Urinary acid excretion in aged subjects

Abstract

Urinary excretion of titratable acid and ammonium was investigated in 50 healthy volunteers in individual decades in subjects aged 20 to 70 years. The examination was made under control conditions and after acute loading with ammonium chloride (0.1 g/kg body weight). Titratable acid excretion calculated per 100 ml of creatinine clearance (UTAV/Ccr · 100) under control conditions increased with age (r = 0.366, p < 0.01). Acute acid loading was associated with a further increase inUTAV/Ccr · 100 which was significant (0.05 to 0.01) in all the examined age groups with the exception of those aged 61–70 years. The changes inUTAV/Ccr 100 paralleled those in fractional phosphate excretion (FEp) both under control conditions and after acid loading (r = 0.591, p < 0.001). Urinary ammonium excretion calculated per 100 ml Ccr (UNH4V/Ccr · 100) under control conditions did not change significantly with age. The increase inUNH4V/Ccr · 100 after acute acid loading (at the time of the lowest urinary pH) was clearly expressed in subjects aged 21–30 years (p < 0.01). In individuals older than 50 years the increase inUNH4V/Ccr · 100 was not significant. Urinary pH under control conditions did not change with age. After acid loading, the average values of urinary pH decreased below 5.0 in all the examined age groups and there was no significant difference between the old and young subjects. The obtained results are in keeping with the assumption that urinary net acid excretion by the aged kidney is predominantly influenced by changes in urinary titratable excretion. The results are compatible with the assumption that the promptness of the aged kidney to respond to an acute acid load is depressed in older subjects. The creation of a high hydrogen-ion concentration gradient is not affected by aging.