Some quantitative uses of drug antagonists. 1958

Importance of receptor expression in the classification of novel ligands at the M2 muscarinic acetylcholine receptor

BACKGROUND AND PURPOSE

Affinity-based, selective orthosteric ligands for the muscarinic acetylcholine receptors (mAChRs) are difficult to develop due to high sequence homology across the five subtypes. Selectivity can also be achieved via the selective activation of a particular subtype or signalling pathway. Promisingly, a prior study identified compounds 6A and 7A as functionally selective and Gi biased compounds at the M2 mAChR. Here, we have investigated the activation of individual G protein subfamilies and the downstream signalling profiles of 6A and 7A at the M2 mAChR.

EXPERIMENTAL APPROACH

G protein activation was measured with the TRUPATH assay in M2 mAChR FlpIn CHO cells. Activity in downstream signalling pathways was determined using the cAMP CAMYEL BRET sensor and assay of ERK 1/2 phosphorylation.

KEY RESULTS

M2 mAChRs coupled to Gɑi1, GɑoA and Gɑs, but not Gɑq, in response to canonical orthosteric agonists. Compounds 6A and 7A did not elicit any G protein activation, cAMP inhibition or stimulation, or ERK 1/2 phosphorylation. Instead, a Schild analysis indicates a competitive, antagonistic interaction of compounds 6A and 7A with ACh in the Gɑi1 activation assay. Overexpression of the M2 mAChR may suggest an expression-dependent activation profile of compounds 6A and 7A.

CONCLUSIONS AND IMPLICATIONS

These data confirm that the M2 mAChR preferentially couples to Gɑi/o and to a lesser extent to Gɑs in response to canonical orthosteric ligands. However, this study was not able to detect Gɑi bias of compounds 6A and 7A, highlighting the importance of cellular background when classifying new ligands.

LINKED ARTICLES

This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

Antihypertensive Potential of Tartaric Acid and Exploration of Underlying Mechanistic Pathways

Tartaric acid is capable of balancing blood pressure. It is the main constituent of antihypertensive agents (grapes and wine) and has not been scientifically explored as an antihypertensive remedy. This study aimed to investigate the antihypertensive effect of a low-dose tartaric acid in vivo and explore underlying mechanisms in vitro. Intravenous administration of tartaric acid at the dose of 50 µg/kg caused a % fall in mean arterial pressure (MAP) in normotensive and hypertensive rats [51.5 ± 1.7 and 63.5 ± 2.9% mmHg]. This hypotensive effect was partially inhibited by atropine (1 mg/kg) and _L-NAME (100 µg/kg) pretreatment. In hypertensive rats, oral administration of tartaric acid (.1, .5, 1, 5, and 10 mg/kg) for 2 weeks resulted in 65 ± 7.3 mmHg MAP at 10 mg/kg. This antihypertensive effect was comparable to the orally administered verapamil (10 mg/kg) for 2 weeks which caused a decrease in MAP 60.4 ± 3.8 mmHg. Tartaric acid relaxed phenylephrine (PE) and High K⁺-induced contractions with EC₅₀ values of .157 (.043-.2) and 1.93 (.07-2) µg/mL in vitro. This endothelium-dependent relaxation was inhibited with atropine (1 µM) and _L-NAME (10 µM) pretreatment. Tartaric acid also suppressed phenylephrine contractions in Ca⁺² free/EGTA medium and on voltage-dependent calcium channels, causing the concentration–response curves toward right. Tartaric acid induced negative inotropic and chronotropic effects with EC₅₀ values of .26 (.14-.4) and .60 (.2-.8) in rat atria. It showed its effect by complete blockade against atropine and partially in propranolol pretreatment. These findings provide scientific basis to low-dose tartaric acid as an antihypertensive and vasodilatory remedy through muscarinic receptor-linked nitric oxide (NO) pathway and Ca⁺² channel antagonist.

The Nonpeptide Agonist MK-5046 Functions As an Allosteric Agonist for the Bombesin Receptor Subtype-3

