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1.
J Pharmacol Exp Ther ; 325(3): 927-34, 2008 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-18322151

RESUMO

Radioligand binding assays remain a common method for quantifying the effects of allosteric modulators at G protein-coupled receptors. The allosteric ternary complex model (ATCM) is the simplest model applied to derive estimates of modulator affinity (K(B)) and cooperativity (alpha), which are necessary for understanding structure-activity relationships. However, the increasing drive toward assay miniaturization in modern drug discovery may lead to conditions where appreciable ligand depletion occurs in the assay. Theoretical simulations investigating the impact of orthosteric radioligand depletion on the estimation of ATCM parameters revealed the following. 1) For allosteric inhibitors, application of the standard ATCM to data obtained under depletion conditions leads to an underestimation of pK(B) and an overestimation of log alpha. 2) For allosteric enhancers, the opposite was noted, but not always; the nonlinear regression algorithm is more likely to struggle to converge to a satisfactory solution of (nondepletion) ATCM parameters in this situation. 3) Application of a novel ATCM that explicitly incorporates orthosteric ligand depletion will yield more reliable model estimates, provided the degree of depletion is not high (< approximately 50%). Subsequent experiments investigated the interaction between [3H]N-methylscopolamine and the allosteric enhancer, alcuronium, or inhibitor, gallamine, in the presence of increasing concentrations of M(2) muscarinic acetylcholine receptor and showed that application of an ATCM that explicitly incorporates radioligand depletion can indeed give more robust estimates of modulator affinity and cooperativity estimates than the standard model. These results have important implications for the quantification of allosteric modulator actions in binding-based discovery assays.


Assuntos
Regulação Alostérica , Modelos Biológicos , Receptor Muscarínico M2/metabolismo , Alcurônio/metabolismo , Animais , Atropina/metabolismo , Células CHO , Carbacol/metabolismo , Membrana Celular/metabolismo , Cricetinae , Cricetulus , Trietiodeto de Galamina/metabolismo , Ligantes , N-Metilescopolamina/metabolismo , Ensaio Radioligante , Trítio
2.
J Pharmacol Exp Ther ; 312(1): 382-90, 2005 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-15333678

RESUMO

The effects of prolonged exposure of M(2) muscarinic acetylcholine receptors (mAChRs), stably expressed in Chinese hamster ovary cells, to the allosteric modulators gallamine, alcuronium, and heptane-1,7-bis (dimethyl-3'-phthalimidopropyl)-ammonium bromide (C(7)/3'-phth) were compared with the effects of the agonist carbachol (CCh) and antagonists atropine and N-methylscopolamine (NMS). Intact cell saturation binding assays using [(3)H]NMS found that pretreatment of the cells for 24 h with CCh caused a significant down-regulation of receptor number, whereas atropine, NMS, and all three allosteric modulators caused receptor up-regulation. Functional assays using a cytosensor microphysiometer to measure whole-cell metabolic rate found no acute effects of gallamine on receptor signaling, whereas atropine seemed to behave as an inverse agonist. Pretreatment of the cells with gallamine (20 microM) or atropine (20 nM) resulted in a significant enhancement of the maximal effect evoked by CCh. In contrast, CCh (100 microM) pretreatment resulted in a significant reduction in maximal receptor signaling capacity. Time-course experiments revealed that the effects of atropine and gallamine on receptor up-regulation are only visualized after at least 12-h ligand exposure, compared with the more rapid effects of CCh, which achieve steady-state down-regulation within 90 min. Additional experiments monitoring CCh-mediated M(2) mAChR internalization in the presence of gallamine revealed that part of the mechanism underlying the effects of the modulator on receptor expression may involve a change in receptor internalization properties. These findings suggest that, like orthosteric ligands, G protein-coupled receptor allosteric modulators also are able to mediate long-term effects on receptor regulation.


Assuntos
Regulação Alostérica , Antagonistas Nicotínicos/farmacologia , Receptor Muscarínico M2/metabolismo , Alcurônio/farmacologia , Animais , Células CHO , Cricetinae , Feminino , Trietiodeto de Galamina/farmacologia , Expressão Gênica/efeitos dos fármacos , Ligantes , Ftalimidas/farmacologia , Compostos de Amônio Quaternário/farmacologia , Receptor Muscarínico M2/efeitos dos fármacos , Fatores de Tempo
3.
Biochem Soc Trans ; 32(Pt 5): 873-7, 2004 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-15494038

RESUMO

Allosteric modulators of G-protein-coupled receptors interact with binding sites that are topographically distinct from the orthosteric site recognized by the receptor's endogenous agonist. Allosteric ligands offer a number of advantages over orthosteric drugs, including the potential for greater receptor subtype selectivity and a more 'physiological' regulation of receptor activity. However, the manifestations of allosterism at G-protein-coupled receptors are quite varied, and significant challenges remain for the optimization of screening methods to ensure the routine detection and validation of allosteric ligands.


