Hypotonic-induced transport pathways in Xenopus laevis erythrocytes: taurine fluxes

Abstract

Taurine fluxes in Xenopus laevis red cells were studied in vitro in media of different tonicities. Both influx and efflux increased 3–10 times reversibly when dilution of the medium exceeded 30%. The absolute values of uptake ranged between 5 and 30 μmol/l cells·h at extracellular taurine concentration of 1 mmol/l, but is poorly selective as almost the same uptake was measured for choline and sucrose. Q 10 of 2.77 and an activation energy of 71.90±7.37 kJ/mol were calculated for the uptake process. Taurine uptake was reduced 50% in the absence of Cl −, whereas the alkali cations (Na +, K +, Li + and Rb +) supported it similarly. Taurine uptake was greatly increased in Ca 2+-free solution, and was inhibited by alkaline pH. The inhibitor of anion exchange protein, 4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid (IC 50=25 μM) and the Cl − channel blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid and [(dihydro-indenyl) oxy] alkanoic acid (IC 50<20 μM) inhibited taurine uptake effectively. Isoproterenol did not affect taurine uptake in isotonic, nor in hypotonic solution. The uptake was reduced slowly to near the original, control level within 15–30 min in hypotonic solutions, indicating deactivation of the hypotonic-induced taurine pathway.