Calcium phosphate precipitation in the sarcoplasmic reticulum reduces action potential-mediated Ca2+ release in mammalian skeletal muscle

During vigorous exercise, P_i concentration levels within the cytoplasm of fast-twitch muscle fibers may reach 30 mM. Cytoplasmic P_i may enter the sarcoplasmic reticulum (SR) and bind to Ca²⁺ to form a precipitate (CaP_i), thus reducing the amount of releasable Ca²⁺. Using mechanically skinned rat fast-twitch muscle fibers, which retain the normal action potential-mediated Ca²⁺ release mechanism, we investigated the consequences of P_i exposure on normal excitation-contraction coupling. The total amount of Ca²⁺ released from the SR by a combined caffeine/low-Mg²⁺ concentration stimulus was reduced by 20%, and the initial rate of force development slowed after 2-min exposure to 30 mM P_i (with or without the presence creatine phosphate). Peak (50 Hz) tetanic force was also reduced (by 25% and 45% after 10 and 30 mM P_i exposure, respectively). Tetanic force responses produced after 30 mM P_i exposure were nearly identical to those observed in the same fiber after depletion of total SR Ca²⁺ by 35%. Ca²⁺ content assays revealed that the total amount of Ca²⁺ in the SR was not detectably changed by exposure to 30 mM P_i, indicating that Ca²⁺ had not leaked from the SR but instead formed a precipitate with the P_i, reducing the amount of available Ca²⁺ for rapid release. These results suggest that CaP_i precipitation that occurs within the SR could contribute to the failure of Ca²⁺ release observed in the later stages of metabolic muscle fatigue. They also demonstrate that the total amount of Ca²⁺ stored in the SR cannot drop substantially below the normal endogenous level without reducing tetanic force responses. muscle fatigue; excitation-contraction coupling