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

Analytical Pharmacology: How Numbers Can Guide Drug Discovery

The unique ways in which pharmacological data compares to mathematical models are described. Examples show that insights into agonist action (prediction of agonism in vivo) and antagonist mechanism of action (orthosteric vs allosteric) can be gained that assist in the candidate selection process for new drugs in drug discovery and development efforts. In addition, the impact of component processes on complex physiological systems can be delineated, such as the effects of the hepatic system on whole body clearance in pharmacokinetics and prediction of drug-drug interactions. Finally, models are instrumental in the procurement of universal drug parameters that can be used in medicinal chemistry-based structure-activity relationships. The revitalization of these ideas under the banner of "Analytical Pharmacology" may serve to re-emphasize these concepts over qualitative description and lead to a better foundation for drug discovery.

International Union of Basic and Clinical Pharmacology. XC. multisite pharmacology: recommendations for the nomenclature of receptor allosterism and allosteric ligands

Allosteric interactions play vital roles in metabolic processes and signal transduction and, more recently, have become the focus of numerous pharmacological studies because of the potential for discovering more target-selective chemical probes and therapeutic agents. In addition to classic early studies on enzymes, there are now examples of small molecule allosteric modulators for all superfamilies of receptors encoded by the genome, including ligand- and voltage-gated ion channels, G protein-coupled receptors, nuclear hormone receptors, and receptor tyrosine kinases. As a consequence, a vast array of pharmacologic behaviors has been ascribed to allosteric ligands that can vary in a target-, ligand-, and cell-/tissue-dependent manner. The current article presents an overview of allostery as applied to receptor families and approaches for detecting and validating allosteric interactions and gives recommendations for the nomenclature of allosteric ligands and their properties.

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M₃ with a small contribution of the M₁ receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M₂ and M₃ with the M₂ outnumbering the M₃ by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M₃ receptor mechanism despite the major presence of the M₂ receptor in smooth muscle. These results are consistent with the conditional role of the M₂ receptor in smooth muscle. That is, the contractile role of the M₂ receptor depends on that of the M₃ so that antagonism of the M₃ receptor eliminates the response of the M₂. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M₃ antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.

In vivo pharmacological resultant analysis reveals noncompetitive interactions between opioid antagonists in the rat tail-withdrawal assay

BACKGROUND AND PURPOSE

Pharmacological resultant analysis is a technique that can detect secondary effects of competitive antagonists in vitro. The utility of pharmacological resultant analysis as a potential tool for the investigation of antagonist interactions in vivo was examined in the present study using two opioid antagonists, naltrexone and CTAP.

EXPERIMENTAL APPROACH

Using the experimental design of pharmacological resultant analysis, the well-characterized opioid antagonist naltrexone was examined in the presence of multiple doses of CTAP to block the antinociceptive effects of morphine in the rat warm-water (55(o)C), tail-withdrawal assay.

KEY RESULTS

Alone, all doses of naltrexone, CTAP, and CTOP examined blocked the antinociceptive effects of morphine. In the presence of fixed doses of 1 or 10 microg CTAP, increasing doses of naltrexone produced dose-dependent shifts to the right in the morphine dose-response curve. However, a lower dose of naltrexone in combination with 1 or 10 mug CTAP failed to alter the morphine dose-response curve. In the presence of a fixed dose of 0.1 mg kg(-1) naltrexone, CTAP doses produced irregular shifts to the right in the morphine dose-response curves.

CONCLUSIONS AND IMPLICATIONS

Resultant analysis was applied and an apparent pK(C) value for CTAP was found to be one log unit higher than the apparent pA(2) value for CTAP, evidence that CTAP may have secondary actions or that a signal transducer function may be altered by the combinations of these antagonists. Taken together, these data suggest pharmacological resultant analysis can reveal novel interactions between antagonists in vivo.

Involvement of 5-hydroxytryptamine (HT)7 receptors in the 5-HT excitatory effects on the rat urinary bladder

OBJECTIVE

To investigate the in vitro and in vivo effects of 5-hydroxytryptamine (5-HT) on the rat urinary bladder and to characterize the receptors involved in mediating these pharmacological effects by using selective antagonists.

