title: Mechanical properties and ATPase activity in glycerinated cardiac muscle of hyperthyroid rabbit.
abstract: Isometric tension, tension transients in response to rapid step stretches in length and ATPase activity were measured at constant levels of various Ca2+ activations in glycerinated right ventricular papillary muscle of L-thyroxine-treated (14 daily injections of 0.2 mg/kg) and control rabbits. The isometric tension increased sigmoidally as Ca2+ was varied from slightly below pCa 7 to about pCa 6 both in thyroxine-treated and control preparations. The maximum isometric tension in thyroxine-treated preparations, however, was only about 66% of that in control. The tension transients were characterized by clear three distinct phases; the first phase of an immediate tension increase coincident with the stretch, the second phase of a rapid quasi-exponential tension decrease and the third phase of a delayed quasi-exponential tension rise. In thyroxine-treated preparations, relative to controls, the time for 63% tension reduction in the second phase decreased from 39.3 +/- 2.8 ms (mean +/- SD, n = 5) to 20.2 +/- 2.0 ms (p less than 0.001) and the time for 63% tension rise in the third phase decreased from 483.9 +/- 14.3 ms to 298 +/- 15.9 ms (p less than 0.001). The ATPase activity increased in a sigmoid fashion with increasing Ca2+ from slightly above pCa 7 to slightly below pCa 6 both in thyroxine-treated and control preparations. However, the tension cost (ATPase activity/tension) was about two times greater in the thyroxine-treated preparations than in controls.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D005990:Glycerol; D006651:Histocytochemistry; D006980:Hyperthyroidism / Q000201:enzymology*; D007537:Isometric Contraction; D007700:Kinetics; D008297:Male; D009206:Myocardium / Q000201:enzymology*; D011817:Rabbits
chemical_list: D000251:Adenosine Triphosphatases; D005990:Glycerol; D002118:Calcium