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 dopami ne, 5-hydroxytryptamine and amine metabolites in nitric oxide deprived hypertensive rats during long-term administration of N-G-nitro-L-argi nine 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 o ccurred were also determined. 2 Twenty four hours after exposure to L- NAME, both systolic (SBP) and diastolic (DBP) blood pressure were incr eased by 20 mmHg; heart rate was slightly decreased. During the next 1 3 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 stabiliza tion averaged 4.4 +/- 0.5, 13.8 +/- 0.3, 37.4 +/- 0.8, 180.0 +/- 2.7 a nd 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 increas ed 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 d ay(-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 admini stration, 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 e vident during the second week of L-NAME administration. 4 In experimen ts performed on homogenates of isolated renal tubules, the decarboxyla tion of L-DOPA to dopamine was dependent on the concentration of L-DOP A 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,, = 1 5 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 equatio n to the net transport of newly-formed dopamine allowed the identifica tion of a saturable (carrier-mediated transfer) and a non-saturable co mponent (diffusion). No significant differences in the diffusional rat e of transfer (0.14 +/- 0.02 mu mol(-1)) were observed between the fou r experimental groups. However, the saturable outward transfer of dopa mine (V-max, in mu mol mg(-1) protein h(-1); K-m in mu M) was higher i n 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 dopam ine 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 conc lusion, the present studies show that the hypertensive response result ing from the long-term administration of L-NAME is accompanied by an i ncreased 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 adapta tional response which results in an enhanced AAAD activity probably in volves a decrease in the generation of nitric oxide.