The isotope effect of tritium in 3H-(-)-adrenaline with very high specific activity.

Initial rates of the extraneuronal uptake of 3H-(-)-adrenaline of very high specific activity were determined in the perfused rat heart, fluorimetrically for unlabelled and by scintillation counting for labeled amine. The presence of nearly four tritium substituents in the adrenaline molecule slowed uptake by a factor of 5.56. The isotope effect of tritium appears to increase in direct proportion to the number of tritium substituents per molecule. It is concluded that 3H-catecholamines of very high specific activity are unsuitable for studies of the uptake and metabolism of 3H-catecholamines.