RESUMEN
The uterine stimulatory effect of the ethanol leaf extract of Newbouldia laevis (Beauv.) Seemann ex Bureau (Bignoniaceae) was evaluated in the presence of some antagonists in vitro in an attempt to elucidate the mechanism of action of the extract. The extract was tested in the presence and absence of phentolamine (4.09 and 40.91 nM), diphenhydramine (4.45 and 44.47 nM), atropine (1.18 and 11.91 nM), and verapamil (2.03 and 20.35 nM). The effect of the antagonists on the extract and on oxytocin used as a reference drug in this study was evaluated. The EC(50) and E(max) were determined and statistically analyzed using one way ANOVA and Dunnett's post hoc test. There was no significant difference in the EC(50) and E(max) of the extract and oxytocin in the presence of phentolamine. Diphenhydramine and atropine significantly inhibited (p <0.01) the extract but both drugs had no effect on oxytocin. However, significant differences (p <0.01) were observed in the EC(50) and E(max) of the extract and oxytocin in the presence of verapamil. These results suggest that the leaf extract of N. laevis contracts the uterus by opening voltage-operated calcium channels and/or by activation of muscarinic receptors.