Oxidized Glutathione Causes Sensitization Of Calcium Release To Inositol 1,4,5-trisphosphate In Permeabilized Hepatocytes
The effects of GSSG on Ca2+ mobilization by Ins(1,4,5)P3 were studied in permeabilized rat hepatocytes. Incubation with GSSG (2 mM) increased the sensitivity to Ins(1,4,5)P3 for Ca2+ release, with no effect on the size of the Ca2+ pool that could be released with maximal concentrations of Ins(1,4,5)P3. GSSG decreased the EC50 for Ins(1,4,5)P3 from a control value of 578 +/- 23 nM to 137 +/- 21 nM. GSSG had no effect on the metabolism of Ins(1,4,5)P3 in permeabilized cells, and sensitization of Ca2+ release was still observed when the poorly metabolizable analogue inositol 1,4,5-trisphosphorothioate was used. GSSG did not affect the ATP-dependent Ca2+ pump or the extent of loading of intracellular Ca2+ pools. In addition, the enhancement of Ins(1,4,5)P3-sensitivity by GSSG occurred under conditions where the Ca2+ pumps were blocked with thapsigargin or by chelation of medium Ca2+ just before Ins(1,4,5)P3 addition. The effect of GSSG was time- and dose-dependent, maximal effects being observed after 5 min incubation with 2 mM-GSSG. Cystine mimicked the GSSG-induced increase in Ins(1,4,5)P3-sensitivity, and the effects could be reversed by dithiothreitol (DTT). DTT, GSH glutathione and cysteine had no effect when added alone. Other agents known to react with protein thiols, including N-ethylmaleimide, p-chloromercuribenzoic acid and Ag+, did not affect the sensitivity to Ins(1,4,5)P3, but were inhibitors of ATP-dependent Ca2+ uptake. The data suggest that the sensitivity of the intracellular Ca2+ pools to release by Ins(1,4,5)P3 can be modulated by the formation of mixed disulphides with GSSG or other oxidized thiols.