The adrenal glands as suppliers of plasma L-Dopa and sources of urinary dopamine.

Dopamine (DA) is a natriuretic hormone synthesized in the kidneys by conversion of filtered 3,4-dihydroxyphenylalanine (L-Dopa), and is activated during hypervolaemia and increased dietary sodium intake. The natriuretic activity of endogenous DA is controversial, however, and the regulation of renal DA synthesis has yet to be explained. It has been suggest that the adrenals may be major suppliers of plasma L-Dopa on the basis of their catecholamine biosynthesis. A study was conducted in rats to elucidate the role of the adrenal glands as dynamic suppliers of L-Dopa to plasma, and thereby as sources of urinary DA. Adrenal venous and systemic arterial plasma concentrations and urinary excretion of L-Dopa, DA and sodium were measured before and during acute isotonic volume expansion (VE; 5% of body weight). One group of animals were acutely adrenalectomized (ADX group) to elucidate the ultimate importance of the adrenals in VE-induced renal sodium and DA excretion. In intact animals, the L-Dopa concentration was 62% higher in adrenal venous than in systemic arterial plasma under control conditions, and 42% higher during VE. The adrenaline concentration was 65 times higher in adrenal venous than in systemic arterial plasma before VE and 56 times higher during VE. The L-Dopa concentration in systemic arterial plasma and the urinary L-Dopa excretion were similar in intact and ADX animals. In intact animals, renal sodium and DA excretion during VE increased more than 13-fold and by 42%, respectively. The corresponding values in ADX animals did not differ from those in the intact animals (more than 14-fold and 36%, respectively). It is concluded that the adrenal glands are only minor suppliers of plasma L-Dopa and minor sources of urinary DA. The regulation of plasma L-Dopa remains to be explained.