AJP - Renal AJP: Endocrinology and Metabolism
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Am J Physiol Renal Physiol 272: F632-F639, 1997;
0363-6127/97 $5.00
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AJP - Renal Physiology, Vol 272, Issue 5 632-F639, Copyright © 1997 by American Physiological Society

ARTICLES

Apical and basal uptake of L-dopa and L-5-HTP and their corresponding amines, dopamine and 5-HT, in OK cells

M. A. Vieira-Coelho and P. Soares-da-Silva
Institute of Pharmacology and Therapeutics, Faculty of Medicine, Porto, Portugal.

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.


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