Cell volume regulation in rat thymocytes

DIOA (dihydroindenyl-oxy-alkanoic acid), a potent inhibitor of the K⁺-CI^- cotransport system, fully blocked regulatory volume decrease (RVD) in swelled rat thymocytes, with an IC₅₀ of 2.2±0.5x10^-5 mol l^-1 (mean ± S.D., n = 4). Conversely, RVD was resistant to quinine, quinidine, apamin, cetiedil, amiloride, bumetanide and DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonate). DIOA-sensitive RVD followed mono-exponential kinetics, with t( 1/2 ) (half-lifetime) of 1-3 min and maximal capacity (C(max)) of about 55% of the initial cell swelling. C(max); and the initial rate of RVD (V(o)) were both linear functions of the increase in cell volume. RVD was: (i) slightly increased by replacing external Cl^- by NO₃ ^-, (ii) reversed by replacing external Na⁺ by K⁺ (in the presence of external Cl^-) and (iii) inhibited by cell K⁺ depletion. All these phenomena were blocked by DIOA (86 μmol l^-1). Increased membrane potassium permeability by valinomycin was unable to accelerate RVD or RVD reversal. In the presence of DIOA, thymocytes responded like osmometers (the relative cell volume was a linear function of the reciprocal of the relative osmolality) in a large range of osmolalities. The results strongly suggest that RVD in rat thymocytes is mediated by the K⁺-Cl^- co-transport system.