Production of urine free dopamine from DOPA; A micropuncture study

1 The formation and outflow of dopamine and its deaminated metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) was studied in cortical fragments of the rat kidney loaded with L-P-3,4-dihydroxyphenylalanine (L-DOPA) or y-glutamyl-L-DOPA (GluDOPA). Dopamine and DOPAC in the tissues and in the effluent were assayed by means of h.p.l.c. with electrochemical detection. 2 In rats given 30 mg kg-' L-DOPA, tissue and outflow levels of both dopamine and DOPAC were 3 fold those observed with a lower dose of L-DOPA (10 mg kg-'). In rats given GluDOPA (16.7 mg kg-') levels of dopamine in renal tissues and in perifusate samples were found to be higher than those obtained with an equimolar dose of L-DOPA (10 mg kg-'); however, no significant difference was observed for DOPAC. The outflow of both dopamine and DOPAC in kidney slices of rats injected with L-DOPA (10 and 30mg kg-') or GluDOPA (16.7 mg kg') was found to decline monophasically with similar slopes of decline. The rate constants of loss (k, min-') of DOPAC (10mg kg-' L-DOPA, k = 0.0070; 30 mg kg-' L-DOPA, k = 0.0087; 16.7 mg kg-' GluDOPA, k = 0.0080) were 2 to 3 fold those of dopamine (10 mg kg-' L-DOPA, k = 0.0027; 30 mg kg-' L-DOPA, k = 0.0034; 16.7 mg kgGluDOPA, k = 0.0030). With both precursors the DOPAC/dopamine ratio in perifusate samples were 2.0 fold those in the tissues. 3 Tissue and outflow levels of dopamine after incubation of renal tissues with L-DOPA, 50 and 100 JM were found to be lower than those observed with GluDOPA (50 and 100 fM). DOPAC/dopamine ratios in tissues and perifusate samples of experiments performed with L-DOPA were significantly higher (P<0.01) than those observed with GluDOPA. The outflow of both dopamine and DOPAC in renal slices incubated with L-DOPA (50 and 100 gM) were found to decline with time, but presented a biphasic shape. DOPAC/dopamine ratios in perifusate samples were 3 fold that in the tissues with both precursors.4 In conclusion, the present results show that both L-DOPA and GluDOPA give origin to substantial amounts of dopamine and the newly-formed amine undergoes considerable deamination to DOPAC. However, dopamine originating from GluDOPA was less deaminated than that resulting from L-DOPA; it appears that this different behaviour may concern aspects related to the formation of the amine and also those related to its deamination and disposition, namely the processes involved in the access of newly-formed dopamine to MAO.

Section: Resultsmentioning

confidence: 99%

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

Pestana¹,

Soares‐da‐Silva²

1994

“…The renal handling of dopamine is expected, however, to involve mainly tubular epithelial cells, namely those of proximal convoluted tubules. These are the cells involved in the synthesis of renal dopamine as a result of the decarboxylation of circulating L-DOPA (Baines & Chan, 1980;Lee, 1982;Suzuki et al, 1984) and a major area in cortical slices is occupied by the renal tubules. The presence of type A and type B MAO in different cellular compartments has been suggested in radioautographic studies showing that MAO-A is homogenously distributed in both the cortex and the medulla, while MAO-B is heterogenously distributed throughout the renal cortex (Saura et al, 1992).…”

Section: Discussionmentioning

confidence: 99%

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

Pestana

Soares‐da‐Silva

1994

1 The influence of pargyline and of selective inhibitors of type A and B monoamine oxidase (MAO), Ro 41-1049 and Ro 19-6327 respectively, on the outflow of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in slices of rat renal cortex loaded with exogenous L-3,4-dihydroxyphenylalanine (L-DOPA) was examined. Dopamine and DOPAC in the tissues and in the effluent were assayed by means of h.p.l.c. with electrochemical detection. 2 The levels of newly-formed dopamine and DOPAC in the perifusate decreased progressively with time. In control conditions, DOPAC/dopamine ratios in the perifusate were 3 to 5 fold those in the tissue and were found to increase progressively with time. The addition of pargyline (100 ItM), produced a marked decrease in the outflow levels of DOPAC (45 to 54% reduction) and significantly increased the levels of dopamine in the effluent (102 to 158% increase); DOPAC/dopamine ratios in the perifusate remained stable throughout the perifusion and were similar to those found in the tissues. The addition of the MAO-A inhibitor Ro 41-1049 to the perifusion fluid also significantly decreased DOPAC outflow (41% to 54% reduction) and increased dopamine outflow (19% to 80% increase). In the presence of Ro 41-1049 DOPAC/dopamine ratios in the perifusate were lower (P<0.01) than in controls; in contrast with the effect of pargyline, this ratio was found to increase (P < 0.01) throughout the perifusion period. Ro 19-6327 did not reduce the outflow of DOPAC, but significantly increased (by 40-60%) that of dopamine. In the presence of Ro 19-6237, the proportion of DOPAC to dopamine in the perifusate was similar to that of controls and significantly increased throughout the perifusion; however, this increase was less than that observed in the control group. 3 When benserazide (50 JAM) was added to the perifusion fluid, the levels of both dopamine and DOPAC in the effluent were similar to those observed in the absence of benserazide. However, in the presence of benserazide, DOPAC/dopamine ratios in the perifusate did not increase with time. In conditions of decarboxylase inhibition, the effects of pargyline, Ro 41-1049 and Ro 19-6327 on dopamine and DOPAC outflow were less pronounced than in experiments conducted in the absence of benserazide. 4 In conclusion, the results presented here show that the fraction of newly-formed dopamine which leaves the compartment where the synthesis has occurred is a constant source for deamination into DOPAC. The results provide evidence favouring the view that MAO-A is the main form of the enzyme involved in this process; however, the data described here suggest that dopamine would also have access to MAO-B.

“…Thus, it might be suggested that decarboxylation of L-DOPA in renal tissues loaded with the amino acid can take place in each of these three cellular elements. However, Baines & Chan (1980) showed that kidney denervation does not affect the formation of dopamine from exogenous L-DOPA; this strongly suggests that most of the decarboxylation of exogenous L-DOPA does occur in non-neuronal structures, namely in tubular epithelial cells (Hayashi et al, 1990). The finding that tissue levels of NA in kidney slices did not change even when 2.5 mM L-DOPA was added to the incubation medium, strongly supports the view that L-DOPA is not decarboxylated inside noradrenergic neurones.…”

Section: Discussionmentioning

confidence: 99%

“…In kidney, a major source of dopamine derives from the decarboxylation of filtered 3,4-dihydroxyphenylalanine (DOPA) in tubular epithelial cells (Baines & Chan, 1980;Lee, 1982;Suzuki et al, 1984). This is a quantitatively important process as tubular epithelial cells have a high aromatic L-amino acid decarboxylase (AAAD) activity (Adam et al, 1986) and plasma levels of DOPA can attain concentrations up to 15 pmol ml 1 (Cuche, 1988).…”

Section: Introductionmentioning

confidence: 99%

Deamination of newly‐formed dopamine in rat renal tissues

Fernandes

Pestana

Soares-da-Silva

1991

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.