Sarcoplasmic reticulum Ca2+ uptake and leak properties, and SERCA isoform expression, in type I and type II fibres of human skeletal muscle

Key points Release and uptake of Ca2+ ions by the sarcoplasmic reticulum (SR) regulates contraction in skeletal muscle. SR Ca2+ uptake and leak properties in human muscle are presently not well defined. The surface membrane of individual human muscle fibres was removed by microdissection, and the rate of SR Ca2+ uptake at different applied [Ca2+] assessed from the amount of Ca2+ accumulated. Ca2+ uptake occurred at lower [Ca2+] in type I fibres than in type II fibres, consistent with the contractile apparatus properties in the respective fibre types. Maximal uptake rate was slightly greater in type II fibres, and approximately two Ca2+ were taken up per ATP hydrolysed. Ca2+ leaking out of the SR ultimately has to be pumped back in again at the cost of ATP usage. SR Ca2+ leakage in human muscle fibres was smaller and regulated differently to that in rat muscle fibres, probably reflecting different contributions to thermogenesis. The Ca2+ uptake properties of the sarcoplasmic reticulum (SR) were compared between type I and type II fibres of vastus lateralis muscle of young healthy adults. Individual mechanically skinned muscle fibres were exposed to solutions with the free [Ca2+] heavily buffered in the pCa range (-log(10)[Ca2+]) 7.3-6.0 for set times and the amount of net SR Ca2+ accumulation determined from the force response elicited upon emptying the SR of all Ca2+. Western blotting was used to determine fibre type and the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) isoform present in every fibre examined. Type I fibres contained only SERCA2 and displayed half-maximal Ca2+ uptake rate at similar to pCa 6.8, whereas type II fibres contained only SERCA1 and displayed half-maximal Ca2+ uptake rate at similar to pCa 6.6. Maximal Ca2+ uptake rate was similar to 0.18 and similar to 0.21 mmol Ca2+ (l fibre)(-1) s(-1) in type I and type II fibres, respectively, in good accord with previously measured SR ATPase activity. Increasing free [Mg2+] from 1 to 3 mm had no significant effect on the net Ca2+ uptake rate at pCa 6.0, indicating that there was little or no calcium-induced calcium release occurring through the Ca2+ release channels during uptake in either fibre type. Ca2+ leakage from the SR at pCa 8.5, which is thought to occur at least in part through the SERCA, was similar to 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 Ca2+ leak observed in rat muscle fibres under the same conditions. The higher affinity of Ca2+ uptake in the type I human fibres can account for the higher relative level of SR Ca2+ loading observed in type I compared to type II fibres, and the SR Ca2+ 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.