Plasma catecholamines in the chronically cannulated sheep fetus: predominance of L-dihydroxyphenylalanine.

Dihydroxyphenylalanine (Dopa) and catecholamines (CA) concentrations were determined in plasma collected from chronically cannulated fetal and maternal sheep from days 125-140 of gestation. Dopa was measured by a radioenzymatic assay, followed by ion exchange chromatography and high performance liquid chromatography. The assay was linear to 2.5 ng Dopa, and its sensitivity was 35-45 pg. Dopamine (DA), norepinephrine, and epinephrine were determined simultaneously by the same radioenzymatic incubation procedure, followed by solvent extraction and two-dimensional thin layer chromatography. The Dopa level in the fetal circulation was 10-25 times higher than that of DA, 5-10 times higher than that of norepinephrine, and 100 times higher than that of epinephrine. Furthermore, Dopa was the only CA that was significantly (P less than 0.05) higher in fetal (3.5-4.5 ng/ml) than in maternal plasma (1-1.5 ng/ml). The data indicate that Dopa is the predominant circulating CA in the sheep fetus. While the physiological importance of this observation is unknown at the present time, fetal Dopa might serve as the source of free DA in fetal urine and/or amniotic fluid.