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

Quantitative pharmacological analysis of antagonist binding kinetics at CRF1 receptors in vitro and in vivo

BACKGROUND AND PURPOSE

A series of novel non-peptide corticotropin releasing factor type-1 receptor (CRF(1)) antagonists were found to display varying degrees of insurmountable and non-competitive behaviour in functional in vitro assays. We describe how we attempted to relate this behaviour to ligand receptor-binding kinetics in a quantitative manner and how this resulted in the development and implementation of an efficient pharmacological screening method based on principles described by Motulsky and Mahan.

EXPERIMENTAL APPROACH

A non-equilibrium binding kinetic assay was developed to determine the receptor binding kinetics of non-peptide CRF(1) antagonists. Nonlinear, mixed-effects modelling was used to obtain estimates of the compounds association and dissociation rates. We present an integrated pharmacokinetic-pharmacodynamic (PKPD) approach, whereby the time course of in vivo CRF(1) receptor binding of novel compounds can be predicted on the basis of in vitro assays.

KEY RESULTS

The non-competitive antagonist behaviour appeared to be correlated to the CRF(1) receptor off-rate kinetics. The integrated PKPD model suggested that, at least in a qualitative manner, the in vitro assay can be used to triage and select compounds for further in vivo investigations.

CONCLUSIONS AND IMPLICATIONS

This study provides evidence for a link between ligand offset kinetics and insurmountable/non-competitive antagonism at the CRF(1) receptor. The exact molecular pharmacological nature of this association remains to be determined. In addition, we have developed a quantitative framework to study and integrate in vitro and in vivo receptor binding kinetic behaviour of CRF(1) receptor antagonists in an efficient manner in a drug discovery setting.

Effect of nimesulide on gastric acid secretion in the mouse stomach in vitro

Nimesulide, a selective inhibitor of cyclooxygenase-2, has been reported to cause less gastric damage, compared to other NSAIDs. We investigated the effect of nimesulide on basal gastric acid secretion, a contributing factor in NSAID-induced gastric damage, and histamine, pentagastrin, 5-methylfurmethide, isobutyl methylxanthine or high K(+) stimulated acid secretion in the isolated mouse stomach. The stomachs, removed from mice, were transferred into an organ bath and continuously perfused. Changes in pH following the addition of secretagogues were measured by a pH electrode system. The effects of nimesulide on basal and secretagogues-stimulated acid secretion were compared to those of indomethacin. Nimesulide (1 microM to 100 microM) produced a rightward concentration-dependent shift and reduction of maximum acid secretion of all the agonist-stimulated acid secretion curves. Indomethacin was only effective at the higher concentration of 100 microM. Compared to their effects singly, nimesulide (20 microM) and famotidine (0.15 microM) together caused a further shift without further reduction in maximum acid output of the histamine-stimulated curve, suggesting that nimesulide was not acting at the histamine H(2)-receptor. Nimesulide concentration-dependent reduction of stimulated acid secretion in the isolated mouse stomach was not by antagonism of the histamine H(2) receptor and is probably beyond the level of adenylate cyclase stimulation. A direct effect on the calcium channel is demonstrated.

Histamine receptor assays

This unit describes three standard in vitro bioassays for studying histamine H₁, H₂ and H₃ receptors in isolated intact tissues removed from the guinea pig. Both the H₁ and H₃ receptor assays are based on preparations of the ileum, whereas the spontaneously beating right atrium assay is used for the H₂-receptor.This unit describes three standard in vitro bioassays for studying histamine H₁, H₂ and H₃ receptors in isolated intact tissue.

Evidence that histamine homologues discriminate between H3-receptors in guinea-pig cerebral cortex and ileum longitudinal muscle myenteric plexus

