RENAL FUNCTIONAL RESERVE IN HEALTHY ELDERLY SUBJECTS

The increase in GFR after an amino acid (AA) load, the so-called renal functional reserve, is impaired in the aged rat. Whether the renal functional reserve predicts the progression of renal disease in humans is controversial, but it is possible that age-related alterations of renal hemodynamics are relevant for the evolution of renal disease in the elderly. We compared renal hemodynamics before and after an AA infusion in 15 healthy normotensive subjects ot young age (seven women, eight men; median age, 26 yr; range, 23 to 32) and in 10 subjects of old age (six women, four men; median age, 70 yr; range, 61 to 82) on normal dietary protein intake. Baseline GFR and effective RPF were measured after 12 h of fasting by the inulin (Cin) and para-aminohippurate (Cpah) steady-state infusion techniques. The renal functional reserve was examined after an overnight AA infusion (7% solution; 83 mL/h). Median basal Cin and Cpah were significantly lower (P < 0.01) in the elderly (102 and 339 mL/min per 1.73 m2) than in the young subjects (122 and 647 mL/min per 1.73 m2), but virtually all GFR values of the elderly were still within the normal range. Median Cin upon infusion of AA was 118 mL/min per 1.73 m2 (range, 98 to 137) in the elderly and 146 (range, 120 to 171) in the young, respectively. Corresponding values of Cpah were 349 mL/min per 1.73 m2 in the elderly versus 689 mL/min per 1.73 m2 in the young. Cin increased significantly (P < 0.01) after the AA load in both young and elderly subjects. The median percent rise of Cin was not significantly different in young (+16%) and elderly (+17%) subjects and was independent of gender. Median Cpah, however, increased significantly (P < 0.01) in the young subjects, but not in the elderly. Median renal vascular resistance, both at baseline (175 versus 83 mm Hg/L per min) and after the AA load (170 versus 77), was significantly higher (P < 0.01) in the elderly as compared with the young subjects. The same was true for filtration fraction. The results document that, in humans, (1) GFR and effective RPF are slightly lower in the elderly in the absence of underlying renal disease, (2) renal vascular resistance and filtration fraction are elevated in old age, and (3) renal functional reserve is demonstrable at least until the age of 80 yr in women and men.