Volume-dependent K+ and Cl- fluxes in rat thymocytes

Hypotonic stress unmasked inward and outward K⁺ and Cl^- movements in rat thymocytes. This KCl flux stimulation was reduced by DIOA (dihydroindenyl-oxyalkanoic acid), but not by DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonate), quinidine, DPAC 144 (5-nitro-2-(2-phenylethyl-amino)-benzoic acid), bumetanide or ouabain. In isotonic media (308 ± 5 mosmol kg^-1, the cells exhibited the following DIOA-sensitive fluxes: (i) a K⁺ efflux of 42.7±17.1 mmol (1 cells. h)^-1 (mean ± S.D., n = 7), (ii) a Cl^- efflux of 68 ± 21 mmol (1 cells. h)^-1 (n = 3), (iii) a Rb⁺ influx of 9.7±3.9 mmol (1 cells.h)^-1)) (n=6) and (iv) a Cl^-1)flux of 9.4±4.1 mmol (1 cells.h)^-1 (n =6). Hypotonic shock (183-200 mosmol kg-l) induced a sevenfold stimulation of DIOA-sensitive K⁺ and Cl^- effluxes and a twofold stimulation of DIOA-sensitive Rb⁺ and Cl^- influxes (with a Rb⁺ to Cl^- stoichiometry of 1.04±0.31; mean±S.D., n=6). The DIOA-sensitive membrane carrier catalysed net outward KCl extrusion (the outward/inward flux ratio was 5-7 in isotonic media and 2O in hypotonic media at 189 mosrnol kg-l). Inhibition of DIOA-sensitive ³⁶CI^- efflux by cell K⁺ depletion suggested coupling of outward K⁺ and Cl^- fluxes. Conversely, inward K⁺ and Cl^- fluxes were found to be uncoupled in NO₃ ^- media and in K⁺-free media. The results clearly show that rat thymocyte membranes possess a 1:1 K⁺-Cl^- co-transport system which is strongly activated by hypotonic shock and catalyses net KCl extrusion.