Decreased sympathetic neuronal uptake in idiopathic orthostatic hypotension

The disappearance rates from plasma of intravenously administered levo-norepinephrine (l-NE), dextro-norepinephrine (d-NE), and isoproterenol (ISO) were measured in normal subjects and in patients with either multiple-system atrophy (MSA) or idiopathic orthostatic hypotension (IOH). The two isomers, l-NE and d-NE, were removed at similar rates in all groups. In normal subjects, the d and l isomers of norepinephrine were cleared more rapidly than ISO. In patients with IOH, the initial rates of disappearance of the NE isomers from plasma were slower than normal and similar to the rate for ISO disappearance. Plasma NE levels, NE clearance, and the apparent release rate of NE into plasma from sympathetic neurons were significantly lower in patients with IOH than in normal subjects. Only the apparent NE secretion rate was related to the baseline plasma NE level. Sympathetic neuronal dysfunction in IOH is attended by a reduction in the clearance of NE. The very low plasma NE levels, in association with the striking reduction in NE clearance, suggest that in IOH there is a marked decrease in NE release. NE clearance and apparent NE secretion rate are normal in MSA, consistent with a central nervous system dysfunction in regulating the sympathetic nervous system. Neuronal uptake of NE in humans does not appear to be stereoselective.