Relaxant responses by optical isomers of ephedrine and methylephedrine in guinea pig tracheal smooth muscle.

The effects of optical isomers of ephedrine and methylephedrine on the guinea pig tracheal smooth muscle were studied. l-Ephedrine markedly caused a graded relaxation of the guinea pig trachea where the tone had been raised spontaneously. A rightward shift of the l-ephedrine concentration-response curve was observed for propranolol and butoxamine, and the pA2 values for propranolol and butoxamine were 8.55 and 6.38, respectively. d-Methylephedrine markedly caused a graded relaxation of the guinea pig trachea contracted with histamine. Propranolol and bupranolol did not affect the relaxant response to d-methylephedrine. d-Methylephedrine competitively antagonized the contractile responses to histamine, and the pA2 value for d-methylephedrine was 5.12. These results suggest that l-ephedrine-induced relaxation of the guinea pig trachea is mediated through beta 2-adrenoceptors, whereas d-methylephedrine relaxes the guinea pig trachea by blocking histamine receptors.