Lack of differences between the neuronal and extraneuronal handling of 3H-7- and 3H-ring-2,5,6-(-)noradrenaline.

The neuronal disposition of the side chain-labelled 3H-7-(-)noradrenaline and the ring-labelled 3H-2,5,6-(-)noradrenaline of relatively high specific activity was compared in the (incubated) rabbit aorta, the extraneuronal one in the (perfused) rat heart. Furthermore, the metabolism by monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT) of the labelled amines was studied in rat heart homogenates. In addition, some of the experiments were also carried out with the side chain-labelled 3H-7,8-(-)noradrenaline that is an inferior substrate of MAO. 1. In agreement with several other studies, the present results revealed the known differences between the two side chain-labelled amines brought about by the poorer deamination of 3H-7,8-(-)noradrenaline. This finding also indicated that the 3H-7-(-)noradrenaline used in this study was of the desired quality, i.e. was labelled exclusively in position 7. 2. In contrast to earlier findings, no differences between the side chain-labelled 3H-7-(-)noradrenaline and the ring-labelled 3H-2,5,6-(-)noradrenaline were observed with regard to neuronal and extraneuronal amine handling (intact tissues) and metabolism by MAO and COMT in rat heart homogenates. Hence, there is no reason to avoid the ring-labelled amine with high specific activity and, consequently, with slightly more than one tritium substituent per catecholamine molecule in experiments designed to analyse the neuronal and extraneuronal fate of (-)noradrenaline.