Actin from Heart Muscle: Isolation, Purification, and Physicochemical Properties.

• A comparison of the properties of cardiac actin with those of skeletal actin has been undertaken in an attempt to explain several fundamental differences between the behavior of heart and skeletal muscle which have recently been elucidated. Most striking among these is the finding that cardiac muscle develops less tension than its skeletal counterpart. Burton has calculated that the force generated by the human ventricular myocardium is less than 30% that developed by the same cross-sectional area of skeletal muscle, and Abbott and Mommaerts'-' have confirmed this calculation by direct measurement. The latter authors also found that the positive inotropie effect resulting from increased rate of stimulation was not associated with prolongation of the active state in cardiac muscle, in contrast to the prolongation expected in skeletal muscle. The positive inotropic effect of adrenergic agents on mammalian cardiac muscle has been attributed by Brady et al. to an increase in shortening velocity without alteration in the