Analysis of the variation in the action of L-365,260 at CCKB/gastrin receptors in rat, guinea-pig and mouse isolated gastric tissue assays

1. Since L-365,260 was first described as a selective antagonist at cholecystokinin (CCK)B/gastrin receptors, we have used it periodically as a reference compound in isolated tissue assays of guinea-pig gastric muscle and lumen-perfused stomachs from mouse and immature rat. L-365,260 behaved as a surmountable antagonist and produced parallel rightward shifts of pentagastrin concentration-effect curves' in each of the replicate experiments. The experiments were performed by several different experimenters in the same laboratories over a five year period. 2. In the isolated, lumen-perfused, immature rat stomach assay, L-365,260 behaved as a simple competitive antagonist (Schild plot slope = 1.00 +/- 0.10, pKB = 7.54 +/- 0.03 from a global analysis of the data) acting at a homogeneous population of receptors in five separate, highly-reproducible, experiments. In contrast, the replicate data sets obtained from the interaction in the isolated, lumen-perfused mouse stomach and guinea-pig gastric muscle assays, over the same period, were not consistent with the presence of a single receptor population. The guinea-pig gastric muscle data were relatively reproducible between experiments but some individual Schild plot slopes and the slope estimated from a global analysis of all the data were significantly less than unity (slope = 0.80 +/- 0.07, pA2 = 8.56 +/- 0.05 from the global analysis). The data obtained in the mouse stomach were significantly more variable than that obtained in the same assay, during the same period, from the interaction between histamine and the H2-receptor antagonist, famotidine. The individual Schild plot slopes ranged from being very flat (0.20) to being not significantly different from unity (1.23) and the pA2 values ranged from 7.68 to 8.70. 3. Overall, the data could be accounted for by assuming the variable expression of two receptor subtypes across the assays. The rat stomach appeared to express a single receptor characterized by a low affinity constant for L-365,260 (pKB approximately 7.5). The guinea-pig gastric muscle and mouse stomach data could be explained by the presence of this receptor and a second one characterized by a high affinity constant for L-365,260 (pKB approximately 8.6). The activity of the two proposed receptor subtypes was consistent between experiments in the guinea-pig and the high affinity receptor appeared to be predominant. In contrast, the mouse stomach data could only be simulated by assuming that the proportion and absolute number of each subtype varied significantly between the replicate experiments. 4. The L-365,260 affinity estimates at the inferred receptor subtypes were indistinguishable from those obtained in a corresponding analysis of the behaviour of L-365,260 in CCKB/gastrin receptor radioligand binding experiments in guinea-pig gastric gland and mouse and rat cerebral cortex preparations.

Analysis of the effects of alpha 1-adrenoceptor antagonists on noradrenaline-mediated contraction of rat small mesenteric artery

1. In this study, we examined the interaction between noradrenaline (NA) and phenylephrine (PE) with seven antagonists (prazosin, tamsulosin, phentolamine, WB-4101, 5-methylurapidil, spiperone and HV 723) in an attempt to characterize the alpha 1-adrenoceptor population of the rat isolated small mesenteric artery (SMA) preparation. 2. Six of the seven antagonists investigated produced concentration-dependent, parallel, rightward shift of the NA concentration-effect (E/[A]) curves. The exception was tamsulosin, which produced significant decrease of the upper asymptote. In the case of 5-methylurapidil and HV723, the Schild plot slope parameters were not significantly different from unity over the range of concentrations used. However, the Schild plot slopes obtained for the other antagonists were all significantly greater than unity, inconsistent with expectations for simple competitive antagonism. 3. HV723, prazosin and tamsulosin were also tested using PE as an agonist. All three antagonists produced concentration-dependent, parallel, rightward shifts of the PE curves and Schild analysis yielded slope parameters not significantly different from unity. The pKB estimates obtained for tamsulosin and prazosin were not significantly different from the pA2 values obtained when NA was used as agonist. In the case of HV723, the 95% confidence intervals for the pKB values yielded with NA and PE did not overlap (pKB = 8.80-9.13 and 8.15-8.77 for NA and PE, respectively). 4. In the absence of evidence to indicate that the steep Schild plots were due to failure to satisfy the basic criteria for quantitative analysis in a one-receptor system, we considered the possibility that the complexity was caused by an action of NA at inhibitory D1 receptors. The selective D1 receptor antagonists, SCH-23390 (10 nM), had no significant effect on the NA E/[A] control curve, but the apparent potency of 100 nM prazosin was reduced by approximately 3.5 fold. 5. This study indicates that the steep Schild plots obtained from the interaction between NA and alpha 1-adrenoceptor antagonists were due to the simultaneous activation of inhibitory D1 receptors by NA. Notwithstanding this complexity, our explanatory model of the system (see Appendix) suggests that the antagonist affinity values estimated in the absence of D1 receptor block were not significantly affected by this other action of NA. The low affinity estimate obtained for prazosin suggests that the pharmacologically-defined alpha IL-subtype operates in the SMA.

