The synaptic noradrenaline concentration in humans as estimated from simultaneous measurements of plasma noradrenaline and dihydroxyphenylglycol (DOPEG).

A linear relationship between plasma DOPEG (ordinate) and plasma noradrenaline (NA; abscissa) was found during orthostasis as well as during NA infusion. The slope of the former (slopeR) was 13 times steeper than that of the latter (slopeI). This difference in slopes suggests that the NA concentration in the synaptic cleft is markedly higher than in plasma. The factor (F) characterizing this concentration difference was obtained from F2 = slopeR/slopeI. It amounted to 3.6.