucosal cells was significantly inhibited by L-NNA. The NO donor, sodium nitroprusside, at a concentration not affecting histamine-induced gastric acid secretion, increased the acid secretory response, and this response was inhibited by famotidine. These results suggest that endogenous NO is involved in the gastric acid secretion via histamine release from histamine-containing cells.

Application of a model to explore interspecies differences in acetylcholine M-receptor-stimulated gastric acid secretion

1. Concentration-effect curves were obtained, in the absence and presence of histamine H2-receptor blockade, to 5-methylfurmethide (5-MeF) and McN-A 343, high efficacy and low efficacy acetylcholine (ACh) M-receptor agonists, respectively, in isolated stomach preparations from the mouse and immature rat and guinea-pig. 2. In the immature guinea-pig assay, the responses to 5-MeF and McN-A 343 were abolished by histamine H2-receptor blockade suggesting that the responses were totally dependent upon gastric mucosal histamine. However, in the mouse and immature rat assays, although the histamine H2-receptor antagonists produced small but significant rightward shifts and, in some cases, depression of the maximum of the agonist concentration-effect curves, a significant secretory response remained, presumed to be due to direct stimulation of oxyntic cells. 3. Previously, by assuming that the histamine H2-receptor blockade alters the mode of agonist-stimulated acid secretion from mainly an indirect action mediated by histamine release to direct stimulation of the oxyntic cell, we applied an operational model of agonism to similar data obtained in the mouse preparation. In that study we were able to account for the behaviour of 5-MeF and McN-A 343 by assuming that the agonists expressed 6 fold higher efficacy, tau in the operational model of agonism, at ACh M-receptors on the histamine-releasing cells than on the oxyntic cells. In this study it was possible to account for the variation in the behaviour of the agonists both between and within assays by simply varying the efficacy expressed by the agonists at each of the cells in the model. The efficacy variation could be due to receptor concentration variation.4. The data and analysis are discussed in terms of contemporary models for the role of histamine in the regulation of gastric acid secretion.

Neurohormonal regulation of histamine release from isolated rabbit fundic mucosal cells

Histamine-containing cells isolated from rabbit fundic mucosa were found in a small cell elutriation fraction (cells with diameter about 9-12 microns) enriched in mucus and endocrine cells and containing less than 1% mast cells (F1 cells). Gastrin (HG-17), pentagastrin and CCK-8 (C-terminal octapeptide of cholecystokinin) dose-dependently stimulated histamine release (EC50, respectively, 0.126 +/- 0.03, 0.92 +/- 0.15 and 0.211 +/- 0.025 nM) and somatostatin inhibited this release. PGE1, PGE2 and PGD2 alone were unable to enhance histamine release even at high concentrations but, when used in combination with gastrin of CCK-8, the release of histamine caused by these peptides was potentiated (about 1.5- to 2-fold). Carbachol also enhanced the liberation of histamine but with a weaker potency and efficacy than the gastrointestinal peptides (EC50: 1.50 +/- 0.06 microM). The use of specific muscarinic antagonists for M1-, M2- and M3-type receptors led us to conclude that an M1 receptor might be involved in the muscarinic-induced stimulation of histamine release. Activators of protein kinase C, 12-O-tetradecanoylphorbol-13-acetate (TPA) and 1-oleyl-2-acetyl-glycerol (OAG) as well as the calcium ionophore, A23187, induced histamine release, whereas agents which increased intracellular cAMP content were devoid of effect.(ABSTRACT TRUNCATED AT 250 WORDS)

A patch clamp study of the nicotinic acetylcholine receptor of bovine adrenomedullary chromaffin cells in culture

