Polyalthic Acid Isolated from Croton reflexifolius has Relaxing Effect in Guinea Pig Tracheal Smooth Muscle

The relaxing activity of Croton reflexifolius H. B. K(Euphorbiaceae) leaves was assessed in isolated guinea pig tracheal rings. The dichloromethane extract of C. reflexifolius was the most active relaxant (EC50 = 118.98 +/- 5.927 mu g/mL), and within this extract polyalthic acid was identified as the main active relaxing agent. Polyalthic acid showed a relaxing effect on tracheal rings precontracted with carbachol (EC50 = 183.71 +/- 3.28 mu M), histamine (6.24 +/- 0.28 mu M), and KCl (195.79 +/- 10.36 mu M). The pretreatment with polyalthic acid did not affect the concentration response curve to histamine, and it reduced the Emax of carbachol without affecting its EC50, thus suggesting that polyalthic acid produces a mild antimuscarinic activity. In addition, neither glibenclamide, L-NAME nor propranolol modified the effect of polyalthic acid, although the latter enabled isoproterenol activity. In conclusion, this study represents the first in which the relaxing effect of C. reflexifolius on tracheal rings of guinea pig was clearly demonstrated. Polyalthic acid, which was the main active agent in this relaxing action, produced a mild antimuscarinic activity in a noncompetitive manner. Its relaxing effect was independent of the contractile agent employed and was not related with beta-adrenergic receptors, K channels, or nitric oxide. Further experiments are needed to clarify the mechanism of action of polyalthic acid.