Maturation and cardiac contractility.

Maturation has been shown to enhance the ability of the myocardium to contract. Whenever developmental changes in the systems that control or modulate myocardial contractility have been sought, they have indeed been found. These include an increase in the amount and organization of the myofilaments, an increase in SR amount, organization, differentiation, Ca-ATPase, phospholamban, and sensitivity to calcium and ryanodine, changes in the sarcolemmal pumps and channels, and changes in the expression of the contractile proteins. The characterization of the control systems that integrate these changes remains to be achieved. The overall gradual increase in myocardial contractility has superimposed on it an acute, sudden increase in ventricular contractility in the hours surrounding birth. In the subsequent neonatal days, the level of inotropy falls, and the reserves in contractility are replenished and expanded. Consequently, disease states that demand an increased use of or negatively affect mechanisms that bring about the neonatal enhancement in ventricular function will have their most malignant effect during the first days of neonatal life.