Assuntos
Desenho de Fármacos , Receptores Acoplados a Proteínas G/química , Alcurônio/farmacologia , Sítio Alostérico , Animais , Sítios de Ligação , Relação Dose-Resposta a Droga , Humanos , Cinética , Ligantes , Modelos Químicos , Ligação Proteica , Estrutura Terciária de Proteína , Receptores Acoplados a Proteínas G/metabolismo , Transdução de Sinais
4.
Recept Channels ; 10(2): 51-60, 2004.
Artigo em Inglês | MEDLINE | ID: mdl-15204035

RESUMO

Allosterism, whereby small molecule ligands produce global changes in the conformations of receptors, is a powerful mechanism for drug effect. This is illustrated by the recent data describing CCR5 antagonists as blockers of HIV infection. Allosteric effects are described in terms of a change in the tertiary conformation of the receptor. This paper outlines some unique features of allosteric antagonists as new drug entities. These include the fact that allosteric ligands have texture in antagonism (not all allosterically blocked receptors are alike), allosteric blockade is probe dependent (not all agonists and radioligands are blocked equally), and the fact that allosteric binding involves a separate site on the receptor may have relevance to duration of effect and selectivity. Dissociation between receptor function and binding also can be encountered with allosteric ligands.


Assuntos
Receptores Acoplados a Proteínas G/metabolismo , Alcurônio/farmacologia , Regulação Alostérica , Sítio Alostérico , Fármacos Anti-HIV/farmacologia , Antagonistas dos Receptores CCR5 , Trietiodeto de Galamina/farmacologia , Infecções por HIV/tratamento farmacológico , Infecções por HIV/prevenção & controle , Humanos , Ligantes , Modelos Moleculares , Receptor Muscarínico M2/antagonistas & inibidores , Receptor Muscarínico M2/química , Receptor Muscarínico M2/metabolismo , Receptores CXCR4/antagonistas & inibidores , Receptores Acoplados a Proteínas G/antagonistas & inibidores , Receptores Acoplados a Proteínas G/química
5.
Recept Channels ; 9(4): 229-40, 2003.
Artigo em Inglês | MEDLINE | ID: mdl-12893536

RESUMO

Allosteric modulation of G protein-coupled receptors has been intensively studied at muscarinic acetylcholine receptors. Findings made with archetypal allosteric agents such as gallamine, alcuronium, and bis(ammonio)alkane-type agents revealed that binding of orthosteric ligands that attach to the acetylcholine site can be allosterically decreased or increased or left unaltered in a subtype-selective fashion. Analyses of structure/activity relationships (SARs) help to elucidate the molecular events underlying the allosteric action and they may pilot the development of new allosteric agents with improved properties and therapeutic perspectives. With a focus on SARs, this review illustrates the principles of muscarinic allosteric interactions, gives an overview of SARs in congeners of archetypal allosteric agents, and considers the topology of M(2) muscarinic allosteric interactions that are characterized by divergent binding modes.


Assuntos
Receptor Muscarínico M2/química , Receptor Muscarínico M2/metabolismo , Alcurônio/análogos & derivados , Alcurônio/metabolismo , Alcanos/metabolismo , Regulação Alostérica , Animais , Humanos , Receptor Muscarínico M2/antagonistas & inibidores , Transdução de Sinais/fisiologia , Relação Estrutura-Atividade
6.
Anaesthesist ; 52(6): 516-21, 2003 Jun.
Artigo em Alemão | MEDLINE | ID: mdl-12835873

RESUMO

The present study evaluates the use of muscle relaxants for rapid-sequence induction (RSI) and different application techniques (pre-curarisation, priming, timing) as a part of a nationwide survey in Germany. In 86.8% of anaesthesia departments succinylcholine is used for RSI and an average of 56.5% of respondents used only succinylcholine for RSI. Of all non-depolarising muscle relaxants rocuronium is the most frequently used alternative. Of the anaesthesia departments 2.6% use rocuronium regularly in patients with increased risk for aspiration of stomach contents; level one centres significantly more than others, 12.9% answered that pre-curarisation techniques were never used, whereas 45.6% use non-depolarising neuromuscular blocking drugs before giving succinylcholine in 80-100% of cases. Priming is not used by 64.4% of respondents, as opposed to 9.8% who utilise this technique regularly. The statements regarding timing are 71.1% and 5.4%, respectively. Alcuronium is used for RSI in departments in which the financial aspect is the primary decision criteria. Despite ist known side-effects and the on-going discussion over the past years, succinylcholine is still the most frequently used muscle relaxants for RSI. Priming is often declined by anaesthetists in Germany, most probably due to the absence of clear advantages and the possibility of severe complications. It is the opinion of the authors that timing but also drugs with a slow onset (e.g., alcuronium and Pancuronium) are obsolete in the context of RSI.


