title: Creatinine reabsorption by the aged kidney.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  METHODS
  Mann-Whitney and Wilcoxon tests were used.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  Our findings indicate that there seems to be a net reabsorption of creatinine in the renal tubules of healthy old persons.
authors.affiliation: Nephrology Division, Hospital Italiano de Buenos Aires, Buenos Aires, Argentina. carlos.musso@hospitalitaliano.org.ar
authors.forename: Carlos G
authors.initials: CG
authors.lastname: Musso
authors.forename: Hernán
authors.initials: H
authors.lastname: Michelángelo
authors.forename: Manuel
authors.initials: M
authors.lastname: Vilas
authors.forename: Juliana
authors.initials: J
authors.lastname: Reynaldi
authors.forename: Bernardo
authors.initials: B
authors.lastname: Martinez
authors.forename: Luis
authors.initials: L
authors.lastname: Algranati
authors.forename: Juan F
authors.initials: JF
authors.lastname: Macías Núñez
mesh_terms: D000042:Absorption; D000328:Adult; D000367:Age Factors; D000368:Aged; D003404:Creatinine / Q000378:metabolism*; D005260:Female; D006801:Humans; D007668:Kidney / Q000378:metabolism*; D008297:Male; D011446:Prospective Studies; D055815:Young Adult
chemical_list: D003404:Creatinine