Although plasma levels of l-dopa are derived substantially from catecholamine-synthesizing tissues, melanocytes-which produce l-dopa as part of the melanin synthetic cascade-also may be a source of circulating l-dopa. We compared plasma l-dopa levels in albino subjects and in Caucasian and Black normal volunteers and patients with essential hypertension. DOPA levels were similar among the subject groups. Among Caucasian normal volunteers, l-dopa levels were negatively correlated with subject age (r = -0.30, P < 0.05), whereas norepinephrine levels tended to increase with subject age (r = 0.25, 0.05 < P < 0.10), so that the l-dopa: norepinephrine ratio was highly negatively correlated with subject age (r = -0.50, P < 0.01). Skin pigmentation does not contribute importantly to plasma l-dopa levels in humans. In contrast with levels of norepinephrine, l-dopa levels appear to decrease during normal aging.
- Sympathetic nervous system