Different apparent modes of inhibition of alpha2A-adrenoceptor by alpha2-adrenoceptor antagonists

G-protein inhibition profile of the reported Gq/11 inhibitor UBO-QIC

UBO-QIC (FR900359) is the only currently available Gq/11 protein inhibitor. However, its characterization has not been published, and we thus set out to do this. Gi, Gs and Gq protein-mediated responses were assessed utilizing endogenous or heterologously expressed receptors in Chinese hamster ovary cells. UBO-QIC, at 1 μM, was an effective inhibitor of the Gq-mediated responses, but was inactive at Gi- and Gs-mediated responses. Gq/11 and G16 responses were additionally compared in HEL92.1.7 cells, showing inhibition of Gq/11 responses. However, UBO-QIC also appeared to inhibit G16. Further studies are required to establish its profile with respect to the different Gq-family proteins.

Adrenoceptor activity of muscarinic toxins identified from mamba venoms

BACKGROUND AND PURPOSE

Muscarinic toxins (MTs) are snake venom peptides named for their ability to interfere with ligand binding to muscarinic acetylcholine receptors (mAChRs). Recent data infer that these toxins may have other G-protein-coupled receptor targets than the mAChRs. The purpose of this study was to systematically investigate the interactions of MTs with the adrenoceptor family members.

EXPERIMENTAL APPROACH

We studied the interaction of four common MTs, MT1, MT3, MT7 and MTα, with cloned receptors expressed in insect cells by radioligand binding. Toxins showing modest to high-affinity interactions with adrenoceptors were additionally tested for effects on functional receptor responses by way of inhibition of agonist-induced Ca²⁺ increases.

KEY RESULTS

All MTs behaved non-competitively in radioligand displacement binding. MT1 displayed higher binding affinity for the human α(2B)-adrenoceptor (IC₅₀ = 2.3 nM) as compared with muscarinic receptors (IC₅₀ ≥ 100 nM). MT3 appeared to have a broad spectrum of targets showing high-affinity binding (IC₅₀ = 1-10 nM) to M₄ mAChR, α(1A)-, α(1D)- and α(2A)-adrenoceptors and lower affinity binding (IC₅₀ ≥ 25 nM) to α(1B)- and α(2C)-adrenoceptors and M₁ mAChR. MT7 did not detectably bind to other receptors than M₁, and MTα was specific for the α(2B)-adrenoceptor. None of the toxins showed effects on β₁- or β₂-adrenoceptors.

CONCLUSIONS AND IMPLICATIONS

Some of the MTs previously found to interact predominantly with mAChRs were shown to bind with high affinity to selected adrenoceptor subtypes. This renders these peptide toxins useful for engineering selective ligands to target various adrenoceptors.

Estimation of the dissociation rate of unlabelled ligand-receptor complexes by a 'two-step' competition binding approach

BACKGROUND AND PURPOSE

Because the in vivo effectiveness of ligands may also be determined by the rate by which they dissociate from their target receptors, drug candidates are being increasingly screened for this kinetic property. The dissociation rate of unlabelled ligand-receptor complexes can be estimated indirectly from their ability to slow the association of subsequently added radioligand molecules.

EXPERIMENTAL APPROACH

We used the 'two-step competition' binding approach consisting of pre-incubating the receptor preparation with a wide range of ligand concentrations, washing off free ligand molecules, adding radioligand and monitoring its receptor binding after a fixed time. Based on the rationale that binding of both ligands is mutually exclusive and that they bind according to the law of mass action to a single class of sites, the unlabelled ligand's dissociation rate can be estimated from the upward shift that the competition curve experiences after washing.

KEY RESULTS

The relevance of the 'two-step competition' approach was explored by computer simulations and by comparing the dissociation behaviour of unlabelled D(2) dopamine and CB(1) cannabinoid receptor antagonists in this and alternative approaches. Besides providing satisfactory estimations of dissociation rates, the method also detects the ability of the unlabelled ligand molecules to be released from 'sinks' such as the cell membrane.

CONCLUSIONS AND IMPLICATIONS

As the 'two-step competition' requires rapid intermediate washing steps and needs radioligand binding to be measured at only one time point, this approach is particularly suited for binding studies on intact plated cells.

