Antagonism of an indirectly acting agonist: block by propranolol and sotalol of the action of tyramine on rat heart

The hemostatic effect study of Cirsium setosum on regulating α1-ARs via mediating norepinephrine synthesis by enzyme catalysis

BACKGROUND

Cirsium setosum (CS) is the aboveground part of Cephalanoplos segetum Kitam. Although it has been used as a hemostatic treatment for thousands of years and is still in use today, the mechanism of CS on regulating ARs is still not clear.

PURPOSE

In this study, we aimed to clarify the mechanism of CS on regulating ARs.

METHODS

We developed a simple method based on UPLC/Q-TOF MS combined adrenergic receptor dual-luciferase reporter assay systems for the rapid determination of active constituents in CS. The mechanism of tyramine, the main active component for regulating ARs, was further investigated by an in vitro norepinephrine biotransformation test and in vivo vaso activity tests.

RESULTS

Two phenethylamine ARs regulators (tyramine and N-methyltyramine) in CS were characterized, and it was found that tyramine could induce vasoconstriction via regulation of α1-ARs by mediating norepinephrine synthesis.

CONCLUSION

The hemostatic effect of CS is associated with tyramine and N-methyltyramine, via regulation of α1-ARs, and the mechanism of tyramine is related to mediating norepinephrine synthesis by enzyme catalysis.

Pharmacological characterization of adenosine receptors on isolated human bronchi

Adenosine induces airways obstruction in subjects with asthma, but the receptor subtype responsible remains unknown. The objectives of this study were to determine the pharmacological profile of adenosine receptor subtypes mediating contraction and to investigate the mechanism in normal and passively sensitized human airway tissues. Contraction of bronchial rings isolated from resected lung tissue of patients with lung carcinoma was measured in response to nonselective adenosine receptor agonists, 5-AMP and 5'-(N-Ethylcarboxamido)adenosine, and A(1) receptor agonist, N(6)-cyclopentyladenosine, in the absence and presence of selective adenosine receptor antagonists. Pharmacological antagonists, chemical ablation of airway sensory nerves using capsaicin, and passive sensitization of tissue with serum from subjects with atopy and asthma was used to investigate the mechanism of contraction. Human bronchial tissue contracted in a concentration-dependent manner to adenosine agonists that showed a rank order of activity of A(1) > A(2B) >> A2(A) = A3. The maximum contractile response to N(6)-cyclopentyladenosine (231.0 ± 23.8 mg) was significantly reduced in tissues chemically treated with capsaicin to desensitize sensory nerves (desensitized: 101.6 ± 15.2 mg; P < 0.05). Passive sensitization significantly augmented the contraction induced by adenosine A(1) receptor activation (sensitized: 389.7 ± 52.8 mg versus nonsensitized; P < 0.05), which was linked to the release of leukotrienes, and not histamine (MK571: 25.5 ± 1.7 mg; epinastine 260.0 ± 22.2 mg versus control; P < 0.05). This study provides evidence for a role for adenosine A(1) receptors in eliciting human airway smooth muscle constriction, which, in part, is mediated by the action of capsaicin sensitive sensory nerves.

CC12, a high-affinity ligand for [3H]cimetidine binding, is an improgan antagonist

Improgan, a chemical congener of cimetidine, is a highly effective non-opioid analgesic when injected into the CNS. Despite extensive characterization, neither the improgan receptor, nor a pharmacological antagonist of improgan has been previously described. Presently, the specific binding of [(3)H]cimetidine (3HCIM) in brain fractions was used to discover 4(5)-((4-iodobenzyl)thiomethyl)-1H-imidazole, which behaved in vivo as the first improgan antagonist. The synthesis and pharmacological properties of this drug (named CC12) are described herein. In rats, CC12 (50-500nmol, i.c.v.) produced dose-dependent inhibition of improgan (200-400nmol) antinociception on the tail flick and hot plate tests. When given alone to rats, CC12 had no effects on nociceptive latencies, or on other observable behavioral or motor functions. Maximal inhibitory effects of CC12 (500nmol) were fully surmounted with a large i.c.v. dose of improgan (800nmol), demonstrating competitive antagonism. In mice, CC12 (200-400nmol, i.c.v.) behaved as a partial agonist, producing incomplete improgan antagonism, but also limited antinociception when given alone. Radioligand binding, receptor autoradiography, and electrophysiology experiments showed that CC12's antagonist properties are not explained by activity at 25 sites relevant to analgesia, including known receptors for cannabinoids, opioids or histamine. The use of CC12 as an improgan antagonist will facilitate the characterization of improgan analgesia. Furthermore, because CC12 was also found presently to inhibit opioid and cannabinoid antinociception, it is suggested that this drug modifies a biochemical mechanism shared by several classes of analgesics. Elucidation of this mechanism will enhance understanding of the biochemistry of pain relief.

