Biol. Pharm. Bull. 30(1) 96—104 (2007)

subfamily Gardennieae, family Rubiaceae. It is widely distributed in tropical and sub-tropical regions, especially in Asia and Africa. Craib reported that R. siamensis CRAIB is synonymous with R. longiflora HOOK f., R. uncata RIDL., Griffithia siamensis MIQ. and Webera siamensis KURZ. It can be found in all parts of Thailand and Burma, especially in humid regions. This plant is known by various local names in Thailand: Khat khao, Khet khao, Khat khao naam and Khat khao thuea. All parts of R. siamensis have been used in Thai folkloric medicine. The fruits have been used for inducing abortion and as an emmenagogue or hematinic, the leaves are claimed to control blood pressure, the root is used for antipyritic and antiscurvy activities, the flowers have been used to stop nosebleeds, and the stem part is used as a hematinic. However, scientific investigations to test these therapeutic claims are very scarce. Aukkanibutra identified the chemical constituents of fruits of the plant. She found that dried fruit of R. siamensis contains: ursolic acid, pseudoginsenoside-RT1, pseudoginsenoside-RP1 and an unidentified saponin. Later, Reanmongkol et al. reported that pseudoginsenoside-RP1, the minor component of the plant fruit, has an antinociceptive activity similar to that of aspirin, a peripherally acting analgesic. However, the activity of the purified compound was much less potent than that of the crude extract. Jansakul et al. tested the activities of the pseudoginsenoside-RT1, isolated by Aukkanibutra, 4) on the cardiovascular system and found that the pseudogensenoside-RT1 (4— 32 mg/kg) caused a decrease in mean arterial blood pressure, with an increased heart rate in a dose-dependent manner in anesthetized rats. However, the mechanisms for these effects have not yet been elucidated due to the limited availability of the substance. This led us to return to study the cardiovascular effects of the crude extract from fresh fruits of R. siamensis (R. siamensis extract). We found that the crude extract (at 4—12 mg/kg) caused an increase in both the mean arterial blood pressure and heart rate in a dose-dependent manner. The effect was more potent than that of the saponin pseudoginsenoside-RT1. It is possible that the fresh fruit of R. siamensis contains some other substances, besides pseudoginsenoside-RT1, that have effects on the cardiovascular system. In the present study, we aimed to investigate the cardiovascular profile of the R. siamensis extract in the rat, as well as to assess the mechanisms involved, and compare the results with those produced by norepinephrine, isoproterenol and epinephrine.