The creatine kinase system is essential for optimal refill of the sarcoplasmic reticulum Ca2+ store in skeletal muscle

Muscle function depends on an adequate ATP supply to sustain the energy consumption associated with Ca2+ cycling and actomyosin sliding during contraction. In this regulation of energy homeostasis, the creatine kinase (CK) circuit for high energy phosphoryl transfer between ATP and phosphocreatine plays an important role. We earlier established a functional connection between the activity of the CK system and Ca2+ homeostasis during depolarization and contractile activity of muscle. Here, we show how CK activity is coupled to the kinetics of spontaneous and electrically induced Ca2+ transients in the sarcoplasm of myotubes. Using the UV ratiometric Ca2+ probe Indo-1 and video-rate confocal microscopy in CK-proficient and -deficient cultured cells, we found that spontaneous and electrically induced transients were dependent on ryanodine-sensitive Ca2+ release channels, sarcoplasmic/endoplasmic reticulum Ca2+-ATPase pumps, extracellular calcium, and functional mitochondria in both cell types. However, at increasing sarcoplasmic Ca2+ load (induced by electrical stimulation at 0.1, 1, and 10 Hz), the Ca2+ removal rate and the amount of Ca2+ released per transient were gradually reduced in CK-deficient (but not wild-type) myotubes. We conclude that the CK/phosphocreatine circuit is essential for efficient delivery of ATP to the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase pumps and thereby directly influences sarcoplasmic reticulum refilling and the kinetics of the sarcoplasmic Ca2+ signals.