Pharmacological effects of ephedrine alkaloids on human alpha(1)- and alpha(2)-adrenergic receptor subtypes

Ephedra species of plants have both beneficial and adverse effects primarily associated with the presence of ephedrine alkaloids. Few reports have appeared that examine the direct actions of ephedrine alkaloids on human subtypes of adrenergic receptors (ARs). In the present study, ephedrine alkaloids were evaluated for their binding affinities on human alpha(1A)-, alpha(1B)-, alpha(1D)-, alpha(2A)-, alpha(2B)-, and alpha(2C)-AR subtypes expressed in HEK and Chinese hamster ovary cells. Cell-based reporter gene assays were used to establish functional activity of ephedrine alkaloids at alpha(1A)-, alpha(2A)-, and alpha(2C)-ARs. The data showed that ephedrine alkaloids did not activate alpha(1)- and alpha(2)-ARs and that they antagonized the agonist-mediated effects of phenylephrine and medetomidine on alpha(1)- and alpha(2)-ARs, respectively. As in the binding studies, 1R,2R- and 1R,2S-ephedrine showed greater functional antagonist activity than the 1S,2R- and 1S,2S-isomers. The rank order of affinity for the isomers was 1R,2R > 1R,2S > 1S,2R > 1S,2S. The rank order of potencies of alkaloids containing a 1R,2S-configuration was norephedrine > or = ephedrine > N-methylephedrine. These studies have demonstrated that orientation of the beta-hydroxyl group on the ethylamino side chain and the state of N-methyl substitution are important for alpha-AR binding and functional activity of the ephedrine alkaloids. In conclusion, the ephedrine isomers and analogs studied did not exhibit any direct agonist activity and were found to possess moderate antagonist activities on cloned human alpha-ARs. The blockade of presynaptic alpha(2A)- and alpha(2C)-ARs may have a pharmacological role in the direct actions of Ephedra alkaloids.