LINKED ARTICLES

This article is part of a themed section on Analytical Receptor Pharmacology in Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2010.161.issue-6.

Neurokinin 1 Receptor Antagonists: Correlation between in Vitro Receptor Interaction and in Vivo Efficacy

We compared the neurokinin 1 receptor (NK(1)R) antagonists aprepitant, CP-99994 [(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine], and ZD6021 [3-cyano-N-((2S)-2-(3,4-dichlorophenyl)-4-[4-[2-(methyl-(S)-sulfinyl)phenyl]piperidino]butyl)-N-methyl]napthamide]] with respect to receptor interactions and duration of efficacy in vivo. In Ca(2+) mobilization assays (fluorometric imaging plate reader), antagonists were applied to human U373MG cells simultaneously with or 2.5 min before substance P (SP). In reversibility studies, antagonists were present for 30 min before washing, and responses to SP were repeatedly measured afterward. The compounds were administered i.p. to gerbils, and the gerbil foot tap (GFT) response was monitored at various time points. The NK(1)R receptor occupancy for aprepitant was determined in striatal regions. Levels of compound in brain and plasma were measured. Antagonists were equipotent at human NK(1)R and acted competitively with SP. After preincubation, aprepitant and ZD6021 attenuated the maximal responses, whereas CP-99994 only shifted the SP concentration-response curve to the right. The inhibitory effect of CP-99994 was over within 30 min, whereas for ZD6021, 50% inhibition still persisted after 60 min. Aprepitant produced maximal inhibition lasting at least 60 min. CP-99994 (3 micromol/kg) inhibited GFT by 100% 15 min after administration, but the effect declined rapidly together with brain levels thereafter. The efficacy of ZD6021 (10 micromol/kg) lasted 4 h and correlated well with brain levels. Aprepitant (3 micromol/kg) inhibited GFT and occupied striatal NK(1)R by 100% for >48 h despite that brain levels of compound were below the limit of detection after 24 h. Slow functional reversibility is associated with long-lasting in vivo efficacy of NK(1)R antagonists, whereas the efficacy of compounds with rapid reversibility is reflected by their pharmacokinetics.

Long-lasting angiotensin type 1 receptor binding and protection by candesartan: comparison with other biphenyl-tetrazole sartans

BACKGROUND

The ability of biphenyl-tetrazole angiotensin type 1 (AT1) receptor antagonists (BTsartans) to block angiotensin II (Ang II)-mediated responses has been extensively investigated in vascular tissues and, more recently, in cell lines expressing the human AT1-receptor. When pre-incubated, BTsartans acted surmountably (shifting the Ang II concentration-response curve to the right) or insurmountably (also decreasing the maximal response). It was shown that their insurmountable behaviour is due to the formation of tight, long-lasting complexes with the receptor. Partial insurmountable antagonism is due to the co-existence of tight and loose complexes. The proportion of insurmountable antagonism, the potency and the dissociation rate of the BTsartans decreases in the order: candesartan > EXP3174 (losartan's active metabolite) > valsartan > irbesartan >> losartan.

OBJECTIVE

It is of interest to explore how tight AT1-receptor binding of BTsartans such as candesartan might contribute to their long-lasting clinical effect.

METHODS

Computer-assisted simulations (COPASI program) were performed to follow the receptor-occupation and protection by different antagonists as a function of time. Free antagonist concentrations were allowed to decrease exponentially with time.

RESULTS

The simulations suggest that slow dissociation does not tangibly prolong receptor occupancy if the free antagonist is eliminated at a slower pace (as is the case for BTsartans). Yet when surmountable and insurmountable antagonists occupy the same amount of receptors, insurmountable antagonists offer appreciably better protection against fluctuations in natural messenger concentration.

CONCLUSION

Slow receptor dissociation and slow antagonist elimination are likely to act in synergy to produce long-lasting receptor protection.

