On the mechanism of bronchodilatation by etafedrine.

Effects of (-)-N-ethylephedrine (etafedrine) were studied on the tracheal chain preparation and the atria of the guinea pig and on the rabbit perfused heart. Contraction of the tracheal chain by acetylcholine or histamine was antagonized by epinephrine (adrenaline), etafedrine and ephedrine, the relative potencies being 91:1:0.3, respectively. Propranolol (5 X 10(-7) mol/l) completely antagonized the bronchodilator effect of 10(-4) mol/l etafedrine on the acetylcholine-evoked contraction. Etafedrine up to 3 X 10(-4) mol/l did not increase heart rate or force of contraction in guinea pig atria. In contrast to tyramine and ephedrine (both 10(-5) mol/l), etafedrine (10(-4) mol/l) failed to release 3H-norepinephrine (noradrenaline) in the perfused rabbit heart. Moreover, the concentration-dependent positive chronotropic and inotropic effects of norepinephrine in isolated atria were slightly, but not significantly enhanced by etafedrine. It is concluded that N-ethylation of ephedrine suppresses the indirect sympathomimetic activity and markedly enhances the efficacy on beta 2-adrenoceptors. Etafedrine appears to be a sympathomimetic agent with a selective activity on beta 2-adrenoceptors.