Selective antagonism activity of alkaloids from bulbs Fritillariae at muscarinic receptors: functional studies.

15 alkaloids were isolated from five Fritillariae species and 6 derivatives were synthesized. Alkaloids having anticholinergic effect on guinea-pig tracheal smooth muscle were screened out and their mechanism was further studied on the cAMP formation in Chinese hamster ovary cells stably expressing human muscarinic M2 receptor (CHO-hM2 cells) and intracellular calcium ([Ca(2+)](i)) transient in Chinese hamster ovary cells stably expressing human muscarinic M3 receptor (CHO-hM3 cells). In normal Krebs-Henseleit (KH) solution, imperialine (15), 3beta-acetylimperialine (16) and sinpeinine A (17) concentration-dependently relaxed 1 microM carbachol-induced contraction of guinea-pig tracheal rings with EC(50) of 4.19, 1.71 and 4.67 microM, respectively. In Ca(2+)-free KH solution, 10 microM 3beta-acetylimperialine (16), imperialine (15) and sinpeinnine A (17) caused 97.42%, 5.45% and 6.55% inhibition, respectively, which indicated that the three components might inhibit muscarinic receptor in different mechanism. Results of muscarinic M2 receptor-inhibited cAMP formation in CHO-hM2 cells showed that imperialine (15) and sinpeinine A (17) could potently elevate the cAMP formation whereas 3beta-acetylimperialine (16) only had weak effect on antagonism of cAMP inhibition. Furthermore, the investigations of muscarinic M3 receptor-stimulated [Ca(2+)](i) transient in CHO-hM3 cells revealed that imperialine (15) and sinpeinine A (17) could not antagonize [Ca(2+)](i) transient, but 3beta-acetylimperialine (16) significantly inhibited [Ca(2+)](i) peak elevation with an IC(50) of 5.26 microM. The functional studies suggest that the mechanism of relaxant action of imperialine (15) and sinpeinine A (17) is due to their selective inhibitory effects on muscarinic M2 receptors and the mechanism of 3beta-acetylimperialine (16) originates from its selective muscarinic M3 receptors antagonism.