Morii H, Takisawa H, Yamamoto T
J Biol Chem. 1985 Sep 25;260(21):11536-41.
ATP-dependent Ca2+ uptake by subfractions of skeletal muscle sarcoplasmic reticulum (SR) was studied with the Ca2+ indicator dye, antipyrylazo III. Ca2+ uptake by heavy SR showed two phases, a slow uptake phase and a fast uptake phase. By contrast, Ca2+ uptake by light SR exhibited a monophasic time course. In both fractions a steady state of Ca2+ uptake was observed when the concentration of free Ca2+ outside the vesicles was reduced to less than 0.1 microM. In the steady state, the addition of 5 microM Ca2+ to the external medium triggered rapid Ca2+ release from heavy SR but not from light SR, indicating that the heavy fraction contains a Ca2+-induced Ca2+ release channel. During Ca2+ uptake, heavy SR showed a constant Ca2+-dependent ATPase activity (1 mumol/mg protein X min) which was about 150 times higher than the rate of Ca2+ uptake in the slow uptake phase. Ruthenium red, an inhibitor of Ca2+-induced Ca2+ release, enhanced the rate of Ca2+ uptake during the slow phase without affecting Ca2+-dependent ATPase activity. Adenine nucleotides, activators of Ca2+ release, reduced the Ca2+ uptake rate. These results suggest that the rate of Ca2+ accumulation by heavy SR is not proportional to ATPase activity during the slow uptake phase due to the activation of the channel for Ca2+-induced Ca2+ release. In addition, they suggest that the release channel is inactivated during the fast Ca2+ uptake phase.
利用钙离子指示剂偶氮胂III，研究了骨骼肌肌浆网（SR）亚组分对ATP依赖的钙离子摄取。重肌浆网对钙离子的摄取呈现两个阶段，即缓慢摄取阶段和快速摄取阶段。相比之下，轻肌浆网对钙离子的摄取呈现单相时间进程。当囊泡外游离钙离子浓度降低至小于0.1微摩尔时，在这两个亚组分中均观察到钙离子摄取的稳态。在稳态下，向外部介质中添加5微摩尔钙离子会触发重肌浆网快速释放钙离子，但轻肌浆网不会，这表明重亚组分含有钙离子诱导的钙离子释放通道。在钙离子摄取过程中，重肌浆网显示出恒定的钙离子依赖性ATP酶活性（1微摩尔/毫克蛋白质×分钟），这比缓慢摄取阶段的钙离子摄取速率高约150倍。钌红是一种钙离子诱导的钙离子释放抑制剂，它在缓慢阶段提高了钙离子摄取速率，而不影响钙离子依赖性ATP酶活性。腺嘌呤核苷酸是钙离子释放的激活剂，降低了钙离子摄取速率。这些结果表明，在缓慢摄取阶段，重肌浆网的钙离子积累速率与ATP酶活性不成正比，这是由于钙离子诱导的钙离子释放通道被激活。此外，这些结果还表明，在快速钙离子摄取阶段，释放通道失活。
