SODIUM-DEPENDENCE AND OUABAIN-SENSITIVITY OF THE SYNTHESIS OF DOPAMINE IN RENAL TISSUES OF THE RAT

1 The present study has examined the influence of sodium chloride (0-160 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 cortex loaded with exogenous L-dihydroxyphenylalanine (L-DOPA). The deamination of newly-formed dopamine into 3,4-dihydroxyphenylacetic acid (DOPAC) was also examined. The assay of L-DOPA, dopamine and DOPAC in kidney slices was performed by high performance liquid chromatography (h.p.l.c.). with electrochemical detection. 2 The accumulation of newly-formed dopamine and DOPAC in kidney slices loaded with L-DOPA (50 and 100-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, respectively, 0.00305 +/- 0.00036 and 0.00328 +/- 0.00029 for dopamine and 0.00672 +/- 0.00072 and 0.00641 +/- 0.00069 for DOPAC. The sodium-dependent formation of dopamine was completely abolished when the experiments were performed in the absence of oxygen. 3 In experiments performed in the presence of 120 mM NaCl, but not in conditions of low sodium (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% reduction; P < 0.05); this effect was more marked at 50 and 100-mu-M L-DOPA. When the experiments were performed in the absence of oxygen the renal production of dopamine and DOPAC became markedly decreased (respectively, 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 to DOPAC in kidney homogenates closely depended on the concentration of L-DOPA added to the medium; ouabain (100, 500 and 1000-mu-M) was found to affect neither the formation of dopamine nor its deamination to DOPAC when homogenates were used instead of kidney slices. 5 In conclusion, the results presented here show that the formation of dopamine in kidney slices loaded with L-DOPA is sodium-dependent and ouabain-sensitive suggesting the involvement of a co-transport system of sodium and DOPA into the tubular epithelial cell.