The investigators' overarching goal is to improve long-term outcomes for end stage renal disease (ESRD) patients. In this study we focus specifically on patients receiving peritoneal dialysis (PD). Volume regulation in PD patients is related to hypertension, heart failure, nutritional status, and survival. Salt (NaCl) is the body's ion transport target to normally regulate volume via the kidneys; however, in hemodialysis (HD) patients the dialyser or in PD patients the peritoneal membrane, must serve that purpose. Determining volume status in PD patients is not easy and monitoring sodium (Na+) is more difficult still. The investigators have developed a novel, noninvasive approach to this problem involving 23Na+ magnetic resonance imaging (Na-MRI). Na+ is stored bound to proteoglycans in mostly the skin. Our technique measures Na+ in skin and skeletal muscle. In this study, we propose to apply this novel technique to PD patients.

Aim 1. To determine Na+ stores in PD patients, to compare Na+ stores to normal controls using Na-MRI technique, and to correlate Na+ stores by Na-MRI with multifrequency bioimpedance measurements and cross-sectional clinical data.

Hypothesis: Na+ stores are increased in PD patients compared to normal controls; they are increased in PD patients with volume expansion and in those patients with high soluble vascular endothelial growth factor receptor-3 (sFlt-4) levels.

Aim 2. To determine the utility of Na-MRI as an assessment of preserving residual renal function in PD patients.

Hypothesis: Extracellular volume expansion as measured by multifrequency bioimpedance was found to have no utility in predicting preservation of residual renal function in PD patients. The investigators hypothesize that Na+ stores as determined by 23Na-MRI will fulfill that function and will be inversely, rather than directly, related.

Inclusion Criteria:

Both subject groups:

• Age 18 to 80 years;
• BMI < 40;
• Ability to give informed consent;
• Life expectancy greater than 6 months.

PD subjects:

• On peritoneal dialysis for greater than 3 months;
• Using glucose lactate-buffered PD solutions with consistent glucose exposure;
• Stable peritoneal prescription (Kt/V > 1.7 or Tccr > 50 ml/week/1.73 m2).

Control subjects:

• Estimated glomerular filtration rate (GFR) ≥ 60 ml/min/1.73m2;
• No proteinuria.

Exclusion Criteria (both subject groups):

• Pregnancy;
• Intolerance to study protocols;
• Severe, unstable, active, or chronic inflammatory disease (congestive heart failure-NY Class IV, active infection, active connective tissue disorder, active cancer or cancer history in the prior 5 years, HIV, liver disease, including active chronic hepatitis B or C);
• Active inflammatory conditions [systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), minimal change disease (MCD)];
• Patients prescribed or being treated with spironolactone;
• History of cirrhosis;
• Poor compliance with dialysis prescription.

Patients on PD and healthy controls