Determining the potency and molecular mechanism of action of insurmountable antagonists

Insurmountable antagonism (maximal response to the agonist depressed) can result from a temporal inequilibrium involving a slow offset orthosteric antagonist or be the result of an allosteric modulation of the receptor. The former mechanism is operative when the antagonist, agonist, and receptors cannot come to proper equilibrium during the time allotted for collection of agonist response (hemi-equilibrium conditions). Allosteric effects (changes in the conformation of the receptor through binding of the allosteric modulator to a separate site) can preclude the agonist-induced production of response, leading to depression of maximal responses. In these cases, the effects on receptor affinity can be observed as well. The first premise of this article is that system-independent estimates of insurmountable antagonist potency can be made with no prior knowledge of molecular mechanism through the use of pA(2) (-log molar concentration of antagonist producing a 2-fold shift of the concentration response curve) measurements The relationship between the pA(2) and antagonist pK(B) (-log equilibrium dissociation constant of the antagonist-receptor complex) is described; the former is an extremely close approximation of the latter in most cases. The second premise is that specially designed experiments are required to differentiate orthosteric versus allosteric mechanisms; simply fitting of data to orthosteric or allosteric theoretical models can lead to ambiguous results. A strategy to determine whether the observed antagonism is orthosteric (agonist and antagonist competing for the same binding site on the receptor) or allosteric in nature is described that involves the detection of the hallmarks of allosteric response, namely saturation and probe dependence of effect.

Drugs from emasculated hormones: the principle of syntopic antagonism

This lecture outlines the early stages in the discovery of adrenaline beta-receptor antagonists and of the histamine H2-receptor antagonists. It ends with a brief personal view about future research.

Acute or chronic effects of cannabinoids on spontaneous or pharmacologically induced yawning in rats

Yawning is a reflex or event that is not fully understood. It is controlled by many neurotransmitters and neuropeptides and can be induced pharmacologically by cholinergic or dopaminergic agonists. Amongst their many actions, cannabinoids acting on cannabinoid (CB(1) or CB(2)) receptors can alter cholinergic and/or dopaminergic activity. This study examined the effects of Delta(8)-tetrahydrocannabinol (Delta(8)-THC) administered acutely (2.5 mg/kg intraperitoneally [ip], 15 min before test) or chronically (5 mg/kg for 30 days followed by 24 h or 7 days of discontinuation) on yawning induced by pilocarpine, a cholinergic agonist (0, 1, 2, 4 or 8 mg/kg ip), or apomorphine, a dopaminergic agonist (0, 20, 40 or 80 microg/kg subcutaneously [sc]). Acute effects of different doses of Delta(9)-tetrahydrocannabinol (Delta(9)-THC: 0, 0.5, 1.25 or 2.5 mg/kg ip) on yawning induced by pilocarpine (2 mg/kg ip) or apomorphine (40 microg/kg sc) were also investigated. Both pilocarpine and apomorphine produced yawning in a dose-related manner. Acute administration of Delta(8)-THC and Delta(9)-THC significantly reduced yawning induced by both pilocarpine and apomorphine. Chronic administration of Delta(8)-THC did not change yawning induced by either agonist 24 h or 7 days after discontinuation of Delta(8)-THC. However, a high frequency of spontaneous yawning was observed 7 days after Delta(8)-THC discontinuation. These results suggest that cannabinoid agonists inhibited yawning induced by cholinergic or dopaminergic agonists. In addition, the increased frequency of spontaneous yawning following cessation of chronic administration of a cannabinoid agonist may be of importance as a withdrawal sign for these drugs.