Assuntos
Anestesia , Relaxantes Musculares Centrais , Alcurônio , Androstanóis , Coleta de Dados , Uso de Medicamentos , Alemanha , Humanos , Fármacos Neuromusculares Despolarizantes , Fármacos Neuromusculares não Despolarizantes , Pneumonia Aspirativa/prevenção & controle , Risco , Rocurônio , Succinilcolina
7.
Anesth Analg ; 97(2): 488-491, 2003 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-12873942

RESUMO

UNLABELLED: Bispectral index (BIS) is an electroencephalographic variable promoted for measuring depth of anesthesia. Electromyographic activity influences surface electroencephalography and the calculation of BIS. In this study, we sought to determine the effect of spontaneous electromyographic activity on BIS. BIS was monitored in three volunteers by using an Aspect A-1000 monitor. The experiment was repeated in one volunteer. Electromyographic activity was recorded. Alcuronium and succinylcholine were administered. No other drugs were used. In parallel with spontaneous electromyographic activity of the facial muscles, BIS decreased in response to muscle relaxation to a minimum value of 33 and, in the repeated measurement, to a minimum value of 9 when total neuromuscular block was achieved. In two volunteers, no total block was achieved. BIS decreased to a minimal value of 64 and 57, respectively. In turn, recovery of BIS coincided with the reappearance of spontaneous electromyographic activity. During the entire experiment, the volunteers had full consciousness. BIS, assessed by software Version 3.31, correlates with spontaneous electromyographic activity of the facial muscles. BIS failed to detect awareness in completely paralyzed subjects. Thus, in paralyzed patients, BIS monitoring may not reliably indicate a decline in sedation and imminent awareness. IMPLICATIONS: The bispectral index (BIS) is an electroencephalographic variable intended for measuring depth of anesthesia. Electromyographic activity influences the calculation of BIS. We found that the administration of a muscle relaxant to unanesthetized volunteers decreases the bispectral index value. Thus, awareness in totally paralyzed patients cannot be excluded.


Assuntos
Estado de Consciência/fisiologia , Eletroencefalografia/efeitos dos fármacos , Eletromiografia , Bloqueio Neuromuscular , Alcurônio , Músculos Faciais/fisiologia , Humanos , Relaxamento Muscular , Fármacos Neuromusculares Despolarizantes , Fármacos Neuromusculares não Despolarizantes , Succinilcolina
8.
J Pharm Biomed Anal ; 28(5): 983-90, 2002 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-12039641

RESUMO

Alcuronium, a neuromuscular blocking drug, was recently introduced to the European Pharmacopoeia. A HPLC method is described to limit the impurities of alcuronium, namely the diallylcaracurine (DAC) and the allyl-Wieland-Gumlich-aldehyde (WCA), to less than 0.5%. Since alcuronium and all impurities are quaternary salts, capillary electrophoresis (CE) is highly suitable to evaluate the impurity profile. Using 12 mM heptakis-(2,6-di-O-methyl)-beta-cyclodextrin in a 50 mM phosphate buffer at pH 5.5 or 50 mM diethanolamine buffer (pH 9.2)-acetonitrile 19:1 containing heptakis-(2,3-O-diacetyl-6-sulfo)-beta-cyclodextrin the impurities could be baseline separated and quantified. The limit of detection for DAC and WCA was found to be in the same range as found with HPLC; thus, less than 0.1% of both DAC and WCA could be detected in the solution for injection in presence of alcuronium. In injection solutions of alcuronium which were stored at higher temperatures three additional, unidentified impurities were detected. In addition, the conversion of alcuronium to DAC, occurring under acidic condition, was monitored by means of the CE method developed.


Assuntos
Alcurônio/análise , Fármacos Neuromusculares não Despolarizantes/análise , Ciclização , Contaminação de Medicamentos , Estabilidade de Medicamentos , Armazenamento de Medicamentos , Eletroforese Capilar , Concentração de Íons de Hidrogênio , Soluções
9.
J Pharmacol Exp Ther ; 301(2): 720-8, 2002 May.
Artigo em Inglês | MEDLINE | ID: mdl-11961078

