Mitochondrial activity of rat kidney during ontogeny

The development of oxidative metabolism was studied from the late fetal to adult stages in mitochondria isolated from rat kidney. We used the oxygen consumption rate, as an index of inner membrane activity and citrate synthase and fumarase activities as an index of matrix activity and cytochrome c oxidase activity as an index of the number of mitochondria. Fumarase and citrate synthase activities displayed different developmental patterns, suggesting that these Krebs cycle enzymes did not mature synchronously. In fetal mitochondria, net oxygen consumption measured in the presence of succinate or glutamate as substrate, was low; it increased during the day after birth and reached adult level between days 10 and 15. During this period, the levels of citrate synthase and cytochrome c oxidase activity did not change significantly in the isolated mitochondrial fraction. However, in fetal and adult kidney homogenates, these levels increased four-fold, suggesting a corresponding increase in the number of mitochondria. Most of these increases occurred during the 15 days after birth. These results suggest that in rat kidney, mitochondrial maturation precedes the maturation of reabsorptive ion transport and does not limit its development.