Errors introduced by a tritium label in position 8 of catecholamines

Summary

The neuronal and extraneuronal disposition of³H-7,8-and³H-7-labelled (−)-noradrenaline and dopamine was compared in in vitro studies.

• 1. In agreement with earlier studies, the present results show that the presence of a tritium label in position 8 (i.e., on the alpha-carbon) has two consequences: a) the rate of deamination declines and b) part of the deamination results in the formation of an unlabelled aldehyde plus tritium water; tritium water is recovered from the OMDA-fraction of the column chromatographic procedure of Graefe et al. (1973).

• 2. Whenever the deamination of a³H-catecholamine is reduced (by tritium in position 8), the intraneuronal³H-catecholamine concentration is increased. This increase, in turn, partly masks the decline in neuronal deamination (rat vas deferens). Irrespective of whether one dertermines the spontaneous efflux, the release of³H-noradrenaline by nerve stimulation or the release of³H-(−)-noradrenaline by the reserpine-like compound Ro 4-1284, the presence of tritium in position 8 distorts the results (experiments with rat vasa deferentia and/or rabbit aorta).

• 3. In the extraneuronal system of the rat heart, two intracellular enzymes inactivate³H-(−)-noradrenaline and³H-dopamine: catechol-O-methyl transferase (COMT) and monoamine oxidase (MAO). Any hindrance of deamination (by tritium in position 8, COMT intact) leads to a shift of the metabolism of the³H-catecholamines from the exclusively deaminated to the exclusively O-methylated metabolites.

• 4. No differences between³H-7,8-and³H-7-labelled catecholamines were found after inhibition of MAO and COMT (extraneuronal accumulation and rate constant for efflux from the extraneuronal compartment III of the rat heart).

• 5. These results indicate that the presence of tritium in position 8 of catecholamines introduces errors, if monoamine oxidase is active. This is important for the interpretation of earlier results, since virtually all samples of “³H-(−)-noradrenaline” and “³H-dopamine nominally labelled in position 7” were contaminated with varying amounts of tritium in position 8.

• 6. In some experiments, also³H-7-(−)-adrenaline was used. For adrenergic nerve endings, the rate of metabolism (deamination) declined in the order:³H-7-dopamine > ³H-7-(−)-noradrenaline > ³H-7-(−)-adrenaline. For the extraneuronal disposition the ranking order was:³H-7-(−)-adrenaline > ³H-7-(−)-noradrenaline=³H-7-dopamine. However, in the extraneuronal disposition of³H-(−)-adrenaline, O-methylation predominated over deamination.