Amphetamine, 3,4-methylenedioxymethamphetamine, lysergic acid diethylamide, and metabolites of the catecholamine neurotransmitters are agonists of a rat trace amine receptor

The trace amine para-tyramine is structurally and functionally related to the amphetamines and the biogenic amine neurotransmitters. It is currently thought that the biological activities elicited by trace amines such as p-tyramine and the psychostimulant amphetamines are manifestations of their ability to inhibit the clearance of extracellular transmitter and/or stimulate the efflux of transmitter from intracellular stores. Here we report the discovery and pharmacological characterization of a rat G protein-coupled receptor that stimulates the production of cAMP when exposed to the trace amines p-tyramine, beta-phenethylamine, tryptamine, and octopamine. An extensive pharmacological survey revealed that psychostimulant and hallucinogenic amphetamines, numerous ergoline derivatives, adrenergic ligands, and 3-methylated metabolites of the catecholamine neurotransmitters are also good agonists at the rat trace amine receptor 1 (rTAR1). These results suggest that the trace amines and catecholamine metabolites may serve as the endogenous ligands of a novel intercellular signaling system found widely throughout the vertebrate brain and periphery. Furthermore, the discovery that amphetamines, including 3,4-methylenedioxymethamphetamine (MDMA; "ecstasy"), are potent rTAR1 agonists suggests that the effects of these widely used drugs may be mediated in part by this receptor as well as their previously characterized targets, the neurotransmitter transporter proteins.

The endogenous cannabinoid agonist, anandamide stimulates sensory nerves in guinea-pig airways

The endogenous cannabinoid agonist, anandamide produced a modest contractile response in guinea-pig isolated bronchus compared with the vanilloid receptor agonist capsaicin. The contractile response to both anandamide and capsaicin was inhibited by the vanilloid receptor antagonist, capsazepine. Furthermore, the NK(2)-selective antagonist, SR48968 but not the NK(1)-selective antagonist, SR140333 inhibited contractile responses to anandamide. The contractile response to anandamide was abolished in tissues desensitized by capsaicin. However, anandamide failed to cross-desensitize the contractile response to capsaicin. The contractile response to anandamide was not significantly altered in the presence of the CB(1) receptor antagonist, SR141716A, nor the amidase inhibitor, phenylmethylsulphonyl fluoride (PMSF) but was significantly increased in the presence of the neutral endopeptidase inhibitor, thiorphan. The cannabinoid agonist, CP55,940 failed to significantly attenuate the excitatory non-adrenergic non-cholinergic (eNANC) response in guinea-pig airways. In contrast, the ORL(1) receptor agonist, nociceptin, significantly inhibited this response. The results demonstrate that anandamide induces a modest contractile response in guinea-pig isolated bronchus that is dependent upon the activation of vanilloid receptors on airway sensory nerves. However, cannabinoid receptors do not appear to play a role in this regard, nor in regulating the release of neuropeptides from airway sensory nerves under physiological conditions.

Acetylcholinesterase-independent action of diisopropyl-flurophosphate in the rat aorta

Recent studies have shown that many organophosphates can bind competitively and noncompetitively to membrane muscarinic receptors. The present study investigated the responses of the rat aortic rings to diisopropyl-flurophosphate (DFP), an organophsophorus cholinesterase inhibitor, and the possible involvement of muscarinic receptors. DFP caused a concentration-dependent contraction when added cumulatively from 10(-8) to 10(-4) M. This contraction was inhibited in a noncompetitive manner by high concentrations of atropine (1.5 x 10(-6) and 1.8 x 10(-6) M) but was unaffected by similar concentrations of selective muscarinic receptor subtype antagonists, pirenzepine, 11-2[2-[(diethylamino)methyl]-1-piperidinyl]acetyl-5, 11-dihydro-6H-rido[2,3-b][1,4]benzodiazepin-6-one (AF-DX116) and 4-Diphenylacetoxy-N-methyl piperidine methiodide (4-DAMP). Phentolamine, an alpha-adrenoceptor antagonist, was able to inhibit the DFP-induced contraction in a noncompetitive manner at a concentration of 10(-7) M. These findings suggested that the DFP-induced contraction in the rat aortic rings was mediated by norepinephrine that was released from sympathetic nerve terminals present in the aortic rings.