MATERIALS AND METHODS

Female Wistar rats (250-350 g) were used for all studies. In vitro, detrusor muscle strips were mounted between two platinum electrodes in organ baths filled with a modified Krebs' solution bubbled with 95% O(2) and 5% CO(2) at 37 degrees C. After equilibration and a contraction to 80 mmol/L KCl, strips were exposed to electrical field stimulation for 30 min and incubated with the antagonist or vehicle for a further 30 min, then a 5-HT concentration-response curve (CRC) was obtained. In vivo, rats were anaesthetized with pentobarbital, and the ureters and urethra ligated, the bladder catheterized and infused with saline. 5-HT (3-100 microg/kg intravenous) dose-dependently increased intravesical pressure (IVP). After administering 5-HT at 30 microg/kg three times at 10 min intervals (controls), one dose of antagonist was perfused for 5 min and, after a further 5 min, 30 microg/kg 5-HT was tested again. This cycle was repeated four times using increasing doses of the antagonist to be tested.

RESULTS

In vitro, 5-HT (0.01-100 micromol/L) induced a concentration-dependent enhancement of the neurogenic response, with a mean (sd) pEC(50) of 6.36 (0.15) and E(max) of 41.1 (4.6)% KCl (eight rats). In unstimulated tissues, 5-HT induced no contractile effect. Selective 5-HT(4), 5-HT(3) and 5-HT(1A) receptor antagonists had no effect on the 5-HT potentiating effects. The potentiating effect of 5-HT was antagonized by mesulergine at 0.3 micromol/L, R(+)lisuride at 0.3 micromol/L and the selective 5-HT(7) receptor antagonist SB-258741 at 0.3 micromol/L. In vivo, in anaesthetized rats, IVP increases induced by repeated doses of 30 microg/kg 5-HT were reproducible. R(+)lisuride (3-100 microg/kg) dose-dependently inhibited the 5-HT-induced increase of IVP. At the maximum dose tested, R(+)lisuride almost totally inhibited the 5-HT effect.

CONCLUSIONS

In rat isolated detrusor muscle the 5-HT(7) receptor antagonists SB-258741, R(+)lisuride and mesulergine blocked the 5-HT potentiating effect with the expected potency. Moreover, in anaesthetized rats, R(+)lisuride abolished 5-HT effects on IVP at doses that antagonize physiological effects known to be mediated by 5-HT(7) receptor activation in several animal species. These results suggest the involvement of 5-HT(7) receptors in the modulation of rat bladder contraction both in vitro and in vivo.

Comparison of the Antimuscarinic Action of p-Fluorohexahydrosiladifenidol in Ileal and Tracheal Smooth Muscle

We investigated the ability of the muscarinic antagonist p-fluorohexahydrosiladifenidol to inhibit muscarinic agonist-induced contractions and phosphoinositide hydrolysis in the guinea pig ileum and trachea. This antagonist displayed higher potency at blocking oxotremorine-M-induced contractions of the ileum compared with those of the trachea. When estimated using a simple model for competitive antagonism, the observed dissociation constant of p-fluorohexahydrosiladifenidol exhibited approximately 12-fold higher potency in the ileum compared with the trachea. We also investigated the ability of p-fluorohexahydrosiladifenidol to affect the inhibition of contraction caused by the known competitive muscarinic antagonist atropine. Using resultant analysis to analyze this interaction, we found that the true dissociation constant of p-fluorohexahydrosiladifenidol for competitively antagonizing oxotremorine-M-induced contractions in the ileum exhibited significantly lower potency than when calculated assuming a simple competitive model. In contrast, resultant analysis showed little difference between the true and observed potencies of p-fluorohexahydrosiladifenidol for antagonizing oxotremorine-M-induced contractions in the trachea. Using a simple competitive model, we found little difference in the observed dissociation constant of p-fluorohexahydrosiladifenidol for antagonizing oxotremorine-M-induced phosphoinositide hydrolysis in guinea pig ileum and bovine trachea. We also noted that p-fluorohexahydrosiladifenidol (0.3-1.0 microM) moderately inhibited histamine-induced contractions of ileum but not of trachea. Our results suggest that p-fluorohexahydrosiladifenidol does not discriminate markedly between M(3) muscarinic receptors in the ileum and trachea and that it may posses a more potent, nonmuscarinic inhibitory effect on contraction in the ileum.

