In vitro effects of thyroid hormones on red blood cell Ca++ -dependent ATPase activity

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Abstract
Thyroid hormones (TH) have been shown to exert a direct stimulatory effect on the Ca++-dependent ATPase from human and other mammalian erythrdcytes. In this in vitro system, T4 has been shown to be more effective than T3. In the present study, TH effects on Ca++-dependent ATPase were investigated, using rabbit and human erythrocyte membranes, after preincubation with 10−10M T4, in the presence or in the absence of exogenous calmodulin (CaM) (5.10−12M to 5.10−9 M). Ca++-dependent ATPase activity was measured as inorganic phosphate (Pi) release from 1 mM ATP. The results showed that basal Ca++-dependent ATPase activity in rabbits was moderately increased by T4 (1.44 ± 0.05 vs 1.32 ± 0.04 μmol Pi/mg protein/90 min, mean ± SE; p4 during the first hour of incubation. The effect of T4 became apparent, however, 1 h after the addition of ATP (ΔT4: 15%). With human membranes, T4 induced a relative stimulation of the Ca++-dependent ATPase of 8–10% (p4 had no effect. These results confirm that Ca++-dependent ATPase activity is increased by T4. The effect of T4 is small, and appears as a late event during incubation with ATP. Stimulation by T4 is expressed in states of low enzyme activation by CaM. In conclusion our results do not favor a direct stimulation of Ca++-dependent ATPase by T4, but suggest an indirect effect on the red blood cell membrane.