Analysis of the activity of alpha 1-adrenoceptor antagonists in rat aorta

1. In this study, the effect of seven alpha 1-adrenoceptor antagonists (tamsulosin, phentolamine, prazosin, WB-4101, 5-methylurapidil, spiperone and HV723) have been examined on the contractile response to noradrenaline (NA) and phenylephrine (PE) in rat isolated aorta. 2. NA and PE, when administered using a cumulative dosing schedule, both produced concentration-dependent contraction of aortic rings. It was possible to fit the individual concentration-effect (E/[A]) curve data to the Hill equation to provide estimates of the curve midpoint location (p[A]50 = 7.74 +/- 0.10 and 7.14 +/- 0.18), midpoint slope (nH = 0.82 +/- 0.03 and 0.99 +/- 0.10) and upper asymptote (alpha = 3.2 +/- 0.3 and 3.1 +/- 0.2 g) parameters for NA and PE, respectively. However, the Hill equation provided a better fit to the E/[A] curve data obtained with another contractile agent, 5-hydroxytryptamine (5-HT) (p[A50] = 6.09 +/- 0.08, nH = 1.49 +/- 0.09, alpha = 2.6 +/- 0.3 g), as judged by calculation of the mean sum of squares of the differences between the observed and predicted values. 3. All of the antagonists investigated produced concentration-dependent inhibition of the contractile responses of the aorta to NA and PE. Although no significant effects on the upper asymptotes of the E/[A] curves of any of the antagonists tested were detected, only tamsulosin and 5-methylurapidil did not have a significant effect on the slope (nH) of the NA and PE E/[A] curves. The other antagonists produced significant steepening of the curves obtained with NA and/or PE. 4. Notwithstanding the fact that one of the basic criteria for simple competitive antagonism at a single receptor class was not always satisfied, the individual log [A]50 values estimated in the absence and presence of antagonist within each experiment were fitted to the competitive model. The Schild plot slope parameters for the antagonism of NA and PE by phentolamine and HV723 were found to be significantly less than unity. The Schild plot slope parameters for the other antagonists were not significantly different from unity. 5. In the absence of evidence to suggest that the deviations from simple competitive antagonism were due to failure to satisfy basic experimental conditions for quantitative analysis, an attempt was made to see whether the data could be accounted for by an existing two-receptor model (Furchgott, 1981). The goodness-of-fit obtained with the two-receptor model was significantly better than that obtained with the one-receptor model. Furthermore, with the exception of the data obtained with phentolamine, the pKB estimates for the two receptors were independent of whether NA or PE was used as agonist. 6. To determine which alpha 1-adrenoceptor subtypes may be associated with those defined by the two receptor model, the mean pKB estimates obtained from the two-receptor model fit were compared with affinities measured by Laz et al. (1994) for rat cloned alpha 1-adrenoceptor subtypes expressed in COS-7 cells. The sum of squared differences of the data points from the line of identity was smallest for both pKB1 and pKB2 in the case of the alpha 1a/d-adrenoceptor (now referred to as alpha 1d-adrenoceptor; Hieble et al., 1995). Therefore, the complexity exposed in this study may be due to the expression of closely-related forms of the alpha 1d-adrenoceptor. However, relatively good matches were also found between pKB1 and alpha 1c and between pKB2 and alpha 1b. Therefore, on the basis of these data, it is not possible to rule out the involvement of all three alpha 1-adrenoceptors. The conflicting reports concerning the characteristics of the alpha 1-adrenoceptor population mediating contraction of the rat aorta may, at least in part, be due to the lack of highly selective ligands and to between-assay variation in the expression of multiple alpha 1-adrenoceptors.

