Ultrastructural and enzymatic development of fetal guinea pig heart

The activities of some enzymes of glycolysis, the citric acid cycle, and amino acid metabolism have been measured in the fetal guinea pig heart over the last third of gestation and correlated with heart ultrastructural development. There is little change in glycolytic enzyme activity except for a two- to threefold increase in phosphofructokinase activity. Mitochondrial content and enzyme activities are low in the early fetal heart, and, although content is similar in the late fetus and adult, enzyme activities increase twofold postnatally, indicating fetal heart mitochondria are incompletely developed. The activities of aspartate and particularly alanine aminotransferase are low in the fetal heart. Over the last third of gestation the myofibrillar content of the fetal myocyte increases twofold to the adult value by term. Associated with this is a fourfold rise in myofibrillar and sarcoplasmic reticulum Ca2+-ATPase activity. Na+-K+-ATPase activity is similar in the late fetal and adult heart but one-third lower in the early fetal heart.