RESUMO

Previous studies on allosteric interactions at muscarinic receptors have often focused on ligand-receptor binding interactions, because ligand binding seemed to reflect functional consequences. The prototypal allosteric agent alcuronium is known to bind with similar affinity to the M(2) subtype of muscarinic acetylcholine receptors whether or not the receptors are occupied by the agonist pilocarpine. To determine allosteric modulation of receptor signaling by alcuronium, the effects of pilocarpine were measured in contracting guinea pig left atria and on G-protein coupling in M(2)-transfected Chinese hamster ovary (CHO) cell membranes. Alcuronium dose-dependently suppressed pilocarpine-induced reduction of isometric contraction force in atria (pIC(50, Alc) = 5.63) without any effect on the EC(50) of pilocarpine, consistent with an allosteric mechanism. In contrast, alcuronium shifted the concentration-effect curve of the agonist oxotremorine M to the right without affecting the maximal effect, in a formally competitive manner (pK(A, Alc) = 5.54). If pilocarpine remained receptor bound in the presence of alcuronium, this indicates that pilocarpine can no longer act as an agonist. In support of this hypothesis, pilocarpine acted as a competitive antagonist against oxotremorine M in the presence of 10 microM alcuronium. Measuring guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding in CHO-M(2) membranes yielded similar results. Alcuronium suppressed pilocarpine-induced stimulation of [(35)S]GTPgammaS binding (pIC(50, Alc) = 5.47) without shift in EC(50), whereas it competitively shifted the response to oxotremorine M (pK(A, Alc) = 5.97). [(3)H]Oxotremorine M binding data corresponded with the functional findings. In conclusion, alcuronium converted the agonist pilocarpine into an antagonist-a novel type of functional allosteric interaction.


Assuntos
Alcurônio/farmacologia , Pilocarpina/farmacologia , Receptores Muscarínicos/metabolismo , Transdução de Sinais/fisiologia , Vasoconstrição/efeitos dos fármacos , Regulação Alostérica , Animais , Função Atrial , Células CHO , Cricetinae , Interações Medicamentosas , Proteínas de Ligação ao GTP/metabolismo , Átrios do Coração/efeitos dos fármacos , Agonistas Muscarínicos/farmacologia , Antagonistas Muscarínicos/farmacologia , Receptor Muscarínico M2 , Receptores Muscarínicos/efeitos dos fármacos , Transfecção
10.
Mol Pharmacol ; 60(4): 761-7, 2001 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-11562438

RESUMO

To clarify the involvement of specific domains of muscarinic receptors in the action of allosteric modulators, muscarinic M(3) receptors (on which allosteric interactions are weak) were genetically modified to become more similar to M(2) receptors (on which allosteric interactions are strong) and were expressed in COS-7 cells. Affinity for allosteric modulator gallamine was enhanced 25- to 50-fold by modifications of the third external loop (o3) and the negative effect of gallamine on the affinity for classical antagonist N-[(3)H]methylscopolamine ([(3)H]NMS) was augmented. Affinity for alcuronium became 3-fold higher after the o3 loop of M(3) receptors was made identical with the o3 loop of M(2) receptors, and alcuronium acquired positive influence on the affinity for [(3)H]NMS. This is the first instance of inducing positive cooperativity on muscarinic receptors by genetic manipulation. Transferring whole o2 loop from M(2) to M(3) receptors substantially enhanced affinities for gallamine and alcuronium without augmenting their negative action on [(3)H]NMS binding. In contrast, effects of simply adding two negative charges into the o2 loop of M(3) receptors were small. Removal of Arg from o1 loop abolished the negative effect of gallamine but not of alcuronium on [(3)H]NMS binding at equilibrium. Data point to an important role of o3 loop in the mechanism of the positive and negative cooperativity between [(3)H]NMS and alcuronium and gallamine, respectively, and in the binding of both modulators to M(2) receptors and reveal independence between mutation-induced changes in the affinity for a modulator and in the magnitude and direction of the allosteric effect of the modulator.


Assuntos
Alcurônio/farmacologia , Trietiodeto de Galamina/farmacologia , N-Metilescopolamina/farmacologia , Receptores Muscarínicos/metabolismo , Regulação Alostérica , Sequência de Aminoácidos , Arginina/genética , Sítios de Ligação , Glicina/genética , Humanos , Dados de Sequência Molecular , Mutação , Antagonistas Nicotínicos/farmacologia , Parassimpatolíticos/farmacologia , Conformação Proteica , Ensaio Radioligante , Receptor Muscarínico M3 , Receptores Muscarínicos/química , Receptores Muscarínicos/efeitos dos fármacos , Receptores Muscarínicos/genética , Homologia de Sequência de Aminoácidos , Trítio
11.
Neurochem Res ; 26(4): 383-94, 2001 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-11495349