Mechanisms involved in the relaxant response of bradykinin in epithelium intact strips of the guinea-pig trachea

Kinins caused graded relaxations in guinea-pig trachea with epithelium under spontaneous or carbachol-induced tone. The order of potency was: [Tyr8]bradykinin > lysyl-bradykinin > bradykinin > methionyl-lysyl- bradykinin. The bradykinin B1 receptor agonist des-Arg9-bradykinin (1 microM) was inactive. Relaxation in response to bradykinin (100 nM) was unaffected by tetrodotoxin (0.3 microM), nicardipine (1 microM), Ca(2+)-free solution without or plus ryanodine (10 microM), propranolol (1 microM), glibenclamide (1 microM), staurosporine (0.3 microM), nickel chloride (100 microM) or [D-p-Cl-Phe6,Leu17]VIP (a vasoactive intestinal peptide receptor antagonist, 0.03 microM), but was partially inhibited by apamin (0.3-1 microM). Both HOE 140 (D-Arg-[Hyp3,Thi5,D-Tic7,Oic8]bradykinin) and NPC 17761 (D-Arg0[Hyp3,D-HypeE(trans-thiophenyl)7,Oic8]bra dykinin) (0.1-1000 nM) caused graded, reversible and selective inhibition of the bradykinin (100 nM) relaxation, with IC50 values of 1.4 and 19.1 nM, respectively. HOE 140 and NPC 17761 (0.1-10 nM) produced a graded shift to the right of the bradykinin concentration-response curves associated with a reduction of the maximum relaxation. The kinin B1 receptor antagonist, des-Arg9-[Leu8]bradykinin (1 microM), was inactive. Thus, bradykinin-induced relaxation in guinea-pig trachea results from activation of bradykinin B2 receptors and can be antagonized with high affinity in a selective and reversible manner, through noncompetitive mechanism, by both HOE 140 and NPC 17761. In addition, the bradykinin response does not involve neural pathways, extracellular Ca2+ influx or mobilization of intracellular Ca2+ stores sensitive to ryanodine, but is modulated by small conductance Ca(2+)-activated K+ channels.

Indirectly acting agonists. A model for the functional interaction of released endogenous double agonists

A theoretical model is presented to describe the functional interaction of an endogenous double agonist released by an indirectly acting agonist. The model takes into account the functional interaction of a released double (dualistic) agonist and describes both functional synergism and antagonism. It was shown that receptor density plays an important role in determining the profiles of concentration-effect curves and that it is necessary that the model should incorporate a parameter which describes receptor density. The model predicted that the shape of the concentration-effect curves may be sigmoid or bell-shaped. The theoretical predictions are comparable with experimental results.

The effects of tyramine on salivary flow rate and protein secretion by rat submandibular glands

The effects of different doses of p-tyramine injected i.v. and i.p. on salivary flow rates and proteins secreted by the submandibular glands of rats were studied with and without various types of autonomic blockers and two enzyme inhibitors. The salivary flow rates and the amounts of protein secreted progressively increased with increasing doses injected both i.v. and i.p., whereas they were dramatically reduced with all autonomic blockers except the lowest doses of beta-blockers, atropine, and yohimbine. Salivation in response to p-tyramine injected i.v. and i.p. was completely abolished by simultaneous injections of both prazosin and propranolol. The concentration of protein was not dose-dependent and was not reduced by yohimbine and phenoxybenzamine at almost all doses used. However, prazosin significantly increased the protein concentration. Protease activities were dose-dependent but were significantly reduced with alpha-blockers other than yohimbine, and with most beta-blockers. The proteins secreted in response to p-tyramine at all doses injected i.v. and i.p. were of the alpha-type except with the lowest dose injected i.p. However, the alpha-type was completely replaced by the beta-type in the presence of all alpha-blockers except yohimbine, but not with beta-blockers, atropine, or two enzyme inhibitors. Pargyline, a monoamine-oxidase inhibitor, but not disulfiram, a dopamine-beta-hydroxylase inhibitor, affected all parameters except the type of protein. Thus, p-tyramine may activate both the alpha 1- and beta 1-adrenoceptors in the submandibular glands of rats directly or indirectly.

