[Effect of thyroid hormones on the calcium pump in sarcoplasmic reticulum]

In vitro thyroxine inhibited accumulation of Ca2+ by fragments of sarcoplasmic reticulum, isolated from rabbit sceletal muscles. The inhibitory effect of thyroxine was responsible for its direct action on Ca2+ dependent ATPase. Half-maximal inhibition of the enzymatic activity occurred at the same concentrations of thyroxine using both membrane-bound and highly purified solubilized forms of Ca2+-ATPase (15--20 micrometer and 10 micrometer of the hormone, respectively). Triiodothyronine was similar in the effect but not diiodothyrosine, which did not possess the hormonal activity. The data obtained suggest that thyroid hormones affect the mechanism of dephosphorylation in ATP-hydrolase reaction.