[Effect of hypokinesia on the contractile function of the myocardium]

Rats were subjected to hypokinesia for two months and the contractile function of isolated papillary muscles was studied. Hypokinesia reduced significantly the isotonic contraction velocity which depended on the ATPase activity of the myofibrils; it also reduced the velocity and index of relaxation which depended on the functional capacity of the Ca++-pump of the sarcoplasmic reticulum. The maximum force of isometric contraction determined by the quantity of actomyosine bridges in the myofibrils did not change after hypokinesia. This complex of changes is contrary to that observed in adaptation to exercise, when the velocity of isotonic contraction and relaxation increases while the force of isometric contraction does not change. The possible mechanism of this stability of the contractile force during adaptation and readaptation of the heart is discussed.