Relationship between urinary excretion of homovanillic acid and norepinephrine metabolites in normal subjects and patients with orthostatic hypotension.

Patients with neurogenic orthostatic hypotension due to multiple system atrophy (MSA) or pure autonomic failure (PAF) excrete lower amounts of homovanillic acid (HVA) than do normal subjects. There is a highly significant correlation between the rates of excretion of HVA and norepinephrine metabolites. The regression line relating excretion of the dopamine and norepinephrine metabolites suggests that about one third of dopamine formed in noradrenergic neurons is converted to norepinephrine and the remainder metabolized, mainly to HVA. About one fourth of urinary HVA appears to be derived from a source independent of norepinephrine; this source is probably brain dopaminergic neurons.