Lamboley C R, Murphy R M, McKenna M J, Lamb G D
Department of Zoology, La Trobe University, Melbourne, Victoria 3086, Australia.
J Physiol. 2014 Mar 15;592(6):1381-95. doi: 10.1113/jphysiol.2013.269373. Epub 2014 Jan 27.
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 (-log10[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 mM 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.
比较了年轻健康成年人股外侧肌I型和II型纤维肌浆网（SR）的钙离子摄取特性。将单个机械去膜肌纤维暴露于游离钙离子浓度在pCa范围（-log10[Ca²⁺]）7.3 - 6.0内且经过重缓冲的溶液中设定时间，然后根据排空SR中所有钙离子后引发的力响应来确定SR净钙离子积累量。采用蛋白质免疫印迹法确定所检测的每根纤维的纤维类型以及肌浆（内质）网钙离子 - ATP酶（SERCA）同工型。I型纤维仅含有SERCA2，在约pCa 6.8时显示出半数最大钙离子摄取率，而II型纤维仅含有SERCA1，在约pCa 6.6时显示出半数最大钙离子摄取率。I型和II型纤维的最大钙离子摄取率分别约为0.18和0.21 mmol Ca²⁺（l纤维）⁻¹ s⁻¹，与先前测得的SR ATP酶活性高度一致。将游离镁离子浓度从1 mM增加到3 mM对pCa 6.0时的净钙离子摄取率无显著影响，表明在两种纤维类型摄取钙离子过程中，通过钙离子释放通道几乎没有或不存在钙诱导的钙释放。在pCa 8.5时，SR的钙离子泄漏（认为至少部分通过SERCA发生）在II型纤维中比在I型纤维中低约2倍，并且几乎不受ADP存在的影响，这与在相同条件下大鼠肌纤维中观察到的更大的SR钙离子泄漏形成鲜明对比。I型人体纤维中钙离子摄取的更高亲和力可以解释与II型纤维相比，I型纤维中观察到的SR钙离子负载相对水平更高的现象，并且人体纤维的SR钙离子泄漏特性表明SERCA的调节方式与大鼠不同，对静息代谢率的贡献相对较小。
