Smooth muscle relaxant activities of compounds isolated from malaysian medicinal plants on rat aorta and guinea-pig ileum

Muscle relaxant activities of six compounds isolated from Malaysian medicinal plants were investigated on the smooth muscles of the rat aorta and longitudinal muscle of the guinea-pig ileum. Except for goniothalamin, the other five compounds exhibited varying degrees of muscle relaxant activities on the two smooth muscles. Dicentrine, isocorydine, altholactone and usnic acid reduced the contractions to KCI and phenylephrine in the rat aorta, whilst atranorin slightly reduced the contractions to phenylephrine but not KCI. In addition, dicentrine and isocorydine markedly reduced the contractile response to phenylephrine in Ca²⁺-free Krebs solution. In the longitudinal muscle of the guinea-pig ileum, altholactone, atranorin, dicentrine and isocorydine inhibited to a greater extent the phasic than the tonic responses to KCI. All four compounds similarly inhibited the contractions induced by ACh. The results suggest that the relaxant activities of the compounds are attributed mainly to inhibition of Ca²⁺ influx via the calcium channels in the membrane of the smooth muscles. Furthermore, the greater sensitivity of dicentrine, isocorydine and altholactone against phenylephrine-induced contractions or KCI-induced phasic contractions are due to their abilities to inhibit intracellular Ca²⁺ release in these muscles.