Giunti Roberta, Gamberucci Alessandra, Fulceri Rosella, Bánhegyi Gábor, Benedetti Angelo
Department of Pathophysiology, Experimental Medicine and Public Health, University of Siena, Siena, Italy.
Arch Biochem Biophys. 2007 Jun 1;462(1):115-21. doi: 10.1016/j.abb.2007.03.039. Epub 2007 Apr 16.
Steady-state levels of calcium ions in endoplasmic reticulum reflect a balance between active inward transport, mediated by MgATP-dependent Ca(2+) pumps, and passive backflux of the ions, through putative "leak channels". We have investigated the efflux of Ca(2+) from rat liver microsomal vesicles, passively pre-equilibrated in the presence radiolabelled Ca(2+). Similarly, we have also evaluated the efflux of a low-Mwt uncharged compound, i.e., sucrose. The results show that two major passive Ca(2+) efflux pathways exist. One appeared to involve the translocon pore, since it was stimulated by the translocon opener puromycin, and also allowed the passage of sucrose. Putative channels likely mediated the other one, since it required counter ion influx and was inhibited by Gd(3+) and La(3+). The latter pathway did not appear to involve inactive Ca(2+) pumps, Bcl2 proteins, or known channels, such as the InsP3 and ryanodine receptors. While sucrose efflux was highly represented in a rough microsomal subfraction--enriched in the translocon component Sec61alpha--the efflux of Ca(2+) was represented both in smooth and in rough microsomes. We conclude that the passive efflux of Ca(2+) from the (liver) ER could be mediated by both the translocon pore and putative Ca(2+) leak channels. However, the relative role of these Ca(2+) efflux pathways in the intact cell as well as the molecular nature of the Ca(2+) leak channel(s) remain to be clarified.
内质网中钙离子的稳态水平反映了由MgATP依赖的Ca(2+)泵介导的主动内向运输与离子通过假定的“渗漏通道”的被动回流之间的平衡。我们研究了在放射性标记的Ca(2+)存在下被动预平衡的大鼠肝微粒体囊泡中Ca(2+)的外流。同样，我们也评估了一种低分子量不带电荷的化合物即蔗糖的外流。结果表明存在两种主要的被动Ca(2+)外流途径。一种似乎涉及转位子孔，因为它受到转位子开放剂嘌呤霉素的刺激，并且也允许蔗糖通过。假定的通道可能介导了另一种途径，因为它需要反离子流入并且受到Gd(3+)和La(3+)的抑制。后一种途径似乎不涉及无活性的Ca(2+)泵、Bcl2蛋白或已知的通道，如InsP3和兰尼碱受体。虽然蔗糖外流在富含转位子成分Sec61α的粗微粒体亚组分中高度存在，但Ca(2+)外流在光滑微粒体和粗微粒体中均有体现。我们得出结论，（肝脏）内质网中Ca(2+)的被动外流可能由转位子孔和假定的Ca(2+)渗漏通道介导。然而，这些Ca(2+)外流途径在完整细胞中的相对作用以及Ca(2+)渗漏通道的分子性质仍有待阐明。
