Knockout mice are important for the elucidation of renal pathophysiology. Gene deletions are frequently lethal during the postnatal development of mice. In such cases, the phenotyping is restricted to the early phase of life when functional maturation of the kidney takes place. Here, we present age-dependent profiles of renal electrolyte excretion in wildtype C57Bl6 mice. Samples were collected during 9 weeks after birth either by puncturing the urinary bladder or from spot urine. Sodium (Na⁺), potassium (K⁺) and magnesium (Mg²⁺) were quantified by atomic absorption spectrometry (Solaar M5, Thermo Scientific), calcium (Ca²⁺) was measured by a colorimetric assay (Biotrend, Köln). Urinary creatinine was quantified according to the Jaffé method. Chloride (Cl^-), phosphate (PO₄^3-) and sulfate (SO₄^2-) were quantified in a different set of samples using an automated ionic chromatography system (IC1600, Dionex).

Adaptation of volume and electrolyte balance in developing mice results in remarkable changes of renal excretion with characteristic profiles of the analyzed ions. Most variations were measured directly after birth and during the third week of life. Analysis and interpretation of excretion data obtained during these critical phases of development should be done very carefully.