RESUMO

Allosteric enhancement of the affinity of muscarinic receptors for their ligands offers a new way to influence cholinergic neurotransmission. The structure of the allosteric binding domain(s) and the features of agonists, antagonists and modulators which determine the occurrence of either positive or negative cooperativity require clarification. We tested interactions between allosteric modulators alcuronium, strychnine and brucine and eight antagonists at muscarinic receptors expressed in CHO cells. In experiments with unlabeled antagonists, all three modulators enhanced the affinity for 4-diphenylacetoxy-N-dimethylpiperidinium (4-DAMP) at the M2 receptors, and strychnine did so also at the M4 receptors. Positive interactions were also observed between alcuronium and L-hyoscyamine (M2) and scopolamine (M2), between strychnine and butylscopolamine (M4), L-hyoscyamine (M2 and M4) and scopolamine (M4), and between brucine and scopolamine (M2). Positive effects of alcuronium, strychnine and brucine on the affinity of the M2 receptors for 4-DAMP have been confirmed by direct measurements of the binding of [3H]-4-DAMP. A comparison of molecular models of several antagonists which are esters revealed that antagonists in which the distance between the N and the carboxyl C atoms corresponds to five chemical bonds are more likely to display positive cooperativity with alcuronium at the M2 receptors than the antagonists in which the N-carboxyl C distance corresponds to four chemical bonds.


Assuntos
Antagonistas Muscarínicos/farmacologia , Piperidinas/farmacologia , Estricnina/análogos & derivados , Alcurônio/farmacologia , Regulação Alostérica , Animais , Células CHO , Cricetinae , Antagonistas Muscarínicos/química , N-Metilescopolamina/metabolismo , Ensaio Radioligante , Estricnina/farmacologia , Trítio
12.
Mol Pharmacol ; 58(6): 1451-60, 2000 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-11093785

RESUMO

A series of ligands that allosterically modulate the binding of classical ligands to muscarinic receptors was evaluated at wild-type and chimeric receptors. All of the ligands studied had highest affinity toward the M(2) subtype and lowest affinity toward the M(5) subtype. The chimeric receptors were mostly M(5) sequence; the amount of M(2) sequence ranged from about 6 to just under 30%. Alcuronium and TMB-8 had much higher affinity for the chimeric receptor that included the M(2) second outer loop of the receptor plus flanking regions of TM4 and TM5 than for any of the other chimeric receptors (the affinities of which remained similar to that of the M(5) subtype). However, this chimera retained the negative cooperativity between alcuronium and the classical antagonist N-methylscopolamine that is characteristic of M(5) (these ligands are positively cooperative at M(2)). Verapamil, tetrahydroaminoacridine, and d-tubocurarine were also sensitive to that chimeric substitution, although verapamil and tetrahydroaminoacridine had even higher affinity for a chimera with M(2) sequence in TM7. None of these ligands shared gallamine's sensitivity to a region of the third outer loop, but studies in which obidoxime reversed the allosteric effects of gallamine and other ligands suggested that they nevertheless compete for a common site. In summary, although the present data are consistent with previous studies that have suggested that allosteric ligands bind to the outermost regions of muscarinic receptors, it appears that different allosteric ligands may derive subtype selectivity from different regions of the receptor.


Assuntos
Alcurônio/farmacologia , Ácido Gálico/análogos & derivados , Ácido Gálico/farmacologia , Receptores Muscarínicos/metabolismo , Acetilcolina/metabolismo , Regulação Alostérica , Animais , Células CHO , Bloqueadores dos Canais de Cálcio/farmacologia , Cricetinae , Relação Dose-Resposta a Droga , Interações Medicamentosas , Ligantes , N-Metilescopolamina/farmacologia , Antagonistas Nicotínicos/farmacologia , Parassimpatolíticos/farmacologia , Receptores Muscarínicos/efeitos dos fármacos , Proteínas Recombinantes de Fusão/efeitos dos fármacos , Proteínas Recombinantes de Fusão/metabolismo
14.
Naunyn Schmiedebergs Arch Pharmacol ; 361(2): 107-12, 2000 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-10685864

RESUMO

Gallamine, alcuronium and W84 (hexane-1,6-bis[dimethyl-3'-phthalimidopropyl-ammonium bromide]) are prototype allosteric modulators of the G-protein coupled muscarinic acetylcholine receptor family, especially of the M2-subtype. In order to probe the specificity of muscarinic allosteric modulation, we checked whether these agents interact with histamine H1-receptors which have a high homology with muscarinic receptors. Binding experiments (38 mM Na2HPO4, 12 mM KH2PO4, pH 7.5) were performed with the H1-receptor antagonist [3H]mepyramine ([3H]MEP) in guinea pig cerebellar homogenates. For the sake of comparison, binding of [3H]N-methylscopolamine ([3H]NMS) at muscarinic M2-receptors was measured in porcine cardiac homogenates under identical conditions. The modulators retarded [3H]NMS dissociation (t1/2 control=1.3 min) concentration-dependently indicating their allosteric action with half-maximum effects for gallamine at EC50,discs=27 microM, for alcuronium at EC50,diss=53 nM, and for W84 at EC50,diss=170 nM. In contrast, [3H]MEP dissociation from H1-receptors (t1/2,control=2.6 min) remained unchanged up to concentrations of 1 mM of the modulators. Equilibrium binding of [3H]NMS (KD=0.46 nM, Bmax=98 fmol/mg protein) was inhibited by gallamine, elevated by alcuronium and left almost unchanged by W84, indicating negative, positive and nearly neutral cooperativity, respectively, with the radioligand. The ternary complex model of allosteric actions yielded the equilibrium dissociation constants K(A) for the binding of the allosteric modulators to free M2-receptors: K(A,gallamine)=100 nM, K(A,alcuronium)=450 nM, K(A,W84)=69 nM. In H1-receptors, more than 1,000-fold higher concentrations than in M2-receptors were required to elicit an effect on the binding of [3H]MEP (KD=1.2 nM, Bmax=205 fmol/mg protein). Half-maximal reduction was observed at 10 mM for gallamine, 1 mM for alcuronium and 92 microM for W84. In conclusion, the muscarinic modulators have little effect on the histamine H1-receptors.


