Myocardial mechanics and oxygen consumption in experimental hyperthyroidism

Myocardial mechanics and oxygen consumption were studied in right ventricular papillary muscles taken from cats (N = 15), pretreated with cristaline L-thyroxine (1 mg/kg/day, i.p.) 8-18 days prior to the examination. Isotonic afterload and isometric contractions were employed. Oxygen consumption was determined polarographically. Data obtained were compared with control studies on papillary muscles taken from euthyroid cats (N = 15). In isotonic afterloaded contractions the extent of shortening was nearly identical in both groups. However, maximal rate of isometric tension development and velocity of isotonic shortening were considerably increased in hyperthyroid myocardium (by 48 and 42%, respectively). Myocardial oxygen consumption was significantly increased in hyperthyroidism (by 47%), primarily because of an increased maximal rate of isometric tension development and-to a small extent-increased isotonic contraction velocity.