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Sarcoplasmic reticulum Ca 2+ uptake and leak properties, and SERCA isoform expression, in type I and type II fibres of human skeletal muscle
Author(s) -
Lamboley C. R.,
Murphy R. M.,
McKenna M. J.,
Lamb G. D.
Publication year - 2014
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2013.269373
Subject(s) - endoplasmic reticulum , skeletal muscle , chemistry , calcium , biophysics , serca , contraction (grammar) , muscle contraction , vastus lateralis muscle , medicine , biochemistry , endocrinology , atpase , biology , enzyme , organic chemistry
Key points Release and uptake of Ca 2+ ions by the sarcoplasmic reticulum (SR) regulates contraction in skeletal muscle. SR Ca 2+ 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 Ca 2+ uptake at different applied [Ca 2+ ] assessed from the amount of Ca 2+ accumulated. Ca 2+ uptake occurred at lower [Ca 2+ ] 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 Ca 2+ were taken up per ATP hydrolysed. Ca 2+ leaking out of the SR ultimately has to be pumped back in again at the cost of ATP usage. SR Ca 2+ leakage in human muscle fibres was smaller and regulated differently to that in rat muscle fibres, probably reflecting different contributions to thermogenesis.Abstract 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. Individual mechanically skinned muscle fibres were exposed to solutions with the free [Ca 2+ ] heavily buffered in the pCa range (–log 10 [Ca 2+ ]) 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+ . Western blotting was used to determine fibre type and the sarco(endo)plasmic reticulum Ca 2+ ‐ATPase (SERCA) isoform present in every fibre examined. 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. 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. Increasing free [Mg 2+ ] 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. 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. 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.