Hypotonicity-activated efflux of taurine and myo-inositol in rat inner medullary collecting duct cells: Evidence for a major common pathway

To further characterize the hypotonicity-activated efflux pathways for the organic osmolytes taurine and myo-inositol in inner medullary collecting duct (IMCD) cells tracer fluxes of taurine and myo-inositol were investigated. The time course of activation of both fluxes after exposure of cells isolated at 600 mosm to a hypotonic medium (300 mosm by omission of sucrose) was identical with a major increase of release within the first 10 min. All 'anion channel blockers' employed proved to be strong inhibitors of both fluxes. Inhibition of myo-inositol efflux by 0.5 mM NPPB and 0.1 mM dideoxyforskolin was not significantly different from that of taurine efflux (87.7+/-11.4 compared to 94.6 +/-4.6% and 98.8+/-2.0 compared to 95.9+/-3.7%). However, SITS (0.5 and 0.01 mM), DIDS (0.5 and 0.01 mM), and niflumic acid (0.5 mM) inhibited myoinositol efflux more strongly than taurine efflux. The respective values were 65.4 +/-4 vs. 42.9+/-3.6% for 0.01 mM SITS, 65.7+/-4.2 vs. 45.8+/-2.0% for 0.01 mM DIDS, and 79.5+/-3.5 vs. 54.2+/-2.5% for 0.5 mM niflumic acid. Taurine as well as myo-inositol efflux were decreased to a similar extent by 10 mM extracellular ATP (26.9+/-6.3 vs. 29.8+/-17.7% inhibition), by 10 mM extracellular cAMP (52.8 +/-9.8 vs. 60.1+/-17.2% inhibition) and by reduction of the intracellular ATP content employing 2-deoxy-D-glucose (31.9+/-5.9 vs. 40.4+/-13.6% inhibition). In polarized primary cell cultures taurine and myo-inositol were released during a hypotonic shock primarily across the basal-lateral membrane, the ratio of basolateral versus apical efflux was 4.1 for taurine and 3.9 for myo-inositol. Apical fluxes were more sensitive to 0.01 mM SITS or DIDS; this was particularly evident for apical myo-inositol efflux which was inhibited by 0.01 mM SITS by 84.1+/-5.9% compared to 43.5+/-13.1% inhibition of the basolateral efflux. Thus, taurine and myo-inositol efflux show to a great extent a similar cellular distribution, intracellular regulation and pharmacological inhibition profile. This similarity suggests that the two osmolytes share an efflux pathway that might be identical with the swelling-activated taurine conductance described previously. Additional minor pathways can, however not be excluded.