Effects of optical isomers of ephedrine (EPH) and methylephedrine (MEP) on histamine H1-receptors and muscarinic receptors in guinea pig ileal muscle were investigated by radioligand binding assay and by measuring the mechanical response to histamine and acetylcholine. EPH and MEP inhibited the specific binding of [3H]mepyramine and [3H]quinuclidinyl benzilate (QNB) to microsomal fractions prepared from this tissue. The rank order of inhibitory potency of [3H]mepyramine and [3H]QNB binding was d-->l-isomer and l-->d-isomer, respectively. Furthermore, the rank order of antagonistic potency in the mechanical response study was the same as that in the binding study. d-MEP competitively antagonized histamine-induced contraction (pA2 value; 5.14). These results suggest that each isomer of EPH and MEP has a distinct affinity for histamine H1-receptors and muscarinic receptors in guinea pig ileal muscle. The antihistaminic and antimuscarinic activity of these compounds may be largely attributed to competition at receptor sites. In addition, it is suggested that d-MEP exhibits a competitive antagonist activity for the histamine H1-receptor.
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