Further characterization of the allosteric site of action of amiodarone at Gq‐linked muscarinic receptors (LB616)

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 M1, M3, and M5 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 (DRON) also modulates Gq‐mediated responses, although in a more discriminating manner. For example, DRON markedly enhances pilocarpine‐stimulated release of arachidonic acid from CHO cells, via the M3 receptor subtype, but does not affect the acetylcholine‐stimulated response. In comparison to these effects at M3, DRON is strongly inhibitory toward both pilocarpine and acetylcholine at the M1 subtype. The effects of DRON are consistent with an interaction at the amiodarone site: DRON inhibits the enhancement of acetylcholine’s response produced by amiodarone at the M3 subtype; and, NEA reverses the enhancement of pilocarpine’s response at M3 produced by either DRON or amiodarone. In studies with the M1‐selective allosteric agonist AC260584, amiodarone enhanced the maximal response observed, whereas DRON was inhibitory. On the other hand, BQCA, the well‐known PAM that dramatically enhances the potency of acetylcholine at the M1 subtype, had no effect on the response profile of AC260584. In summary, DRON acts at Gq‐linked muscarinic receptors in a manner that complements amiodarone and provides an additional tool with which to investigate this novel allosteric site. Grant Funding Source : Supported by PHS R01 005214 to JE