Allosteric ligands of various G-protein-coupled receptors are being increasingly described and are providing important advances in the development of ligands with novel selectivity and efficacy. These unusual properties allow expanded opportunities for pharmacologic studies and treatment. Unfortunately, no allosteric ligands are yet described for the bombesin receptor family (BnRs), which are proposed to be involved in numerous physiologic/pathophysiological processes in both the central nervous system and peripheral tissues. In this study, we investigate the possibility that the bombesin receptor subtype-3 (BRS-3) specific nonpeptide receptor agonist MK-5046 [(2S)-1,1,1-trifluoro-2-[4-(1H-pyrazol-1-yl)phenyl]-3-(4-[[1-(trifluoromethyl)cyclopropyl]methyl]-1H-imidazol-2-yl)propan-2-ol] functions as a BRS-3 allosteric receptor ligand. We find that in BRS-3 cells, MK-5046 only partially inhibits iodine-125 radionuclide (125I)-Bantag-1 [Boc-Phe-His-4-amino-5-cyclohexyl-2,4,5-trideoxypentonyl-Leu-(3-dimethylamino) benzylamide N-methylammonium trifluoroacetate] binding and that both peptide-1 (a universal BnR-agonist) and MK-5046 activate phospholipase C; however, the specific BRS-3 peptide antagonist Bantag-1 inhibits the action of peptide-1 competitively, whereas for MK-5046 the inhibition is noncompetitive and yields a curvilinear Schild plot. Furthermore, MK-5046 shows other allosteric behaviors, including slowing dissociation of the BRS-3 receptor ligand 125I-Bantag-1, dose-inhibition curves being markedly affected by increasing ligand concentration, and MK-5046 leftward shifting the peptide-1 agonist dose-response curve. Lastly, receptor chimeric studies and site-directed mutagenesis provide evidence that MK-5046 and Bantag-1 have different binding sites determining their receptor high affinity/selectivity. These results provide evidence that MK-5046 is functioning as an allosteric agonist at the BRS-3 receptor, which is the first allosteric ligand described for this family of receptors. SIGNIFICANCE STATEMENT: G-protein-coupled receptor allosteric ligands providing higher selectivity, selective efficacy, and safety that cannot be obtained using usual orthosteric receptor-based strategies are being increasingly described, resulting in enhanced usefulness in exploring receptor function and in treatment. No allosteric ligands exist for any of the mammalian bombesin receptor (BnR) family. Here we provide evidence for the first such example of a BnR allosteric ligand by showing that MK-5046, a nonpeptide agonist for bombesin receptor subtype-3, is functioning as an allosteric agonist.

Lipidated Calcitonin Gene-Related Peptide (CGRP) Peptide Antagonists Retain CGRP Receptor Activity and Attenuate CGRP Action In Vivo

Signaling through calcitonin gene-related peptide (CGRP) receptors is associated with pain, migraine, and energy expenditure. Small molecule and monoclonal antibody CGRP receptor antagonists that block endogenous CGRP action are in clinical use as anti-migraine therapies. By comparison, the potential utility of peptide antagonists has received less attention due to suboptimal pharmacokinetic properties. Lipidation is an established strategy to increase peptide half-life in vivo. This study aimed to explore the feasibility of developing lipidated CGRP peptide antagonists that retain receptor antagonist activity in vitro and attenuate endogenous CGRP action in vivo. CGRP peptide analogues based on the archetypal CGRP receptor antagonist, CGRP_8-37, were palmitoylated at the N-terminus, position 24, and near the C-terminus at position 35. The antagonist activities of the lipidated peptide analogues were tested in vitro using transfected Cos-7 cells expressing either the human or mouse CGRP receptor, amylin subtype 1 (AMY₁) receptor, adrenomedullin (AM) receptors, or calcitonin receptor. Antagonist activities were also evaluated in SK-N-MC cells that endogenously express the human CGRP receptor. Lipidated peptides were then tested for their ability to antagonize endogenous CGRP action in vivo using a capsaicin-induced dermal vasodilation (CIDV) model in C57/BL6J mice. All lipidated peptides except for the C-terminally modified analogue retained potent antagonist activity compared to CGRP_8-37 towards the CGRP receptor. The lipidated peptides also retained, and sometimes gained, antagonist activities at AMY₁, AM₁ and AM₂ receptors. Several lipidated peptides produced robust inhibition of CIDV in mice. This study demonstrates that selected lipidated peptide antagonists based on αCGRP_8-37 retain potent antagonist activity at the CGRP receptor and are capable of inhibition of endogenous CGRP action in vivo. These findings suggest that lipidation can be applied to peptide antagonists, such as αCGRP_8-37 and are a potential strategy for antagonizing CGRP action.

Histamine exerts both direct H2-mediated and indirect catecholaminergic effects on heart rate in pythons

The vertebrate heart is regulated by excitatory adrenergic and inhibitory cholinergic innervations, as well as non-adrenergic non-cholinergic (NANC) factors that may be circulating in the blood or released from the autonomic nerves. As an example of NANC signaling, an increased histaminergic tone, acting through stimulation of H₂ receptors, contributes markedly to the rise in heart rate during digestion in pythons. In addition to the direct effects of histamine, it is also known that histamine can reinforce the cholinergic and adrenergic signaling. Thus, to further our understanding of the histaminergic regulation of the cardiovascular response in pythons, we designed a series of in vivo experiments complemented by in vitro experiments on sinoatrial and vascular ring preparations. We demonstrate the tachycardic mechanism of histamine works partly through a direct binding of cardiac H₂ receptors and in part through a myocardial histamine-induced catecholamine release, which strengthens the sympathetic adrenergic signaling pathway.

