SOME PHARMACOLOGICAL STUDIES OF A HYPOTENSIVE FRACTION FROM DERRIS SCANDENS

C. JANSAKUL, A. SRICHANBARN AND A. SAELEE

ABSTRACT: Bioassay directed fractionation of water extract from dried stems of Derris scandens Benth. resulted in the isolation of a hypotensive fraction. In the in vivo preparation, intravenous injection of the hypotensive fraction (0.04-1.6 mg/kg) caused a decrease in both mean arterial blood pressure and heart rate of anesthetized rats in a dose dependent manner. Pre-treatment of the animals with atropine (1.5 mg/kg, i. v.) did not alter the hypotensive and the negative chronotropic activities of the hypotensive fraction. Pre-treatment of the animal with -adrenergic receptor antagonist, propranolol (0.6 mg/kg), caused significantly reduced negative chronotropic activities with no change in hypotensive activities of the hypotensive fraction. In the in vitro preparation, the hypotensive fraction caused vasodilatation of endothelium-intact thoracic aortic rings (preconstricted with phenylephrine). This effect was abolished by pre-incubation of blood vessels with NG-nitro-L-arginine (300 mM), a specific nitric oxide synthase inhibitor, or removing of the vascular endothelium. For the isolated atria, the hypotensive fraction caused a decrease in atrial contraction rate in a dose-dependent manner. In addition, pre-incubation of the atria with the hypotensive fraction (0.1 mg/ml) for 10 min caused lowering in the positive chronotropic effect of the isoproterenol. These results suggest that the mechanisms involved for the hypotensive activity of the hypotensive fraction may operate in both ways, a direct effect by inhibiting the -adrenergic receptor at the atria, and an indirect effect causing the blood vessel to dilate by stimulated release of nitric oxide from the vascular endothelial cells.

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Department of Physiology, Faculty of Science, Prince of Songkla University, Songkla 90112, Thailand.

Received 7 August 1997