Mitochondrial creatine kinase functional development in post-natal rat skeletal muscle. A combined polarographic/P-31 NMR study

Mitochondrial creatine kinase (Mi-CK) function in viable mitochondria from developing rat skeletal muscle was assessed both by polarographic measureme nts of creatine-induced respiration and P-31 NMR spectroscopy measurements of phosphocreatine (PCr) synthesis. Creatine-induced respiration was observ ed in very young rats and increased by 50% to 35 days of age. PCr synthesis was present in 7 day old animals and increased by 300% reaching levels mea sured in 35 day and adult muscle. Unlike reports showing Mi-CK enzymatic ac tivities but no mitochondrial function in several situations, a concomitant progression of enzymatic activity and mitochondrial function was evidenced during the developmental stages of skeletal muscle Mi-CK in altricious ani mals. These results correlated with the progressive pattern of muscle diffe rentiation during development of motricity in such animals. The observation that Mi-CK is functional in skeletal muscle mitochondria very early after birth, strongly favors the notion that adaptations in skeletal muscle of Mi -CK knock-out mice occur early.