1. Acetylcholine-induced currents recorded from bovine adrenal medullary chromaffin cells maintained in culture were studied during pressure or ionophoretic applications of ACh, using the 'whole-cell' and 'outside-out' configurations of the patch clamp technique. In standard salines, ACh evoked whole-cell currents of -38 pA to -1 nA at -60 mV, which had a reversal potential (EACh) of -7.1 +/- 0.6 mV. The ACh current-voltage relationship was characteristically linear at negative holding potentials and biphasic at positive holding potentials, displaying a region of almost zero slope conductance between 0 and +40 mV followed by a region of positive slope conductance at more positive potentials. 2. Relative permeation to cations was examined. Substitution of external Na+ by sucrose resulted in a -42 mV shift of EACh for a 10-fold reduction in [Na+]o. Using isotonic substitutions, the permeability ratios (relative to Na+) for monovalent cations were determined to be 1.32 +/- 0.02 for Cs+ (n = 11), 1.03 +/- 0.02 for Li+ (n = 8) and 0.18 +/- 0.02 for Tris+ (n = 7). Elevated external Ca2+ salines were found to shift EACh to more positive potentials, especially in the presence of low external Na+. 3. The nicotinic agonists nicotine, tetramethylammonium and lobeline evoked inward currents in bovine chromaffin cells. In contrast, decamethonium and the muscarinic agonist, methacholine, had no effect. 4. The nicotinic antagonists mecamylamine, trimetaphan, (+)-tubocurarine and hexamethonium caused dose-dependent reductions in the amplitude of ACh-evoked inward currents. The estimated IC50's were 0.25, 0.33, 0.63 and 2.2 microM respectively, for cells voltage clamped at -60 mV. High concentrations (> 2 microM) of the muscarinic antagonist, atropine, also produced a dose-dependent reduction in the amplitude of ACh-induced currents. 5. Inhibition by trimetaphan was voltage independent. With the other drugs the antagonism was voltage sensitive, increasing with membrane hyperpolarization. The voltage sensitivity was most marked for hexamethonium. Neither hexamethonium nor mecamylamine were found to depress ACh-evoked outward currents at concentrations which severely depressed inward currents. In addition to its antagonist actions, (+)-tubocurarine activated unitary currents in these cells and on isolated membrane patches. 6. The results indicate that nicotinic ion channels of bovine chromaffin cells have a similar ionic selectivity to monovalent cations, but that Ca2+ ions permeate the channels to a greater degree than at the motor endplate. The ACh current-voltage relationship resembles that described for other types of 'neuronal' nicotinic receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the muscarine receptor type on paracrine cells activated by McN-A-343 in the mouse isolated stomach

An attempt was made to characterize the muscarine receptor type(s) involved in acid secretion in the mouse isolated stomach when stimulated by the muscarinic agonist McN-A-343. A series of 8 muscarinic antagonists was used with preference for M1 receptors (telenzepine and pirenzepine), M1 and M2 receptors (secoverine), M2 receptors (AF-DX 116 and himbacine) and M1 and M3 receptors (p-F-HHSiD and HHSiD). BTM-1086 was used as a high affinity antagonist at the M1 receptor however with little selectivity. Receptor type preferences were determined in binding experiments with [3H]telenzepine in cortical membranes (M1) and [3H]N-methylscopolamine in atrial (M2) or salivary gland (M3) membranes, derived from guinea pigs. No antagonist with M3 preference could be identified in the binding studies. A fixed antagonist concentration of 1 mumol/l was used to antagonize acid secretion stimulated by 10 mumol/l McN-A-343. By plotting the percentage inhibition obtained in the functional test against the Ki values determined in binding experiments for each antagonist at M1, M2 and M3 binding sites, an affinity-inhibition curve could only be constructed when based on the antagonist affinities to the M1 receptor. No statistically significant fit was found using antagonist affinities to the M2 or M3 receptor. Thus, in accordance with the presumed M1 selectivity of the agonist McN-A-343, the rank order of potencies of different antagonists point to the M1 nature of the muscarine receptor which stimulates acid secretion in the mouse isolated stomach upon activation by McN-A-343.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation by McN-A-343 and blockade by telenzepine of acid secretion in the mouse isolated stomach at histamine-liberating cells

(1) In the lumen-perfused mouse stomach in vitro, potential sites of gastric antisecretory action of the muscarine M1-receptor antagonist telenzepine were investigated. Acid secretion was stimulated by the muscarinic agonist McN-A-343 (1-1000 mumol/l). Neither basal nor McN-A-343-stimulated acid secretion was affected by 1 mumol/l TTX indicating that neuronal structures were probably not involved. (2) Acid secretion stimulated by 10 mumol/l McN-A-343 was inhibited by telenzepine (0.1-1.4 mumol/l) and cimetidine (10-140 mumol/l). Neither of the antagonists affected basal acid secretion. TTX had no inhibitory influence on the antagonist effect of telenzepine and cimetidine. (3) Compound 48/80 (100 mumol/l), which depletes histamine stores, initially mimicked but subsequently prevented the effect of McN-A-343. Prenylamine (50 mumol/l), which prevents histamine release, also abolished the secretagogue effect of subsequently administered McN-A-343. (4) Up to concentrations greater than 100 mumol/l, McN-A-343 did not stimulate acid production in rabbit isolated fundic glands and guinea-pig isolated parietal cells. Thus, parietal cells are not directly stimulated by McN-A-343. (5) Based on the site of action of the agonist McN-A-343 in the mouse isolated stomach and its failure to stimulate parietal cells from different species directly, it is concluded that telenzepine blocks, in the mouse isolated stomach, muscarine receptors located on paracrine cells to reduce endogenous histamine release.

