Deficiency of renal dopaminergic-dependent natriuretic response to acute sodium load in black salt-sensitive subjects in contrast to salt-resistant subjects

Objective To evaluate the involvement of the renal dopaminergic system in t he natriuretic responses to acute saline load in salt-resistant (SR) and sa lt-sensitive (SS) black normotensive (NT) and hypertensive (HT) subjects. Design and methods We studied the relationship between the urinary excretio n of dopa, dopamine (DA) and its metabolite DOPAC and the natriuretic respo nses to acute volume expansion (2 I NaCl 0.9% over 2 h) in 20 black NT subj ects (12 SR and 8 SS) and 19 black HT subjects (10 SS and 9 SR). Subjects r eceived a low salt (LS) diet (40 mmol sodium/day) for 1 week and a high sal t (HS) diet (300 mmol sodium/day) for 1 week; the sequence of the dietary r egimens was randomized. Comparisons were made between the results before th e saline infusion (baseline) and the results 2 h after the infusion. Results In all the groups saline infusion induced significant increases in urinary volume (ml/4 h) of two- to three-fold and in urinary sodium excreti on (mmol/4 h h) of three- to ten-fold; these increases were significantly g reater during the HS diet than during the LS diet. Saline infusion signific antly increased the mean arterial pressure (MAP) by 5 mmHg in HT-SS subject s and by 4-5 mmHg in NT-SS subjects, but the MAP did not changed in the NT- SR and HT-SR groups. Under the LS diet, saline infusion changed the DA excr etion (in nmol/4 h) by -49 +/- 89 in HT-SS subjects, by 17 +/- 52 in NT-SR subjects, by 235 +/- 72 in HT-SR subjects and by 220 +/- 86 in NT-SR subjec ts (P < 0.05 between SR and SS subjects). The saline infusion-induced chang es in DA excretion correlated significantly with the increases in urinary s odium excretion (r = 0.71, P < 0.01) in the NT-SR and NT-SR subjects under the LS diet, but not in the SR groups on the HS diet nor in the SS groups ( HT and NT) on either diet. Saline infusion significantly reduced the DA/dop a ratio in SS (NT and HT) but not SR (NT and HT) subjects, whereas the DA/D OPAC (dihydroxyphenylacetic acid) ratios were similar in all the groups. Conclusions The urinary dopaminergic system may participate in the natriure tic responses to acute sodium load only in SR subjects (NT and HT) and only under LS diets, but not in SR subjects (NT and HT). This strongly suggests that black NT- and HT-SS subjects have an underlying impairment in the act ivity of the renal dopaminergic system which may be associated with a reduc ed decarboxylation of dopa into DA. (C) Lippincott Williams & Wilkins.