Inhibition of endothelium-dependent vasorelaxation by arginine analogues: a pharmacological analysis of agonist and tissue dependence

1. Isolated rings of rabbit external jugular vein (RbJV) and rat thoracic aorta (RA) were used to study the effect of the NO synthase inhibitor L-NG-nitroarginine methyl ester (L-NAME) on muscarinic and 5-hydroxytryptamine (5-HT) receptor-stimulated, endothelium-dependent vascular relaxations. 2. In RbJV relaxations produced by the endothelial 5-HT receptor agonist alpha-methyl-5-HT were potently and non-surmountably inhibited by L-NAME (10 microM), whereas acetylcholine relaxations in this tissue were unaffected by this concentration of inhibitor. By contrast, acetylcholine relaxations in RA were virtually abolished by 10 microM L-NAME. In each case an equivalent concentration of D-NAME was without effect on agonist-induced relaxations. 3. The different effect of L-NAME on acetylcholine relaxations in RbJV and RA was not due to muscarinic receptor differences. Affinity estimates for acetylcholine (pKA = 6.12 +/- 0.09; 6.09 +/- 0.08 respectively) and for 4-diphenyl-acetoxy-N-methylpiperidine methobromide (4-DAMP, pKB = 9.01 +2- 0.012; 9.24 +/- 0.16 respectively) indicated that the receptors in both tissues belong to the same M3 class. Tissue differences resulting from the release of a cyclo-oxygenase product or a glibenclamide-sensitive K(+)-channel-linked hyperpolarizing factor were also ruled out by selective inhibition of these pathways. 4. When phenoxybenzamine was used to reduce the efficacy of acetylcholine in RbJV so that it behaved as a partial agonist in this tissue, L-NAME (10 microM) now produced non-surmountable inhibition of relaxation responses. In untreated tissues the same concentration of L-NAME also profoundly inhibited responses produced by butyrylcholine and pilocarpine, both of which behave as partial agonists at the M3 receptor in RbJV. 5. A simple model was developed which describes the theoretical behaviour of receptor-stimulated synthesis and release of NO. The model predicts that competitive inhibition of NO formation results in parallel displacements of the agonist response curve in the case of high efficacy agonist, but right-shift with concomitant depression of the curve maximum in the case of low efficacy agonists. Simulations based on the model showed reasonable agreement with the experimental data. 6. It is concluded that analogues of L-arginine demonstrate tissue- and agonist-dependence in terms of their ability to inhibit receptor-mediated events involving the liberation of NO. This behaviour can reflect differences in agonist efficacy in the receptor systems being studied, a possibility that should be ruled out before apparent resistance to inhibition is taken as evidence for the involvement of heterogeneous endothelium-derived relaxing factors (EDRFs).

Drugs from emasculated hormones: the principle of syntopic antagonism

This lecture illustrates the early stages in the planning and discovery of propranolol, an adrenaline beta-adrenergic receptor antagonist, and cimetidine, a histamine H2-receptor antagonist--the first examples of clinically useful drugs from each of these classes. The significance of selective agonists, partial agonists, and syntopic antagonists and the importance of the bioassay and the use of molar models in the drug discovery process are discussed. For the future, an outline of potential developments in hormone-receptor concepts is offered leading to the conclusion that progress may depend on improvements in bioassays and related molar modeling.

Drugs from emasculated hormones: the principle of syntopic antagonism

This lecture outlines the early stages in the discovery of adrenaline beta-receptor antagonists and of the histamine H2-receptor antagonists. It ends with a brief personal view about future research.

Beta-blockade antagonism of tyramine-induced rise in blood pressure

The effect beta-adrenoceptor blockade on the pressor response to tyramine has been investigated in 6 healthy volunteers, each submitted to an i.v. tyramine pressor test before and after 7 days of propranolol 40 mg b.d. or indenolol 60 mg o.d. Tyramine was given as i.v. boluses of 1-6 mg, alternating with saline, in a randomized, single blind fashion. Prior to treatment tyramine caused a temporary, dose-dependent increase in systolic and diastolic blood pressure, whilst the heart rate remained unaffected. Both propranolol and indenolol reduced the pressor response to tyramine, as shown by a significant increase in ED15, i.e. the dose of tyramine required to increase systolic blood pressure by 15%.

