The aim of the study was to detect differences between salt-sensitive and salt-resistant hypertensive patients in the response of the renin-aldosterone axis, plasma noradrenaline and atrial natriuretic peptide to high salt intake. 2. Fifty essential hypertensive patients followed 2 weeks of a standard diet with 20 mmol of NaCl daily, supplemented by placebo tablets for the first 7 days and by NaCl tablets for the last 7 days, in a single-blind fashion. Salt sensitivity was defined as a significant rise (P < 0.05) in 24 h mean blood pressure obtained by ambulatory blood pressure monitoring from the low- to the high-salt period. Biochemical and hormonal measurements were performed on the last day of both periods. 3. Twenty-two (44%) patients fulfilled criteria of salt-sensitive hypertension, whereas the remaining 28 (56%) were considered salt-resistant. High salt intake promoted a significant decrease (P < 0.05) in plasma creatinine, potassium, glucose, cholesterol, low-density lipoprotein-cholesterol, triacylglycerols, uric acid and plasma renin activity, and a significant increase in plasma atrial natriuretic peptide and 24 h urinary calcium excretion. The direction of these changes did not differ between salt-sensitive and salt-resistant patients. Salt-resistant hypertensive patients exhibited a significant decrease in plasma aldosterone induced by high salt intake (from 446 +/- 35 to 226 +/- 35 pmol/l; P < 0.001), whereas this parameter was not significantly modified in salt-sensitive patients (from 485 +/- 76 to 364 +/- 83 pmol/l; P not significant). Salt-sensitive patients showed an increase in plasma noradrenaline after high salt intake (from 1.15 +/- 0.11 to 1.56 +/- 0.14 nmol/l; P < 0.05), whereas salt-resistant patients presented a decrease in this parameter (from 1.48 +/- 0.08 to 1.12 +/- 0.08 nmol/l; P < 0.05). The change in plasma noradrenaline was directly correlated with the change in mean blood pressure induced by high salt intake (r = 0.479; P = 0.003). 4. We conclude that the increase in blood pressure induced by high salt intake in salt-sensitive patients is associated with a stimulation of the sympathetic nervous system and a blunted decrease in plasma aldosterone. Conversely, changes in renal function, electrolyte excretion and plasma concentrations of atrial natriuretic peptide induced by high salt intake seem to be similar in both salt-sensitive and salt-resistant patients.
