Xu L, Meissner G
Department of Biochemistry & Biophysics, University of North Carolina, Chapel Hill, North Carolina 27599-7260, USA.
Biophys J. 1998 Nov;75(5):2302-12. doi: 10.1016/S0006-3495(98)77674-X.
The cardiac muscle sarcoplasmic reticulum Ca2+ release channel (ryanodine receptor) is a ligand-gated channel that is activated by micromolar cytoplasmic Ca2+ concentrations and inactivated by millimolar cytoplasmic Ca2+ concentrations. The effects of sarcoplasmic reticulum lumenal Ca2+ on the purified release channel were examined in single channel measurements using the planar lipid bilayer method. In the presence of caffeine and nanomolar cytosolic Ca2+ concentrations, lumenal-to-cytosolic Ca2+ fluxes >/=0.25 pA activated the channel. At the maximally activating cytosolic Ca2+ concentration of 4 microM, lumenal Ca2+ fluxes of 8 pA and greater caused a decline in channel activity. Lumenal Ca2+ fluxes primarily increased channel activity by increasing the duration of mean open times. Addition of the fast Ca2+-complexing buffer 1,2-bis(2-aminophenoxy)ethanetetraacetic acid (BAPTA) to the cytosolic side of the bilayer increased lumenal Ca2+-activated channel activities, suggesting that it lowered Ca2+ concentrations at cytosolic Ca2+-inactivating sites. Regulation of channel activities by lumenal Ca2+ could be also observed in the absence of caffeine and in the presence of 5 mM MgATP. These results suggest that lumenal Ca2+ can regulate cardiac Ca2+ release channel activity by passing through the open channel and binding to the channel's cytosolic Ca2+ activation and inactivation sites.
心肌肌浆网Ca2+释放通道（兰尼碱受体）是一种配体门控通道，被微摩尔浓度的胞质Ca2+激活，被毫摩尔浓度的胞质Ca2+失活。使用平面脂质双层方法进行单通道测量，研究了肌浆网腔Ca2+对纯化的释放通道的影响。在存在咖啡因和纳摩尔浓度的胞质Ca2+的情况下，腔到胞质的Ca2+通量≥0.25 pA会激活该通道。在4 microM的最大激活胞质Ca2+浓度下，8 pA及更高的腔Ca2+通量会导致通道活性下降。腔Ca2+通量主要通过增加平均开放时间的持续时间来增加通道活性。向双层的胞质侧添加快速Ca2+络合缓冲剂1,2-双（2-氨基苯氧基）乙烷四乙酸（BAPTA）会增加腔Ca2+激活的通道活性，这表明它降低了胞质Ca2+失活位点的Ca2+浓度。在没有咖啡因且存在5 mM MgATP的情况下，也可以观察到腔Ca2+对通道活性的调节。这些结果表明，腔Ca2+可以通过穿过开放通道并与通道的胞质Ca2+激活和失活位点结合来调节心脏Ca2+释放通道的活性。