Exposure and characterization of the action of noradrenaline at dopamine receptors mediating endothelium-independent relaxation of rat isolated small mesenteric arteries

1. Previously, we reported that noradrenaline (NA), in addition to its alpha 1-adrenoceptor-mediated contractile effect, may relax the rat small mesenteric artery (SMA) in order to account for steep Schild plots obtained with compounds classified as alpha 1-adrenoceptor antagonists. In this study, a relaxant action of NA has been exposed in the rat isolated, endothelium-denuded SMA precontracted by the thromboxane A2-mimetic, U46619. 2. NA, but not the selective alpha 2-adrenoceptor agonist, UK14304, produced concentration-dependent contraction of the SMA (pEC50 = 5.7 +/- 0.1). After precontraction with 0.1 microM U46619, 10 nM-30 microM NA produced a further contraction (pEC50 = 6.1 +/- 0.2), while higher concentrations of NA produced small, but significant, relaxant responses. 3. In the presence of 1 microM prazosin, 0.1-30 microM NA produced concentration dependent relaxation (pIC50 = 5.9 +/- 0.1) after precontraction with 0.1 microM U46619. The NA relaxation concentration-effect curve was completely inhibited by 1 microM of the beta 1/beta 2-adrenoceptor antagonist, timolol. However, when the concentration of prazosin was increased by 10 fold (10 microM), NA once again produced concentration-dependent relaxation (pIC50 = 4.5 +/- 0.2). This relaxation concentration-effect curve was not blocked by a 10 fold higher concentration of timolol (10 microM), nor by the presence of idazoxan (10 microM), cyanopindolol (10 microM), NG-nitro-L-arginine methyl ester (L-NAME, 100 microM), indomethacin (10 microM) or sulpiride (1 microM). However, haloperidol (10 microM) and (+/-)-SCH-23390 (10 nM) produced significant inhibition of the relaxation, suggesting the involvement of dopamine D1 receptors. 4. Dopamine also produced concentration-dependent relaxation following U46619 precontraction (pIC50 = 5.4 +/- 0.1) which was significantly inhibited by haloperidol and (+)-SCH-23390. Pretreatment with 10 microM phenoxybenzamine for 60 min produced a significant inhibition of the dopamine and NA relaxation curves and application of the operational model of agonism yielded estimates of the affinity (pKA = 5.3 +/- 0.2 and 4.4 +/- 0.2) and efficacy (log gamma = 0.06 +/- 0.11 and 0.01 +/- 0.10) for dopamine and NA, respectively, at D1 receptors. 5. HV723 (0.1 and 1 microM), a ligand that yielded a Schild plot slope parameter of unity as an antagonist of NA in the contractile assay, produced concentration-dependent inhibition of the NA-mediated relaxation (pA2 approximately 8). 6. The results of this study indicate that NA can activate D1 receptors mediating relaxation in the rat SMA at concentrations which were encountered in our previous receptor classification experiments using competitive alpha 1-adrenoceptor antagonists.

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.

Pharmacological analysis of the interaction between purinoceptor agonists and antagonists in the guinea-pig taenia caecum

