Sorenson M M, Coelho H S, Reuben J P
J Membr Biol. 1986;90(3):219-30. doi: 10.1007/BF01870128.
Oxalate-supported Ca accumulation by the sarcoplasmic reticulum (SR) of chemically skinned mammalian skeletal muscle fibers is activated by MgATP and Ca2+ and partially inhibited by caffeine. Inhibition by caffeine is greatest when Ca2+ exceeds 0.3 to 0.4 microM, when free ATP exceeds 0.8 to 1 mM, and when the inhibitor is present from the beginning of the loading period rather than when it is added after Ca oxalate has already begun to precipitate within the SR. Under the most favorable combination of these conditions, this effect of caffeine is maximal at 2.5 to 5 mM and is half-maximal at approximately 0.5 mM. For a given concentration of caffeine, inhibition decreases to one-half of its maximum value when free ATP is reduced to 0.2 to 0.3 mM. Varying free Mg2+ (0.1 to 2 mM) or MgATP (0.03 to 10 mM) has no effect on inhibition. Average residual uptake rates in the presence of 5 mM caffeine at pCa 6.4 range from 32 to 70% of the control rates in fibers from different animals. The extent of inhibition in whole-muscle homogenates is similar to that observed in skinned fibers, but further purification of SR membranes by differential centrifugation reduces their ability to respond to caffeine. In skinned fibers, caffeine does not alter the Ca2+ concentration dependence of Ca uptake (K0.5, 0.5 to 0.8 microM; Hill n, 1.5 to 2.1). Reductions in rate due to caffeine are accompanied by proportional reductions in maximum capacity of the fibers, and this configuration can be mimicked by treating fibers with the ionophore A23187. Caffeine induces a sustained release of Ca from fibers loaded with Ca oxalate. However, caffeine-induced Ca release is transient when fibers are loaded without oxalate. The effects of caffeine on rate and capacity of Ca uptake as well as the sustained and transient effects on uptake and release observed under different conditions can be accounted for by a single mode of action of caffeine: it increases Ca permeability in a limited population of SR membranes, and these membranes coexist with a population of caffeine-insensitive membranes within the same fiber.
草酸盐支持的化学去膜哺乳动物骨骼肌纤维肌浆网（SR）的钙积累由MgATP和Ca2+激活，并部分受咖啡因抑制。当Ca2+超过0.3至0.4微摩尔、游离ATP超过0.8至1毫摩尔，以及从加载期开始就存在抑制剂而非在草酸盐已开始在SR内沉淀后添加时，咖啡因的抑制作用最大。在这些条件的最有利组合下，咖啡因的这种作用在2.5至5毫摩尔时最大，在约0.5毫摩尔时为最大作用的一半。对于给定浓度的咖啡因，当游离ATP降至0.2至0.3毫摩尔时，抑制作用降至其最大值的一半。改变游离Mg2+（0.1至2毫摩尔）或MgATP（0.03至10毫摩尔）对抑制作用无影响。在pCa 6.4时，5毫摩尔咖啡因存在下的平均残余摄取率为不同动物纤维对照率的32%至70%。全肌肉匀浆中的抑制程度与去膜纤维中观察到的相似，但通过差速离心进一步纯化SR膜会降低其对咖啡因的反应能力。在去膜纤维中，咖啡因不会改变钙摄取的Ca2+浓度依赖性（K0.5，0.5至0.8微摩尔；希尔系数n，1.5至2.1）。由于咖啡因导致的速率降低伴随着纤维最大容量的成比例降低，并且这种情况可以通过用离子载体A23187处理纤维来模拟。咖啡因会诱导草酸盐负载的纤维持续释放钙。然而，当纤维在没有草酸盐的情况下加载时，咖啡因诱导的钙释放是短暂的。咖啡因对钙摄取速率和容量的影响以及在不同条件下观察到的对摄取和释放的持续和短暂影响可以通过咖啡因的单一作用模式来解释：它增加了有限数量的SR膜中的钙通透性，并且这些膜与同一纤维内对咖啡因不敏感的膜群体共存。
