Myocardial cell heterogeneity in the human heart with respect to myosin ATPase activity.

The PH sensitivity of the Ca2+-activated myosin ATPase in atrial, ventricular and conduction tissue of human hearts has been established. Heterogeneity with respect to ATPase activity is shown not only to exist between the atrial, the ventricular myocardium and the conduction system but also within both the ordinary atrial and ventricular myocardium and within the conduction system. These observations are related to the polymorphism of the myosin molecule and suggest that fibre types with different contractile properties co-exist in the human heart.