Modulation of Tissue Sodium in Hemodialysis Patients

To evaluate effects of moderate reduction of dialysate Na+ concentration on tissue Na+ content the investigators intend to recruit 40 hemodialysis patients, who will be offered a therapeutic change of their dialysate Na+ concentration. After detection of tissue Na+ content using 23Na-MRI technique, the applied dialysate \[Na+\] will be initially increased stepwise by 2 mmol/l per week from 138 to 142 mmol/l and maintained for a period of 5 weeks. After another 23Na-MRI measurement, dialysate \[Na+\] will then be lowered stepwise by 1-2 mmol/l per week to a minimum of 135 mmol/l, which will be also maintained for a period of 5 weeks followed by a final 23Na-MRI assessment. Hypothesis: Reduction of dialysate Na+ concentration will decrease tissue sodium storage. Additionally, the investigators will assess changes in body fluid distribution by bioimpedance spectroscopy. Furthermore, vascular compliance in response to the modulation of dialysate \[Na+\] and its correlation with tissue Na+ will be assessed. To investigate the immune response to tissue Na+ accumulation, the osmostress-induced as well as pro-inflammatory immune cell response of isolated monocytes will be quantified.