ASSESSMENT OF RENAL DOPAMINERGIC SYSTEM ACTIVITY IN THE NITRIC OXIDE-DEPRIVED HYPERTENSIVE RAT MODEL

1 The present paper reports changes in the urinary excretion of dopamine, 5-hydroxytryptamine and amine metabolites in nitric oxide deprived hypertensive rats during long-term administration of N-G-nitro-L-arginine methyl ester (L-NAME). Aromatic L-amino acid decarboxylase (AAAD) activity in renal tissues and the ability of newly-formed dopamine to leave the cellular compartment where the synthesis of the amine has occurred were also determined. 2 Twenty four hours after exposure to L-NAME, both systolic (SBP) and diastolic (DBP) blood pressure were increased by 20 mmHg; heart rate was slightly decreased. During the next 13 days both SBP and DBP increased progressively reaching 170 +/- 3 and 116 +/- 3 mmHg, respectively. 3 Baseline urinary excretion of L-DOPA, dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3-MT) and homovanillic acid (HVA) during the 4 day period of stabilization averaged 4.4 +/- 0.5, 13.8 +/- 0.3, 37.4 +/- 0.8, 180.0 +/- 2.7 and 206.1 +/- 6.7 nmol day(-1), respectively. The urinary excretion of L-DOPA, dopamine and DOPAC, but not that of 3-MT and HVA, were increased from day 6-8 of L-NAME administration onwards (L-DOPA, up to 13.4 +/- 2.1; dopamine, up to 23.0 +/- 1.6; DOPAC, up to 62.8 +/- 3.7 nmol day(-1)). Baseline daily urinary excretion of 5-hydroxytryptamine and 5-hydroxyindolacetic acid (5-HIAA) averaged 73.5 +/- 1.1 and 241.7 +/- 5.4 nmol day(-1), respectively. During the first week of L-NAME administration, the urinary excretion of both 5-hydroxytryptamine and 5-HIAA did not change significantly; however, as was found with dopamine and DOPAC, changes in the urinary excretion of 5-hydroxytryptamine were evident during the second week of L-NAME administration. 4 In experiments performed on homogenates of isolated renal tubules, the decarboxylation of L-DOPA to dopamine was dependent on the concentration of L-DOPA used (10 to 5000 mu M) and saturable at 1000 mu M. AAAD activity as determined in homogenates (V-max, in nmol mg(-1) protein h(-1); K-m in mu M) was significantly (P < 0.01) higher in rats given L-NAME for 14 days (V-max = 25 +/-:2; K-m = 72 +/- 10) than in control rats (V-max = 14 +/- 1; K-m = 63 +/- 7), rats given L-NAME for 7 days (V-max,, = 15 1 1; K-m,= 69 +/- 5) and rats given L-NAME plus L-arginine (V-max = 13 +/- 1; K-m = 60 +/- 3) for 14 days. 5 A considerable amount of the total dopamine formed from added L-DOPA in kidney slices escaped into the incubation medium. The application of the Michaelis-Menten equation to the net transport of newly-formed dopamine allowed the identification of a saturable (carrier-mediated transfer) and a non-saturable component (diffusion). No significant differences in the diffusional rate of transfer (0.14 +/- 0.02 mu mol(-1)) were observed between the four experimental groups. However, the saturable outward transfer of dopamine (V-max, in mu mol mg(-1) protein h(-1); K-m in mu M) was higher in control animals (V-max = 2.3 +/- 0.2; K-m = 568 +/- 67) than that in rats treated with L-NAME for 14 days (V-max = 0.8 +/- 0.02; K-m = 241 +/- 21), but similar to that observed in rats receiving L-NAME plus L-arginine (V-max = 2.4 +/- 0.2; K-m = 618 +/- 61); the saturable dopamine outward rate of transfer id rats given L-NAME for 7 days (V-max = 3.9 +/- 0.2; K-m = 1006 +/- 32) was higher than in controls. 6 In conclusion, the present studies show that the hypertensive response resulting from the long-term administration of L-NAME is accompanied by an increased urinary excretion of dopamine and 5-hydroxytryptamine, which appears to follow an enhanced activity of renal AAAD. The observation that the increased AAAD activity can be reversed by the administration of L-arginine to L-NAME-treated rats favours the view that the adaptational response which results in an enhanced AAAD activity probably involves a decrease in the generation of nitric oxide.