RX 821002 as a tool for physiological investigation of alpha(2)-adrenoceptors

RX 821002 is the 2-methoxy congener of idazoxan. In binding and tissue studies it behaves as a selective antagonist of alpha(2)-adrenoceptors, with at least 5 times greater affinity for these receptors than any other binding site. It does not select between the different types of alpha(2)-receptor. Although this drug probably has no future as a therapeutic agent, it remains a good probe for physiological activity at alpha(2)-adrenoceptors in animal experiments. A particularly useful feature of this compound is its lack of binding at I(1) and I(2) imidazoline receptors. However, it has relatively high affinity for 5-HT(1A) receptors (at which it acts as an antagonist) and a tendency to behave as an inverse agonist at alpha(2A)-adrenoceptors in some cell culture systems. These potential drawbacks may be overcome by careful design of experiments, and the greater selectivity of RX 821002 renders it much superior to yohimbine or idazoxan as a tool for probing physiological actions at alpha(2)-receptors. It can be compared favorably with other selective antagonists such as atipamezole. In physiological studies, RX 821002 augments norepinephrine release in the frontal cortex and increases drinking behavior in rat. In rabbit, intrathecal administration of this drug enhances somatic and autonomic motor outflows, showing that tonic adrenergic descending inhibition of withdrawal reflexes and sympathetic pre-ganglionic neurons is strong in this species. The potentiation of reflexes may be considered a pro-nociceptive action. In the same model, RX 821002 antagonizes the inhibitory effects of the mu opioid fentanyl, indicating that exogenous opioids synergize with endogenously released norepinephrine in the spinal cord. Thus, the careful use of RX 821002 has revealed several aspects of the physiological activity of alpha(2)-adrenoceptors in rabbit spinal cord and rat brain. We recommend that RX 821002 and/or compounds with similar selectivity for alpha(2)-adrenoceptors (atipamezole, MK-912, RS-79948) should be used in preference to yohimbine or idazoxan in all future studies of this type.

Agonist trafficking of G(i/o)-mediated alpha(2A)-adrenoceptor responses in HEL 92.1.7 cells

1. The ability of 19 agonists to elevate Ca(2+) and inhibit forskolin-induced cyclic AMP elevation through alpha(2A)-adrenoceptors in HEL 92.1.7 cells was investigated. Ligands of catecholamine-like- (five), imidazoline- (nine) and non-catecholamine-non-imidazoline-type (five) were included. 2. The relative maximum responses were similar in both assays. Five ligands were full or nearly full agonists, six produced 20 - 70% of the response to a full agonist and the remaining eight gave lower responses (< 20%) so that their potencies were difficult to evaluate. 3. Marked differences in the potencies of the agonists with respect to the two measured responses were seen. The catecholamines were several times less potent in decreasing cyclic AMP than in increasing Ca(2+), whereas the other, both imidazoline and ox-/thiazoloazepine ligands, were several times more potent with respect to the former than the latter response. For instance, UK14,304 was more potent than adrenaline with respect to the cyclic AMP response but less potent than adrenaline with respect to the Ca(2+) response. 4. All the responses were sensitive to pertussis toxin-pretreatment. Also the possible role of PLA(2), beta-adrenoceptors or ligand transport or metabolism as a source of error could be excluded. The results suggest that the active receptor states produced by catecholamines and the other agonists are markedly different and therefore have different abilities to activate different signalling pathways.

From 'captive' agonism to insurmountable antagonism: demonstrating the power of analytical pharmacology

1. Mathematical modelling is useful in pharmacology, allowing the investigator to obtain insights into the biological processes under study that may not always be intuitively obvious. Examples are presented in this review using the pharmacology of the muscarinic acetylcholine receptor (mAChR) agonist xanomeline. 2. Xanomeline possesses a novel mode of action that involves persistent binding to the M1 mAChR, yielding a fraction of agonist in the receptor compartment that continually activates the receptor, despite extensive washout, as assessed in functional assays measuring the cumulative production of M1 mAChR-mediated L-[3H]-citrulline. This persistent effect was reversed by the antagonist atropine, but re-established upon the removal of atropine. Thus, xanomeline may represent the first 'captive' agonist of the mAChR. 3. Atropine was equally potent at reversing the effect of persistently bound xanomeline and preventing the effect of added xanomeline. Application of standard quantitative equilibrium models of agonist-antagonist interaction to these data suggested that the interaction between xanomeline and atropine satisfied the criteria of competitivity in each case. 4. Subsequent real-time assays of M1 mAChR-mediated intracellular calcium mobilization found that atropine inhibited the effects of xanomeline in an insurmountable manner. 5. The discrepancy between the modes of antagonism in the various functional assays could be reconciled in a dynamic receptor model of antagonism within a transient response system and subsequent Monte Carlo simulations allowed for the development of an optimized analytical procedure to quantify antagonist potency under such conditions of response fade. 6. These types of studies exemplify the diagnostic and integrative features of analytical pharmacology.

