Some problems encountered in attempting to estimate catecholamine turnover using labelled tyrosine

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Abstract
Rats were given [3H]tyrosine intravenously. The decrease of [3H]noradrenaline (3H-NA) and [3H]dopamine (3H-DA) from 2 to 6 h after the [3H]tyrosine injection was estimated in the cerebral cortex, brain stem, caudate nucleus, spinal cord and adrenal medulla. Half-lives were estimated with or without the tyrosine hydroxylase inhibitor H 44/68 or the dopamine-β-hydroxylase inhibitor FLA-63, given 2 h after the [3H]tyrosine. The influence of the inhibitors on endogenous noradrenaline was also studied. H 44/68 did not change the half-life of 3H-NA in the central nervous system but FLA-63 reduced it considerably. FLA-63 reduced endogenous noradrenaline more than did H 44/68. The mechanism responsible for the faster turnover of noradrenaline after FLA-63 has not yet been elucidated. In the adrenal medulla the [3H]adrenaline + 3H-NA level rose significantly from 2 to 6 h after the injection of [3H]tyrosine. The synthesis inhibitors prevented this rise, indicating that appreciable amounts of labelled precursors remain for a long time after the injection of the labelled tyrosine. Some problems encountered with the different methods for turnover determinations are discussed on the basis of these data.