Assuntos
Receptores Histamínicos H1/efeitos dos fármacos , Receptores Muscarínicos/efeitos dos fármacos , Alcurônio/farmacologia , Regulação Alostérica , Animais , Sítios de Ligação , Trietiodeto de Galamina/farmacologia , Cobaias , Isoindóis , N-Metilescopolamina/metabolismo , Ftalimidas/farmacologia , Pirilamina/metabolismo , Receptores Histamínicos H1/metabolismo , Receptores Muscarínicos/metabolismo
15.
Naunyn Schmiedebergs Arch Pharmacol ; 362(6): 512-9, 2000 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-11138843

RESUMO

Muscarinic M2 acetylcholine receptors contain an allosteric site that is probably located at the entrance of the ligand binding pocket above the orthosteric binding site. With the orthosteric area not occupied, allosteric agents might gain access to this site. The interaction of allosteric agents with orthoster-occupied receptors is known to depend on the buffer conditions in an alloster-specific fashion. Utilizing the buffer-dependent potency shift as an indicator, we aimed to find out for two rod-like shaped and flexible allosteric agents whether or not there is evidence for a switch in the site of attachment in free compared with [3H]N-methylscopolamine ([3H]NMS)-occupied porcine heart M2 receptors. These agents are the bispyridinium compounds WDuo3 (1,3-bis[4-(phthalimidomethoxyimino-methyl)-pyridinium-1-yl] propane dibromide) and Duo3 (4,4'-bis-[(2,6-dichloro-benzyloxy-imino)-methyl]-1,1'-propane-1,3-diyl-bis-pyridinium dibromide). The prototype allosteric agents gallamine and alcuronium were included. Inhibition of [3H]NMS association was taken to reflect alloster interaction with free receptors, inhibition of [3H]NMS dissociation indicated binding to [3H]NMS-occupied receptors. In Na,K,Pi buffer (4 mM Na2HPO4, 1 mM KH2PO4, pH 7.4 at 23 degrees C) compared with Mg,Tris,Cl,Pi buffer (45 mM Tris-HCl, 2.6 mM MgHPO4, pH 7.3 at 37 degrees C) WDuo3 underwent the same loss of potency for the interaction with either free or [3H]NMS-liganded receptors. The loss of potency was quantified by a potency ratio (PR), i.e. the ratio between the concentrations of the modulator leading to a half-maximal delay of [3H]NMS association or dissociation, respectively, in Mg,Tris,Cl,Pi compared with Na,K,Pi. For WDuo3 the ratios were PRass=27 and PRdiss=22, respectively. For Duo3, the interaction with free and [3H]NMS-occupied receptors only slightly depended on the composition of the incubation medium: PRass=1.3, PRdiss=2.8. In contrast to the other agents, the concentration-effect curves of which had slope factors nH not different from unity, the curves of Duo3 were steep (nH about -1.6). For alcuronium the shift factors amounted to PRass=29 and PRdiss=25, for gallamine to PRass=216 and PRdiss=159. In conclusion, there was a wide variation between the allosteric agents with regard to the respective buffer dependence of action. Yet, for a given allosteric agent, the interaction with either free or [3H]NMS-occupied receptors was always characterized by the same buffer-dependent shift. Thus, even the applied rod-shaped allosteric agents do not appear to switch to the orthosteric site in free compared with orthoster-occupied M2 receptors.