1. In the absence of adenosine uptake inhibition, adenosine produced a concentration-dependent (threshold 30 microM) relaxation of the 5-methylfurmethide pre-contracted guinea-pig taenia caecum. The relaxation was not blocked by 8-phenyltheophylline (8-PT, 3 microM) or 1,3-dipropyl, 8-cyclopentylxanthine (DPCPX, 30 microM). 2. In the presence of the adenosine uptake inhibitor, dipyridamole (Dip, 3 microM), a biphasic adenosine concentration-effect curve was obtained (threshold 0.3 microM). The time course of the responses to adenosine in the absence of Dip was similar to that of the second phase responses in the presence of Dip and occurred over the same adenosine concentration-range. 5'-(N-ethyl) carboxamido-adenosine (NECA) concentration-effect curves (in the absence of Dip) were also biphasic. Only the first phases of the concentration-effect curves obtained with NECA and adenosine (plus Dip) were inhibited by 8-PT. The pA2 values for 8-PT of 6.7 and 7.0 versus adenosine and NECA, respectively, were consistent with actions at P1-purinoceptors. There was a trend towards an increase in the upper asymptote of the first phase of the NECA curve in the presence of increasing concentrations of 8-PT. The A1-purinoceptor selective antagonist, DPCPX, also blocked only the first phase of the NECA concentration-effect curve and produced a significant increase in the upper asymptote. The pA2 value (6.8) obtained was consistent with activation of A2-subtype P1-purinoceptors by the low concentrations of NECA. 3. There was no correlation between A1-purinoceptor affinity and the propensity to cause the increase in the upper asymptote of the first phase of the NECA concentration-effect curves amongst a series of 9-methyl adenine analogues, suggesting that the amplification was not due to inhibition of an underlying A1-purinoceptor-mediated contractile response.4. DPCPX (10 microM) produced a significant increase in the upper asymptote of the NECA concentration effect curve, but had no effect on isoprenaline curves whereas the phosphodiesterase inhibitor Ro20-1724 (30 microM) produced a significant increase in the upper asymptote of both NECA and isoprenaline concentration-effect curves. Therefore the amplification of the first phase responses by DPCPX did not appear to be due to phosphodiesterase inhibition.5. It was not possible to conclude whether second phase responses to adenosine and NECA were mediated by intracellular or extracellular sites of action. However, if intracellular sites of action were involved then adenosine did not apparently gain access by the Dip-sensitive uptake system.

Interaction between methotrexate and indomethacin on a human normal haemopoietic cell line

1. The interaction between methotrexate and indomethacin has been examined, at a physiological folate concentration (20 nM), on a human normal lymphoblast-like cell line (RPMI 1788) in vitro. 2. Indomethacin (1 microgram ml-1) increased the reduction of lymphoblast growth caused by methotrexate (10-80 ng ml-1). 3. Indomethacin (0.1 and 1 microgram ml-1) potentiated the cytotoxicity of methotrexate (20 and 40 ng ml-1) after 4 days in culture. 4. Indomethacin (0.4 micrograms ml-1) reduced the accumulation of tritium in lymphoblasts incubated with [3H]-methotrexate after 30 min; therefore initial drug accumulation was not responsible for the potentiation seen after 4 days. 5. If indomethacin increases the killing of human cancer cells by methotrexate in vivo, with a smaller potentiation on lymphoblasts, this combination may be beneficial in treating human malignancy.

The use of receptor desensitization to analyse CCKA and CCKB/gastrin receptors coupled to contraction in guinea-pig stomach muscle

1. The results of previous studies have been in conflict with respect to the involvement of specific cholecystokinin (CCKA) and CCKB/gastrin receptors in guinea-pig gastric muscle. Here, in an in vitro, guinea-pig gastric muscle assay, pentagastrin (PG) and tetragastrin (TG) behaved as high potency agonists and produced symmetrical concentration-effect curves. In contrast, cholecystokinin-octapeptide (CCK-8), while also behaving as a high potency agonist, produced flat asymmetrical curves. Unlike recent data reported using this tissue (Boyle et al., 1993), the CCKA receptor-selective antagonist, devazepide (3, 10, 30 nM) produced a rightward shift of the upper region of the CCK-8 curve rendering it biphasic. The lower phase was abolished by the CCKB/gastrin receptor-selective antagonist, L-365260 (300 nM) indicating that the contractile effects of CCK-8 in this tissue are mediated by both receptor types. 2. L-365260 produced a concentration-dependent, parallel rightward displacement of PG concentration-effect curves. However, a flat Schild plot slope parameter (0.77 +/- 0.06) was obtained. Therefore, an empirical pA2 value of 8.64 +/- 0.21 was estimated from the smallest dose ratio. This value is consistent with published values characteristic of an interaction at CCKB/gastrin receptors. 3. TG (1 microM) was used to densensitize selectively the CCKB/gastrin receptors in the gastric muscle assay and thereby expose a population of receptors capable of responding to subsequent stimulation by CCK-8 but not by PG. The selectivity of TG for CCKB/gastrin- over CCKA receptors was demonstrated by its low efficacy compared to CCK-8 in the guinea-pig gallbladder assay, a tissue shown previously to contain a homogeneous population of CCKA receptors. In TG-desensitized gastric muscle, CCK-8 concentration-effect curves were symmetrical and could be displaced in a simple parallel fashion by devazepide at nanomolar concentrations consistent with an interaction at CCKA receptors (pKB approximately 10). 4. These results indicate that the guinea-pig gastric muscle contains both CCKA- and CCKB/gastrin receptors and the effects of CCK-8 are mediated via both of these receptors. Notwithstanding the complexity of the behaviour of L-365260, it was possible to obtain a reasonable description of the system using a simple 2-receptor model in which the effects of individual receptor activation were assumed to be additive. The absence of a simple competitive interaction of PG with L-365260 may indicate, for example, non-homogeneity of CCKB/gastrin receptors or lack of concentration equilibrium between the bath and the receptor biophase.

