Regulation of Na+ and K+ contents in rat thymocytes

Ca2+ mobilization induced by ouabain in thymocytes involves intracellular and extracellular Ca2+ pools

Immune dysfunction has been reported in hypertensive rats, and circulating levels of ouabain are increased in some experimental models of hypertension. Ouabain is an inhibitor of the Na+/K+-ATPase capable of diverse effects on cells of the immune system, but its mode of action on these cells is still unknown. The levels of cytoplasmic calcium ions play an important role in cell signaling, and ouabain may induce an increase in intracellular calcium indirectly through the Na+/Ca2+ exchanger. The current work examined the possibility that this drug could be exerting its effects on thymocytes through calcium mobilization and an increase in the cytosolic calcium concentration. Intracellular calcium was evaluated by using Balb-c mouse thymocytes loaded with FURA-2. Both intracellular and extracellular calcium pools were mobilized by ouabain (3 to 1000 nmol). The influx of extracellular calcium depended on the Na+/Ca2+ exchanger and on voltage-dependent calcium channels, as it was inhibited by amiloride and benzamil, consistent with the inhibition of the Na+/K+ pump. In addition, the increase of calcium from intracellular stores was extremely rapid. Furthermore, an increase in cytosolic calcium levels was obtained with the combination of ouabain and thapsigargin, which was greater than that seen with either drug alone. Our data suggest that low concentrations of ouabain may be acting on thymocytes through a mechanism different from the traditional inhibition of the Na+/K+-ATPase, as the cytosolic calcium rise was partly dependent on the release from intracellular stores.

Plasma membrane depolarization without repolarization is an early molecular event in anti-Fas-induced apoptosis

The movement of intracellular monovalent cations has previously been shown to play a critical role in events leading to the characteristics associated with apoptosis. A loss of intracellular potassium and sodium occurs during apoptotic cell shrinkage establishing an intracellular environment favorable for nuclease activity and caspase activation. We have now investigated the potential movement of monovalent ions in Jurkat cells that occur prior to cell shrinkage following the induction of apoptosis. A rapid increase in intracellular sodium occurs early after apoptotic stimuli suggesting that the normal negative plasma membrane potential may change during cell death. We report here that diverse apoptotic stimuli caused a rapid cellular depolarization of Jurkat T-cells that occurs prior to and after cell shrinkage. In addition to the early increase in intracellular Na(+), (86)Rb(+) studies reveal a rapid inhibition of K(+) uptake in response to anti-Fas. These effects on Na(+) and K(+) ions were accounted for by the inactivation of the Na(+)/K(+)-ATPase protein and its activity. Furthermore, ouabain, a cardiac glycoside inhibitor of the Na(+)/K(+)-ATPase, potentiated anti-Fas-induced apoptosis. Finally, activation of an anti-apoptotic signal, i.e. protein kinase C, prevented both cellular depolarization in response to anti-Fas and all downstream characteristics associated with apoptosis. Thus cellular depolarization is an important early event in anti-Fas-induced apoptosis, and the inability of cells to repolarize via inhibition of the Na(+)/K(+)-ATPase is a likely regulatory component of the death process.

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 (l cells.h)-1 (mean +/- S.D., n = 7), (ii) a Cl- efflux of 68 +/- 21 mmol (l cells.h)-1 (n = 3), (iii) a Rb+ influx of 9.7 +/- 3.9 mmol (l cells.h)-1 (n = 6) and (iv) a Cl- influx of 9.4 +/- 4.1 mmol (l cells.h)-1 (n = 6). 3. Hypotonic shock (183-200 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 36Cl- 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.