Muscarinic agonist properties of the hydrobutanol extract from aerial parts of Waltheria viscosissima St. Hil. (Sterculiaceae) in rats.

The cardiovascular effects of the hydrobutanol phase of the ethanolíc extract from the aerial parts (HBWV) of Waltheria viscosissima A. St. Hil. (Sterculiaceae) were tested in rats by using a combined (in vivo and in vitro) approach. HBWV (5, 7.5, 10, 15 and 20 mg/kg, randomly) induced a significant and dose-dependent hypotension and bradycardia in conscious freely moving normotensive rats. Both hypotensive and bradycardic effects evoked by a submaximal dose of HBWV (10 mg/kg) were inhibited by pre-treatment of the animals with atropine (2 mg/kg, i.v.). In anaesthetized animals, electrocardiogram recordings revealed second and third degree sinoatrial and atrioventricular blockade induced by the extract (10 mg/kg, i.v.), which were inhibited by cardiac muscarinic blockade (atropine, 2 mg/kg, i.v.). In isolated rat aortic rings, increasing concentrations of HBWV (50, 100, 200 and 400 micrograms/mL) were able to antagonize the contractile effects of noradrenaline (1 microM). This effect was inhibited by pre-incubation of the aortic rings with atropine (1 microM), by removal of the vascular endothelial tissue or by nitric oxide synthase blockade. These results suggest that both cardiac and peripheral actions induced by HBWV are probably mediated by stimulation of cardiac and endothelial muscarinic receptors, respectively.