Assessment of renal dopaminergic system activity in the nitric oxide‐deprived hypertensive rat model

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Abstract
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 NGnitro‐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 20mmHg; heart rate was slightly decreased. During the next 13 days both SBP and DBP increased progressively reaching 170± 3 and 116±3mmHg, respectively.3 Baseline urinary excretion of L‐DOPA, dopamine, 3,4‐dihydroxyphenylacetic acid (DOPAC), 3methoxytyramine (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 5hydroxytryptamine 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 μm) and saturable at 1000 μm. AAAD activity as determined in homogenates (Vmax, in nmol mg−1 protein h−1; Km in μm) was significantly (PVmax = 25 ± 2; Km = 72 ± 10) than in control rats (Vmax =14 ± 1; Km = 63 ± 7), rats given L‐NAME for 7 days (Vmax = 15 ± 1; Km = 69 ± 5) and rats given L‐NAME plus L‐arginine (Vmax =13 ± 1; Km = 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 μmol−1) were observed between the four experimental groups. However, the saturable outward transfer of dopamine (Vmax, in μmol mg−1 protein h−1; Km in μm) was higher in control animals (Vmax = 2.3 ± 0.2; Km = 568 ± 67) than that in rats treated with L‐NAME for 14 days (Vmax = 0.8 ± 0.02; Km = 241 ±21), but similar to that observed in rats receiving L‐NAME plus L‐arginine (Vmax = 2.4 ± 0.2; Km = 618 ± 61); the saturable dopamine outward rate of transfer in rats given L‐NAME for 7 days (Vmax = 3.9 ± 0.2; Km = 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.