1) There is evidence for an influence of a thyme extract on beta (2)-receptors by both binding studies and biological effects: As can be derived from the shift of the propranolol displacement curve (nM), ingredients of the thyme extract slightly interact with beta (2)-receptors in rat lung tissue. This effect is indirect since no full range competition curve was reached. 2) An at least indirect interaction with beta (2)-receptors in rat uteri and trachea is revealed by a decreased antagonism of propranolol on the relaxing effect of isoprenaline by the plant extract. 3) An additional mechanism is presumed because at high extract concentrations isoprenaline-induced relaxation is complete, whereas the displacement of propranolol at beta (2)-receptors is only weak. 4) Thyme extract has an indirect (modulatory) effect on the beta (2)-receptor system. 5) Mucociliary clearance is improved in vivo. Its mechanism has still to be elucidated.

Thyme extract is known to posses spasmolytic effects on isolated trachea and to increase ciliary activity. A small part of this effect is mediated via beta (2) receptors, but other receptors are supposed to be involved. Endothelin has a pathophysiological impact in asthma with respect to hyper-reagibility and contracting isolated trachea smooth muscles and was, therefore, investigated. Thyme extract inhibited endothelin-induced contraction. Emax decreased depending on the concentration of thyme extract indicating a non-competitive inhibition. Bosentan, an endothelin receptor antagonist (positive control), acted as a competitive inhibitor. The interaction/combination of thyme extract and bosentan was additive but not supra-additive indicating an interaction with the endothelin system. Thymol (one of the major compounds of thyme extract) did not interact with the endothelin system. It is concluded that thyme extract may help in diseases related to endothelin hyper-reagibility of the bronchus system such as asthma and COPD (chronic obstructive pulmonary disease), although thymol is not involved in this effect.