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.