Free and total dopamine in human plasma: effects of posture, age and some pathophysiological conditions

Although dopamine (DA) has an important role also outside the central nervous system, the physiological significance of DA in the peripheral plasma is not clearly known. In the present study, we assayed the levels of free and total (free plus conjugated) DA in the human plasma to examine the effect of posture, age and some pathophysiological conditions. Nine healthy volunteers were subjected for head up tilt study. For the examination of the effect of age and some pathophysiological conditions, venous blood samples were taken after overnight fast in 64 control subjects who have no obvious disease and patients who had essential hypertension or congestive heart failure (CHF). The plasma free adrenaline (AD), noradrenaline (NA) and DA were determined by a HPLC-diphenylethylenediamine method and total DA was determined by the same HPLC method after acid hydrolysis. After tilting neither free nor total DA changed significantly despite the increased plasma AD and NA levels. Plasma free NA, DA and total DA levels were correlated positively with age in control subjects but not in the hypertensive nor CHF patients. Patients with CHF showed higher levels of plasma free AD, NA and DA and freeDA/totalDA ratio than did in control subjects. We suspect that plasma DA increased with age partially due to change in renal catecholamine handling and cardiac function.