Background: The heterogeneity among patients with essential hypertension is known. We observed in an earlier study a large spread in the values of fractional excretion of sodium in a group of subjects with mild essential hypertension after ambulation, suggesting possible subgroups among them. We defined as the retainer (R) group that which presented a reabsorption index (RI = fractional excretion of sodium during supine/fractional excretion of sodium after ambulation) < 2.5 and the non-retainer (NR) group as that whose RI < 2.5. We analyzed at what level the reabsorption of Na was produced and the differences between the possible groups.
Patients and methods: We studied 51 mild essential hypertensive patients, 22 men and 29 women, in two consecutive periods -recumbent, 90 minutes; ambulation, 90 minutes-. We calculated the clearance of creatinine and lithium, fractional proximal and distal reabsorption of Na, plasma renin activity (PRA), plasma aldosterone (ALDO) and elimination of PGE2 and kallikrein in the urine; the plasma catecholamines at the end of the recumbent position and after 10 minutes in the upright position. The study was taken after at least ten days without treatment and following a diet with free Na intake.
Results: We did not encounter significant differences between the retainer (n = 19) and non-retainer (32) group with regard to age, sex, body mass index or elimination of Na/24 h. The lower natriuresis in ambulation in the R group compared with the NR group is due to a higher fractional reabsorption of sodium, proximal (83.7 +/- 4.9% vs 79 +/- 5.2; p < 0.01) as well as distal (96.8 +/- 2 vs 95.3 +/- 2%; p < 0.05). The increment in the PRA was greater in the R group (1.3 +/- 1.4 vs 0.8 +/- 0.8; p < 0.05) after ambulation; in the R group showed a lower index -increment in ALDO/increment in PRA- in response to postural change although without significancy. We observed a lower elimination of PGE2 and kallikrein in the R group as compared with NR group (p < 0.05). We did not find differences with regard to plasma catecholamines.
Conclusions: Ambulation allows to distinguish two groups of essential hypertensive patients according to natriuresis. The lower natriuresis in ambulation on the part of the R group could be due to the increase in PRA and therefore of the angiotensin II at the level of the proximal tubule, and to a lower activity of natriuretic hormones (PGE21 kallikrein) at the distal level.