Dietary Salt in Postural Tachycardia Syndrome

Study Day 1 * Start 150 milliequivalents (mEq) Na+/day diet (POTS patients as inpatients; healthy control subjects with clinical research center (CRC)- provided outpatient diet) * Start a 24h urine collection (for Na+, K+, Cr, fractionated catecholamines) * Blood work * Blood volume - carbon monoxide rebreathing Study Day 2 * Complete 24h urine * 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; water ad lib Study Day 3 - 5 * Continue STUDY DIET; water ad lib * On Day 5, a 24 hr holter combined ECG monitor will be placed on the subjects. Study Day 6 * Continue STUDY DIET; water ad lib * Remove 24h Holter combined ECG monitor and BP monitor from subject * Start a 24h urine collection (for Na+, K+, Cr, fractionated catecholamines) * Complete questionnaires * Nothing by mouth (NPO) after midnight for study next day Study Day 7 (BIG DAY) * Awaken early (\~6am) to void (still collecting 24h 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 3 tablespoons of blood in each body position to measure hormones that regulate blood pressure and blood volume * Hormones to verify the subject's phase of menstrual cycle * Serum/plasma aliquots for future analysis * 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 * Autonomic Function Test with Cardiac Output and Brief Tilt * The subject will be tilted up to 60-75 degrees head-up tilt for up to 10 minutes to measure the changes in heart rate and blood pressure and symptoms with upright challenge. * Blood volume - carbon monoxide rebreathing * 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 increase 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 to the first phase, the second phase is the remaining level)