1. The binding of the selective histamine H3-receptor agonist ([3H]-R-alpha-methylhistamine) to sites in guinea-pig cerebral cortex and ileum longitudinal muscle myenteric plexus has been characterized and a comparison made of the apparent affinities of a series of H3-receptor ligands. 2. Saturation analysis suggested that [3H]-R-alpha-methylhistamine labelled a homogeneous population of histamine H3-receptors in guinea-pig cerebral cortex (pKD=9.91+/-0. 07; nH=1.07+/-0.03; n=5) and ileum longitudinal muscle myenteric plexus (pKD=9.75+/-0.21; nH=0.97+/-0.02; n=5). There was no significant difference in the estimated affinity of [3H]-R-alpha-methylhistamine in the two tissues. The cerebral cortex had a significantly higher receptor density (3.91+/-0.37 fmol mg-1 tissue) than the ileum longitudinal muscle myenteric plexus (0. 39+/-0.11 fmol mg-1). 3. Overall, the apparent affinities of compounds, classified as H3-receptor ligands, in cerebral cortex and ileum longitudinal muscle myenteric plexus were well correlated (r=0. 91, P<0.0001) and consistent with the cerebral cortex and ileum longitudinal muscle myenteric plexus expressing histamine H3-receptor population(s) that are pharmacologically indistinguishable by the majority of histamine H3-receptor ligands. However, it was evident that the homologues of histamine within this group of compounds could discriminate between the receptor populations in the two tissues. Thus, the estimated affinity of five imidazole unbranched alkylamines (histamine, homohistamine, VUF4701, VUF4732 and impentamine) were significantly higher in the guinea-pig cerebral cortex than in the ileum longitudinal muscle myenteric plexus assay.

Analysis of alpha 1L-adrenoceptor pharmacology in rat small mesenteric artery

1. To illuminate the controversy on alpha 1A- or alpha 1L-adrenoceptor involvement in noradrenaline-mediated contractions of rat small mesenteric artery (SMA), we have studied the effects of subtype-selective alpha 1-adrenoceptor agonists and antagonists under different experimental conditions. 2. The agonist potency order in rat SMA was: A61603 >> SKF89748-A > cirazoline > noradrenaline > ST-587 > methoxamine. Prazosin antagonized all agonists with a low potency (pA2: 8.29-8.80) indicating the involvement of alpha 1L-rather than alpha 1A-adrenoceptors. 3. The putative alpha 1L-adrenoceptor antagonist JTH-601, but not the alpha 1B-adrenoceptor antagonist chloroethylclonidine (10 microM) antagonized noradrenaline-induced contractions of SMA. The potency of the selective alpha 1D-adrenoceptor antagonist BMY 7378 against noradrenaline (pA2 = 6.16 +/- 0.13) and of the selective alpha 1A-adrenoceptor antagonist RS-17053 against noradrenaline (pKB = 8.35 +/- 0.10) and against the selective alpha 1A-adrenoceptor agonist A-61603 (pKB = 8.40 +/- 0.09) were too low to account for alpha 1D- and alpha 1A-adrenoceptor involvement. 4. The potency of RS-17053 (pKB/pA2's = 7.72-8.46) was not affected by lowering temperature, changing experimental protocol or inducing myogenic tone via KCl or U46619. 5. Selective protection of a putative alpha 1A-adrenoceptor population against the irreversible action of phenoxybenzamine also failed to increase the potency of RS-17053 (pA2 = 8.25 +/- 0.06 against A61603). 6. Combined concentration-ratio analysis demonstrated that tamsulosin, which does not discriminate between alpha 1A- and alpha 1L-adrenoceptors, and RS-17053 competed for binding at the same site in the SMA. 7. In summary, data obtained in our experiments in rat SMA indicate that the alpha 1-adrenoceptor mediating noradrenaline-induced contraction displays a distinct alpha 1L-adrenoceptor pharmacology. This study does not provide evidence for the hypothesis that alpha 1L-adrenoceptors represent an affinity state of the alpha 1A-adrenoceptor in functional assays. Furthermore, there is no co-existing alpha 1A-adrenoceptor in the SMA.

Effects of histamine H2 receptor agonists and antagonists on the isolated guinea pig gallbladder