Partial to complete antagonism by putative antagonists at the wild-type alpha(2C)-adrenoceptor based on kinetic analyses of agonist:antagonist interactions

1. Activation of the recombinant human alpha(2C)-adrenoceptor (alpha(2C) AR) by (-)-adrenaline in CHO-K1 cells transiently co-expressing a chimeric G(alpha q/i1) protein induced a rapid, transient Ca(2+) response with a high-magnitude followed by a low-magnitude phase which continued throughout the recorded time period (15 min). 2. Activation of the alpha(2C) AR by various alpha(2) AR agonists revealed the following rank order of high-magnitude Ca(2+) response [E(max) (%) versus 10 microM (-)-adrenaline]: UK 14304 (102+/-4)=talipexole (101+/-3)=(-)-adrenaline (100)=d-medetomidine (98+/-1)>oxymetazoline (81+/-4) reverse similarclonidine (75+/-5). 3. The methoxy- (RX 821002) and ethoxy-derivatives (RX 811059) of idazoxan and the dexefaroxan analogue atipamezole were fully effective as antagonists of both the high- and the low-magnitude Ca(2+) response. However, though acting as full antagonists of the high-magnitude response, the further putative alpha(2) AR antagonists idazoxan (27%), SKF 86466 (29%) and dexefaroxan (59%) reversed the low-magnitude response only partially. 4. In conclusion, kinetic analyses of agonist : antagonist interactions at the alpha(2C) AR demonstrate a wide spectrum of partial to complete antagonism of the low-magnitude Ca(2+) response for structurally related alpha(2) AR ligands.

Analysis of Apparent Noncompetitive Responses to Competitive H(1)-Histamine Receptor Antagonists in Fluorescent Imaging Plate Reader-Based Calcium Assays

We have examined the utility of high throughput fluorescent imaging plate reader (FLIPR)-based calcium assays for pharmacological characterization of G-protein coupled receptors (GPCRs) using recombinant and native human H1-histamine receptors (H(1)-HR), expressed in HEK293 and HeLa S3 cells, respectively, as model systems. For stably transfected HEK293 cell lines, the potency of histamine for elevating intracellular calcium increased (pD(2), 7.13 and 7.86) with increased H(1)-HR density (about 0.8 and 14 pmol/mg protein, respectively), though histamine binding affinities were similar. The classic H(1)-HR competitive antagonists diphenhydramine and chlorpheniramine appeared noncompetitive by causing depressions of the maximal histamine responses along with rightward shifts of histamine concentration-response curves, thus precluding Schild analysis. Applying the generalized Cheng-Prusoff equation to antagonist concentration-response curves for inhibition of fixed histamine concentrations yielded apparent pK(b) values that were consistent among recombinant and native receptors at different expression levels. These pK(b) values for diphenhydramine and chlorpheniramine (e.g., 7.83 and 8.77, respectively) were in good agreement with binding pK(i) values (e.g., 7.98 and 8.52, respectively). Apparent antagonist affinities determined from FLIPR calcium and competition binding assays were also consistent for the competitive antagonists mepyramine, tripelennamine, and promethazine. In phosphoinositide hydrolysis assays, chlorpheniramine exhibited insurmountable inhibition of histamine calcium responses, although to a lesser extent than that observed in calcium assays; pK(b) values were similar. These results demonstrate that competitive antagonist potencies can be attained from FLIPR-derived data by application of the generalized Cheng-Prusoff equation, despite apparent noncompetitive antagonism under these assay conditions. Apparent noncompetitive antagonist effects may in part be attributable to a lack of equilibrium of histamine and antagonists with H(1)-HR within the short duration of rapid transient effects of histamine on intracellular calcium.

