Renal sodium transport in healthy elderly and elderly with systolic hypertension

Abstract

It is accepted that lithium clearance expresses the delivery of sodium and water. This study was designed to investigate renal sodium transport in healthy and hypertensive elderly using lithium clearance (Cli) and fractional lithium clearance (FEli). The clearance studies were performed and analyzed in young healthy volunteers (n = 29, 26.2 ± 3.7 years), elderly with normal blood pressure (n = 49, 67.8 ± 6.1 years) and elderly with systolic hypertension (n = 37, 68.5 ± 7.2 years, blood pressure ≥ 160/95 mmHg). FENa of healthy elderly group was greater than that of young group (1.36 ± 0.49% vs. 1.06 ± 0.54%, P < 0.05), but there was no difference between the healthy elderly group and hypertensive elderly group. In the healthy elderly with normal GFR (subgroup 1, n = 27), FELi was still significantly higher than that of young group (26.1 ± 10.0 vs. 18.6 ± 4.0%, P < 0.001). The absolute proximal reabsorption of sodium (APRNa) and the fractional proximal reabsorption of sodium (FDRNa) in subgroup 1 were obviously lower than that of the young group (11.9 ± 1.8 vs. 13.3 ± 3.1 mmol/min, P < 0.05 and 76.5 ±6.4% vs. 81.5 ± 4.0%, P < 0.005). On the other hand, no significant difference was found in absolute or fractional distal reabsorption of sodium (ADRNa or FDRNa) between subgroup 1 and the young group. ADRNa/filtered sodium load (FLNa) in subgroup 1 was significantly higher than that of the young group (22.6 ± 6.8% vs. 17.6 ± 3.9%, P < 0.05), whereas there were no differences in CLi, FELi, APRNa, ADRNa, FDRNa or ADRNa/FLNa between the healthy elderly group and the aged systolic hypertensive group. It is concluded that in ageing, sodium reabsorption in the proximal nephron is reduced alone with a relative enhancement of sodium reabsorption in distal nephron, which might be a compensation in sodium balance of old persons. Abnormalities of sodium reabsorption were unlikely to be responsible for systolic hypertension in the elderly.