Role of myoplasmic phosphate in contractile function of skeletal muscle: studies on creatine kinase-deficient mice

1. Increased myoplasmic inorganic phosphate (P-i) has been suggested to have an important role in skeletal muscle fatigue, especially in the early phase. In the present study we used intact fast-twitch muscle cells from mice completely deficient in creatine kinase (CK-/-) to test this suggestion. These CK-/- muscle cells provide a good model since they display a higher P-i concentration in the unfatigued state and fatigue without significant increase of P-i. 2. Tetanic contractions (350 ms duration) were produced in intact single muscle fibres. The free myoplasmic [Ca2+] ([Ca2+](i)) was measured with the fluorescent indicator indo-1. The force-[Ca2+](i) relationship was constructed from tetani at different frequencies. 3. Compared with wild-type fibres, CK-/- fibres displayed lower force in 100 Hz tetani and at saturating [Ca2+](i) (i.e. 100 Hz stimulation during caffeine exposure), higher tetanic [Ca2+](i) during the first 100 ms of tetanic stimulation, reduced myofibrillar Ca2+ sensitivity when measurements were performed 100-200 ms into tetani, and slowed force relaxation that was due to altered cross-bridge kinetics rather than delayed Ca2+ removal from the myoplasm. 4. In wild-type fibres, a series of 10 tetani resulted in reduced tetanic force, slowed force relaxation, and increased amplitude of [Ca2+](i) tails after tetani. None of these changes were observed in CK-/- fibres. 5. Complementary experiments on isolated fast-twitch extensor digitorum longus muscles showed a reduction of tetanic force and relaxation speed in CK-/- muscles similar to those observed in single fibres. 6. In conclusion, increased P-i concentration can explain changes observed in the early phase of skeletal muscle fatigue. Increased P-i appears to be involved in both fatigue-induced changes of cross-bridge function and SR Ca2+ handling.