Antimuscarinic action of liriodenine, isolated from Fissistigma glaucescens, in canine tracheal smooth muscle

The antimuscarinic properties of liriodenine, isolated from Fissistigma glaucescens, were compared with methoctramine (cardioselective M₂ antagonist) and 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, smooth muscle selective M₃ antagonist) by radioligand binding tests, functional tests and measurements of second messenger generation in canine cultured tracheal smooth muscle cells. Liriodenine, pirenzepine, methoctramine and 4-DAMP displaced [³H]-N-methyl scopolamine ([³H]-NMS) binding in a concentration-dependent manner with K values of 2.2 ± 0.4 x 10^-6, 3.3 ± 0.7 x 10^-7, 8.9 ± 2.3 x 10^-8 and 2.3 ± 0.6 x 10^-9 M, respectively. The curves for competitive inhibition of [³H]-NMS with liriodenine, methoctramine and 4-DAMP were best fitted according to a two site model of binding, but pirenzepine was best fitted according to a model with one site. Liriodenine and 4-DAMP displayed a high affinity for blocking tracheal contraction (pK(B) = 5.9 and 9.1, respectively) and inositol phosphate formation (pK(B) = 6.0 and 8.9, respectively), but a low affinity for antagonism of cyclic AMP inhibition (pK(B) = 4.7 and 7.8, respectively). Methoctramine blocked cyclic AMP inhibition with a high affinity (pK(B) = 7.4), but it antagonized tracheal contraction and inositol phosphate formation with a low affinity (pK(B) = 6.1 and 6.0, respectively). In conclusion, both M₂ and M₃ muscarinic receptor subtypes coexist in canine tracheal smooth muscle and are coupled to the inhibition of cyclic AMP formation and phosphoinositide breakdown, respectively. The antimuscarinic characteristics of liriodenine are similar to those of 4-DAMP. It may act as a selective M₃ receptor antagonist in canine tracheal smooth muscle.