MITOCHONDRIAL GENE-EXPRESSION IN RAT-HEART AND LIVER DURING GROWTH AND DEVELOPMENT

The levels of mitochondrial enzyme activities involved in respiration and oxidative phosphorylation and of specific mitochondrial gene trans cripts were examined in rat heart and liver tissues during early growt h, development, and aging. Increases were shown in cardiac respiratory complex activities I, III, IV, and V and ATPase6 and CoxII transcript levels during the transition from neonate to young adult. This increa sed mitochondrial gene expression is not associated with a proportiona te increase in mitochondrial number. In contrast, no significant chang es in liver mitochondrial activities or transcripts were detected duri ng this transition. Marked reductions in the activities of complexes I , III, IV, and V and in ATPase6 and COXII transcripts were demonstrate d in older adult as compared with young adult cardiac tissue with no c oncomitant reduction in cardiac citrate synthase activity and content, and mtDNA copy number. No decline was noted in liver mitochondrial en zyme activity levels and transcripts of old adult rats. These findings suggest that cardiac mitochondrial gene expression is developmentally regulated at a pretranslational level. The pattern of increasing mito chondrial gene expression in the young adult and decreasing gene expre ssion in the aging heart stands in clear contrast to liver mitochondri al gene expression or nuclear-encoded genes such as citrate synthase.