Apical and basal uptake of L-dopa and L-5-HTP and their corresponding amines, dopamine and 5-HT, in OK cells.
The present study defined the kinetic characteristics of L-3,4-dihydroxyphenylalanine (L-dopa) and L-5-hydroxytryptophan (L-5-HTP) transport and their decarboxylation products, dopamine and 5-hydroxytryptamine (5-HT), respectively, in OK cells. The apical uptake of both L-dopa and 5-HTP was temperature dependent and stereoselective. Nonlinear analysis of the saturation curves revealed for L-dopa and L-5-HTP Michaelis constants of 13.8 and 29.1 microM, respectively, and maximal velocities of 21 and 29 nmol.mg protein-1.6 min-1, respectively, L-5-HTP inhibited the apical uptake of L-dopa with a half-maximal inhibitory constant of 29.1 microM. The accumulation of L-dopa and L-5-HTP from the basolateral cell border was dependent on the concentration used and saturable at nearly 50 microM. The accumulation of dopamine and 5-HT was found to be saturable only when the substrates were applied from the basolateral cell border. These results demonstrate that the L-dopa and L-5-HTP transporters in OK cells are located in both the apical and basolateral cell borders, whereas the uptake of dopamine and 5-HT is a saturable process only when the substrates are applied from the basolateral cell border.
Vieira-Coelho
M A
MA
Institute of Pharmacology and Therapeutics, Faculty of Medicine, Porto, Portugal.
Soares-da-Silva
P
P
D006916:5-Hydroxytryptophan / Q000378:metabolism*; D000818:Animals; D001545:Benserazide / Q000378:metabolism; D001692:Biological Transport; D016764:Cell Polarity; D002478:Cells, Cultured; D004298:Dopamine / Q000378:metabolism; D005260:Female; D007668:Kidney / Q000378:metabolism*; D007700:Kinetics; D007980:Levodopa / Q000378:metabolism*; D009893:Opossums; D012701:Serotonin / Q000378:metabolism; D014867:Water / Q000378:metabolism; D014882:Water-Electrolyte Balance
D014867:Water; D012701:Serotonin; D007980:Levodopa; D001545:Benserazide; D006916:5-Hydroxytryptophan; D004298:Dopamine