Complexity in biological signaling systems

Biological signaling pathways interact with one another to form complex networks. Complexity arises from the large number of components, many with isoforms that have partially overlapping functions; from the connections among components; and from the spatial relationship between components. The origins of the complex behavior of signaling networks and analytical approaches to deal with the emergent complexity are discussed here.

Application of an allosteric ternary complex model to the technique of pharmacological resultant analysis

A simple ternary complex model of drug-receptor interaction has been used to extend the procedure of pharmacological resultant analysis, enabling the quantitation of interactions between allosteric modulators and orthosteric antagonists. Equations derived in the theoretical treatment were used to analyse functional data for the interaction between the allosteric modulator gallamine and the orthosteric antagonist scopolamine, with oxotremorine as the agonist, at rat tracheal muscarinic acetylcholine receptors. Quantitative estimates of the affinity of gallamine for the allosteric site (pKz = 4.7) and the extent of negative, heterotropic co-operativity between gallamine and scopolamine (alpha' = 13.1) were obtained. Furthermore, an alternative direct, model-fitting approach, that does not rely on the determination of concentration ratios, was also developed, and yielded similar results. It is suggested that the approach presented in this paper is useful for quantifying interactions between orthosteric antagonists and allosteric modulators, particularly when the extent of co-operativity is low or the modulators possess multiple pharmacological properties, or both.

SDZ NVI 085, an alpha 1A-adrenoceptor agonist with 5-HT2A receptor antagonist properties

(+/-)-SDZ NVI 085 (3,4,4a,5,10,10a-hexahydro-6-methoxy-4-methyl-9- methylthio-2H-naphth [2,3-b]-1,4-oxazine hydrochloride), an alpha 1-adrenoceptor agonist, produced a concentration-dependent relaxation (pIC50 of 7.2 +/- 0.1) in the isolated caudal artery of rat precontracted with serotonin (5-hydroxytryptamine, 5-HT, 1 microM). (+/-)- SDZ NVI 085 had no effect upon caudal arteries precontracted with vasopressin or U46619 (9,11-dideoxy-11 alpha, 9 alpha-epoxymethano-prostaglandin F2 alpha). In other studies, (+/-)-SDZ NVI 085 shifted 5-HT concentration-effect curves to the right, in a concentration-dependent manner, and Schild regression gave a pA2 estimate of 8.0 (slope of 1.0). Experiments using pharmacological resultant analysis indicated a syntopic interaction of (+/-)-SDZ NVI 085 with ketanserin (a 5-HT2 receptor antagonist) toward 5-HT-induced contractions. It is concluded that (+/-)-SDZ NVI 085 behaves as a reversible competitive 5-HT2A receptor antagonist, a property which may be of importance regarding its pharmacological effects in vivo.

Mode of antagonism of methoctramine, AF-DX 116 and hexahydrosiladifenidol in guinea-pig left atrium and ileum: comparison of Schild and resultant analysis

1. Methoctramine, AF-DX 116 and hexahydrosiladifenidol (HHSiD) are the muscarinic antagonists most widely used to study muscarinic receptor subtypes. 2. The present study was undertaken to examine the mode of antagonism of these compounds in guinea-pig left atrium and ileum by comparison of the Schild and resultant analysis. With this method the effect of various concentrations of the test antagonist on the antagonism produced by specific concentrations of a reference antagonist was measured and the equilibrium dissociation constant of the test antagonist-receptor complex estimated. Atropine was used for comparative purposes and scopolamine as the reference antagonist. 3. At the cardiac level the affinity values obtained by Schild and resultant analysis for methoctramine and AF-DX 116, as for atropine, are very similar: these results indicate that the two cardio-selective antagonists and the non-selective antagonist, atropine, bind at a common site with the reference antagonist scopolamine. The resultant plot for the ileo-selective HHSiD has a slope considerably less than unity: this finding might indicate that this antagonist binds to a site different from that of scopolamine and it should be considered like an allosteric antagonist. 4. At the ileal level the affinity values obtained by Schild and resultant analysis are identical for the ileo-selective antagonist HHSiD as for atropine but not for methoctramine and AF-DX 116. This indicates a mutual binding site with scopolamine for HHSiD and atropine but not for the two cardio-selective antagonists. However, it is worth emphasizing that the difference between affinity values obtained by Schild and resultant analysis is seen when relatively high concentrations are required: a dual mode of interaction (both competitive and allosteric) could be involved.