Assuntos
Colinérgicos/metabolismo , N-Metilescopolamina/metabolismo , Receptores Muscarínicos/metabolismo , Alcurônio/metabolismo , Alcurônio/farmacologia , Animais , Sítios de Ligação , Ligação Competitiva , Soluções Tampão , Colinérgicos/farmacologia , Relação Dose-Resposta a Droga , Trietiodeto de Galamina/metabolismo , Trietiodeto de Galamina/farmacologia , Cinética , Antagonistas Muscarínicos/metabolismo , Miocárdio/metabolismo , N-Metilescopolamina/farmacologia , Agonistas Nicotínicos/metabolismo , Agonistas Nicotínicos/farmacologia , Compostos de Piridínio/metabolismo , Compostos de Piridínio/farmacologia , Receptor Muscarínico M2 , Suínos , Trítio
16.
Cent Afr J Med ; 46(1): 9-13, 2000 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-14674200

RESUMO

OBJECTIVE: To investigate the effects of Heteromorpha trifoliata on rat uterine and skeletal muscle. DESIGN: Laboratory based study using experimental animals. Investigating the effects of the plant extract and agonists on isolated muscle preparations. SETTING: Department of Physiology, University of Zimbabwe. SUBJECTS: 28 Sprague-Dawley rats. MAIN OUTCOME MEASURES: Amplitude of contraction of uterine smooth muscle and skeletal muscle. RESULTS: Experiments were performed on the isolated rat uterus preparation in which strips of myometrium were placed in tissue baths filled with Kreb's solution. The aqueous extract of the root bark of Heteromorpha trifoliata ("dombwe") contracted the rat uterus. The contractions were not antagonised by atropine but were blocked by both cyproheptadine and verapamil. In addition, "dombwe" induced a contracture of the rat diaphragm muscle in the presence of alcuronium. CONCLUSIONS: The contractile effects on the uterus appear to involve stimulation of 5-HT2 receptors leading to an increase in calcium influx into the smooth muscle cell. Promotion of calcium influx could also explain the effects observed on the skeletal muscle preparation since the contracture induced by "dombwe" occurred in the presence of the nicotinic antagonist, alcuronium. In view of the effects of "dombwe" on other smooth muscle preparations (from previous work) it appears that the pharmacological profile of the crude aqueous extract of the root bark of Heteromorpha trifoliata is complex and suggestive of the presence of more than one active ingredient.


Assuntos
Apiaceae/fisiologia , Medicina Tradicional Africana , Contração Muscular/efeitos dos fármacos , Músculo Esquelético/efeitos dos fármacos , Miométrio/efeitos dos fármacos , Fitoterapia/métodos , Extratos Vegetais/farmacologia , Contração Uterina/efeitos dos fármacos , Alcurônio/farmacologia , Animais , Atropina/farmacologia , Bloqueadores dos Canais de Cálcio/farmacologia , Ciproeptadina/farmacologia , Avaliação Pré-Clínica de Medicamentos , Feminino , Técnicas In Vitro , Masculino , Antagonistas Muscarínicos/farmacologia , Antagonistas Nicotínicos/farmacologia , Casca de Planta/fisiologia , Extratos Vegetais/agonistas , Extratos Vegetais/antagonistas & inibidores , Ratos , Ratos Sprague-Dawley , Receptores 5-HT2 de Serotonina/efeitos dos fármacos , Antagonistas da Serotonina/farmacologia , Verapamil/farmacologia , Zimbábue
18.
Mol Pharmacol ; 54(5): 899-906, 1998 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-9804625

RESUMO

We investigated whether alcuronium, an allosteric modulator of muscarinic acetylcholine receptors, can induce receptor-mediated activation of Go proteins in liposomal membranes incorporating purified M2 receptors and Go proteins and whether its action is affected by the receptor/Go protein (R/Go) ratio. The binding of guanosine-gamma-[35S]thiotriphosphate ([35S]GTPgammaS) served as the indicator of G protein activation. It was stimulated by empty receptors at high receptor densities, and the dose-response curve was shifted to the left by the agonist carbachol and to the right by the antagonist atropine. At an R/Go ratio of 300:100, the rate of [35S]GTPgammaS binding was the same in the presence or absence of 0. 1 mM carbachol. Alcuronium increased the binding of [35S]GTPgammaS at R/Go ratios of <3:100 and diminished it at R/Go ratios of >10:100, similar to previous observations on intact cells expressing muscarinic receptors at different densities. The apparent biphasicity of alcuronium action indicates that the allosteric modulator has at least two effects on muscarinic receptor/G protein interaction but its mechanistic basis is unclear. The "active state" of muscarinic receptors induced by alcuronium probably is different from that induced by carbachol. Changes in the densities of receptors and Go proteins had little effect on the kinetics of [35S]GTPgammaS binding and on receptor affinity for carbachol, provided the R/Go ratio was kept constant. This suggests that the receptors and G proteins are located in microdomains in which their concentrations remain constant, despite variations in the amounts of lipidic membranes in the system.


