Introduction Aspalathus linearis (Fabaceae) commonly known as Rooibos, is an indigenous South African plant. Local people of the Clanwilliam region, in north of Cape Town process its leaves to prepare herbal tea [1]. Rooibos tea is gaining popularity due to many health properties as evident by a rapidly growing number of its drinkers throughout the world, including China, Japan, Germany, England, Poland, Malaysia, South Korea, USA and of course South Africa. The total domestic and international sales of Rooibos in 1999 amounted to 6150 tons, of which 1800 tons (29%) were exported to 31 countries. In next year, the Arif-ullah Khan Anwarul Hassan Gilani Selective bronchodilatory effect of Rooibos tea (Aspalathus linearis) and its flavonoid, chrysoeriol j Summary Background Rooi- bos tea (Aspalathus linearis) is commonly used for hyperactive gastrointestinal, respiratory and cardiovascular disorders. Aim of study The aqueous extract of Rooibos tea (RT) was studied for the possible bronchodilator, anti- spasmodic and blood pressure lowering activities in an attempt to rationalize some of its medicinal uses. Methods Isolated tissue preparations, such as rabbit jeju- num, aorta and guinea-pig trachea and atria were set up in appro- priate physiological salt solutions and aerated with carbogen. For in vivo studies rats were anesthe- tized with pentothal sodium and blood pressure was measured through carotid artery cannula- tion. Results In jejunum, RT caused a concentration-dependent relaxation of low K + (25 mM)- induced contractions, with mild effect on the contractions induced by high K + (80 mM). In presence of glibenclamide, the relaxation of low K + -induced contractions was prevented. Similarly, cromakalim caused glibenclamide-sensitive inhibition of low K + , but not of high K + , while verapamil did not differentiate in its inhibitory effect on contractions produced by the two concentrations of K + . Like in jejunum, RT caused glibencla- mide-sensitive relaxation of low K + -induced contractions in tra- chea and aorta, but with a 20 times higher potency in trachea. In atria, RT was least potent with weak inhibitory effect on atrial force and rate of contractions. RT caused a dose-dependent fall in arterial blood pressure in rats under anesthesia. Among the tested pure compounds of Rooibos, chrysoeriol showed selective bronchodilator effect. Chrysoeriol (luteolin 3¢-methyl ether) is a bioactive flavonoid known for antioxidant, antiin- flammatory, antitumor, antimi- crobial, antiviral, and free radical scavenging activities. Conclusion These results indicate that the bronchodilator, antispasmodic and blood pressure lowering ef- fects of Rooibos tea are mediated predominantly through K ATP channel activation with the selec- tive bronchodilatory effect. This study provides a sound mecha- nistic basis for the wide medicinal use of Rooibos tea, with the ther- apeutic potential to be developed for congestive respiratory ailments. j Key words Rooibos tea – K ATP channel activator – airway selectivity – hypotensive – chrysoeriol Received: 22 June 2006 Accepted: 17 August 2006 Published online: 1 November 2006 ORIGINAL CONTRIBUTION Eur J Nutr (2006) 45:463–469 DOI 10.1007/s00394-006-0620-0 EJN 620 A. Khan Æ A.H. Gilani, PhD (&) Dept. of Biological and Biomedical Sciences The Aga Khan University Medical College Karachi 74800, Pakistan Tel.: +92-21/486-4571 Fax: +92-21/493-4294 or /494-2095 E-Mail: anwar.gilani@aku.edu