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

Patients with Postural Tachycardia Syndrome (POTS) may not adequately expand their plasma volume in response to a high sodium diet. Mechanisms involved in the regulation of plasma volume, such as the renin-angiotensin-aldosterone system and renal dopamine (DA), may be impaired in POTS and may respond inappropriately to changes in dietary sodium. The investigators propose that the changes in urinary sodium and dopamine excretion caused by consuming low-sodium and high-sodium diets will be different between patients with POTS and healthy volunteers. The purpose of this study is to determine (1) whether changes in dietary sodium level appropriately influence sodium excretion in POTS; (2) whether changes in dietary sodium level appropriately influence DA excretion in POTS; (3) whether a high dietary sodium level appropriately expands plasma volume in POTS; and (4) whether patients with POTS have improvements in their orthostatic tachycardia and symptoms as a result of a high dietary sodium level.

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)

Eligibility Criteria:
Inclusion Criteria:

* Premenopausal patients with POTS and healthy volunteers, 18-50 years old, who are non-smokers and free of medications with the potential to influence blood pressure
* Patients diagnosed with postural tachycardia syndrome by the Vanderbilt Autonomic Dysfunction Center
* Patients who Increase heart rate ≥30 beats/min with position change from supine to standing (10 minutes)
* For patients, chronic symptoms consistent with POTS that are worse when upright and get better with recumbence
* Only female participants are eligible. Since 80-90% of POTS patients are female, and there can be differences in measures with the menstrual cycle, including a small number of males might introduce a significant amount of noise.
* Able and willing to provide informed consent

Exclusion Criteria:

* Smokers
* Overt cause for postural tachycardia, i.e., acute dehydration
* Significant cardiovascular, pulmonary, hepatic, or hematological disease by history or screening results
* Positive pregnancy test or breastfeeding
* Hypertension defined as BP\>145/95 off medications when supine or needing antihypertensive medication
* Other factors which in the investigator's opinion would prevent the participant from completing the protocol, including poor compliance during previous studies or an unpredictable schedule
* Unable to give informed consent