Abstract:
Renin angiotensin system plays a major role in blood pressure regulation. The inhibition of angiotensin converting enzyme is a major action of the drug used for the treatment of hypertension. Presently, there have been substantial reports from various countries demonstrating that several medicinal plants had angiotensin converting enzyme inhibition (ACEI) activity. The present study aimed to evaluate the in vitro ACEI activity of Thai medicinal plants that had pharmacological activity and/or claimed in traditional medicine to lower blood pressure, by using fluorometric assay. In addition, the mangrove plants with antioxidant activity were also investigated for this activity. From 49 specimens of 30 plant species, the ACEI activity of greater than 50 % were determined in 9 specimens of 6 species of ethyl acetate extract of mangrove plants and 4 specimens from 3 species of water extract of medicinal plants. Among the water extract of medicinal plants, Alstonia scholaris (L.) R.Br. or Sattabans bark, which had the highest ACEI activity (52.89 %) was used to examine the antihypertensive and vasodilating actions. The antihypertensive action was examined in two-kidney, one-clip hypertensive rats. The blood pressure was measured by tail cuff method every 15 minutes for 90 minutes after single oral feeding with A. scholaris bark extract at the doses of 0.75, 1.5, 3 and 6 g of dried bark/kg compared to captopril 25 mg/kg. The isolated aortic rings of these hypertensive rats were used to examine the effect of the extract at the doses of 12.7, 25, 51, 102 mg of dried bark/ml on contractile response to norepinephrine (NE) compared to captopril 0.42 mg/ml. This was also examined in the isolated aortic ring from normal rats. The A. scholariss bark extract had significant antihypertensive effects in a
dose-independent manner with lesser effect compared to captopril. The extract slightly decreased heart rate, while captopril had no effect. In addition, the extract also decreased contractile response of isolated aortic ring to NE with the greater effect in those from normal than hypertensive rats. The results from this study indicated that some Thai medicinal plants and mangrove plants had ACEI activity, and A. scholariss bark water extract with ACEI activity can decrease blood pressure and caused vasodilatation in hypertensive rats.