Alterations of mechanical parameters in chemically skinned preparations of rat myocardium as a function of isoenzyme pattern of myosin.

Myofibrillar ATPase activity, maximum unloaded shortening velocity, and isometric tension development were evaluated in left ventricular preparations of 5-week-old rats with a high endogeneous level of thyroid hormones and hypothyroid rats after 4-week treatment with propylthiouracil (PTU). The range of possible alterations of the above functional parameters was defined in relation to myosin isoenzyme distribution. The mechanical behaviour of the ventricular preparations was investigated in native myocardium as well as in the glycerinated state. The essential results of the present study is that alterations of myofibrillar ATPase activity and mechanical Vmax, evaluated in glycerinated preparations, are limited to a well-defined range of similar magnitude for both functional parameters: 32-40% of maximum values (obtained from rat myocardium with homogeneous myosin V1). Isometric tension was only insignificantly decreased in glycerinated preparations of the PTU-treated group. The alteration in the apparent maximum shortening velocity of native myocardium (V0) was of the same magnitude as changes in Vmax of chemically skinned preparations. Physical training revealed a shift in the direction of V1-type myosin with increased ATPase activity and shortening velocity; aging and pressure overload showed an opposite effect. The documented mechanical alterations do not contradict an adaptational interpretation of the myosin isoenzyme redistribution in pressure-induced hypertrophy.