Pharmacological analysis of ecto-ATPase inhibition: evidence for combined enzyme inhibition and receptor antagonism in P2X-purinoceptor ligands

1. Previous studies have shown that suramin and FPL 66301 are competitive antagonists at the P2X-purinoceptor in the rabbit ear artery. Those studies employed alpha,beta-methylene ATP, a poorly hydrolysable ATP analogue, as the agonist. In this study these compounds have been tested using ATP as the agonist. 2. Suramin, in the concentration range 30-1000 microM, potentiated the contractile effects of ATP, producing a 3-fold leftward shift of the ATP E/[A] curves. FPL 66301, in the concentration range 100-1000 microM, produced a significant but small (approximately 3-fold) rightward shift of the ATP curves. These results are in marked contrast with previous studies using alpha,beta-methylene ATP in which 30-fold rightward shifts were achieved using the same concentration ranges of suramin and FPL 66301. 3. Suramin and FPL 66301 were tested as ecto-ATPase inhibitors in a human blood cell assay. Suramin inhibited the enzyme with a pIC50 of 4.3, FPL 66301 with a pIC50 of 3.3. 4. The pharmacological data were analysed using a theoretical model describing the action of a compound with dual enzyme inhibitory and receptor antagonistic properties on the effects of an agonist susceptible to enzymatic degradation. The model was found to fit the data well using the known pKB estimates for suramin and FPL 66301 and similar relative (but not absolute) pK1 estimates to those obtained for the compounds in the enzyme assay. 5. From this analysis it was concluded that the limited shifts of ATP E/[A] curves produced by suramin and FPL 66301 were the result of 'self-cancellation' of the potentiating (enzyme inhibitory) and rightward-shifting (receptor antagonistic) properties.6. The analysis also indicated that the presence of ecto-ATPase activity in the rabbit ear artery preparation has a marked effect on the apparent potency of ATP. The experimental p[A50] was 3.4,whereas the 'true' value, that is the value which would be obtained in the absence of ecto-ATPase activity, was 6.0, some 400-fold higher.7 Two conclusions are drawn from this study. Firstly, caution must be exercised in the use of suramin and FPL 66301 as tools for receptor classification. Absence of overt antagonism by these compounds when metabolically unstable agonists are used could lead to erroneous claims for receptor subtypes.Secondly, the agonist potency order currently used to designate P2X- purinoceptors may require modification.

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.

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.

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.

Positive cooperative interaction of quaternary anticholinergics with functional muscarinic receptors in bovine tracheal smooth muscle

The interaction of quaternary anticholinergics with muscarinic receptors in bovine tracheal smooth muscle strips was investigated because some of these compounds have shown anomalous (biphasic) behaviour in radioligand displacement studies, in contrast to their tertiary analogues. It was found that ipratropium, N-methylscopolamine, oxyphenonium and N-methyldeptropine give Schild plots with slopes significantly greater than unity (up to 2.0) in contrast to 4-DAMP methobromide and thiazinamium, and the tertiary analogues atropine and scopolamine. However, in guinea pig tracheal smooth muscle, ipratropium and N-methyldeptropine behaved as classic antagonists with Schild slopes of unity. The high Schild plot slopes in bovine tracheal smooth muscle could not be solely explained by inadequate equilibration of the antagonists, since increased incubation times (3 or 5 h instead of 30 min) still brought about slopes significantly greater than unity, or by the presence of an atropinesterase in the tissue. However, by using combinations of atropine with ipratropium or oxyphenonium it could be demonstrated that these quaternary antagonists interact with muscarinic M3 receptors in bovine but not in guinea pig tracheal smooth muscle in a positive cooperative fashion.

