Dietary Sodium's Effect on Urinary Sodium and Dopamine Excretion in Patients With Postural Tachycardia Syndrome

Study Day 1 * Start 150 mEq Na+/day diet (POTS patients as inpatients; healthy control subjects with Clinical Research Center(CRC)- provided outpatient diet); consume 1.5-2 liters of water per day * Start a 24hour (24hr) urine collection (for sodium (Na+), potassium (K+), creatinine (Cr), fractionated catecholamines) * Blood work Study Days 2-5 * Continue 24hr urine collection * Start STUDY DIET (10 mEq Na+/day or 300 mEq Na+/day in a random order) after 3 meals of 150 mEq Na+/day are complete; consume 1.5-2 liters of water per day * On Day 5, a 24 hr Holter combined ECG monitor and BP monitor will be placed on the subjects. Study Day 6 * Continue STUDY DIET; consume 1.5-2 liters of water per day * Remove 24hr Holter combined ECG monitor and BP monitor from subject * Continue 24hr urine collection (for Na+, K+, Cr, fractionated catecholamines) * Admit to CRC in afternoon (healthy control subjects only, as POTS patients will have already been admitted). Each subject will spend the night in the CRC and remain supine * Nothing by mouth (NPO) after midnight for study next day Study Day 7 * Awaken early (\~6am) to void (still collecting 24hr urine) * Patient returns to bed, IV catheter inserted * Posture Study (in morning; between 7-8am ideally) * Blood pressure and heart rate will be measured while supine and then while standing for up to 30 minutes * We will draw blood in each body position to measure electrolytes and hormones that regulate blood pressure and blood volume * Subjects will rate symptoms during supine period and at end of stand using Vanderbilt Orthostatic Symptoms Score (VOSS) * Total Blood Volume (DAXOR)- using injection of iodinated I-131 tagged human serum albumin nominally 25 micro-Ci of radiation blood samples drawn through IV catheter before injection and for \~30 minutes post-injection (total - 25 ml) * This will be done after supine assessment, but before standing the subject up * Exercise Capacity Test (in the afternoon) Will estimate maximal oxygen consumption (VO2 max). This test will be conducted on a stationary bicycle. Effort will be gradually increased while expired air is measured during exhaustive physical work. All procedures are repeated at least a month later with the 2nd level of dietary salt. (Randomized to high or low salt in the first phase, the second phase is the remaining level)