Salt (NaCl) intake is implicated in causing hypertension and cardiovascular disease, the commonest cause of death worldwide. The investigators recently established that Na+ is stored in tissues, bound to glycosaminoglycans (GAGs) in skin and muscle. The resulting local hypertonicity leads to immune cell-driven induction of local tissue electrolyte clearance via modulation of cutaneous lymph capillary density. To visualize these complex processes in man directly, the investigators established Na+ magnetic resonance imaging (23Na-MRI) and investigated Na+ stores in hemodialysis (HD) patients. Hemodialysis patients were sodium-"overloaded" and HD treatment lowered tissue Na+ stores in this study. The observed effects were highly variable and independent of Na+ or water removal from the body during a dialysis session. Tissue Na+ mobilization correlated with circulating vascular endothelial growth factor-C (VEGF-C). The investigators believe that excessive Na+ storage is a reversible condition and therefore susceptible for therapeutic interventions. The investigators hypothesize that lowering dialysate Na+ concentration may favorably affect accelerated tissue Na+ accumulation in hemodialysis patients. Besides, improved tissue Na+ storage, osmostress-induced as well as pro-inflammatory immune cell response should be affected by such a revised dialysis management.

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.

Inclusion Criteria:

• Chronic Kidney Disease Stage 5D, hemodialysis performed for at least 6 months, three times hemodialysis per week, signed informed consent

Exclusion Criteria:

• Pregnancy, severe heart failure (NYHA III - IV), severe liver disease (CHILD C), acute infection, pacemaker or other non-MRI suitable conditions, hyponatremia < 132 mmol/l