title: Effects of thyroxine pretreatment and calcium on the isolated perfused rat heart.
abstract: Phosphorylase a activity was the same in isolated perfused hearts from euthyroid and thyroxine-pretreated rats. Perfusion with 3.6 mM Ca2+ caused an increase in phosphorylase a in hearts from euthyroid as well as those from thyroxine-pretreated animals, but the Ca2+-induced stimulation of phosphorylase activity was similar in both groups over the time course studied. Greater conversion of phosphorylase b to a occurred with 7.2 mM than with 3.6 mM Ca2+ in both groups, but once again thyroxine pretreatment did not significantly influence the conversion of phosphorylase b to a. Isometric systolic tension increased in response to 3.6 mM and 7.2 mM Ca2+ in hearts from normal and thyrotoxic rats, but thyroxine pretreatment did not appreciably alter the nature of this response. While spontaneous heart rate was higher in hearts from thyroxine-pretreated rats, perfusion with 3.6 mM or 7.2 mM Ca2+ had no significant effect on heart rate in hearts from euthyroid or thyrotoxic rats.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D006321:Heart / Q000187:drug effects*; D006339:Heart Rate / Q000187:drug effects; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000201:enzymology; D006005:Phosphorylases / Q000378:metabolism; D051381:Rats; D013974:Thyroxine / Q000494:pharmacology*
chemical_list: D006005:Phosphorylases; D013974:Thyroxine; D002118:Calcium