Effects of deuterium substitution on the chronotropic responses to some sympathomimetic amines in the isolated rat atria

Summary In spontaneously beating rat atria the potencies for the chronotropic effects of the following deuterated phenylethylamine derivatives were higher than the potencies of the corresponding non-substituted (protio-) amines: ,,d₂--phenylethylamine; ,,,-d₄-p-tyramine; ,,,-d₄-m-tyramine; ,,-d₃-p-octopamine. In contrast, ,,-d₃-noradrenaline and ,,-d₃-m-octopamine were equipotent with the corresponding protio-amines. Experiments performed in atria depleted of endogenous noradrenaline by pretreatment with reserpine and in atria exposed to the monoamine oxidase (MAO) inhibitor pargyline indicated: a. p-octopamine had both direct and indirect effects, but the chronotropic responses to p-octopamine in tissues with normal MAO activity depended mostly on the direct action of the amine; deuterium substitution enhanced the indirect component of action of p-octopamine; b. m-octopamine possessed considerable indirect effects while d₃-m-octopamine behaved as an amine of direct action. The substitution of deuterium for hydrogens in the -carbon of the alkyl-side chain of phenylethylamines decreases the rate of deamination by MAO. Therefore, the results obtained with all the amines, except for m-octopamine and ,,p-d₃-m-octopamine, could be interpreted in terms of the direct, indirect or mixed action of those compounds and/or of the influence that MAO activity has on the chronotropic responses to these amines. The results obtained with protio-and deuterio-m-octopamine suggested that deuterium substitution, either at the - or the -carbon, can alter some other mechanisms in addition to the enzymatic deamination.Career Investigator on leave of absence from the Consejo de Investigaciones Cientificas y Técnicas, ININFA, Junín 956, 5°P, RA-1113 Buenos Aires, ArgentinaSend offprint requests to S. M. Celuch