![]() 2009a), including in human muscle (Talmadge et al. The Ca 2+ is subsequently sequestered back into the SR by the sarco(endo)plasmic reticulum Ca 2+-ATPase (SERCA), with SERCA1 being the predominant or sole isoform present in adult mammalian type II fibres and SERCA2a the predominant or sole isoform in type I fibres (Lytton et al. 2009a) and can be released extremely rapidly into the cytoplasm through the ryanodine receptor–Ca 2+ release channels (RyRs) in the SR terminal cisternae (Franzini-Armstrong & Protasi, 1997). The Ca 2+ in the SR is predominantly bound to calsequestrin (CSQ), a high capacity, low affinity Ca 2+-binding protein (MacLennan & Wong, 1971 Beard et al. In skeletal muscle the release and reuptake of Ca 2+ by the sarcoplasmic reticulum (SR) governs force production by the contractile apparatus, and hence the SR properties are major determinants of muscle function and performance. The higher affinity of Ca 2+ uptake in the type I human fibres can account for the higher relative level of SR Ca 2+ loading observed in type I compared to type II fibres, and the SR Ca 2+ leakage characteristics of the human fibres suggest that the SERCAs are regulated differently from those in rat and contribute comparatively less to resting metabolic rate. Ca 2+ leakage from the SR at pCa 8.5, which is thought to occur at least in part through the SERCA, was ∼2-fold lower in type II fibres than in type I fibres, and was little affected by the presence of ADP, in marked contrast to the larger SR Ca 2+ leak observed in rat muscle fibres under the same conditions. Increasing free from 1 to 3 m m had no significant effect on the net Ca 2+ uptake rate at pCa 6.0, indicating that there was little or no calcium-induced calcium release occurring through the Ca 2+ release channels during uptake in either fibre type. ![]() Maximal Ca 2+ uptake rate was ∼0.18 and ∼0.21 mmol Ca 2+ (l fibre) –1 s –1 in type I and type II fibres, respectively, in good accord with previously measured SR ATPase activity. Type I fibres contained only SERCA2 and displayed half-maximal Ca 2+ uptake rate at ∼pCa 6.8, whereas type II fibres contained only SERCA1 and displayed half-maximal Ca 2+ uptake rate at ∼pCa 6.6. Western blotting was used to determine fibre type and the sarco(endo)plasmic reticulum Ca 2+-ATPase (SERCA) isoform present in every fibre examined. Individual mechanically skinned muscle fibres were exposed to solutions with the free heavily buffered in the pCa range (–log 10) 7.3–6.0 for set times and the amount of net SR Ca 2+ accumulation determined from the force response elicited upon emptying the SR of all Ca 2+. The Ca 2+ uptake properties of the sarcoplasmic reticulum (SR) were compared between type I and type II fibres of vastus lateralis muscle of young healthy adults.
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