Functional assessment of recombinant human alpha(2)-adrenoceptor subtypes with cytosensor microphysiometry

We applied the Cytosensor Microphysiometry system to study the three human alpha(2)-adrenoceptor subtypes, alpha(2A), alpha(2B) and alpha(2C), expressed in Chinese hamster ovary (CHO) cells, and assessed its potential in the quantitative monitoring of agonist activity. The natural full agonist, (-)-noradrenaline, was used to define agonist efficacy. The imidazole derivative dexmedetomidine was a potent full agonist of all three receptor subtypes. The imidazolines clonidine and UK 14,304 (5-bromo-N-(4, 5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) appeared to be partial agonists at alpha(2B)-adrenoceptors (E(max) approximately 60% of (-)-noradrenaline) but full agonists at alpha(2A)- and alpha(2C)-adrenoceptors. The responses mediated by all three alpha(2)-adrenoceptor subtypes were partly inhibited by the sodium-hydrogen (Na(+)/H(+)) exchange inhibitor, MIA (5-(N-methyl-N-isobutyl)-amiloride). The agonist responses were totally abolished by pretreatment with pertussis toxin in cells with alpha(2A)- and alpha(2C)-adrenoceptors, and partly abolished in cells with alpha(2B)-adrenoceptors. The residual signal in alpha(2B)-cells was sensitive to the intracellular Ca(2+)chelator, BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester). Cholera toxin (which acts on G(s)-proteins) had no effect on the agonist responses. The results suggest that the extracellular acidification responses mediated by all three human alpha(2)-adrenoceptor subtypes are dependent on Na(+)/H(+)exchange and G(i/o) pathways, and that alpha(2B)-adrenoceptors are capable of coupling to another, G(i/o)-independent and Ca(2+)-dependent signaling pathway.

The assessment of antagonist potency under conditions of transient response kinetics

The muscarinic acetylcholine receptor antagonists, atropine and pirenzepine, produced an apparent insurmountable antagonism of muscarinic M(1) receptor-mediated intracellular Ca(2+) mobilization in Chinese hamster ovary (CHO) cells when tested against the agonists carbachol or xanomeline. Each antagonist caused a dextral shift of the agonist concentration-response curves with depression of the maximum response that was incomplete (i.e., saturated) and which varied with the pairs of agonist and antagonist. Equilibrium competition binding assays found no deviation from simple, reversible competitive behavior for either antagonist. The relative rates of dissociation of unlabeled atropine and pirenzepine were also assessed in radioligand kinetic studies and it was found that atropine dissociated from the receptor approximately 8-fold slower than pirenzepine. Numerical dynamic simulations suggested that the insurmountability of antagonism observed in the present study was probably a kinetic artifact related to the measurement of transient responses to a non-equilibrated agonist in the presence of a slowly dissociating antagonist. Importantly, the patterns of antagonism observed included a saturable depression of agonist maximal response, a mode of antagonism that is incompatible with the previously described phenomenon of hemi-equilibrium states. Monte Carlo simulations indicated that reasonable, semi-quantitative estimates of antagonist potency could be determined by a minor modification of standard methods, where equieffective agonist concentrations, rather than EC(50) values, are compared in the absence and presence of antagonist. Application of the latter approach to the functional data yielded estimates of antagonist potency that were in excellent agreement with those derived from the equilibrium binding assays, thus indicating that the present method can be useful for quantifying antagonist potency under non-equilibrium conditions.

Pseudo-noncompetitive antagonism of M1, M3, and M5 muscarinic receptor-mediated Ca2+ mobilization by muscarinic antagonists

Muscarinic receptors M1, M3, and M5 were expressed in Sf9 cells. Three different patterns of inhibition of Ca2+ elevations could be resolved for the subtype nonselective muscarinic receptor antagonists: (i) a right shift of the agonist dose-response curve, (ii) a right shift of the agonist dose-response curve and a depression of the maximum signal, and (iii) an intermediate pattern where the antagonist apparently behaved more competitively at higher concentrations. A simulation performed assuming that these differences are due to differences in the dissociation rates of the antagonists reproduced all three different modes of inhibition; the novel intermediate pattern (iii) is suggested to be caused by an intermediate antagonist dissociation rate. A direct correlation between the type of inhibition and the measured dissociation rate of the antagonists was also observed. Functional selectivity between receptor subtypes based on the dissociation constants is suggested based on the results.