Histamine H2 receptor-mediated responses were examined on cholecystokinin-octapeptide (CCK-8)-precontracted guinea pig gallbladder in vitro, testing histamine and a series of specific histamine H2 receptor agonists and antagonists. Among the antagonists tested, zolantidine and compound SKF 92857 were previously shown to antagonize H2 receptor-mediated responses in the heart, but not in the stomach. Histamine, in the presence of the H2 receptor antagonist mepyramine (10 microM), and the H2 receptor agonists dimaprit, impromidine and amthamine, inhibited CCK-8 (3 nM)-induced contractions in a concentration-dependent fashion, with the following rank orders of potency: impromidine > amthamine > histamine > dimaprit (pD2 values were 6.73 +/- 0.04, 5.44 +/- 0.07, 4.64 +/- 0.04 and 3.71 +/- 0.05, respectively). The maximal relaxation produced by dimaprit was significantly lower than that produced by histamine, as well as by impromidine and amthamine, which behaved as full agonists. All the H2 receptor antagonists examined were able to inhibit amthamine-induced relaxation. Famotidine (pA2 = 7.09 +/- 0.26), zolantidine (pA2 = 6.54 +/- 0.11), compound SKF 92857 (pA2 = 6.58 +/- 0.13) and aminopotentidine (pA2 = 6.86 +/- 0.06) competitively antagonised the amthamine-induced effect, while iodoaminopotentidine produced surmountable antagonism only at low concentrations (1 microM, pA2 = 6.83 +/- 0.21). Finally, the slowly-dissociable antagonist loxtidine caused a non-parallel displacement to the right of the concentration--response curve to amthamine (pKB = 7.55 +/- 0.24), with a significant depression of the maximal response to the agonist, even at the lowest effective concentration (0.3 microM). In conclusion, histamine H2 receptors in guinea pig gallbladder resemble those of the heart, as regards their sensitivity to zolantidine and compound SKF 92857, which, by contrast, were unable to antagonize histamine effects at gastric H2 receptors in different experimental models.

Cardiac and gastric effects of histamine H2 receptor antagonists: no evidence for a correlation between lipophilicity and receptor affinity

1. A series of histamine H2 receptor antagonists with different lipophilicity were tested in cardiac and gastric assays in order to reveal possible differences in receptor affinity. Lipophilicity of the compounds was expressed as CLOG P (theoretically-determined logarithm of octanol:water partition coefficient) and log k' (logarithm of capacity factor, experimentally-determined by reverse-phase high performance liquid chromatography). 2. Aminopotentidine (APT) and iodoaminopotentidine (I-APT), which are both lipophilic compounds, behaved as insurmountable antagonists of histamine responses in rat isolated gastric fundus (pKB = 6.20 +/- 0.16 and 6.89 +/- 0.19, respectively) and guinea-pig isolated papillary muscle (pKB = 6.34 +/- 0.37 and 6.81 +/- 0.26, respectively). They were approximately as effective as ranitidine (RAN) in reducing histamine-induced acid secretion in the anaesthetized rat, ID50 values being 0.018 +/- 0.02, 0.020 +/- 0.03 and 0.036 +/- 0.01 mumol kg-1 i.v. for APT, I-APT and RAN, respectively. Both APT and I-APT had a significantly longer duration of action than RAN. 3. The hydrophilic compound, SK&F 92857, was inactive up to 10 microM in modifying histamine-induced acid secretion in the isolated rat stomach. In the papillary muscle, low concentrations (0.1-1 microM) of this compound produced a competitive antagonism of the histamine responses (pA2 value = 7.38 +/- 0.11), while a higher concentration (10 microM) significantly reduced the maximal response to histamine. 4. RAN competitively antagonized histamine effects with a comparable affinity in cardiac and gastric preparations (pA2 values were 6.42 +/- 0.09 and 6.78 +/- 0.38 in heart and stomach, respectively). 5. Results obtained in this study clearly showed that the discrepancies between gastric and cardiac effects observed for some H2 antagonists are not explained solely by differences in lipophilicity of compounds. Moreover, the significant correlation found between CLOG P and log k' parameter, which takes into account, besides their lipophilicity, the ionization of the molecules, suggests that ionization has a similar influence for all the molecules on the partition between the lipophilic and aqueous phase.

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.

Multiconformational composite molecular potential fields in the analysis of drug action. I. Methodology and first evaluation using 5-HT and histamine action as examples

The quality of molecular electrostatic maps generated by non-quantum mechanical methods has been improved using extended electron distributions. Further simplification has been achieved by distilling these maps down to their energy extrema. A new means of defining surface interaction has been added and the resulting composite map has been plotted for a limited number of low-lying conformers of a series of agonists and antagonists of the H2 and H3 receptors and 5-HT1A and 5-HT1D receptors. The results from the cross-comparison of these maps indicate their ability to distinguish the specific receptor. Interesting consequences of the method are that structural overlay is irrelevant, that several conformations may contribute to the overall binding pattern and that lesser pharmacological activities may be deduced from the results.

