Effect of Crude Extracts from Leonotis nepetaefolia (Labiatae) on Rat and Guinea-pig Smooth Muscle and Rat Cardiac Muscle

Abstract

Abstract The actions of hydroalcoholic and tea extracts of stems of Leonotis nepetaefolia on agonist-induced and electrically-evoked contractions have been analysed in-vitro in rat uterus and left atrium and in guinea-pig ileum and trachea. The tea extract (500–2000 μg mL−1) caused parallel and graded rightward shifts of concentration-response curves to bradykinin and BaCl2 in the rat isolated uterus, but antagonized responses to prostaglandin F2α in a typically non-competitive manner. The hydroalcoholic extract also caused rightward displacements of the curves to bradykinin, acetylcholine (ACh), angiotensin II, oxytocin and BaCl2 and reduced their maximal contractile effects. Both extracts (30–3000 μg mL−1) relaxed uterine preparations precontracted with KCl (80 Mm), the hydroalcoholic extract being about 2-fold more potent than the tea extract. The relaxant response to the former was unaffected by propranolol (1 μm) or forskolin (10 Nm), but was potentiated 2-fold by 3-isobutyl-1-methylxanthine (10 μm). In the guinea-pig ileum the hydroalcohplic extract shifted the ACh- and bradykinin-induced contractile curves to the right and markedly inhibited their maximal effects, whereas the tea extract caused a typical non-competitive antagonism of ACh-induced contractile responses. In field-stimulated ileal strips, both extracts (3–3000 μg mL−1) caused contractions and inhibited twitch responses. Guinea-pig tracheal rings precontracted with carbachol (0·3 μm) were relaxed only by concentrations of either extract in excess of 1000 μg mL−1, an action that was unaffected by propranolol (0·1 μm) or by indomethacin (1 μm). Prior incubation of tracheal rings with the hydroalcoholic extract (1000 μg mL−1), which failed to affect responses to ACh, significantly potentiated the relaxations induced by isoprenaline or theophylline (3-fold) of preparations contracted with carbachol. Finally, the hydroalcoholic extract (0·1–1000 μg mL−1) caused a graded long-lasting inotropic effect in the rat electrically-stimulated left atrium. Altogether, these findings provide some experimental support for the reputed folk medicinal anti-asthmatic and anti-diarrhoeal properties of this plant. It is suggested that the pharmacological effects of the extracts of L. nepetaefolia may result from potentiation of the cAMP system.