Differences among muscarinic agonists in M1 receptor-mediated nonselective cation channel activation and TASK1 channel inhibition in adrenal medullary cells

Muscarinic receptor stimulation induces depolarizing inward currents and catecholamine secretion in adrenal medullary (AM) cells from various mammals. In guinea-pig AM cells muscarine and oxotremorine at concentrations ≤ 1 μM produce activation of nonselective cation channels with a similar potency and efficacy, whereas muscarine at higher concentrations produces not only nonselective cation channel activation, but also TASK1 channel inhibition. In rat AM cells, the muscarinic M₁ receptor is involved in TASK1 channel inhibition in response to muscarinic agonists, and the efficacy of oxotremorine is half that of muscarine. These pharmacological findings might indicate that different muscarinic receptor subtypes are responsible for the regulation of nonselective cation and TASK1 channel activities. The present study aimed to determine the muscarinic receptor subtypes involved in nonselective cation channel activation in guinea-pig and mouse AM cells. The inward current evoked by 1 μM muscarine was completely suppressed by 100 μM quinine, whereas 30 μM muscarine-induced inward currents were comprised of quinine-sensitive and -insensitive components. The electrophysiological and pharmacological properties of the muscarine-induced currents indicated that the quinine-sensitive and insensitive components are due to nonselective cation channel activation and TASK1 channel inhibition, respectively. Muscarine at 30 μM failed to induce any current in AM cells treated with muscarinic toxin 7 or genetically deleted of the M₁ receptor. The K_D value of VU0255035 against the muscarinic receptor mediating nonselective cation channel activation was 17.5 nM. These results indicate that the M₁ receptor mediates nonselective cation channel activation as well as TASK1 channel inhibition.

Repurposing antimalarial aminoquinolines and related compounds for treatment of retinal neovascularization

Neovascularization is the pathological driver of blinding eye diseases such as retinopathy of prematurity, proliferative diabetic retinopathy, and wet age-related macular degeneration. The loss of vision resulting from these diseases significantly impacts the productivity and quality of life of patients, and represents a substantial burden on the health care system. Current standard of care includes biologics that target vascular endothelial growth factor (VEGF), a key mediator of neovascularization. While anti-VGEF therapies have been successful, up to 30% of patients are non-responsive. Therefore, there is a need for new therapeutic targets, and small molecule inhibitors of angiogenesis to complement existing treatments. Apelin and its receptor have recently been shown to play a key role in both developmental and pathological angiogenesis in the eye. Through a cell-based high-throughput screen, we identified 4-aminoquinoline antimalarial drugs as potent selective antagonists of APJ. The prototypical 4-aminoquinoline, amodiaquine was found to be a selective, non-competitive APJ antagonist that inhibited apelin signaling in a concentration-dependent manner. Additionally, amodiaquine suppressed both apelin-and VGEF-induced endothelial tube formation. Intravitreal amodaiquine significantly reduced choroidal neovascularization (CNV) lesion volume in the laser-induced CNV mouse model, and showed no signs of ocular toxicity at the highest doses tested. This work firmly establishes APJ as a novel, chemically tractable therapeutic target for the treatment of ocular neovascularization, and that amodiaquine is a potential candidate for repurposing and further toxicological, and pharmacokinetic evaluation in the clinic.

Synthesis and biological evaluation of compact, conformationally constrained bifunctional opioid agonist - neurokinin-1 antagonist peptidomimetics

A reported mixed opioid agonist - neurokinin 1 receptor (NK1R) antagonist 4 (Dmt-D-Arg-Aba-Gly-(3',5'-(CF3)2)NMe-benzyl) was modified to identify important features in both pharmacophores. The new dual ligands were tested in vitro and subsequently two compounds (lead structure 4 and one of the new analogues 22, Dmt-D-Arg-Aba-β-Ala-NMe-Bn) were selected for in vivo behavioural assays, which were conducted in acute (tail-flick) and neuropathic pain models (cold plate and von Frey) in rats. Compared to the parent opioid compound 33 (without NK1R pharmacophore), hybrid 22 was more active in the neuropathic pain models. Attenuation of neuropathic pain emerged from NK1R antagonism as demonstrated by the pure NK1R antagonist 6. Surprisingly, despite a lower in vitro activity at NK1R in comparison with 4, compound 22 was more active in the neuropathic pain models. Although potent analgesic effects were observed for 4 and 22, upon chronic administration, both manifested a tolerance profile similar to that of morphine and cross tolerance with morphine in a neuropathic pain model in rat.

Biofunctional studies of new 2-methoxyphenylpiperazine xanthone derivatives with α₁-adrenolytic properties

BACKGROUND

The aim of this study was to assess the selectivity of the studied xanthone derivatives for α1-adrenoceptor subtypes (α1A, α1B, α1D, α1L) in functional experiments in order to verify if they possess any selectivity for a distinct subtype of α1-adrenoceptor. Moreover, several pharmacological tests were carried out to assess whether they reveal other than α1-adrenoceptor blocking properties such as: antagonistic for 5-HT2 receptors, vasorelaxant or spasmolytic.

