This study is being replaced by another protocol.

One goal of this study is to determine whether people with different amounts of dopamine-beta-hydroxylase (DBH) have different responses to events that affect blood pressure and heart rate. We will also study whether increasing dietary salt improves symptoms during upright posture. Finally, we will examine whether the dietary sodium level influences serum DBH activity and whether DBH level influences the response to sodium. DBH levels and the function of the sympathetic or involuntary nervous system will be assessed in normal volunteers and in patients with postural tachycardia syndrome (POTS).

Inclusion Criteria:

• Upright plasma norepinephrine > 600 pg/ml, or
• An increase in heart rate of > 30 beats per minute upon standing, and
• Withdrawal of all medications for at least 3 days, and
• No associated medical illness, and
• Age 18 - 60 years old.

Exclusion Criteria:

• Older than 60 years of age
• Routinely taking medications affecting the autonomic nervous system
• Any chronic illness (cardiac, pulmonary, endocrine, gastrointestinal, rheumatologic) other than POTS
• Anemia (Hct < 30)
• Women of childbearing age who are pregnant or nursing
• Unable to give informed consent