Mechanical properties and ATPase activity in glycerinated cardiac muscle of hyperthyroid rabbit

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.