VOLUME-DEPENDENT K+ AND CL- FLUXES IN RAT THYMOCYTES

1. Hypotonic stress unmasked inward and outward K+ and Cl- movements in rat thymocytes. This KCl flux stimulation was reduced by DIOA (dihydroindenyl-oxy-alkanoic 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. 2. 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 (I cells. h)-1 (mean +/- S.D., n = 7), (ii) a Cl- efflux of 68 +/- 21 mmol (I cells. h)-1 (n = 3), (iii) a Rb+ influx of 9.7 +/- 3.9 mmol (I cells. h)-1 (n = 6) and (iv) a Cl- influx of 9.4 +/- 4.1 mmol (I cells. h)-1 (n = 6). 3. Hypotonic shock (1 83-200 mosmol kg-1) 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). 4. The DIOA-sensitive membrane carrier catalysed net outward KCl extrusion (the outward/inward flux ratio was 5-7 in isotonic media and 20 in hypotonic media at 189 mosmol kg-1). Inhibition of DIOA-sensitive Cl-36- 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. 5. 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.