Comparative analysis of the vagal stimulation of gastric acid secretion in rodent isolated stomach preparations

1. Electrical field stimulation produced a tetrodotoxin-sensitive, frequency-dependent, release of acid from isolated, lumen-perfused, stomach preparations from mouse, immature rat and guinea-pig. 2. In the guinea-pig and mouse preparations, the frequency-dependent response was abolished by hexamethonium, acetylcholine (ACh) muscarinic (M) and histamine H2-receptor blockade, consistent with the hypothesis that the vagal ACh acts indirectly by stimulating the release of endogenous histamine. 3. In contrast, in the rat preparation the frequency-dependent response was partially refractory to all of these inhibitors. However, a combination of H2- and ACh M-receptor blockade did abolish the effect. 4. We conclude that vagal-stimulated acid secretion in the rat, unlike the other two species, behaves as though there is a direct innervation of the oxyntic cells by either cholinergic or noncholinergic neurones.

Comparative study of the control of basal acid output from rodent isolated stomachs

1. Isolated, lumen-perfused, whole stomach preparations from mouse and immature rat produced a stable basal acid output which, although not blocked by histamine H2-, acetylcholine M- or CCKB/gastrin receptor antagonists, was almostly completely blocked by the H+/K(+)-ATPase inhibitor, omeprazole, and the metabolic inhibitor, sodium thiocyanate (NaSCN). 2. Fully-defined concentration-effect curves could be obtained on both assays with the phosphodiesterase inhibitor, isobutyl methylxanthine (IBMX) and with dibutyryl cyclic AMP. 3. On the rat stomach assay, histamine H2-receptor blockade had no effect on the IBMX curve. In contrast, the IBMX response in the mouse was abolished by histamine H2-receptor blockade. On both assays responses to dibutyryl cyclic AMP were resistant to H2-receptor blockade. 4. In the absence of suprathreshold endogenous histamine, it is argued that H+/K(+)-ATPase mediated basal acid secretion from the mouse stomach assay is regulated by something other than cyclic AMP.

2-Naphthalenesulphonyl L-aspartyl-(2-phenethyl)amide (2-NAP)--a selective cholecystokinin CCKA-receptor antagonist

1. The in vitro pharmacological characterization of the sodium salt of 2-naphthalenesulphonyl 1-aspartyl-(2-phenethyl)amide [2-NAP], a hydrophilic compound derived from the C-terminal aspartate-phenylalanine dipeptide of cholecystokinin (CCK), is described. 2. 2-NAP behaved as a competitive antagonist of sulphated cholecystokinin octapeptide (CCK-8) at CCKA-receptors in both intact tissue bioassays (guinea-pig gall bladder, pancreas and ileum, human and rabbit gall bladder) and a radioligand displacement assay (guinea-pig pancreatic cells). The mean pKB, over assays, was 6.5. 3. Compared to the other assays, the rabbit gall bladder assay gave a significantly higher pKB estimate [7.0] for 2-NAP and a significantly lower estimate [8.9] for devazepide (formerly L-364,718 and MK-329), a well-characterized CCKA-receptor antagonist; these anomalous results suggest that a different class of CCKA-receptors may be involved. 4. 2-NAP, was found to be highly selective, having at least 300 fold greater affinity for CCKA-receptors than for 50 other pharmacological loci, including gastrin/CCKB, as estimated by bioassay or radioligand displacement.

