P. Soaresdasilva et Mh. Fernandes, SODIUM-DEPENDENCE AND OUABAIN-SENSITIVITY OF THE SYNTHESIS OF DOPAMINE IN RENAL TISSUES OF THE RAT, British Journal of Pharmacology, 105(4), 1992, pp. 811-816
1 The present study has examined the influence of sodium chloride (0-1
60 mM) and ouabain (100 and 500-mu-M), an inhibitor of the enzyme Na- K+ ATPase, on the synthesis of dopamine in slices of rat renal corte
x loaded with exogenous L-dihydroxyphenylalanine (L-DOPA). The deamina
tion of newly-formed dopamine into 3,4-dihydroxyphenylacetic acid (DOP
AC) was also examined. The assay of L-DOPA, dopamine and DOPAC in kidn
ey slices was performed by high performance liquid chromatography (h.p
.l.c.). with electrochemical detection. 2 The accumulation of newly-fo
rmed dopamine and DOPAC in kidney slices loaded with L-DOPA (50 and 10
0-mu-M) was found to be dependent on the concentration of NaCl in the
medium. A similar picture could be observed for DOPAC. The fractional
rate of accumulation (k; mM NaCl-1) was at 50 and 100-mu-M L-DOPA, res
pectively, 0.00305 +/- 0.00036 and 0.00328 +/- 0.00029 for dopamine an
d 0.00672 +/- 0.00072 and 0.00641 +/- 0.00069 for DOPAC. The sodium-de
pendent formation of dopamine was completely abolished when the experi
ments were performed in the absence of oxygen. 3 In experiments perfor
med in the presence of 120 mM NaCl, but not in conditions of low sodiu
m (20 mM NaCl in the medium), ouabain (100 and 500-mu-M) was found to
inhibit the accumulation of newly-formed dopamine and DOPAC (14-57% re
duction; P < 0.05); this effect was more marked at 50 and 100-mu-M L-D
OPA. When the experiments were performed in the absence of oxygen the
renal production of dopamine and DOPAC became markedly decreased (resp
ectively, 40% and 77% reduction; P < 0.05) and ouabain (100 and 500-mu
-M) was no longer able to reduce the accumulation of both newly-formed
dopamine and DOPAC. 4 The formation of dopamine and its deamination t
o DOPAC in kidney homogenates closely depended on the concentration of
L-DOPA added to the medium; ouabain (100, 500 and 1000-mu-M) was foun
d to affect neither the formation of dopamine nor its deamination to D
OPAC when homogenates were used instead of kidney slices. 5 In conclus
ion, the results presented here show that the formation of dopamine in
kidney slices loaded with L-DOPA is sodium-dependent and ouabain-sens
itive suggesting the involvement of a co-transport system of sodium an
d DOPA into the tubular epithelial cell.