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

BACKROUND: 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.