The half-life of endothelium-derived relaxing factor released from bovine aortic endothelial cells in culture

1. The half-life of endothelium-derived relaxing factor (EDRF) in Krebs solution was determined by bioassay in vitro. 2. A column of bovine aortic endothelial cells grown on microcarrier beads in suspension culture was perfused with Krebs solution. EDRF was released from these cells by sequential treatment with increasing concentrations of bradykinin (0.01-100 nM). EDRF was detected by the relaxation of an endothelium-denuded ring segment of dog coronary artery. 3. Complete bradykinin concentration-relaxation curves were determined in the absence or presence of coils of tubing that increased the transit time (delay) between the cell column and the assay tissue. An estimate of the falls in concentration, and hence of the half-life of EDRF, was obtained from the shift of the bradykinin concentration-relaxation curves. 4. Mass-action equations were used to model the relationship between the indirectly acting agonist bradykinin and the relaxation via EDRF. The modelling adequately predicted the consequences of different transit delay times (0-4 half-lives). 5. This new analysis of half-life of an active intermediate emphasizes the measurement of changes in concentration with increasing transit time rather than a fall in tissue response. 6. The half-life of EDRF in Krebs solution is 41 s.

A model for agonists acting directly as well as indirectly

A theoretical model is presented to describe the receptor interaction of a drug which possesses indirect as well as direct pharmacological action. Sets of theoretical log concentration-effect curves predicted by this model are presented. The model predicts that the shape of the log concentration-effect curves may be sigmoid (S-shaped), biphasic (bell-shaped) or composed of a combination of two sigmoid curves, joined together by a plateau. The theoretical predictions are comparable with the results obtained from an experimental study.

Pharmacologic analysis of LY188695 (KB-2413), 1-(2-ethoxyethyl)-2-(4-methyl-1-homopiperazinyl)-benzimidazole difumarate, a potent histamine1 receptor antagonist

LY188695 was evaluated both in vitro and in vivo in the guinea pig to determine its pharmacologic profile. The compound antagonized histamine-induced contractions of ileum, aorta, and trachea with pKB values of 9.9, 9.9, and 9.2 respectively. In the lung parenchymal strip, LY188695 caused a rightward shift of the histamine concentration-response curve with a reduction in the maximal response at all antagonist concentrations tested. The reason for this effect is unknown, but it was not due to a nonspecific depressant action of the compound on the parenchyma. Selectivity was shown by its inactivity against leukotriene D4, bradykinin, prostaglandin F2 alpha, acetylcholine, norepinephrine, and serotonin on various guinea pig and rat smooth muscles. Similarly, H2 receptor-mediated relaxation of the rat uterus was unaltered by LY188695. Increases in total pulmonary impedance caused by i.v. histamine to anesthetized guinea pigs were reduced by as little as 3 micrograms/kg given orally 1 hour prior to histamine challenge. In this system, LY188695 was 15 times more potent than chlorpheniramine and 100 times more potent than terfenadine. Similar responses elicited by acetylcholine were not antagonized by LY188695. A duration of action greater than 4 hours was observed in this model. Ovalbumin given i.v. to sensitized guinea pigs increased total pulmonary impedance which was markedly decreased after oral administration of 30 or 100 micrograms/kg LY188695. These results indicate that LY188695 is a very potent antagonist of H1-mediated responses and suggest that this agent might be useful in disease states characterized by an overproduction of histamine.

