Interaction of ebeinone, an alkaloid from Fritillaria imperialis, at two muscarinic acetylcholine receptor subtypes

The ability of the alkaloid, ebeinone, isolated from Fritillaria imperialis, to act at muscarinic M(2) and M(3) acetylcholine receptors was investigated. In functional studies with guinea-pig left atrium, ebeinone was found to be ca. 10-fold more active as an antagonist of responses to carbachol (CCh) than in either guinea-pig ileum or trachea. Estimates of dissociation constants (K-B values) in the three tissues were 77.3, 931.1 and 547.0 nM, respectively. Inhibition binding studies in rat atria with the non-selective antagonist [H-3]N-methylscopolamine ([H-3]NMS) showed ebeinone to have a K-I value of 80.9 nM. Comparison of ebeinone with pancuronium, another steroid-like compound with a similar K-B value at the muscarinic M(2) receptor, found both compounds able to retard the dissociation rate of [H-3]NMS in atria, indicating an allosteric mode of interaction at the M(2) receptor. It is concluded that ebeinone exhibited a higher affinity for muscarinic M(2) receptors than for M(3) receptors in the guinea-pig and that it interacted allosterically at rat atrial M(2) receptors.