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.