Gerdes U, Nakhla A M, Møller J V
Biochim Biophys Acta. 1983 Oct 12;734(2):180-90. doi: 10.1016/0005-2736(83)90116-5.
Passive Ca2+ permeability of sarcoplasmic reticulum vesicles has been studied after maximal loading with Ca2+ (150-200 nmol/mg protein) in the presence of Ca2+, MgATP and an ATP generating system of limited capacity. Outflow of accumulated Ca2+ in the non-energized state of the system was studied by depletion of the medium of one of the substrates, either MgATP (by complete consumption) or Ca2+ (by complexation with EGTA). It was found that Ca2+ outflow under these conditions is relatively slow and independent of the medium concentration of Ca2+ (5 X 10(-9)-5 X 10(-5) M) or MgATP (0.7-730 microM). Outflow curves were steep at the beginning of the outflow phase (30-60 nmol/min per mg protein), and outflow proceeded at a much lower rate below 100 nmol Ca2+/mg protein. Outflow could be completely inhibited by La3+. The Ca2+ release curves are not compatible with simple diffusion, and cannot be accounted for by Ca2+ binding inside the vesicles. Neither are our observations consistent with permeation mediated via the Ca2+ translocation sites involved in active transport. We suggest that non-energized Ca2+ outflow may proceed by a process of ion-exchange through negatively charged, water-filled channels in the membrane, the properties of which are altered by a high intravesicular concentration of Ca2+.
在存在钙离子、镁 - 三磷酸腺苷（MgATP）以及有限容量的三磷酸腺苷生成系统的情况下，使肌浆网囊泡最大程度地加载钙离子（150 - 200纳摩尔/毫克蛋白质）后，对其被动钙离子通透性进行了研究。通过耗尽介质中的一种底物来研究系统在非激活状态下积累钙离子的流出情况，该底物可以是MgATP（通过完全消耗）或钙离子（通过与乙二醇双四乙酸（EGTA）络合）。结果发现，在这些条件下钙离子流出相对缓慢，且与介质中钙离子（5×10⁻⁹ - 5×10⁻⁵摩尔/升）或MgATP（0.7 - 730微摩尔/升）的浓度无关。流出曲线在流出阶段开始时很陡（30 - 60纳摩尔/分钟·毫克蛋白质），当低于100纳摩尔钙离子/毫克蛋白质时，流出速率要低得多。镧离子（La³⁺）可完全抑制流出。钙离子释放曲线不符合简单扩散，也不能用囊泡内的钙离子结合来解释。我们的观察结果也与通过参与主动运输的钙离子转运位点介导的渗透作用不一致。我们认为，非激活状态下的钙离子流出可能是通过膜中带负电荷的充满水的通道进行离子交换的过程，囊泡内高浓度的钙离子会改变这些通道的性质。
