Binding of dexetimide and levetimide to [3H](+)pentazocine- and [3H]1,3-di(2-tolyl)guanidine-defined sigma recognition sites

σ2-Receptor Ligand-Mediated Inhibition of Inwardly Rectifying K+ Channels in the Heart

The sigma(2)-receptor agonist, ifenprodil, was suggested as an inhibitor of G protein-coupled inwardly rectifying potassium channels. Nevertheless, an analysis of the role of sigma(2) receptors in cardiac electrophysiology has never been done. This work aims i) to identify the roles of cardiac sigma(2) receptors in the regulation of cardiac K(+) channel conductances and ii) to check whether sigma(2)-receptor agonists exhibit class III antiarrhythmic properties. The sigma(2)-receptor agonists ifenprodil, threo-ifenprodil, LNP250A [threo-8-[1-(4-hydroxyphenyl)-1-hydroxy-propan-2-yl]-1-phenyl-1,3,8-triazaspiro[4,5]decane-4-one] (a derivative of ifenprodil devoid of alpha(1)-adrenergic and N-methyl-d-aspartate glutamate receptor-blocking properties), and 1,3-di(2-tolyl)guanidine were used to discriminate the effects linked to sigma(2) receptors from those of the sigma(1) subtype, induced by (+/-)-N-allylnormetazocine (SKF-10,047). The sigma(2)-receptor antagonist 3-alpha-tropanyl-2(pCl-phenoxy)butyrate (SM-21) was employed to characterize sigma(2)-mediated effects in patch-clamp experiments. In rabbits, all sigma(2)-receptor agonists reduced phenylephrine-induced cardiac arrhythmias. They prolonged action potential duration in rabbit Purkinje fibers and reduced human ether-a-go-go-related gene (HERG) K(+) currents. (+)-SKF-10,047 was completely inactive in the last two tests. The effects of threo-ifenprodil were not antagonized by SM-21. In HERG-transfected COS-7 cells, SM-21 potentiated the ifenprodil-induced blockade of the HERG current. These data suggest that sigma(2)-receptor ligands block I(Kr) and that this effect could explain part of the antiarrhythmic properties of this ligands family. Nevertheless, an interaction with HERG channels not involving sigma(2) receptors seems to share this pharmacological property. This work shows for the first time that particular caution has to be taken toward ligands with affinity for sigma(2) receptors. The repolarization prolongation and the early-afterdepolarization can be responsible for "torsades de pointe" and sudden cardiac death.

Allosteric modulation of ligand binding to [3H](+)pentazocine-defined sigma recognition sites by phenytoin

The allosteric modulation of sigma recognition sites by phenytoin (diphenylhydantoin) has been demonstrated by the ability of phenytoin to stimulate binding of various [3H] sigma ligands, as well as to slow dissociation from sigma sites and to shift sigma sites from a low- to a high-affinity state. Phenytoin stimulated the binding of the sigma 1- selective ligand [3H](+)pentazocine in a dose-dependent manner. Stimulation of binding at a final concentration of 250 microM phenytoin was associated with a decrease in the KD. The affinities of the sigma reference compounds caramiphen, dextromethorphan, dextrophan, (+)3-PPP and (+)SKF-10,047 were three- to eight-fold higher, while the affinities of benzetimide, BMY-14802, carbetapentane, DTG and haloperidol were unchanged in the presence of 250 microM phenytoin. The relative sensitivity of sigma compounds to allosteric modulation by phenytoin is not a property of all sigma ligands, and may provide an in vitro basis for distinguishing actions of sigma compounds and predicting sigma effects in vivo.