Effects of 6-hydroxydopamine on noradrenaline metabolism linked to neuronal uptake1 and extraneuronal uptake1 in dental pulp in vitro.

The study was undertaken to determine the relative roles of neuronal and extraneuronal uptake1 in the metabolism of 3H-noradrenaline in human dental pulp. Rabbit dental pulp was used as a reference since it was already known that normetanephrine (NMN) formation in this tissue utilised extraneuronal uptake1. Slices of pulp were preincubated in the absence and presence of 6-hydroxydopamine (1.6 mmol/l, for 10 or 20 min at pH 4.5) and subsequently incubated with 3H-noradrenaline (0.18 mumol/l for 30 min at pH 7.4). The principal metabolites formed were normetanephrine in rabbit pulp and deaminated catechols (dihydroxymandelic acid and dihydroxyphenylglycol) in human pulp. In both tissues 6-hydroxydopamine strongly inhibited formation of the deaminated catechols, but was without effect on normetanephrine formation. It is concluded that: i) in vitro 6-hydroxydopamine does not influence the metabolic process which is dependent on extraneuronal uptake1, namely normetanephrine formation in rabbit dental pulp, and ii) the deaminated catechols are largely neuronal in origin in human pulp. Attention is drawn to an unusual feature of the neuronal metabolism in human pulp, namely the appearance of dihydroxymandelic acid as the principal metabolite.