Autoimmune Basis for Postural Tachycardia Syndrome

Inclusion Criteria: * 18-50 years old * Postural Tachycardia Syndrome: Heart rate increase \>30 bpm from supine within 10 min of standing, in the absence of orthostatic hypotension (\>20/10 mmHg fall in blood pressure), with chronic symptoms (\> 6 months), and in the absence of other acute cause of orthostatic tachycardia. * Able and willing to provide informed consent * Female premenopausal subjects must utilize adequate birth control and willingness to undergo serum beta-hCG testing * The subject must understand and be able to comply with the study procedures and restrictions. Exclusion Criteria: * Hypertension (\>150 mmHg systolic and \>100 mmHg diastolic) based on history or findings at screening. * Orthostatic hypotension (consistent drop in blood pressure \>20/10 mmHg with 10 min stand) * Pregnancy * Cardiovascular disease, such as myocardial infarction within 6 months, angina pectoris, significant arrhythmia (sinus tachycardia is not excluded), deep vein thrombosis, pulmonary embolism * History of serious neurologic disease * History or presence of significant immunological or hematological disorders * Clinically significant gastrointestinal impairment that could interfere with dietary compliance or drug absorption * Impaired hepatic function (aspartate amino transaminase and/or alanine amino transaminase \>1.5 x upper limit of normal range) * Impaired renal function (serum creatinine \>1.5 mg/dL) * Hematocrit \<28% * Current or concurrent disease that could affect the absorption, action or disposition of the drug, or clinical or laboratory assessments. * Any underlying or acute disease requiring regular medication that could possibly pose a threat to the subject or make implementation of the protocol or interpretation of the study results difficult * Inability to comply with the protocol Healthy control subjects will be healthy, non-smoking and on no chronic medications at the time of the study. Healthy control subjects will be group-matched to the POTS patients for age and gender. We will attempt to study female patients in the first half of their menstrual cycle to minimize cyclical variability.