Lack of involvement of alpha-adrenoceptors in sympathetic neural vasoconstriction in the hindquarters of the rabbit

The hypothesis that sympathetic nerves in arterial blood vessels activate excitatory receptors distinct from alpha-adrenoceptors was investigated in vivo in the rabbit. In anaesthetized, ganglion-blocked rabbits, graded stimulation of the lumbar sympathetic nerve chains caused graded hind limb vasoconstriction. The responses to single pulses and short trains of stimuli were unaffected by benextramine (10 mg kg-1) and the longer trains were enhanced. Phenoxybenzamine (5 mg kg-1) slightly reduced the responses to short trains of stimuli and did not affect the responses to long trains. The dose-response curve to intra-arterial noradrenaline (after beta-adrenoceptor blockade) was shifted rightwards about ten fold by benextramine (10 mg kg-1) and by phenoxybenzamine (5 mg kg-1). In conscious rabbits the vasoconstriction caused by the nasopharyngeal reflex initiated by smoke inhalation was unaffected by benextramine (10 mg kg-1). Small mesenteric arteries (less than 250 microns) taken from untreated rabbits responded to noradrenaline with a threshold concentration of about 1 microM. Similar tissues from benextramine (10 mg kg-1)-treated rabbits were unresponsive to noradrenaline at concentrations up to 300 microM. However, these tissues were able to respond to potassium and angiotensin II. Aortic ring segments taken from the same rabbits were only about ten fold less sensitive to noradrenaline than segments from control rabbits. These results are in accord with the hypothesis that sympathetic nerves activate non-alpha-receptors in the vasculature of the rabbit.

Antagonism of receptor-activated biological effects mediated by second messenger pathways

It is generally held that many biologically active compounds produce their effects through a sequence of events that are initiated when the substances combine with selective receptors located on the cell surface membrane. Activation of these receptors produces a stimulus that is somehow transmitted intracellularly. The transduction between the stimulus and the response is now known to be mediated, in many systems, by an intracellular intermediary or second messenger. A model describing the relation of agonist concentration, receptor occupation, and biological response in such a system is herein extended to include antagonist binding at two target sites the cell-surface receptor and the receptor for the second messenger. It is demonstrated that the shift of an agonist's dose-response curve is characteristic of the site of antagonism and that analysis of this shift can reveal the existence of a second messenger pathway or, if this is known, the site of action of the antagonist.

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

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

The quantification of relative efficacy of agonists

While much pharmacological effort has been expended in the measurement, quantification, and comparison of agonist affinity for the classification of drugs and drug receptors, inordinately less emphasis has been placed on the quantification of the other property of agonists, namely intrinsic efficacy. This is unfortunate as the existing studies of the relative intrinsic efficacy of agonists show this to be a most useful scale for the classification of agonists and the prediction of tissue responses. This paper will review some of the theories that describe efficacy on a molecular level, the methods of measuring relative efficacy, and the factors in these procedures which can lead to artifacts and misleading information for the classification of drug receptors. Lastly, the value of the quantification of efficacy will be discussed in terms of the design of agonists for therapeutic advantage.

Quantification of H2-agonism by clonidine and dimaprit in an adenylate cyclase assay

In homogenates of guinea-pig ventricle clonidine and dimaprit both stimulate adenylate cyclase and exhibit "bell-shaped' E/[A] curves. The two properties (stimulatory and inhibitory) could be resolved using a theoretical model assuming a "down line' auto-inhibitory mechanism. In the case of clonidine a further depressive property could be seen in the presence of high concentrations of the selective H2-receptor antagonist tiotidine which is not explicable in terms of this model. The results suggest that clonidine has a direct agonistic effect on H2-receptors in guinea-pig heart. However, like dimaprit, clonidine appears to be a partial agonist because its expression is confounded by a secondary inhibitory property(ies).

A comparison of the blocking effect of alpha-adrenoceptor antagonist drugs towards directly and indirectly acting agonists on the rat vas deferens

The antagonistic effect of dihydroergocristine and phentolamine towards the directly acting amine noradrenaline and the indirectly acting amine tyramine on the rat vas deferens was evaluated. It has been demonstrated that both alpha-adrenoceptor blocking drugs reduced the effect of the indirectly acting amine at concentrations lower than those needed to antagonize the action of noradrenaline. When concentrations of the alpha-adrenoceptor blocking drugs effective against noradrenaline were employed a qualitative difference in the antagonism towards the directly and indirectly acting amines has also been shown. It appears that in the rat vas deferens the effects induced by tyramine-released noradrenaline are more easily affected by alpha-adrenoceptor antagonist drugs than those induced by exogenously applied noradrenaline.