Evidence that the apparent complexity of receptor antagonism by angiotensin II analogues is due to a reversible and syntopic action

1. The interactions between angiotensin II (AII), two non-peptide antagonists DuP 753 and IMI, and eight peptide analogues of AII were investigated on the rabbit isolated aorta assay. DuP 753 and IMI behaved as simple competitive antagonists (pKB values 8.4 and 6.8, respectively). To different degrees, all the AII-peptide analogue interactions failed to meet the basic criteria for simple competition. In addition to rightward shift, the most significant feature was a concentration-dependent saturable depression of the upper asymptote of the AII concentration-effect curves. 2. 'Washout' and combined dose-ratio analysis experiments, in which DuP 753 was used as a reference antagonist, indicated that the profile of peptide antagonism was solely due to a reversible and syntopic action at the AII receptor. 3. By use of an operational model of agonism (Black & Leff, 1983) as a starting point, it was possible to account for the data with a new model which describes reversible receptor occupancy and occupied receptor-determined, saturable reduction in the efficacy of AII. Model-fitting gave estimates of pKB values for the peptide analogues and agonist affinity and efficacy parameters for AII. 4. The model was successfully tested by applying it to qualitatively similar results obtained in a cross-tissue analysis on guinea-pig aorta, ileum and stomach. 5. A 'molecular' interpretation of the efficacy changes, based on the concepts of receptor internalisation and expression, is offered.

Combined dose-ratio analysis of cholecystokinin receptor antagonists, devazepide, lorglumide and loxiglumide in the guinea-pig gall bladder

1. Interactions between cholecystokinin octapeptide (CCK-8) and CCKA-receptor antagonists derived from benzodiazepines (devazepide) and glutamic acid (lorglumide and loxiglumide) have been examined in an improved bioassay using the guinea-pig, isolated, gall bladder preparation. 2. The presence of CCKB-receptors in the assay was provisionally-ruled out on the basis of the low potency of pentagastrin in the assay. By applying analyses of both agonism and antagonism, pentagastrin was shown to behave as a partial agonist at the CCKA-receptor. 3. Devazepide, lorglumide and loxiglumide behaved as simple competitive antagonists of CCKA-receptors and pKB values of 9.98, 7.59 and 7.07 were estimated, respectively. 4. Application of a combined dose-ratio analysis to the interactions between CCK-8 and combinations of devazepide/lorglumide and devazepide/loxiglumide indicated that these molecules behave as syntopic, competitive, antagonists at the CCKA-receptor. 5. We conclude that the guinea-pig gall bladder assay contains a homogeneous population of CCKA-receptors and offer an explanation for the differences between our results and those obtained recently by Maubach et al. (1991) which were taken as preliminary evidence for CCKA-receptor heterogeneity.

Histamine dependence of pentagastrin-stimulated gastric acid secretion in rats

Does gastrin stimulate gastric acid secretion by direct action on oxyntic cells, by releasing histamine, or by being potentiated by histamine? Previous studies in the mouse pointed to gastrin-regulated histamine release. Guinea pig and rat are well known to vary in their sensitivity to histamine. Therefore, the effects of histamine and pentagastrin were compared quantitatively on isolated, lumen-perfused, stomach preparations from these species in the absence and presence of histamine H2-receptor blockade. The loss of potency of histamine in the rat was mirrored by a loss of potency of pentagastrin consistent with the idea that pentagastrin acts by releasing histamine. In the rat, a well-defined pentagastrin curve was obtained in the presence of histamine H2-receptor block as though pentagastrin acts both directly on the oxyntic cell and indirectly by releasing histamine. It was not necessary to invoke a potentiating interaction between histamine and pentagastrin at the oxyntic cell; the two effects appeared simply to add. Potentiation was observed, however, between other combinations of stimuli, for example, between vagal nerve and pentagastrin stimulation. The physiological consequences of these results are discussed.

