Excretion and metabolism of dopa and dopamine by isolated perfused rat kidney.

The renal metabolism and handling of dopa and dopamine were studied with the use of the isolated perfused rat kidney to elucidate the source of urinary dopamine. Dopa added to the perfusate was rapidly metabolized, and a significant amount of urinary dopamine excretion was observed, whereas dopa was not detected in the urine. When the perfusate dopa concentration was near the plasma dopa concentration in normal rats, urinary dopamine excretion was comparable with physiological values. Carbidopa, a dopa decarboxylase blocker, added to the perfusate markedly reduced the urinary excretion of dopamine. When the physiological concentration of dopamine was added to the medium, the perfusate dopamine rapidly decreased and urinary dopamine excretion was much less than that seen in normal rats. In studies with nonfiltering kidneys, a marked decrease of dopa and slight increase of dopamine in the perfusate were observed. It is suggested that the process of glomerular filtration and tubular reuptake of dopa is not essential for renal dopamine formation. These data indicate that urinary free dopamine is mainly derived from plasma dopa, which is converted by dopa decarboxylase in the kidney.