The renal handling of dopamine originating from L-dopa and gamma-glutamyl-L-dopa.

Abstract

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-beta-3,4-dihydroxyphenylalanine (L-dopa) or gamma-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-1 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-1). In rats given GluDOPA (16.7 mg kg-1) 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-1); 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 30 mg kg-1) or GluDOPA (16.7 mg kg-1) was found to decline monophasically with similar slopes of decline. The rate constants of loss (k, min-1) of DOPAC (10 mg kg-1 L-DOPA, k = 0.0070; 30 mg kg-1 L-DOPA, k = 0.0087; 16.7 mg kg-1 GluDOPA, k = 0.0080) were 2 to 3 fold those of dopamine (10 mg kg-1 L-dopa, k = 0.0027; 30 mg kg-1 L-DOPA, k = 0.0034; 16.7 mg kg-1 GluDOPA, 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 MicroM were found to be lower than those observed with GluDOPA (50 and 100 MicroM). 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 MicroM) 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.