METHODS

The influence on α1A-adrenoceptors was examined in biofunctional studies employing isolated rat vas deferens, on α1B-adrenoceptors in guinea-pig spleen, on α1D-adrenoceptors in rat aorta, and on α1L-adrenoceptors in rabbit spleen. Affinity for 5-HT2 receptors was measured in radioligand binding assay, whereas antagonistic potency for 5-HT2 receptors was studied on isolated rat aorta. Vasorelaxant effect of tested compounds was assessed in functional study employing rat aorta, whereas direct spasmolytic activity was investigated using the isolated rabbit small intestine.

RESULTS

The present study provides evidences that the tested 2-methoxyphenylpiperazine xanthone derivatives are non-selective α1-adrenoceptor blockers. However, at higher concentrations the direct spasmolytic effect could enhance their hypotensive activity. The obtained results indicate that the studied xanthones possessed weak calcium entry blocking activity, as well as antagonistic properties for 5-HT2A receptors.

CONCLUSIONS

The results of the present study support the idea that the hypotensive activity of the studied compounds is related to their α1-adrenolytic properties.

Prophylactic antioxidant potential of gallic Acid in murine model of sepsis

Present study is to investigate the effect of Gallic acid pretreatment on survival of septic animals and oxidative stress in different organs like lungs, liver, kidney, spleen, and vascular dysfunction of mice. Sepsis was induced by cecal ligation and puncture (CLP) in healthy adult male albino mice (25–30 g) and was divided into 3 groups each consisting of 6 animals, that is, sham-operated (SO group (Group I), SO + sepsis (Group II), and Gallic acid + sepsis (Group III)). Group III animals were pretreated with Gallic acid at the dose rate of 20 mg/kg body weight for 2 days before induction of sepsis. Animals were sacrificed on 8th day and the tissue samples were obtained for further investigation on lipid peroxidation (LPO), malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GSH). Gallic acid pretreatment significant (P < 0.05) reduces kidney, spleen, liver, and lungs' malondialdehyde level in septic mice. However, it fails to improve reduced glutathione level in all given organs, while, Gallic acid pretreated mice showed significant improvement in SOD activity of kidney and spleen when compared to septic mice. Finally, the beneficial effects of Gallic acid pretreatment in sepsis are evident from the observations that Gallic acid partially restored SOD and catalase activity and completely reversed lipid peroxidation. Further studies are required to find out the possible mechanisms underlying the beneficial effects of Gallic acid on large population.

Functional significance of muscarinic receptor expression within the proximal and distal rat vagina

Information regarding the role of cholinergic nerves in mediating vaginal smooth muscle contraction is sparse, and in vitro studies of the effects of muscarinic agonists on vaginal smooth muscle are discrepant. The goal of this study was to determine the expression of muscarinic receptors in the vaginal wall of the rat. In addition, we sought to determine the effect of the muscarinic receptor agonist carbachol on contractility and inositol phosphate production of the proximal and distal rat vaginal muscularis. RT-PCR analysis indicated that both M(2) and M(3) receptor transcripts were expressed within the proximal and distal rat vagina. Carbachol dose-dependently (10(-7)-10(-4) M) contracted the rat vaginal muscularis with a greater maximal contractile response in the proximal vagina (P < 0.01) compared with the distal vagina. The contractile responses of the rat vaginal muscularis to carbachol were dose dependently inhibited by the M(3) antagonist para-fluoro-hexahydrosiladefenidol, and a pK(B) of 7.78 and 7.95 was calculated for the proximal and distal vagina, respectively. Inositol phosphate production was significantly increased in both regions of the vagina following 20-min exposure to 50 muM carbachol with higher levels detected in the proximal vagina compared with the distal (P < 0.05). Preliminary experiments indicated the presence of M(2) and M(3) receptors in the human vaginal muscularis as well as contraction of human vaginal muscularis to carbachol, indicating that our animal studies are relevant to human tissue. Our results provide strong evidence for the functional significance of M(3) receptor expression in the vaginal muscularis.

Pharmacologic responses of the mouse urinary bladder

The aim of the study was to determine pathways involved in contraction and relaxation of the mouse urinary bladder. Mouse bladder strips were set up in gassed Krebs-bicarbonate solution and responses to various drugs and electrical field stimulation were obtained. Isoprenaline (b-receptor agonist) caused a 63% inhibition of carbachol precontracted detrusor (EC50=2nM). Carbachol caused contraction (EC50=0.3µM), responses were antagonised more potently by 4-DAMP (M3-antagonist) than methoctramine (M2-antagonist). Electrical field stimulation caused contraction, which was inhibited by atropine (60%) and less by guanethidine and α,β-methylene-ATP. The neurogenic responses were not potentiated by inhibition of nitric oxide synthase. Presence of an intact urothelium significantly depressed responses to carbachol (p=0.02) and addition of indomethacin and L-NNA to remove prostaglandin and nitric oxide production respectively did not prevent the inhibitory effect of the urothelium. In conclusion, b-receptor agonists cause relaxation and muscarinic agonists cause contraction via the M3-receptor. Acetylcholine is the main neurotransmitter causing contraction while nitric oxide has a minor role. The mouse and human urothelium are similar in releasing a factor that inhibits contraction of the detrusor muscle which is unidentified but is not nitric oxide or a prostaglandin. Therefore, the mouse may be used as a model to study the lower urinary tract.

