Lamboley, Cedric ORCID: 0000-0002-4008-4929, Murphy, RM, McKenna, Michael ORCID: 0000-0001-9998-0093 and Lamb, GD (2014) Sarcoplasmic reticulum Ca2+ uptake and leak properties, and SERCA isoform expression, in type I and type II fibres of human skeletal muscle. The Journal of Physiology, 592 (6). pp. 1381-1395. ISSN 0022-3751 (print) 1469-7793 (online)

Abstract

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 (–log10[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 ∼pCa 6.8, whereas type II fibres contained only SERCA1 and displayed half-maximal Ca2+ uptake rate at ∼pCa 6.6. Maximal Ca2+ uptake rate was ∼0.18 and ∼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 ∼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.

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