Guo X, Becker P L
Department of Physiology, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
J Biol Chem. 1997 Jul 4;272(27):16984-9. doi: 10.1074/jbc.272.27.16984.
Cyclic ADP-ribose (cADPr) has been shown to release intracellular Ca2+ from sea urchin eggs and a variety of vertebrate cell types, although its mechanism of action remains elusive. We employed the caged version of cADPr to study the [Ca2+] transient kinetics in intact sea urchin eggs for insights into how cADPr gates Ca2+ release. Ca2+ release triggered by photolytic production of cADPr was initially slow, with an effective delay of several hundred milliseconds before the onset of a rapid Ca2+ release phase. In contrast, Ca2+ release induced by photolysis of caged inositol 1,4,5-trisphosphate was immediate in onset and roughly an order of magnitude faster. The delay before cADPr-induced Ca2+ release was eliminated when the [Ca2+] was step-elevated coincident with the photoliberation of cADPr and greatly prolonged in the presence of exogenous Ca2+ buffers. Thus, the slow onset of Ca2+ release does not reflect an intrinsically slow rate by which cADPr gates release channels. Rather, a [Ca2+] rise from resting levels is needed to achieve more than minimal cADPr activity. Full release of Ca2+ by cADPr in intact sea urchin eggs requires a positive Ca2+ feedback.
环磷酸腺苷核糖（cADPr）已被证明能从海胆卵和多种脊椎动物细胞类型中释放细胞内的钙离子，尽管其作用机制仍不清楚。我们使用了cADPr的笼锁形式来研究完整海胆卵中的钙离子瞬变动力学，以深入了解cADPr如何开启钙离子释放。由cADPr光解产生引发的钙离子释放最初很慢，在快速钙离子释放阶段开始前有几百毫秒的有效延迟。相比之下，由笼锁型肌醇1,4,5-三磷酸光解诱导的钙离子释放瞬间开始，速度快约一个数量级。当钙离子浓度与cADPr光解同时阶跃升高时，cADPr诱导的钙离子释放前的延迟消除，而在外源钙离子缓冲剂存在时则大大延长。因此，钙离子释放的缓慢起始并不反映cADPr开启释放通道的内在缓慢速率。相反，需要钙离子浓度从静息水平升高才能实现超过最低限度的cADPr活性。在完整海胆卵中，cADPr完全释放钙离子需要正钙离子反馈。
