Levels of plasma (p-) noradrenaline (NA), the sympathetic neurotransmitter, have been used to indicate activity of the neuronal sympathoadrenal component, while adrenaline (Adr) levels indicate activity of the hormonal adrenomedullary component of the sympathoadrenal system (Christensen 1991, Goldstein 1995, Christensen & Norsk 2000). Based upon the absence of an arterio-venous increase in p-DOPA concentration in sympathectomized limbs and a decrease in p-DOPA after inhibition of tyrosine hydroxylase (TH) in dogs, it was concluded that DOPA can pass across sympathetic neuronal membranes to reach the general circulation and furthermore, that p-DOPA may be related to regional rate of tyrosine hydroxylation (Goldstein et al 1987a). P-DOPA only demonstrated minimal changes during stimuli that produced significant changes in p-NA. Due to partly parallel changes of p-NA and p-DOPA, however, it was believed that p-DOPA reflect the rate of catecholamine synthesis and that p-DOPA was a simple and direct index of TH activity in vivo (Eisenhofer et al 1988, Goldstein & Eisenhofer 1988, Garty et al 1989b). It was inferred that p-DOPA levels may be an index of sympathetic activity.
Department of Internal Medicine and Endocrinology, Herlev University Hospital, Herlev.
Correspondence: Ebbe Eldrup, Bolbrovænge 29, DK-2960 Rungsted Kyst.