In vitro effect of hydrocortisone on calcium distribution in rat thymocytes

Chlorotetracycline (CTC) was used as a fluorescence indicator to investigate the intracellular membrane-bound Ca2+ redistribution in the hydrocortisone (HC)-treated rat thymocytes. The effect of HC (0.1-1 microM) on the thymocytes incubated in Ca2+ containing media for 60-120 min increased 1.5-fold the quantity of membrane-bound Ca2+ in the plasma membrane. When thymocytes were incubated in Ca(2+)-free media, no HC-induced changes in the Ca2+ pool of plasma membrane were revealed. In the presence of inhibitors of respiration and oxidative phosphorylation, the CTC fluorescence intensity in the HC-treated cells decreased to a greater extent than in the control, thereby indicating an increase in the mitochondrial Ca2+ pool. Using inhibitors of endoplasmic reticulum Ca(2+)-ATPase (vanadate, BHQ). HC was shown to decrease the non-mitochondrial Ca2+ pool. Calmodulin blockers (triphthasine and R24) slightly decreased the CTC fluorescence intensity in the HC-treated cells as compared to the control. HC was found to inhibit the calmodulin-mediated Ca2+ accumulation in the thymocyte membrane. Based on the experiments using AlF4, we concluded that HC stimulates the activity of G-proteins through the receptor-mediated mechanism. A possible role of the observed Ca2+ redistribution between the thymocyte intracellular compartments, specifically between mitochondria and reticulum, is discussed.