Assuntos
Alcurônio/farmacologia , Atropina/farmacologia , Carbacol/farmacologia , Proteínas de Ligação ao GTP/metabolismo , Guanosina 5'-O-(3-Tiotrifosfato)/metabolismo , Agonistas Muscarínicos/farmacologia , Antagonistas Muscarínicos/farmacologia , Antagonistas Nicotínicos/farmacologia , Quinuclidinil Benzilato/farmacologia , Receptores Muscarínicos/metabolismo , Animais , Humanos , Cinética , Lipossomos , Radioisótopos de Enxofre , Suínos , Trítio
19.
Br J Pharmacol ; 124(6): 1213-8, 1998 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-9720793

RESUMO

1. Radioligand binding experiments indicate that the affinity of muscarinic receptors for their agonists may be enhanced by allosteric modulators. We have now investigated if brucine can enhance the inhibitory effects of muscarinic receptor agonists on the electrically evoked release of [3H]acetylcholine ([3H]ACh) from superfused slices of rat striatum. 2. The evoked release of [3H]ACh was inhibited by all agonists tested (i.e., furmethide, oxotremorine-M, bethanechol and oxotremorine). 3. Brucine enhanced the inhibitory effects of furmethide, oxotremorine-M and bethanechol on the evoked [3H]ACh release without altering the inhibitory effect of oxotremorine. 4. Alcuronium was applied for comparison and found to diminish the inhibitory effect of furmethide on the evoked [3H]ACh release. 5. The results demonstrate that it is possible both to enhance and diminish the functional effects of muscarinic receptor agonists by allosteric modulators. 6. The direction of the observed effects of brucine and alcuronium on [3H]ACh release fully agrees with the effects of these modulators on the affinities of human M4 receptors for furmethide, oxotremorine-M, bethanechol and oxotremorine, as described by Jakubik et al. (1997). This supports the view that the presynaptic muscarinic receptors responsible for the autoinhibition of ACh release in rat striatum belong to the M4 muscarinic receptor subtype.


Assuntos
Acetilcolina/antagonistas & inibidores , Alcurônio/farmacologia , Corpo Estriado/efeitos dos fármacos , Agonistas Muscarínicos/farmacologia , Estricnina/análogos & derivados , Acetilcolina/metabolismo , Animais , Betanecol/farmacologia , Corpo Estriado/metabolismo , Humanos , Técnicas In Vitro , Masculino , Oxotremorina/farmacologia , Compostos de Amônio Quaternário/farmacologia , Ratos , Ratos Wistar , Estricnina/farmacologia , Trítio
20.
Clin Exp Pharmacol Physiol ; 25(3-4): 185-94, 1998.
Artigo em Inglês | MEDLINE | ID: mdl-9590567

RESUMO

1. An allosteric interaction occurs when the binding of a ligand to its site on a receptor is able to modify the binding of another ligand to a topographically distinct site on the same receptor and vice versa. The muscarinic cholinoceptors represent the best-studied examples of allosteric phenomena among the G-protein-coupled receptor superfamily. 2. The simplest model describing allosteric interactions at muscarinic cholinoceptors is the ternary complex model, which allows for a three-way interaction between the receptor, a classical (orthosteric) ligand and an allosteric modulator. The interaction may be quantified using the dissociation constant of each ligand for its respective binding site on the free receptor and the 'co-operativity factor' alpha. This latter term is the ratio of affinities of a ligand for the occupied versus the unoccupied receptor and is a measure of the magnitude of the cooperativity between two concomitantly bound ligands. 3. Identification of allosteric phenomena requires the utilization of both radioligand binding and functional approaches. Manifestations of allosterism include: (i) a limited ability to influence radioligand binding as the concentration of the latter is increased; (ii) alterations in the dissociation rate of orthosteric ligands; (iii) curvilinear Schild regressions; and (iv) nonadditivity of agonist/orthosteric antagonist/allosteric modulator combination concentration ratios. 4. Allosteric modulators of muscarinic cholinoceptors represent a diverse range of compounds. Some of the most studied agents include gallamine, alcuronium and the bis-ammonium compounds, C7/3'-phth and W84. Alcuronium has proven a most useful pharmacological tool, as it has been shown to display both positive and negative co-operativity, depending on the receptor subtype and orthosteric ligand involved in the interaction. 5. Evidence has accumulated pointing to the existence of a common allosteric binding site on the muscarinic cholinoceptors, located close to the orthosteric site, but at a more extracellular level. However, the possibility of more than one accessory binding site on various receptor subtypes cannot be excluded. 6. Allosteric modulators offer a number of potential therapeutic advantages, including a ceiling level to their effects and the possibility of 'absolute selectivity' of action, based on the degree of co-operativity rather than the affinity of the modulator for any one receptor subtype.


Assuntos
Receptores Muscarínicos/metabolismo , Alcurônio/química , Regulação Alostérica , Compostos de Bis-Trimetilamônio/química , Previsões , Trietiodeto de Galamina/química , Humanos , Isoindóis , Modelos Químicos , Antagonistas Muscarínicos/química , Fármacos Neuromusculares não Despolarizantes/química , Receptores Muscarínicos/química
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