Bepridil and cetiedil reversibly inhibit thyroid hormone stimulation in vitro of human red cell Ca2+-ATPase activity.
Thyroid hormone (10(-11) to 10(-10) M) stimulates plasma membrane Ca2+-ATPase activity in vitro in various tissues, including the human red cell (RBC), by a calmodulin-requiring mechanism. Bepridil and cetiedil are Ca2+ antagonists with an intracellular (calmodulin-antagonist) site of action, as well as an effect on the calcium channel in excitable tissues. We have studied the actions of bepridil and cetiedil on Ca2+-ATPase in a channel-free membrane (RBC) to determine effectiveness of these agents as inhibitors of thyroid hormone action on the enzyme. Dose-response studies showed that thyroid hormone stimulation of Ca2+-ATPase activity in vitro was significantly inhibited by as little as 2 x 10(-5) M bepridil and cetiedil. IC50 values of bepridil and cetiedil for thyroid hormone response of the enzyme were 5 x 10(-5) and 2 x 10(-5) M, respectively, whereas IC50s of these agents for enzyme activity in the absence of thyroid hormone were both 10(-4) M. Progressive addition of purified rat testis calmodulin in vitro (10-150 ng calmodulin/mg membrane protein) restored hormone responsiveness in the presence of bepridil and cetiedil. Binding of labeled thyroid hormone by RBC membranes was unaffected by bepridil and cetiedil (up to 2 x 10(-4) M). Thus, bepridil and cetiedil are Ca2+ antagonists that reversibly inhibit thyroid hormone action on human RBC Ca2+-ATPase by a calmodulin-dependent mechanism. Thyroid hormone effect on Ca2+-ATPase is more susceptible to bepridil and cetiedil inhibition than is basal enzyme activity.
Dube
M P
MP
Department of Medicine, State University of New York at Buffalo School of Medicine.
Davis
F B
FB
Davis
P J
PJ
Blas
S D
SD
D001381:Azepines / Q000494:pharmacology*; D015764:Bepridil; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002147:Calmodulin / Q000378:metabolism / Q000494:pharmacology; D002462:Cell Membrane / Q000201:enzymology / Q000378:metabolism; D004912:Erythrocytes / Q000187:drug effects / Q000201:enzymology* / Q000648:ultrastructure; D006801:Humans; D011759:Pyrrolidines / Q000494:pharmacology*; D013974:Thyroxine / Q000037:antagonists & inhibitors*
D001381:Azepines; D002147:Calmodulin; D011759:Pyrrolidines; C006955:cetiedil; D015764:Bepridil; D000252:Calcium-Transporting ATPases; D013974:Thyroxine