Rabbit myocardial membrane Ca2+-adenosine triphosphatase activity: stimulation in vitro by thyroid hormone

The in vitro stimulation of human and rabbit erythrocyte membrane Ca2+-ATPase activity by physiological concentrations of thyroid hormone has recently been described. To extend these observations to a nucleated cell model, Ca2+-ATPase activity in a membrane preparation obtained from rabbit myocardium has been studied. Activity of 5'-nucleotidase in the preparation was increased 26-fold over that of myocardial homogenate, consistent with enrichment by sarcolemma. Mean basal enzyme activity in membranes from nine animals was 20.8 +/- 3.3 mumol Pi mg membrane protein-1 90 min-1, approximately 20-fold the activity described in rabbit red cell membranes. Exposure of heart membranes in vitro to L-thyroxine (T4) (10(-10)M) increased Ca2+-ATPase activity to 29.2 +/- 3.8 mumol Pi (P less than 0.001). Dose-response studies conducted with T4 showed that maximal stimulatory response was obtained at 10(-10) M). Hormonal stimulation was comparable for L-T4 and triiodo-L-thyronine (T3) (10(-10) M). Tetraiodothyroacetic acid was without biological activity, whereas triiodothyroacetic acid and D-T4, each at 10(-10) M, significantly decreased enzyme activity compared to control (basal) levels. The action of L-T4 on myocardial membrane Ca2+-ATPase activity was inhibited by trifluoperazine (100 microM) and the naphthalenesulfonamide W-7 (50-100 microM), compounds that block actions of calmodulin, the protein activator of membrane-associated Ca2+-ATPase. Radioimmunoassay revealed the presence of calmodulin (1.4 micrograms/mg membrane protein-1) in the myocardial membrane fraction and 0.35 micrograms/mg-1 in cytosol. Myocardial Ca2+-ATPase activity, apparently of sarcolemmal origin, is thus thyroid hormone stimulable. The hormonal responsiveness of this calcium pump-associated enzyme requires calmodulin.