Contractile effect of Sclerocarya birrea (A Rich) Hochst (Anacardiaceae) (Marula) leaf aqueous extract on rat and rabbit isolated vascular smooth muscles

BACKGROUND Sclerocarya birrea (Anacardiaceae) is traditionally used for treating hypertension. The pharmacological effects of S birrea leaf aqueous extract (SBE) on rabbit and rat vascular smooth muscles were investigated in this study. METHODS Fresh S birrea leaves (1 kg) were air dried at 26 ± 1°C, milled, macerated in 2.5 l of distilled water for 48 hours, filtered, and the filtrate was concentrated in a rotary evaporator. Rat isolated portal vein preparations, as well as rabbit isolated endothelium-denuded and endothelium-intact descending thoracic aortic ring preparations were mounted in 30-ml Ugo Basile organ baths under physiological conditions, and challenged with SBE (50-400 mg/ml). The contractile effects of SBE and/or other reference drugs on the isolated vascular smooth muscle preparations were recorded by means of Ugo Basile's force-displacement transducers and Gemini recorders. RESULTS SBE (50-400 mg/ml) caused a significant, concentration-dependent upward shift in baseline tone in the aortic ring preparations (p < 0.01-0.001). Indomethacin (20 µM) markedly attenuated the contractile effects of SBE in both the endothelium-intact and -denuded aortic rings, while N(G)-nitro-(L)-arginine methyl ester (L-NAME, 100 µM) significantly (p < 0.05) increased the contractile tension of the endothelium-intact aortic rings. Verapamil (1-3 µg/ml) partially inhibited the contractile effects of SBE. SBE also elicited significant (p < 0.05-0.01) increases in the amplitude of the myogenic contractions of the portal veins. These contractions were abolished by verapamil (1-3 µg/ml) in a concentration-dependent manner, while prazosin (1-3 µg/ml) did not affect the SBE-induced contractions. CONCLUSION SBE possessed spasmogenic effects on vascular smooth muscle preparations in vitro. It may induce and/or exacerbate hypertension, contrary to the folkloric, ethnomedical use of S birrea.