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 c onversion of filtered 3,4-dihydroxyphenylalanine (L-Dopa), and is acti vated during hypervolaemia and increased dietary sodium intake. The na triuretic activity of endogenous DA is controversial, however, and the regulation of renal DA synthesis has yet to be explained. It has been suggested that the adrenals may be major suppliers of plasma L-Dopa o n the basis of their catecholamine biosynthesis. A study was conducted in rats to elucidate the role of the adrenal glands as dynamic suppli ers of L-Dopa to plasma, and thereby as sources of urinary DA. Adrenal venous and systemic arterial plasma concentrations and urinary excret ion of L-Dopa, DA and sodium were measured before and during acute iso tonic volume expansion (VE; 5% of body weight). One group of animals w ere acutely adrenalectomized (ADX group) to elucidate the ultimate imp ortance of the adrenals in VE-induced renal sodium and DA excretion. I n intact animals, the L-Dopa concentration was 62% higher in adrenal v enous than in systemic arterial plasma under control conditions, and 4 2% higher during VE. The adrenaline concentration was 65 times higher in adrenal venous than in systemic arterial plasma before VE and 56 ti mes higher during VE. The L-Dopa concentration in systemic arterial pl asma and the urinary L-Dopa excretion were similar in intact and ADX a nimals. In intact animals, renal sodium and DA excretion during VE inc reased more than 13-fold and by 42%, respectively. The corresponding v alues in ADX animals did not differ from those in the intact animals ( more than 14-fold and 36%, respectively). It is concluded that the adr enal glands are only minor suppliers of plasma L-Dopa and minor source s of urinary DA. The regulation of plasma L-Dopa remains to be explain ed.