Expression of the ADP/ATP carrier and expansion of the mitochondrial (ATP + ADP) pool contribute to postnatal maturation of the rat heart.

The role of the ADP/ATP carrier (AAC), a key protein of the mitochondrial ATP-generating system, is not clear during postnatal rat heart development. To elucidate this role, the phosphorylating respiration (state 3), the activity and the content of AAC, the size of the exchangeable mitochondrial (ATP + ADP) pool and the control of AAC over respiration at state 3 were measured in mitochondria isolated from rat hearts at various postnatal ages. There was a 5-fold increase in the AAC activity from newborn to aged rat hearts, which was paralleled by a 1.5-fold increase in state 3 respiration. At birth, the AAC and the F0F1-ATP synthase exerted about 80% of the control over phosphorylating respiration (state 3: flux control coefficients 0.39 +/- 0.04 and 0.38 +/- 0.08). The strong increase in the AAC activity was partly caused by the doubling of the protein content. In addition, the turnover number of AAC increased by a factor of 2.5 due to the expansion of the (ATP + ADP) pool from 3.4 +/- 0.9 to 10.6 +/- 1.5 nmol.mg protein-1. The data strongly indicate that the increase in the AAC activity is an essential step in the postnatal maturation of rat heart mitochondria.