Kinetic study of the tubular dopamine outward transporter in the rat and dog kidney

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Abstract
1 The present study has determined the kinetic characteristics of the outflow of dopamine of renal origin in slices of rat and dog renal cortex loaded with exogenous l‐dihydroxyphenylalanine (l‐DOPA 5 to 5000 μm). 2 In both dog and rat renal tissues the production of dopamine was found to be dependent on the concentration of l‐DOPA used and reached its maximum at 2500 μm l‐DOPA. The decarboxylation of l‐DOPA in rat cortical slices (16.4 ± 2.6 to 1479.2 ± 85.2 nmol g−1) was 6 fold that in the dog (2.2 ± 0.4 to 252.1 ± 21.2 nmol g−1). In the rat kidney a large amount (approximately 50%) of the dopamine (5.2 ± 0.6 to 743.4 ± 58.3 nmol g−1) was found to escape into the incubation medium, whereas in dog renal slices the amount of newly‐formed dopamine escaping into the incubation medium (0.7 ± 0.2 to 46.5 ± 9.3 nmol g−1) was less than 25% of the total amount of the amine formed. 3 The application of the Michaelis‐Menten equation to the net transport of newly‐formed dopamine has allowed the identification of a saturable (carrier‐mediated transfer) and a non‐saturable component (diffusion). The Vmax (nmol g−1 15 min−1) and Km (nm) values for the saturable component were, respectively, 340 ± 41 and 396 ± 45 in the rat kidney and 112 ± 16 and 319 ± 35 in the dog kidney. In both rat and dog renal tissues, the magnitude of the non‐saturable component was found to be of minor importance up to a concentration of 250 nmol g−1 of dopamine to be transported. At high concentrations of the amine (greater then 250 nmol g−1), only attainable in rat kidney slices, most of the dopamine was found to leave the compartment where the synthesis did occur through a non‐saturable transport system. 4 In conclusion, the results presented here show that the outflow of newly‐formed dopamine in both dog and rat kidney slices loaded with exogenous l‐DOPA follows Michaelis‐Menten kinetics with a saturable component and a non‐saturable one, the latter assuming particular importance only at higher concentrations of the amine.