Correlation between log POCT/H2O and pKB estimates for a series of muscarinic and histamine H2-receptor antagonists

1. With histamine used as agonist, pKB values were estimated for seventeen histamine H2-receptor antagonists on assays involving acid secretion by the mouse isolated stomach and contraction frequency of the guinea-pig right atrium. 2. With the exception of oxmetidine, SK&F 94,826 and SK&F 94,206 on the right atrium assay, the compounds behaved as simple competitive antagonists on both assays. Although the former three compounds produced concentration-dependent, parallel, displacement of the histamine concentration-effect curves, subsequent analysis indicated Schild plot slope parameters significantly less than unity. However, the application of a combined dose-ratio analysis indicated that their antagonistic behaviour did not differ from expectations for simple competition at dose-ratios of approximately 20, and pKB values were estimated on this basis. 3. In accordance with previously reported data, pKB values were found to be consistently lower on the stomach than atrial assays. The pKB value for tiotidine was underestimated to the same extent on the stomach assay when impromidine was used as agonist. 4. The removal of the serosal muscle from the mouse stomach, achieved by using an isolated, perfused, mucosal sheet preparation, did not significantly affect the underestimation of the pKB value for metiamide. 5. Linear regressional analysis indicated a significant, positive, correlation between lipophilicity (log POCT/H2O) of the antagonists and the degree of antagonist pKB value underestimation on the gastric secretion assay.

Muscarinic activity of McN-A-343 and its value in muscarinic receptor classification

The affinity and potency of McN-A-343 (4-(m-chlorophenyl-carbamoyloxy) -2-butynyltrimethylammonium chloride) has been assessed at a range of M1 and M2 muscarinic receptors. McN-A-343 was shown to act as a full agonist at M2 receptors present in the guinea-pig isolated taenia caeci (-log EC50 = 5.14). McN-A-343 exhibited no agonist action in the guinea-pig ileum, atria, bladder or trachea. McN-A-343 was not selective in terms of affinity since its dissociation constants at M1 and M2 binding sites in the rat cerebral cortex and myocardium respectively, were very similar (cortical pPKi = 5.05; myocardial pKi = 5.22). The selectivity previously reported for the compound may be due to differences in intrinsic efficacy and/or tissue receptor reserve. Based on differential antagonist affinities, the muscarinic receptor profile of the taenia caeci, trachea and bladder was similar to that observed in the ileum, but dissimiliar to that observed in the atria.

Modulation of GABAA receptor activity by alphaxalone

The modulation of the gamma-aminobutyric acidA (GABAA) receptor by alphaxalone has been investigated by use of voltage-clamp recordings from enzymatically isolated bovine chromaffin cells maintained in cell culture. Alphaxalone (greater than 30 nM) reversibly and dose-dependently potentiated the amplitude of membrane currents elicited by locally applied GABA (100 microM). The potentiation was not associated with a change in the reversal potential of GABA-evoked currents and was not influenced by the benzodiazepine receptor antagonist, Ro15-1788 (300 nM). At relatively high concentrations (greater than 1 microM), alphaxalone directly elicited a membrane current. It is concluded that such currents result from GABAA receptor activation since they were reversibly suppressed by bicuculline (3 microM), dose-dependently enhanced by phenobarbitone (100-500 microM), and had a similar reversal potential (approximately 0 mV) to currents elicited by GABA. Additionally, on outside-out membrane patches, alphaxalone activated single channel currents with amplitudes and a reversal potential similar to those evoked by GABA. Alphaxalone (30 nM-1 microM) had no effect upon the amplitude of membrane currents elicited by locally applied acetylcholine (ACh) (100 microM). However, higher concentrations of alphaxalone (10-100 microM) reversibly suppressed ACh-evoked currents, the IC50 for blockade being 20 microM. The beta-hydroxy isomer of alphaxalone, betaxalone (100 nM-1 microM), did not potentiate GABA-induced currents, nor did higher concentrations of the steroid (10-100 microM) directly evoke a membrane current. However, over the latter concentration range, betaxalone suppressed the amplitude of currents elicited either by GABA or ACh. The relevance of the present results to the anaesthetic action of alphaxalone is discussed together with the broader implications of steroidal modulation of the GABAA receptor.

