THYROID-HORMONES AND ACTIVE CALCIUM-TRANSPORT OF INSIDE-OUT RED-CELL MEMBRANE-VESICLES

Thyroid hormones may influence the active transport of Ca2+ across the cell membrane. To test the physiologic relevance of this mechanism, we used inside-out human red cell membrane vesicles as a model of the cell membrane Ca2+ pump. We monitored by spectrophotometric methods the kinetics of the uptake of Ca2+ in the presence of 10-5-10-10 m thyroid hormones or their analogues. Vesicles freed of calmodulin and protein inhibitor(s) of the Ca2+ pump were also obtained. The results are as follows: (1) Thyroxine inhibits the active Ca2+ uptake; (2) this effect antagonizes that of soluble calmodulin; and (3) triiodothyronine and other analogues of the thyroid hormones are less active than thyroxine. We conclude that the thyroid hormones may influence cell Ca2+ homeostasis by direct action on the Ca2+ pump.