Occurrence and characterization of a low-affinity choline uptake mechanism in the internal tissues of the cestode Hymenolepis diminuta.

[3H]Choline was taken up by tissue slices of the cestode Hymenolepis diminuta by sodium-dependent and sodium-independent mechanisms. The sodium-dependent uptake was saturable, against an apparent concentration gradient, and by analysis of the kinetics of uptake could be delineated into a high-affinity choline uptake (HAChU) mechanism (Kt of approximately 2.0 microM; Vmax of approximately 0.15 pmol/mg wet weight tissue/min), and a low-affinity choline uptake (LAChU) mechanism (Kt of approximately 20.0 microM and a Vmax of approximately 2.0 pmol/mg wet weight tissue/min). Unlike the HAChU system, the LAChU system was unaffected by potassium. Furthermore, the responses of the LAChU and the HAChU transporters to pharmacological agents were distinctive, indicating that these transporters are two separate entities. Moreover, the entry or exit of choline via the LACh transporter was dependent upon the direction of the sodium gradient. Furthermore, the majority of released radiolabel from preloaded [3H]choline was associated with choline and was via a proteinaceous transporter. The present study is the first to provide direct evidence for a partial chemosmotic coupling of a LAChU system to the sodium electrochemical gradient.