Effect of water loading on enzymuria and cellular content of urine in normal human subjects

We have studied urinary N-acetyl-beta-D-glucosaminidase (NAG), lactate dehy drogenase (LDH), glutamate dehydrogenase (GDH) and alkaline phosphatase (AL P) activities and the cellular content of urine in normal healthy individua ls over a 3-h period following hydration. Diuresis was induced progressivel y by an oral water load and the subjects were initially hydrated with 7.5 m l/kg of water (prehydration) and 1 h later with a further 15 ml/kg water (r ehydration). Measurements were made before any hydration procedure, 1 h aft er prehydration and over 3 h following rehydration. The excretion rates of all the enzymes investigated were significantly elevated upon hydration whe n the activity was expressed relative to the flow rate (U/h) or urinary cre atinine concentration (U/mg CR). However, if the enzyme activities were exp ressed in terms of concentration, (U/l) then LDH and NAG activities fell si gnificantly with increased urinary flow but those of ALP and GDH were surpr isingly unchanged. There was a high correlation between urinary enzyme acti vity expressed per mg creatinine and the urine flow rate. The activity of a ll four urinary enzymes depended on the urinary flow rate, but in the case of LDH and NAG part of the activity appeared to be independent of flow rate . These findings demonstrate that the most appropriate way to express the a ctivity of some urinary enzymes is in terms of concentration (U/l) rather t han flow rate (U/h). Investigators using these enzymes as markers of renal injury should bear in mind the effects of urine flow rate on these physiolo gical markers. Med Sci Res 27:541-544 (C) 1999 Lippincott Williams & Wilkin s.