Dronedarone Modulates M1 and M3 Muscarinic Receptors with Subtype Selectivity, Functional Selectivity, and Probe Dependence.

We have previously reported that amiodarone interacts with a novel allosteric site on muscarinic receptors. Amiodarone's most striking effect is to enhance the maximal response elicited by muscarinic agonists at the M-1, M-3, and M-5 receptors. Furthermore, the quaternary analog N-ethylamiodarone (NEA) is inhibitory at these receptors and appears to compete with amiodarone at that allosteric site. In the present studies, we show that dronedarone also modulates Gq-mediated responses at M-1 and M-3, although in a more discriminating manner. For example, dronedarone markedly enhances pilocarpine-stimulated release of arachidonic acid from CHO cells, via the M-3 receptor subtype, but does not affect the acetylcholine-stimulated response. Such probe-dependent effects are diagnostic of an allosteric interaction. In comparison to these effects at M-3, dronedarone is strongly inhibitory toward both pilocarpine and acetylcholine at the M-1 subtype. The effects of dronedarone are consistent with an interaction at the amiodarone site: dronedarone inhibits the enhancement of acetylcholine's response produced by amiodarone at the M-3 subtype; also, NEA reverses the enhancement of pilocarpine's response at M-3 produced by either dronedarone or amiodarone. In studies with the M-1-selective allosteric agonist 4-[3-(4-butylpiperidin-1-yl)propyl]-7-fluoro-4H-benzo[1,4] oxazin-3-one (AC-260584), amiodarone enhanced the maximal response observed, whereas dronedarone was inhibitory. On the other hand, benzyl quinolone carboxylic acid, the well-known allosteric ligand that dramatically enhances the potency of acetylcholine at the M-1 subtype, had no effect on the response profile of AC-260584. In summary, dronedarone acts at M-1 and M-3 muscarinic receptors in a manner that complements amiodarone and provides an additional tool with which to investigate this novel allosteric site. (C) 2016 S. Karger AG, Basel.