Therefore, in the present study we performed an intervention study in patients with CKD stage III-IV and healthy control subjects during baseline conditions and in response to an acute 3% hypertonic saline infusion to test the following hypothesis; 1) the tubular handling of sodium and water is different in patients with CKD compared to healthy subjects and 2) Different sodium and water handling by the kidney is reflected by varying changes in urinary excretion of AQP2, NKCC2 and/or ENaC. The gene discussed is AQP2; the disease is chronic kidney disease.