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Decline in glomerular filtration rate (GFR) is one of several changes in renal physiology in the elderly. Renal reserve (RR) is the kidney′s capacity to increase its basal GFR by at least 20% after a protein overload. Even though it has already been reported that RR is preserved in healthy old people, there is no information whether RR is also preserved in the healthy very old one (older than 74 of age), which we decided to study and report our findings in this paper. We studied RR in 16 healthy persons divided into three age groups: young: 20–40 years old (n: 5): 64–74 years old (n: 6) and oldest old: >74 years old (n: 5). Renal reserve test was performed by assessing creatinine clearance with cimetidine before and after an oral protein load. Statistical analysis was performed using ANOVA test. Even though renal reserve response was present in all age groups, its magnitude (delta GFR) was significantly higher in the healthy young group (103.6 ± 53 ml/min) compared to the old one (34.1 ± 40 ml/min) (P = 0.002), while it was significantly lower in the healthy oldest old (20.7 ± 0.7 ml/min) group compared to the other two groups (P = 0.002). Renal reserve is preserved in healthy very old people, but its magnitude decreases significantly with age.

The handling of renal creatinine in human beings has classically been described as the result of two particular physiological processes: glomerular filtration and proximal tubular secretion. However, there are particular physiological situations in which tubular creatinine reabsorption has been documented, such as in the case of healthy newborns and premature babies. We performed a prospective study in order to evaluate if there is tubular creatinine reabsorption in healthy elderly people. We studied prospectively nine healthy volunteers, four of them young (20–33 years old) and the remaining five, old (65–73 years old). Since creatinine is secreted in the proximal tubules, and its secretion can be completely blocked by cimetidine administration, a creatinine clearance with cimetidine reliably represents the glomerular filtration rate. Therefore, if the ratio creatinine clearance (Ccr)/creatinine clearance with cimetidine (CcrWC) is higher than one, this would indicate net creatinine secretion, whereas a ratio lower than one would indicate a net renal creatinine tubular reabsorption; a ratio equal to one indicates creatinine filtration. Finally, the Ccr, CcrWC, and Ccr/CcrWC ratios were compared between the young and old group. Mann-Whitney and Wilcoxon tests were used. As expected, creatinine clearance in the elderly was significantly lower than in the young [Ccr: 74.4 ml/min (47.9–100.9) (old) vs. 153.8 ml/min (108.3–199.2) (young), p = 0.014]. Similarly, the creatinine clearance with cimetidine (CcrWC) was significantly lower in the elderly compared to the young [CcrWC: 81.8 ml/min (69.2–94.5) (old) vs. 122.5 ml/min (82.6–162.4) (young), p = 0.028]. The ratio of Ccr/CcrWC was 0.9 in the elderly vs. 1.26 in the young (p = 0.014), indicating net creatinine reabsorption in the elderly and net creatinine secretion in the young. Our findings indicate that there seems to be a net reabsorption of creatinine in the renal tubules of healthy old persons.

Furosemide test is a simple and useful test of renal physiology used to evaluate the capability of the collecting ducts to secrete potassium under the effect of serum aldosterone. Its behaviour pattern has been established in children and young adults but not described in very old healthy people, which we explored in this study.Twenty-six healthy volunteers on a standard Western diet (50 mmol of K/day) were studied: 20 of them were young (between 17 and 40 years old) and the rest were very old (between 75 and 85 years old). They suffered from no diseases and were not on any medication. Before, during the test and 180 min after a single dose of intravenous furosemide (1 mg/kg), urine and blood samples were obtained for creatinine and electrolytes levels. From these data we calculated fractional excretion (FE) of electrolytes; serum aldosterone was measured pre and post furosemide infusion. Statistical analysis was performed by applying Student's t-test.There was no significant difference regarding pre-furosemide (basal) FE of potassium between the very old and young group. Post-furosemide average FE of potassium was significantly lower in the very old group (27.4 +/- 2%) compared with the young group (35.4 +/- 9%) (P = 0.04). Even though there was no significant difference in post-furosemide peak FE of potassium value, it was reached later in the very old (120 min) compared with the young (30 min). Serum aldosterone levels were significantly higher post furosemide in both groups: 18.3 +/- 12.2 ng/dl (pre) versus 32.5 +/- 18.6 ng/dl (post) in the young (P = 0.007) and 69.8 +/- 13.7 ng/dl (pre) versus 113.3 +/- 54.8 ng/dl (post) in the very old (P = 0.04). Furthermore, all serum aldosterone values (pre and post furosemide) were significantly higher in very old people compared with young people (P0.001). Basal fractional excretion of sodium and chloride were slightly higher in the very old group compared with the young group (P = 0.05). Average post-furosemide FE of sodium and chloride were slightly and significantly lower in the very old (P = 0.05 and P = 0.03), respectively. However, there was no significant difference in peak post-furosemide FE of sodium and chloride values, which were reached later in the very old (120 min) compared with the young (30 min).Furosemide test showed a significantly lower average post-furosemide FE of potassium value, delayed post-furosemide peak FE of Na, K and Cl and a hormonal pattern of aldosterone resistance in very old people.