Effect of thyroxine on calcium distribution in rat thymocytes in vitro

Chlorotetracycline (CTC) was used as a fluorescence probe to study the redi stribution of intracellular membrane-bound Ca2+ in the thyroxine (T-4)-trea ted rat thymocytes. Incubation of thymocytes in the presence of 1-100 nM T- 4 for 30 min resulted in a twofold increase in the amount of EGTA-accessibl e plasma membrane-bound Ca2+ as compared to that in the Ca2+-free media. The T-4-induced decrease in CTC fluorescence was more pronounced in the pre sence of respiration and oxidative phosphorilation in inhibitors. The mitoc hondrial Ca2+ pool was shown to increase. The nonmitochondrial Ca2+ pool decreased after 30-min incubation in the pre sence of 1 nM T-4 and increased when 100 nM T-4 was used under the same con ditions. Without incubation, different concentration of T-4 stimulated the decrease in the Ca2+ pool of the endoplasmic reticulum (ER) compared to the control cells, wich was demonstrated using inhibitors of the ER Ca2+-ATP-a se (vanadate, BHQ). Calmodulin blockers (triftazin and R-24) caused a significant decrease (ove r 50%) in CTC fluorescence in the T-4-treated thymocytes. It suggests that T-4 can act as in vitro stimulator of calmodulin-dependent Ca2+ accumulatio n in thymocyte membranes. The results of our experiments with AlF4- suggest that T-4 stimulates the activity of G-proteins by the receptor-mediated me chanism.