title: Rabbit myocardial membrane Ca2+-adenosine triphosphatase activity: stimulation in vitro by thyroid hormone.
abstract: 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.
mesh_terms: D000818:Animals; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000037:antagonists & inhibitors / Q000502:physiology; D002462:Cell Membrane / Q000201:enzymology; D005260:Female; D009206:Myocardium / Q000201:enzymology*; D011817:Rabbits; D012508:Sarcolemma / Q000201:enzymology; D013963:Thyroid Hormones / Q000494:pharmacology*; D013974:Thyroxine / Q000494:pharmacology; D014284:Triiodothyronine / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: D002147:Calmodulin; D013963:Thyroid Hormones; D014284:Triiodothyronine; C010642:3,3',5-triiodothyroacetic acid; D000252:Calcium-Transporting ATPases; D013974:Thyroxine