Deamination of newly‐formed dopamine in rat renal tissues

Abstract

1 The present study has examined the formation of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in slices of the rat renal cortex and the renal medulla loaded with exogenous L-fl-3,4-dihydroxyphenylalanine (L-DOPA). The effects of pargyline and of two selective inhibitors of monoamine oxidase (MAO) types A and B, respectively Ro 41-1049 and Ro 19-6327, on the deamination of newlysynthesized dopamine in kidney slices incubated with exogenous L-DOPA were also tested. The assay of L-DOPA, dopamine, noradrenaline and DOPAC was performed by means of h.p.l.c. with electrochemical detection. 2 Incubation of renal slices with exogenous L-DOPA resulted in a concentration-dependent accumulation of dopamine and DOPAC; the tissue levels of newly-formed dopamine and DOPAC in slices of the renal medulla were 6-8% of those in cortical slices. 3 Pargyline (0.1 mM) produced a marked decrease (84% reduction) in the formation of DOPAC in kidney slices loaded with 1.0mM L-DOPA; this effect was accompanied by a 17% increase in the accumulation of dopamine. Similar effects were obtained at higher concentrations of pargyline (0.5 and 1.0mM). At 5.0 and 10.0mm pargyline, a marked decrease (46 and 76% reduction) in the accumulation of newlyformed dopamine was observed. 4 The accumulation of dopamine and DOPAC was found to be time-dependent in experiments in which tissues were incubated with 5 and 1IOPM L-DOPA for 5, 10, 20 and 30min. Pargyline (0.1 mM) produced an increase in the accumulation of dopamine at all incubation periods and decreased the formation of DOPAC. 5Ro 41-1049 (50, 100 and 250 nM) was found to produce a concentration-dependent increase of newlyformed dopamine (16-66% increase) and reduced DOPAC formation (44-89% reduction). Ro 19-6327 (50, 100 and 250nM), was found not to affect the accumulation of newly-formed dopamine, but significantly reduced the formation of DOPAC. 6 It is concluded that deamination of newly-formed dopamine in kidney slices loaded with L-DOPA constitutes an important mechanism of amine inactivation. The results presented also suggest that most of the MAO, located inside the compartment where the synthesis of dopamine occurs, is of the A type.

Effect of type A and B monoamine oxidase selective inhibition by Ro 41–1049 and Ro 19–6327 on dopamine outflow in rat kidney slices

“…Of course it could be argued also that the lack of effect of MAO inhibitors in reducing the tissue accumulation of DOPAC is a concentration-related problem. In our hands, however, when added to the incubation medium before loading with L-DOPA, all the three MAO inhibitors, at these very same concentrations, were found to decrease DOPAC formation (100pM pargyline, 84% reduction; 250 nM Ro 41-1049, 86% reduction; 250 nM Ro 19-6327, 79% reduction) Fernandes et al, 1991). The deamination of newly-formed dopamine in kidney slices loaded with exogenous L-DOPA is rather unlikely to occur in a neuronal compartment.…”

“…Although this does not completely rule out the possibility that some of the renal dopamine might have access to neuronal MAO, there is evidence to suggest that the neuronal uptake of newly-formed dopamine in kidney slices is most probably an irrelevant process. When levels of dopamine in kidney slices increase up to 480 nmol g-' (i.e., 6000 fold the levels of endogenous dopamine), as has found to occur in the presence of 2.5 mM L-DOPA, the levels of endogenous noradrenaline (1.3 nmol g') in kidney slices do not change (Fernandes et al, 1991).…”

“…Previous studies in kidney slices loaded with L-DOPA have shown that both MAO-A and MAO-B are involved in the deamination of newly-formed dopamine and that MAO-B would be particularly involved in the deamination of the amine outside the compartment where the synthesis takes place Fernandes et al, 1991). In these experimental conditions, the cellular structures potentially involved in the handling of renal dopamine may theoretically include all those present in the renal cortical slices of the rat kidney: glomeruli, tubules, vascular tissues and support connective tissue.…”

“…2.0 - Fernandes et al, 1991;. Of course it could be argued also that the lack of effect of MAO inhibitors in reducing the tissue accumulation of DOPAC is a concentration-related problem.…”

The renal handling of dopamine originating from l‐DOPA and γ‐glutamyl‐l‐DOPA

“…Firstly, most of renal MAO appears to be located extraneuronally (Caramona & Soares-da-Silva, 1990;Soares-da-Silva et al, 1992a). Secon&b ly, it has been shown that the levels of noradrenaline in kidney slices did not change when up to 2500fiM L-DOPA was added to the incubation medium, thus suggesting that the dopamine formed in renal tissues loaded with L-DOPA has no access to nerve terminals (Fernandes et al, 1991). If the released dopamine is in fact subject to an uptake process into the tubular cells, it might be suggested that this is most likely to occur at the basolateral cell border of tubular cells, namely when using kidney slices.…”