Co-expression of neuropeptide Y Y1 and Y5 receptors results in heterodimerization and altered functional properties

Centrally administered neuropeptide Y (NPY) produces anxiolytic and orexigenic effects by interacting with Y1 and Y5 receptors that are colocalized in many brain regions. Therefore, we tested the hypothesis that co-expression of Y1 and Y5 receptors results in heterodimerization, altered pharmacological properties and altered desensitization. To accomplish this, the carboxyl-termini of Y1 and Y5 receptors were fused with Renilla luciferase and green fluorescent protein and the proximity of the tagged receptors assessed using bioluminescent resonance energy transfer. Under basal conditions, cotransfection of tagged Y1 receptor and Y5 produced a substantial dimerization signal that was unaffected by the endogenous, nonselective agonists, NPY and peptide YY (PYY). Selective Y5 agonists produced an increase in the dimerization signal while Y5 antagonists also produced a slight but significant increase. In the absence of agonists, selective antagonists decreased dimerization. In functional studies, Y5 agonists produced a greater inhibition of adenylyl cyclase activity in Y1/Y5 cells than cells expressing Y5 alone while NPY and PYY exhibited no difference. With PYY stimulation, the Y1 antagonist became inactive and the Y5 antagonist exhibited uncompetitive kinetics in the Y1/Y5 cell line. In confocal microscopy studies, Y1/Y5 co-expression resulted in increased Y5 signaling following PYY stimulation. Addition of both Y1 and Y5 receptor antagonists was required to significantly decrease PYY-induced internalization. Therefore, Y1/Y5 co-expression results in heterodimerization, altered agonist and antagonist responses and reduced internalization rate. These results may account for the complex pharmacology observed when assessing the responses to NPY and analogs in vivo.

Comparison of 2 endothelin-receptor antagonists on in vitro responses of equine palmar digital arterial and venous rings to endothelin-1

The goals of this study were to determine the concentration-response (C-R) relationship of endothelin-1 (ET-1), compare 2 ET-receptor antagonists and determine the antagonist concentrations that block the vasomotor effects of ET-1, and compare the effectiveness of ET-1 and previously studied vasoconstrictors in equine palmar digital arterial and venous rings in vitro. Vessel rings from 8 nonlaminitic horses were placed in Tyrode's solution, 1 side fixed to the floor of an organ bath and the other side fixed to a force-displacement transducer. Two separate studies were conducted: (I) incubation with a single ET-receptor antagonist (PD142893 or PD145065 at a concentration of 10(-7), 10(-6), or 10(-5) M), followed by determination of an ET-1 C-R curve (using concentrations of 10(-10) to 10(-6) M) for medial vessel rings; and (II) comparison of ET-1 with norepinephrine and histamine (10(-10) to 10(-6) M) and comparison of contractile responses of medial and lateral vessel rings. In study I, ET-1 administration caused pronounced and sustained concentration-dependent contraction of vessel rings; these contractile responses were decreased by 10(-5) M PD142893 and were completely blocked by 10(-5) M PD145065. Venous rings had greater apparent maximum contraction in response to ET-1 than arterial rings. In study II, the relative sensitivity of norepinephrine was found to be equivalent to that of ET-1, whereas that of histamine was lower. No significant differences were observed between responses of medial versus lateral vessel rings. Thus, ET-1 is a potent vasoconstrictor of equine palmar digital arteries and veins, and the ET-receptor antagonist PD145065 is more effective than PD142893 in inhibiting these contractile effects in vitro.

Piboserod (SB 207266), a selective 5-HT4 receptor antagonist, reduces serotonin potentiation of neurally-mediated contractile responses of human detrusor muscle