Synthesis and histamine H2-receptor antagonist activity of 4-(1-pyrazolyl)butanamides, guanidinopyrazoles, and related compounds

A series of 4-(1-pyrazolyl)butanamides, pyrazolylalkyl cyanoguanidines, and related compounds with diverse functional groups (e.g. nitro, amino, guanidino groups) in the 3-position of the pyrazole ring was prepared via 4-(3-nitro-1-pyrazolyl)butanenitrile (5) and the corresponding carboxylic acid 7 as central intermediates. The amides 9a-d were prepared from the primary amines 8a-d which represent partial structures of the H2-receptor antagonists roxatidine, cimetidine, ranitidine, and famotidine. The roxatidine-derived 4-(3-nitro-1-pyrazolyl)butanamide (9a) proved to be the compound with the highest H2-receptor antagonist activity of 23 compounds tested at the isolated guinea pig right atrium preparation, achieving about 6 times famotidine's or 160 times cimetidine's potency. By contrast, in Ghosh-Schild rats 9a did not inhibit histamine-stimulated gastric acid secretion at a dosage of 0.1 mumol/kg i.v. Compounds 20a (the 3-(trifluoroethyl-guanidino)pyrazole analogue of 9a, 12a (the cyanoguanidine analogue) and N-(4-[3-(trifluoroethylguanidino)-1-pyrazolyl]butyl)cyanogua nidine (29), which are about as active as famotidine in the atrium, turned out to be very potent inhibitors of gastric acid secretion as well (e.g., 29: 74% inhibition at 0.025 mumol/kg). These compounds are comparable to famotidine in the rat stomach and by far superior to cimetidine and ranitidine in this test system.

Examination of the role of inhibition of cyclic AMP in alpha 2-adrenoceptor mediated contractions of the porcine isolated palmar lateral vein

1. We have examined the effect of elevation of cellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) on alpha 1- and alpha 2-adrenoceptor-mediated contraction of the isolated palmar lateral vein of the pig. Cellular cyclic AMP was increased by either inhibition of phosphodiesterase by rolipram, or direct activation of adenylyl cyclase by forskolin. 2. Noradrenaline (1 nM-10 microM) caused concentration-dependent contractions of the porcine isolated palmar lateral vein (pD2 7.32 +/- 0.07, n = 10). The selective alpha 1-adrenoceptor antagonist, prazosin (0.1 microM) and the selective alpha 2-adrenoceptor antagonist, rauwolscine (1 microM) caused a 10 fold rightward displacement of the concentration-response curve and a combination of the two antagonists caused a 200 fold rightward displacement of the concentration-response curve. The selective alpha 2-adrenoceptor agonist, UK-14304, also produced concentration-dependent contractions of the palmar lateral vein (pD2 7.70 +/- 0.15, n = 5), but the maximum response was 55.5 +/- 7.6% (n = 5) of that produced by noradrenaline. Prazosin (0.1 microM) failed to affect responses to UK-14304 but rauwolscine, 1 microM, caused a 200 fold rightward displacement. The estimated pKB value for rauwolscine (8.28 +/- 0.19, n = 10) is consistent with inhibition of alpha 2-adrenoceptors. Thus, the porcine isolated palmar lateral vein has a population of alpha 1- and alpha 2-adrenoceptors capable of producing a contraction. 3. Rolipram, 10 micro M, and forskolin, 1 micro M, caused a 2-3 fold rightward displacement of the noradrenaline concentration-response curve (CRC), but 1,9-dideoxyforskolin, 1 micro M, a forskolin analogue which does not activate adenylyl cyclase, failed to produce a significant inhibition of noradrenaline induced contractions. The combination of forskolin (1 micro M) and rolipram (10 micro M) were additive, producing a 20 fold rightward displacement of the noradrenaline CRC.4. Responses to noradrenaline were similarly affected by a combination of rolipram (10 micro M) and prazosin (0.1 micro M) (isolation of alpha 2-adrenoceptors) and the combination of rolipram (10 micro M) and rauwolscine(1 micro M) (isolation of alpha l-adrenoceptors), resulting in a 100 fold rightward displacement of the noradrenaline CRC. Although forskolin inhibited both alpha l- and alpha 2-adrenoceptor-mediated contractions,the effects produced were not similar. In particular, noradrenaline, 0.3-3 micro M, produced a significant contraction in the presence of forskolin (1 micro M) and prazosin (0.1 micro M) (an alpha 2-adrenoceptor-mediated response) but not in the presence of forskolin (1 micro M) and rauwolscine (1 micro M) (an alpha l-adrenoceptor mediated response).5. Five minute exposure to either rolipram (10 micro M) or forskolin (1 micro M) elevated [3H]-cyclic AMP of the porcine isolated palmar lateral vein by approximately 70% and 150-200%, respectively. Neither noradrenaline (1 nM- 100 micro M) nor UK-14304 (1 nM- 100 micro M) affected basal levels of [3H]-cyclic AMP,but both produced a concentration-dependent inhibition of forskolin-stimulated [3H]-cyclic AMP accumulation with a pKi of 7.43 +/- 0.1 (n = 3) and 7.97 +/- 0.18 (n = 3), respectively. The effect of noradrenaline against forskolin-stimulated [3H]-cyclic AMP accumulation was reversed by rauwolscine(1 micro M) but not by prazosin (0.1 micro M). In contrast, alpha 2-adrenoceptor activation did not affect rolipram induced elevation of [3H]-cyclic AMP.6. These findings indicate that M2-adrenoceptor contractions of the porcine isolated palmar lateral vein are not produced by reduction in cellular cyclic AMP per se. It is proposed that this response involves a novel signal transduction mechanism. However, when cellular cyclic AMP has been elevated by agents that stimulate adenylyl cyclase, rather than through inhibition of phosphodiesterase, the ability of alpha 2-adrenoceptors to inhibit cyclic AMP formation may be of functional importance in vascular smooth muscle.

