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U Burgmer

Publications and source records attributed to U Burgmer.

3 recordsLinked to original sources
PubMed1998

Interaction of Mg2+ with the allosteric site of muscarinic M2 receptors.

Mg2+-ions have been suspected to attenuate the inhibitory effect of allosteric modulators on the dissociation of orthosteric ligands from muscarinic M2 receptors. It was aimed to gain more insight into the molecular events underlying the effect of Mg2+. The interaction of Mg2+ with the allosteric model compounds W84 (hexane-1,6-bis [dimethyl-3'-phthalimidopropylammonium bromide]) and Chin3/6 (hexane-1 ,6-bis[dimethyl-3'-¿4-oxo-2-phenyl-3,4-dihydro-2H-quinazolin-1-yl propylammonium bromide]) was studied in porcine heart muscarinic receptors, the primary binding site of which was occupied by the ligand [3H]N-methylscopolamine ([3H]NMS). The incubation buffer was composed of 4 mM Na2HPO4 and 1 mM KH2PO4 (pH 7.4, 23 degrees C). The retardation of [3H]NMS dissociation (control t1/2=5.6 min) induced by the allosteric test compounds was diminished by 3 mM Mg2+ to a greater extent than to be expected with regard to its contribution to the ionic strength of the buffer solution. Concentration-effect curves for the allosteric retardation of [3H]NMS dissociation by W84 (half maximal effective concentration EC0.5=24 nM in the absence of Mg2+) and by Chin3/6 (EC0.5=28 nM) were shifted by Mg2+ to the right in a parallel fashion. The curve-shift was compatible with a competitive interplay between Mg2+ and the modulators. The pKb-values as a measure of the antagonistic potency of Mg2+, however, differed depending on the modulator, i.e. pKb=3.4 with W84 and pKb=2.8 with Chin3/6. Mg2+ itself was capable of slowing the dissociation of [3H]NMS; the maximal retardation of [3H]NMS dissociation was about 3 fold, the concentration-effect relationship was compatible with a two-site model using the above-mentioned pKb-values as affinity constants. Since the equilibrium-binding of [3H]NMS remained unchanged up to a Mg2+-concentration of 3 mM, the cation appears to inhibit the association and dissociation of [3H]NMS to the same extent in this concentration range. Taken together, the findings indicate that Mg2+ may bind to the allosteric region of muscarinic M2 receptors and that more than one site is involved in this interaction. The sites of action may represent divalent cation binding sites.

Allosteric Site↗
PubMed1996

[Simvastatin].

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Anticholesteremic Agents↗
PubMed1996

Search for lead structures to develop new allosteric modulators of muscarinic receptors.

Various compounds are known to allosterically modulate the binding of ligands to muscarinic receptors. Most of these compounds have another predominant pharmacological action. Identification of the potent representatives should be useful for the development of allosteric modulators that are specific and highly active. For various reasons, a direct comparison of allosteric potencies on the basis of literature data is difficult. Therefore, a series of compounds was compared with regard to the allosteric delay of the dissociation of N-[3H]methylscopolamine from porcine heart M2 receptors under the following assay conditions: "Na,K,Pi buffer", 4 mM Na2HPO4, 1 mM KH2PO4, pH 7.4, 23 degrees C; "Mg,Tris,Cl,Pi buffer', 50 mM Tris-HCl, 3 mM MgHPO4,pH 7.3, 37 degrees C. Generally, the allosteric potency of the compounds was higher in the Na,K,Pi buffer, compared with the Mg,Tris,Cl,Pi buffer. However, the extent of the potency shift differed, ranging from approximately 2-fold for tacrine to approximately 100-fold for gallamine. The concentration retarding radioligand dissociation to half of the control rate (EC50) served as a measure of allosteric potency. Under both assay conditions, alcuronium was the most potent compound (EC50,Na,K,Pi = 4 nM and EC50,Mg,Tris,Cl,Pi = 55 nM), followed by alkane-bisammonium and bispyridinium compounds containing phthalimido moieties. Gallamine showed intermediate potency (EC50 values of 180 nM and 16,000 nM in Na,K,Pi buffer and Mg,Tris,Cl,Pi buffer, respectively). Obidoxime and hexamethonium, both known to antagonize allosteric actions, revealed submaximal efficacy and low potency (EC50,Na,K,Pi of approximately 100,000 nM). The relevance of these results, regarding the identification of lead structures for the development of new allosteric modulators, is discussed.

Alcuronium↗