Taurine transport systems in the ciliate protozoanTetrahymena pyriformis

Summary

Tetrahymena pyriformis cultivated in the presence of 1 mM taurine prior to transfer of the cells to non-nutrient medium express an enhanced capacity for concentrative taurine uptake and for taurine diffusion compared to cells grown without added taurine. The unidirectional taurine influx in taurine-grown cells comprises a saturable component with K_m -257μM, V_max = 21 n-moles · g dry wt⁻¹ sd min⁻¹, and a diffusion component with a diffusion constant of 0.20 ml · g dry wt⁻¹ · min⁻¹. At extracellular taurine concentrations <30μM, 20% of the influx is via the saturable system and 80% is via the diffusion system. 19% of the influx in Na⁺-dependent, Cl⁻-independent, and not inhibitable with structural analogues to taurine, suggesting that the transport system responsible for the saturable component in Tetrahymena is different from the Na⁺- and Cl⁻-dependent taurine translocating system (theβ-system) described in vertebrate cells. The unidirectional taurine influx is reduced by 80% by 1mM DIDS (inhibitor of anion exchange and anion channels) and by 1 mM MK196 (indachrinone, inhibitor of anion channels) indicating that taurine diffusion inTetrahymena is via a channel, which is permanently active and which resembles the swelling-induced “taurine channel” seen in mammalian cells. Taurine influx is stimulated by the forskolin analogue 1,9-dideoxyforskolin and by arachidonic acid, and this stimulation is in both cases sensitive to DIDS and MK196.