Pharmacological analysis of agonist-antagonist interactions at acetylcholine muscarinic receptors in a new urinary bladder assay

1. Agonist-antagonist interactions at acetylcholine (ACh) muscarinic receptors have been analysed by use of an improved urinary bladder assay, isolated and intact, from the mouse. With 5-methylfurmethide as agonist, validated cumulative concentration-effect curves were obtained in less than 7 min by re-dosing before the response plateaux began to fade. 2. The pKB value estimated for pirenzepine was 6.76. The pKB values estimated for atropine and N-methylatropine from data obtained at concentrations which produced dose-ratios greater than 20 and 60 were 8.90 and 9.58, respectively. 3. The deviation from simple competitive behaviour at low dose-ratios with atropine and N-methylatropine was consistent with the operation of saturable antagonist removal processes. The deviation observed with atropine was corrected by pre-incubation with methylbutyrate, an alternative substrate for 'atropine esterase'. 4. The simple competitive behaviour of N-methylatropine was restored following pre-incubation with the neuronal choline uptake blocker hemicholinium-3 (HC-3). However, the pKB estimated for N-methylatropine under these conditions was low. This latter result could be accounted for by the observed behaviour of HC-3 as a competitive antagonist of ACh muscarinic receptors (pKB = 4.01). 5. We conclude that the modified mouse urinary bladder assay is suitable for the quantitative analysis of muscarinic receptor interactions. In addition, we postulate the existence of a previously undescribed uptake mechanism for quaternary muscarinic receptor antagonists.

Interpretation of agonist affinity estimations: the question of distributed receptor states

In the receptor-transducer model of pharmacological agonism, rejection of the traditional assumption that receptor molecules are in vast excess of transducer molecules permits the receptors to become distributed among unbound, bound and complexed states. Under these conditions, agonist affinities are liable to be overestimated when the method of irreversible receptor antagonism is used. Graphical tests have been developed to detect distribution, and these were applied to experimental data from the interaction between 5-HT and phenoxybenzamine on aortic tissue. Significant receptor distribution was not detected by the method. However, in the model it was assumed that there was a linear relation between the concentration of ternary complex and pharmacological effect. If this relation was replaced with a saturable one the effect of receptor distribution would be masked. The implications for pharmacologists and medicinal chemists are discussed.

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.

Resultant action of cimetidine in a cardiac adenylate cyclase assay: its elucidation by concentration-ratios analysis

Cimetidine is widely used as a ligand for the classification of histamine H2-receptors in peripheral and central tissues. It demonstrates apparently simple competitive antagonism in isolated tissue assays and in brain adenylate cyclase assays. In this study its action in a cardiac adenylate cyclase assay was compared with its effects in the isolated guinea pig right atrial preparation. Using either histamine or impromidine, cimetidine expressed similar affinity (pKB = 6.1) in the intact tissue assay. In washed homogenates of guinea pig ventricle cimetidine appeared to antagonize competitively the stimulation by histamine of adenylate cyclase but the estimated pKB was 6.7. In contrast, the pKB for tiotidine was similar in both assays (7.7). Application of a recently developed concentration-ratios method indicated that cimetidine expressed a resultant action in the cyclase assay consisting of competitive H2-receptor antagonism and an additional inhibitory property involving a postreceptor action. The concentration-ratios analysis allowed the purely competitive component of cimetidine action to be quantified in the cyclase assay giving a pKB of 6.0.

Analysis of competitive antagonism when this property occurs as part of a pharmacological resultant

In this paper, pharmacological resultant is defined as the net effect of a single compound resulting from the simultaneous expression of two or more specific actions. The principles of concentration-ratio analysis are extended to develop a method for detecting and quantifying competitive antagonism when this property is a component of a pharmacological resultant. The method is general to the extent that it allows analysis of competitive antagonism in combination with all types of post-receptor intervention. Essentially it depends on the altered expression of competition by a reference antagonist. It incorporates tests for validating its application and it is independent of agonist concentration-effect curve shape: in these respects the method is analogous to Schild plot-analysis of simple competition. The methodology for the practical application of the analysis is exemplified by studying the net effect of a combination of a phosphodiesterase inhibitor (isobutylmethylxanthine) and histamine H2-receptor antagonist (metiamide) on histamine-stimulated tachycardia in guinea-pig, isolated, right atrium. Cimetidine was used as the reference antagonist. The equation used in this analysis is similar in form to one recently described by Hughes & Mackay (1985) to elucidate the situation when competitive antagonism occurs in combination with functional interactions. The relation between their method and the present analysis is discussed.

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.