APICAL AND BASAL UPTAKE OF L-DOPA AND L-5-HTP AND THEIR CORRESPONDINGAMINES, DOPAMINE AND 5-HT, IN OK CELLS

The present study defined the kinetic characteristics of L-3,4-dihydro xyphenylalanine (L-dopa) and L-5-hydroxytryptophan (L-5-HTP) transport and their decarboxylation products, dopamine and 6-hydroxytryptamine (5-HT), respectively, in OK cells. The apical uptake of both L-dopa an d 5-HTP was temperature dependent and stereoselective. Nonlinear analy sis of the saturation curves revealed for L-dopa and L-5-HTP Michaelis constants of 13.8 and 29.1 mu M, 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 mu M. The accumulation of L-dopa and L-5-HTP from the basolateral cell border was dependent on the concentration used and s aturable at nearly 50 mu M. The accumulation of dopamine and 5-HT was found to be saturable only when the substrates were applied from the b asolateral cell border. These results demonstrate that the L-dopa and L-5-HTP transporters in OK cells are located in both the apical and ba solateral cell borders, whereas the uptake of dopamine and 5-HT is a s aturable process only when the substrates are applied from the basolat eral cell border.