Respiratory Enzymes in the Heart and Liver of the Prenatal and Postnatal Rat.

1. Heart and liver tissue samples were obtained from rats in various developmental stages from the 12-day-old embryo to the 120-day-old postnatal animal. 2. The body, heart and liver weights and percentage protein in the liver and heart of the prenatal and postnatal rat were determined. 3. The activities of NADH-, NADPH- and succinate-cytochrome c reductases and cytochrome oxidase were determined also. 4. The specific activities of all the enzymes increased in both heart and liver during late foetal development (16 days to term). The NADH- and succinate-cytochrome c-reductase activities in the heart increased threefold during the neonatal period (0 to 25 days post partum) and then remained constant to 120 days. All reductase activities increased in the liver three- to six-fold during the neonatal period. Cytochrome-oxidase activity in both tissues increased sixfold during this time but plateaued in the liver at 12 days rather than 25 days. 5. A sex difference was observed in NADH-cytochrome c-reductase activity in the liver. Up to 25 days post partum the activity was the same in both sexes, but from that time on the activity continued to increase in the female but remained unchanged in the male. 6. NADPH-cytochrome c-reductase activity increased only in the liver. 7. These results indicate that different electron-transport pathways predominate according to the tissue, developmental stage and sex of the animal.