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 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.

Analysis of the CCKB receptor antagonism of virginiamycin in guinea-pig ileum longitudinal myenteric plexus

1. Virginiamycin, a macrolide reported to bind selectively to CCKB/gastrin receptors has been studied in a functional test, namely cholecystokinin-induced contraction of guinea-pig ileum myenteric plexus (LMMP). 2. Virginiamycin (1-10 microM) antagonized the selective CCKB agonist cholecystokinin tetrapeptide (CCK-4). The antagonism appeared not to be competitive as the highest concentration (10 microM) caused a reduction of its maximal effect. An apparent pA2 of 6.64 +/- 0.06 (s.e.) could be estimated if this depression was ignored. The selective CCKB antagonist, L-365,260 (0.01-0.3 microM) antagonized competitively the CCK-4 induced contraction and a pKB of 8.60 +/- 0.16 (s.e.) was estimated. 3. The combined dose-ratio analysis for virginiamycin, tested at 3 and 10 microM in association with 0.03 and 0.1 microM L-365,260, respectively, resulted in observed log dose-ratios of 1.39 and 1.53. That was consistent with both antagonists acting on the same receptor in LMMP. 4. These data, represent the first evidence of the antagonism of virginiamycin in a functional assay and they support the hypothesis of homogeneity between CCKB receptors in the CNS and in peripheral tissues.

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.

Expression of a cloned rat histamine H2 receptor mediating inhibition of arachidonate release and activation of cAMP accumulation

A DNA, cloned after screening a rat genomic bank with probes derived from the sequence of a putative dog histamine H2 receptor [Gantz, I., Schäffer, M., Delvalle, J., Logsdon, C., Campbell, V., Uhler, M. & Yamada, T. (1991) Proc. Natl. Acad. Sci. USA 88, 429-433], was used to prepare a probe for Northern blot analysis and to transfect Chinese hamster ovary (CHO) cells. Distribution of the gene transcripts in guinea pig tissues was consistent with that of H2 receptors. Transfected CHO cells expressed a high density of sites binding [125I]iodoaminopotentidine, a selective H2-receptor ligand. These sites were characterized as typical H2 receptors by using a series of competing agents that displayed apparent dissociation constants closely similar to corresponding values at a reference biological system. In transfected cells, histamine stimulated, with high potency and large receptor reserve, the accumulation of cAMP. In addition, in the same cells, histamine potently inhibited the release of arachidonic acid induced either by stimulation of constitutive purinergic receptors or by application of a Ca2+ ionophore. This inhibition was independent of either cAMP or Ca2+ levels. The results suggest that a single H2 receptor may be linked not only to adenylyl cyclase activation but also to reduction of phospholipase A2 activity. Because H1 receptors have been reported to stimulate arachidonic acid release, inhibition of this release, an unexpected signaling pathway for H2 receptors, may account for the opposing physiological responses elicited in many tissues by stimulation of these two receptors subtypes.