The aim of this study is to evaluate the potency of piboserod (SB 207266), a selective 5-HT(4) receptor antagonist, at inhibiting the 5-HT(4)-mediated potentiating effect of serotonin (5-HT) on the neurally-mediated contractile responses of human detrusor strips to electrical field stimulations (EFS). Strips of human detrusor muscle were mounted in Krebs-HEPES buffer under a resting tension of 500 mg and EFS (20 Hz, 1 ms duration at 300 mA for 5 s) was applied continuously at 1 min intervals. After stabilization of the EFS-induced contractions, concentration-response curves to 5-HT (0.1 nM-100 microM) were constructed in the absence or presence of 1 or 100 nM of piboserod. The experiments were performed in the presence of methysergide (1 microM) and ondansetron (3 microM) to block 5HT(1)/5HT(2) and 5-HT(3) receptors, respectively. 5-HT potentiated the contractile responses to EFS of human bladder strips in a concentration-dependent manner, with a maximum mean of 60.0+/-19.9% of the basal EFS-evoked contractions. Piboserod did not modify the basal contractions but concentration-dependently antagonized the ability of 5-HT to enhance bladder strip contractions to EFS. In presence of 1 and 100 nM of piboserod, the maximal 5-HT-induced potentiations were reduced to 45.0+/-7.9 and 38.7+/-8.7%, respectively. A mean apparent antagonist dissociation constant value (K(B)) of 0.56+/-0.09 nM was determined. These data show the ability of piboserod to antagonize with high potency the enhancing properties of 5-HT on neurally-mediated contractions of isolated human bladder strips. Therefore, the 5-HT(4) receptor might represent an attractive pharmacological target for the treatment of overactive bladder.

5-HT2A receptor antagonist properties of cyamemazine in rat and guinea pig smooth muscle

5-HT(2A) receptor antagonism seems to explain the low incidence of extrapyramidal side effects with atypical neuroleptics. Whether the neuroleptic cyamemazine, which at low doses is also devoid of extrapyramidal side effects, possesses 5-HT(2A) receptor antagonist properties is unknown. Cyamemazine was tested for its ability to antagonize 5-HT(2A)-mediated responses in isolated rat aorta and guinea pig trachea and to displace [3H]ketanserin specifically bound to rat brain membranes. In isolated rat aorta, cyamemazine potently and competitively antagonized serotonin-dependent contractions (pA(2)=8.82+/-0.26, n=7; Schild's slope=1.02+/-0.29). In this test, cyamemazine was of similar potency as ketanserin (pA(2)=8.23). In isolated guinea pig trachea, cyamemazine reduced maximum contractile responses to serotonin with pIC(50)=7.92+/-0.35, (n=4), whereas ketanserin exhibited a pIC(50)=8.79. Finally, cyamemazine displaced [3H]ketanserin specifically bound to rat brain membranes with pK(i)=8.76+/-0.53 (n=3). In conclusion, cyamemazine behaves as a potent antagonist at 5-HT(2A) receptors, which compares well with the reference compound, ketanserin. Whether this 5-HT(2A) receptor antagonist action of cyamemazine can explain its low incidence of extrapyramidal side effects deserves further investigation.

Serotonin antagonist profiling on 5HT2A and 5HT2C receptors by nonequilibrium intracellular calcium response using an automated flow-through fluorescence analysis system, HT-PS 100

Characterization of the potencies of agonists and antagonists in cell-based assays can be complicated by nonequilibrium conditions of functional response. We assessed the potencies of a series of serotonin (5HT) antagonists by inhibition of intracellular calcium response in HEK 293 cells expressing 5HT(2A) or 5HT(2C) receptors. An automated system, HT-PS 100, was used to profile the antagonists in two experimental setups: coadministration of agonist and antagonist to cells and preincubation of the cells with antagonist prior to agonist administration. We showed that the antagonist potencies (pIC(50) values) determined in the preincubation configuration were close to or exceeded those measured in the coadministration configuration. Closeness of the potencies determined in the two configurations supposedly reflected a rapid antagonist-receptor equilibration, whereas a significantly higher preincubation potency implied slow antagonist dissociation from the receptor. Schild analysis of the inhibition of serotonin-induced cell response by a competitive 5HT(2A) antagonist, spiperone, showed a typical competitive inhibition pattern when both the agonist and antagonist were applied simultaneously. Contrary to this, an insurmountable diminishing of the maximal cell response to serotonin was observed when the cells were preincubated with spiperone. We conclude that a combination of the coadministration and preincubation experimental setups is necessary for appropriate mechanistic interpretation and quantitative assessment of the antagonist activity when using transient functional readouts.

A competitive interaction model predicts the effect of WAY-100,635 on the time course of R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin-induced hypothermia

The objective of this investigation was to characterize quantitatively the pharmacodynamic interaction between N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxamide (WAY-100,635) and R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (R-8-OH-DPAT) in vivo. The 8-OH-DPAT-induced change in body temperature was used as a pharmacodynamic endpoint. Four groups of rats each received 1 mg/kg 8-OH-DPAT in 5 min during computer-controlled infusions of physiological saline or WAY-100,635, targeted at steady-state concentrations of 20, 85, and 170 ng/ml. Body temperature was monitored continuously with a telemetric system, and frequent blood samples were obtained to determine the pharmacokinetics of both drugs. Large differences in pharmacokinetics were observed between WAY-100,635 and R-8-OH-DPAT, reflected in values of the terminal elimination half-life of 33 and 143 min, respectively. Infusion of WAY-100,635 had no influence on the pharmacokinetics of R-8-OH-DPAT. With regard to the pharmacodynamics, clear antagonism of the R-8-OH-DPAT-induced hypothermia was observed. The complex pharmacological effect versus time profiles of R-8-OH-DPAT were analyzed on the basis of an indirect physiological response model with set point control coupled to a competitive interaction model for an agonist and antagonist acting at a common receptor. This model converged, yielding precise estimates of the pharmacodynamic parameters of both WAY-100,635 and R-8-OH-DPAT, which were independent of the infusion rate of WAY-100,635. The estimated in vivo binding constant of WAY-100,635 was 0.98 ng/ml (2.3 nM), which is very similar to the reported value from in vitro receptor binding assays. The findings of this investigation show that, in contrast to earlier reports in the literature, WAY 100,635 behaves as a pure competitive antagonist at the 5-hydroxytryptamine(1A) receptor in vivo.