Pharmacological analysis of the inhibition by pirenzepine and atropine of vagal-stimulated acid secretion in the isolated stomach of the mouse

The muscarinic receptors involved in the vagal stimulation of gastric acid secretion in the mouse isolated stomach assay have been examined by analysing the effects of pirenzepine and atropine on fully-defined frequency-effect curves. Both atropine and pirenzepine produced concentration-dependent inhibition of vagal-stimulated acid secretion in a manner consistent with a model describing competitive antagonism of endogenous acetylcholine, which was assumed to be released by vagal stimulation. The results obtained are quite compatible with the hypothesis that vagal stimulation involves muscarinic receptors which are homogeneous with those previously found on histamine and oxyntic cells in the mouse stomach assay. These results find no evidence for muscarinic receptor heterogeneity and reinforce the hypothesis that the selectivity of pirenzepine in vivo relative to atropine is due to the loss of atropine into the gastric secretion.

Pharmacological analysis of muscarinic receptors coupled to oxyntic cell secretion in the mouse stomach

In the light of recent attempts to subclassify muscarinic receptors, agonist-antagonist interactions at muscarinic receptors have been re-examined using improved techniques, on the mouse, isolated, lumen-perfused stomach gastric acid assay. Using 5-methylfurmethide as the muscarinic agonist, the pKB estimated for atropine was significantly lower on the stomach assay (7.78) than on the guinea-pig trachea (8.93). However pKB values for N-methylatropine, the quaternary ammonium derivative of atropine, at concentrations producing dose-ratios above 20 on the stomach assay (pKB = 9.67), and over the full concentration range studied on the trachea (pKB = 9.69) were not significantly different. The deviation from simple competitive behaviour at low dose-ratios with N-methylatropine in the stomach assay is consistent with the effects of a saturable uptake mechanism for quaternary ammonium compounds. The pKB values for pirenzepine on the stomach (6.67) and the trachea (6.87) were not significantly different suggesting that pirenzepine behaves more like N-methylatropine in terms of expressed affinity. We conclude that the oxyntic cell muscarinic receptors are homogeneous with those in the guinea-pig trachea. An initial exploration suggests that there is a relationship between the lipophilicity (log P) of the antagonists and the degree of apparent underestimation of antagonist affinity in the stomach assay. This supports the hypothesis that the underestimation of antagonist affinity is due to the loss of antagonist into the gastric secretion from the receptor compartment. Apparently, relatively selective inhibition of acid secretion, compared to atropine, could be explained without the need to postulate heterogeneity of muscarinic receptor populations.

Pharmacological analysis of the pentagastrin-tiotidine interaction in the mouse isolated stomach

The pentagastrin-tiotidine interaction has been analysed, using improved techniques, in the mouse isolated, lumen-perfused, stomach assay. For comparison and quantification of the H2-receptor blocking activity of tiotidine, histamine-tiotidine interactions have also been analysed in the mouse stomach and guinea-pig isolated right atrial preparation. Tiotidine behaved as a competitive antagonist of histamine both in the guinea-pig right atrium (pKB 7.57) and mouse stomach (pKB 6.96). The difference in pKB was attributed to the loss of tiotidine into the gastric secretion. On the stomach assay, pentagastrin concentration-effect curves were significantly flatter with lower maximal responses than those obtained to histamine. In addition the profile of inhibition observed with tiotidine was different in that the pentagastrin curve maxima were depressed with only a small concomitant dextrad shift. A mathematical model has been developed which accounts for the differences in agonist concentration-effect curves and describes in a quantitative manner the expectations for the competitive antagonism of endogenous histamine assumed to be released by pentagastrin. Fitting of the pentagastrin-tiotidine data to this model provided a reasonable goodness-of-fit. The results are discussed in terms of the role of endogenous histamine in gastrin-stimulated acid secretion. We conclude that the results are consistent with the hypothesis that pentagastrin stimulates acid secretion via the release of endogenous histamine under the present experimental conditions.