A possible correlation between ATPase activity and Na content of embryonic chick heart

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Abstract
The water content of embryonic chick ventricles decreases gradually from 88% at 2 days of incubation to 83% shortly after hatching. Ventricular protein nitrogen decreases during early development and then increases rapidly at midfetal age until hatching, after which a more gradual increase continues. Magnesium-activated ATPase activity of whole ventricle homogenates at pH 7.0–7.5 is much greater than is the calcium-activated enzyme throughout development. A very marked increase in Mg-ATPase occurs between 4 and 7 days of incubation; a lesser increase occurs just prior to and after hatching. Calcium-ATPase increases gradually throughout development. Magnesium-ATPase has been identified with both isolated mitochondrial and microsomal fractions; Ca-ATPase is exclusively associated with the myosin fraction. Myokinase occurs in the supernatant. Optimal pH's are given for the enzymes. It is suggested that the initial rapid fall in ventricular Na content between 2 and 7 days of incubation is directly related to two events: disappearance of the cardiac jelly and establishment of an effective Na extrusion mechanism.