Promiscuous or heterogeneous muscarinic receptors in rat atria? II. Antagonism of responses to carbachol by pirenzepine

Carbachol produces both negative and positive inotropy in rat left atria. It is not clear whether these two effects are mediated by two separate cell surface muscarinic receptors or a single receptor interacting with two coupling proteins in the cell membrane. Pirenzepine, known to selectively block some biochemical muscarinic responses, was used in this study to block the biphasic response to carbachol in rat left atria. The negative inotropy to carbachol was blocked by pirenzepine, and Schild analysis indicated a -log dissociation constant (pKb) for the pirenzepine-receptor complex of 6.2. However, the Schild analysis may have been complicated by positive inotropy observed with pirenzepine. This positive inotropic effect was sensitive to blockade by other muscarinic antagonists. In atria from rats pretreated with pertussis toxin, carbachol produced a positive inotropic effect. Schild analysis with pirenzepine for antagonism of this response indicated a -log equilibrium dissociation constant for the pirenzepine-receptor complex of 6.7, significantly different from that for antagonism of negative inotropy. This ostensibly suggested a difference in the receptors mediating these responses. In view of the possible complicating effects of the positive inotropic effects of pirenzepine in this assay, an alternative method for the measurement of pirenzepine affinity was utilized. Resultant analysis was used to measure the pKb for pirenzepine antagonism of negative inotropy to carbachol. This method had the advantage of cancelling the positive inotropy to pirenzepine. Under these circumstances, pirenzepine had a pKb of 6.9, a value not significantly different from for antagonism of the positive inotropy to carbachol. The relevance of these findings is discussed in terms of a single promiscuous muscarinic receptor or heterogeneous receptors in this tissue. These data do not support the hypothesis that two separate receptors mediate these two effects.

Drugs and receptors. An overview of the current state of knowledge

This paper reviews the theoretical concepts and methods utilised with isolated tissues to characterise drugs and drug receptors. Specifically the impact, on the in vitro measurement of agonist affinity and relative efficacy, of the idea that receptors bind to transduction proteins in the lipid bilayer of the cell membrane is discussed. The effects of ternary complex formation of agonist-receptor equilibria raise theoretical objections to the measurement of agonist receptor equilibrium dissociation constants. Possible 'promiscuity' of receptors with respect to the G-proteins with which they can interact makes classification of receptors by agonists suspect. The use of Schild analysis for the measurement of antagonist affinity and subsequent classification of receptors is considered in the light of recent data showing that estimates calculated with this method are heterogeneous. Resultant analysis for the detection of allosteric effects is also discussed. Lastly, the impact of molecular biology on the drug and drug receptor classification process is considered, as well as the effects of pathological processes on drug action at the receptor level.

Mechanism of action of H2-antagonists on histamine- or dimaprit-stimulated H2-receptors of spontaneously beating guinea-pig atrium

Cimetidine, ranitidine and famotidine are antagonists of the histamine H2-receptors on the spontaneously beating right atrium of the guinea pig. When analyzed by the classical Schild method their pA2-values are respectively: 6.3, 6.8 and 7.7 with dimaprit as agonist and 5.8, 6.5 and 7.7 with histamine as agonist. Radioligand-displacement studies with [3H]-tiotidine as radioligand resulted in pKd values for cimetidine, ranitidine and famotidine of 6.3; 6.9 and 8.2 respectively. In dimaprit-stimulated atria all antagonists added at concentrations above their Kd values depressed the maximal increase in frequency. In the presence of histamine this effect was much less pronounced and only visible at concentrations of ranitidine and famotidine around 10.Kd. The rightward shift of the curves as well as the decrease in Emax are reversible but the dissociation constants of the antagonists are small (less than 10(-3) s-1). The spontaneously beating right atrium showed receptor reserve for histamine and virtually no receptor reserve for dimaprit. The results have been interpreted in a model in which H2-antagonists act mainly by competing with the agonist for the histamine receptor site but have in addition a distinct affinity for a secondary site on the receptor. Occupation of this site by the antagonist prevents building up of the stimulus elicited by the agonist and thus decreases the Emax. In systems with receptor reserve (histamine) the effect of antagonist binding to the secondary binding site is seen only at high concentration of antagonist while in absence of receptor reserve (dimaprit) the depression of Emax is directly visible. Simulations of the model show that the affinity of this secondary binding site is 50-(famotidine) to 100-(cimetidine and ranitidine) fold lower than for the agonist binding site.

Isobolic determination of "ratio-linear" parameters in pharmacological stimulus-response models

Two response models of fundamental importance in pharmacology are introduced, which are partly-specified functions of experimental variables, unknown parameters and random disturbances. An analysis is carried out of the extent of determination of the parameters from knowledge of isobol structure alone, with results that are more powerful than might have been expected. Moreover, such knowledge is exhaustive, unless untestable assumptions are admitted. The paper is not concerned with statistical methods for estimating the determined parameters.