Prostanoid receptor assays

Prostanoids, which include the prostaglandins (PGs) and thromboxanes (TXs), interact with a specific family of G-protein coupled receptors, of which there are known to be five distinct types, DP, EP, FP, IP and TP, each particularly sensitive to one of the five natural prostanoids, PGD₂, PGE₂, PGF₂(, PGI₂ and TXA₂, respectively. Of these, it is known that the EP receptor comprises four well-characterized subtypes: EP₁, EP₂, EP₃ and EP₄. These receptor subtypes are widely distributed throughout mammals and other species, and show particularly high levels of expression in smooth muscle and blood platelets. Despite the fact that few of these preparations express a single receptor type/subtype in isolation, a range of useful smooth muscle and platelet assays for the various prostanoid receptors are available and are presented in this unit.

Characterization of CGRP(1) receptors in the guinea pig basilar artery

The purpose of the present study was to characterise receptors mediating calcitonin gene-related peptide (CGRP)-induced relaxation of guinea pig basilar artery. This was done by investigating vasomotor responses in vitro and performing autoradiographic binding studies. We also intended to study the importance of an intact endothelium. Agonist studies showed that peptides of the CGRP family induced relaxation of the guinea pig basilar artery with the following order of potency: human beta-CGRP=human alpha-CGRP>>adrenomedullin=[acetamidomethyl-Cys(2,7)]alpha-human CGRP ([Cys(ACM)(2,7)]CGRP)=amylin. These data are in concord with those of the autoradiographic binding studies that showed displacement of [125I] human alpha-CGRP binding with the following order of potency: human alpha-CGRP=human beta-CGRP>>adrenomedullin=human alpha-CGRP-(8-37)>>Cys(ACM)(2,7)]CGRP. In blockade experiments, the relaxant responses to human alpha- and human beta-CGRP were competitively blocked by the CGRP(1) receptor antagonist human alpha-CGRP-(8-37), while those of adrenomedullin and amylin were blocked non-competitively. In order to examine whether amylin induced relaxation via amylin or CGRP receptors, we studied the antagonistic effect of amylin-(8-37) on the weak relaxant response to amylin and found that it was not blocked by amylin-(8-37). These findings, together with the finding that the CGRP(2) receptor agonist [Cys(ACM)(2,7)]CGRP only induced a weak relaxation in the highest concentrations examined, suggest that the CGRP family of peptides mediate relaxation by CGRP(1)-type receptors. Removal of the endothelium, the addition of N(G)-nitro-L-arginine methyl ester (L-NAME), methylene blue or indomethacin did not affect the concentration-response curves of the CGRP analogues, neither in the presence nor in the absence of human CGRP-(8-37). The study shows the presence of a relaxant CGRP(1) receptor on the smooth muscle cells of guinea pig basilar artery. Various endothelial factors did not influence relaxant responses.

The pharmacological potency of various AT(1) antagonists assessed by Schild regression technique in man

RATIONALE

A quantitative technique was used to compare the pharmacological potency in healthy volunteers of angiotensin II receptor antagonists (AIIA): candesartan cilexetil, losartan, irbesartan, valsartan, and telmisartan.

METHODS

In a randomised, double-blind, parallel-group (4x12 subjects) study, single oral doses of candesartan cilexetil 4, 8 and 16 mg, losartan potassium 25, 50 and 100 mg, valsartan 40, 80 and 160 mg, and irbesartan 75, 150 and 300 mg were administered on three consecutive days. Telmisartan 20, 40 and 80 mg was similarly evaluated in 12 volunteers in an open amendment. Angiotensin II (Ang II) antagonistic effects were determined in vivo from rightward shifts in Ang II dose-response curves for diastolic blood pressure (BP) and dose ratios were calculated. Apparent K(i)-doses, i.e. doses (in mg) required to induce a two-fold shift in Ang II dose-response curves (equivalent to approx. 50% blockade of receptors) were determined, using Schild regression analysis.

