CREATINE-KINASE ACTIVITY IN RAT SKELETAL-MUSCLE RELATES TO MYOSIN PHENOTYPE DURING DEVELOPMENT

Creatine kinase (CK) has been implicated in the maintenance of skeleta l muscle intracellular energy supply via its ATP buffering capacity. W e examined the postnatal expression of CK activity and isoform phenoty pe in four skeletal muscles [diaphragm (DIA), intercostal (IC), extern al abdominal oblique (EAO), and the soleus (SOL)] of the rat. Moreover , we correlated CK activity during development with postnatal changes in myosin heavy chain (MHC) phenotype, the latter an index of relative changes in the energetic demands of muscle contractile proteins. CK a ctivity was lowest in the immediate newborn period and increased in al l muscles during postnatal development; the highest levels of CK activ ity were observed in the adult IC and EAO. CK activity did relate to t he MHC phenotype as indexed by the ratio of adult MHC isoform content (slow + IIa + IIx + IIb) to developmental MHC isoform content (slow neonatal; r(2) = 0.93, P < 0.001). Stepwise regression revealed that t ype IIb, MHC expression alone accounted for 79% of the developmental v ariance in CK activity. We conclude that CK activity increases during postnatal development in a muscle specific fashion and relates to the energetic demands of the muscle contractile proteins as reflected by M HC isoform composition. We speculate that the role of CK as an energy buffer is greatest in muscles expressing the IIb MHC isoform.