Is field (vagal) stimulation of gastric acid secretion mediated by M1 or non-M1 muscarinic receptors? A methodical problem exemplified in the mouse stomach in vitro

1. Highly controversial claims have been put forward in the literature as to the involvement of either M1 or non-M1 muscarinic receptors in the field (vagal) stimulation of gastric acid secretion. This mini-review considers three available sets of data obtained in the mouse isolated, lumen-perfused stomach. 2. While pA2 values seemed to favour non-M1 receptors, a comparison between M1 selective versus non-selective antagonists on the one hand and field stimulation versus bethanechol stimulation on the other clearly pointed to M1 receptors being involved. 3. In the present review we discuss a novel experimental approach supporting the latter concept of M1 receptors. This novel approach provides a simple though provocative way to deal with the particular difficulties in determining pA2 values in the acid-secreting gastric mucosa. It is based on the rank order in the effects of different antagonists relative to their receptor type-dependent affinities, when employed at a fixed concentration, rather than on their pA2 values.

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.

Structure-activity relationships of histamine H2-agonists, a new class of positive inotropic drugs

The cimetidine-like moiety in impromidine was replaced by either alternative partial structures known from H2-antagonists or by H2-nonspecific lipophilic groups. The most potent H2-agonists were found in a series of compounds structurally derived from the H1-antagonist pheniramine. Arpromidine (N1-[3-(4-fluorophenyl)-3-(2-pyridinyl)propyl]-N2-[3-(1H-imidazol-4- yl)propyl]guanidine) may be considered a new lead for the development of "cardiohistaminergics". This guanidine combines both about 100 times the potency of histamine in the isolated guinea-pig atrium (pD2 = 8.0) and H1-antagonistic activity (pA2 = 7.65) in the range of pheniramine. Analogues difluorinated in 3,4-(BU-E-75) or 3,5-position (BU-E-76) or chlorinated in 3,4-position (BU-E-64) are up to 160 times more potent H2-agonists than histamine. In contrast to other types of guanidines, in the arpromidine series the order of potency found in guinea-pig atria was in good agreement with the results from isolated perfused guinea-pig hearts. In particular, the two-fold halogenated arpromidine analogues proved to be more potent positive inotropic agents than impromidine with lower stimulating effects on heart rate and reduced arrhythmogenic properties.

Receptor classification using antagonists without determining pA2 values

Albeit time-consuming, the most rigorous method of classification of receptor subtypes is the determination of pA2 values. However, this methodology requires the knowledge of absolute concentrations of both agonists and antagonists--something that may not necessarily be calculable under certain experimental conditions. Wolfgang Kromer offers a different method for classification using antagonists which is not dependent on absolute affinities, and which has the additional advantage of speed.

Histamine receptors: subclasses and specific ligands

In this review the three main types of histamine receptors are discussed together with their specific ligands. For the classical H1-receptors much emphasis is put on the mechanism by which the receptor is stimulated. For the H1- and H2-receptor the review includes information on the several models available for establishing agonistic or antagonistic activity. In the section on the H3-receptor the ligands are discussed as well as the possible physiological role of this receptor. In the final paragraphs some less well defined activities are presented.

Electrophysiological studies on the effects of opiates on dorsal horn nociceptive neurones at 51 atmospheres absolute

The purpose of this study was to investigate the effects, if any, of high pressure on opiate analgesia. The interaction of high pressure with intrathecal morphine (2.5-15 micrograms) and pethidine (10-145 micrograms) was studied on the responses of 51 dorsal horn neurones in the intact rat under urethane anaesthesia to electrical stimulation applied to their receptive fields. Two types of response to the addition of the opiates were found. Cells were either rapidly and maximally inhibited by the lowest dose of morphine (2.5 micrograms) or pethidine (10 micrograms) or slowly inhibited in a dose-dependent manner with an ED50 of 13.6 nmol for morphine and 401 nmol for pethidine. Pressure did not significantly affect the time-response curves for these two types of response but did change the relative numbers of each type recorded. The number of cells totally inhibited by the lowest of drug concentrations was increased for morphine at pressure but decreased for pethidine.