RESULTS

All treatments dose-dependently attenuated increases in diastolic BP induced by infusion of exogenous Ang II. Candesartan cilexetil appeared to have a more pronounced increase in effect following cumulative dosing. At 24 hours, apparent K(i)-doses were: candesartan cilexetil 6 mg, irbesartan 123 mg, valsartan 93.5 mg, and telmisartan 54 mg. It was not possible to determine an apparent K(i)-dose for losartan at 24 hours.

CONCLUSION

Consistent with results from experimental pharmacology, candesartan cilexetil displayed the highest pharmacological potency (i.e. antagonistic activity per mg substance) of the AIIAs tested. Apparent K(i)-doses at 24 hours were within the dose range recommended for clinical use in patients with hypertension.

Coexpression of rat P2X2 and P2X6 subunits in Xenopus oocytes

Transcripts for P2X(2) and P2X(6) subunits are present in rat CNS and frequently colocalize in the same brainstem nuclei. When rat P2X(2) (rP2X(2)) and rat P2X(6) (rP2X(6)) receptors were expressed individually in Xenopus oocytes and studied under voltage-clamp conditions, only homomeric rP2X(2) receptors were fully functional and gave rise to large inward currents (2-3 microA) to extracellular ATP. Coexpression of rP2X(2) and rP2X(6) subunits in Xenopus oocytes resulted in a heteromeric rP2X(2/6) receptor, which showed a significantly different phenotype from the wild-type rP2X(2) receptor. Differences included reduction in agonist potencies and, in some cases (e.g., Ap(4)A), significant loss of agonist activity. ATP-evoked inward currents were biphasic at the heteromeric rP2X(2/6) receptor, particularly when Zn(2+) ions were present or extracellular pH was lowered. The pH range was narrower for H(+) enhancement of ATP responses at the heteromeric rP2X(2/6) receptor. Also, H(+) ions inhibited ATP responses at low pH levels (<pH 6.3). The pH-dependent blocking activity of suramin was changed at this heteromeric receptor, although the potentiating effect of Zn(2+) on ATP responses was unchanged. Thus, the rP2X(2/6) receptor is a functionally modified P2X(2)-like receptor with a distinct pattern of pH modulation of ATP activation and suramin blockade. Although homomeric P2X(6) receptors function poorly, the P2X(6) subunit can contribute to functional heteromeric P2X channels and may influence the phenotype of native P2X receptors in those cells in which it is expressed.

Equilibrium and kinetic measurements of muscarinic receptor antagonism on living cells using bead injection spectroscopy

Bead injection spectroscopy (BIS) techniques are introduced for automated measurement of pharmacological antagonism by functional assay. Chinese hamster ovary cells that express the rat type 1 muscarinic receptor are cultured on microbeads and used as a renewable biological target for muscarinic receptor antagonist ligands. A flow injection instrument is used to reproducibly sample and capture the cells in a jet ring chamber. The effect of the antagonist pirenzepine on the carbachol-induced intracellular calcium response of the cells is measured with a fluorescence microscope photometry system. The BIS functional assay is used to quantify both equilibrium and kinetic pharmacological values for pirenzepine. In addition, two muscarinic receptor antagonists (pirenzepine and atropine) are assayed to compare their relative efficacy at diminishing the calcium response. Due to the precision of the automated fluid/bead handling protocols, and reproducibility of the measured calcium response, the quantification of useful pharmacological information from living cells by BIS techniques is demonstrated.

Coexpression of rat P2X2 and P2X6 subunits in Xenopus oocytes

Transcripts for P2X(2) and P2X(6) subunits are present in rat CNS and frequently colocalize in the same brainstem nuclei. When rat P2X(2) (rP2X(2)) and rat P2X(6) (rP2X(6)) receptors were expressed individually in Xenopus oocytes and studied under voltage-clamp conditions, only homomeric rP2X(2) receptors were fully functional and gave rise to large inward currents (2-3 microA) to extracellular ATP. Coexpression of rP2X(2) and rP2X(6) subunits in Xenopus oocytes resulted in a heteromeric rP2X(2/6) receptor, which showed a significantly different phenotype from the wild-type rP2X(2) receptor. Differences included reduction in agonist potencies and, in some cases (e.g., Ap(4)A), significant loss of agonist activity. ATP-evoked inward currents were biphasic at the heteromeric rP2X(2/6) receptor, particularly when Zn(2+) ions were present or extracellular pH was lowered. The pH range was narrower for H(+) enhancement of ATP responses at the heteromeric rP2X(2/6) receptor. Also, H(+) ions inhibited ATP responses at low pH levels (<pH 6.3). The pH-dependent blocking activity of suramin was changed at this heteromeric receptor, although the potentiating effect of Zn(2+) on ATP responses was unchanged. Thus, the rP2X(2/6) receptor is a functionally modified P2X(2)-like receptor with a distinct pattern of pH modulation of ATP activation and suramin blockade. Although homomeric P2X(6) receptors function poorly, the P2X(6) subunit can contribute to functional heteromeric P2X channels and may influence the phenotype of native P2X receptors in those cells in which it is expressed.