Prentki M, Corkey B E, Matschinsky F M
J Biol Chem. 1985 Aug 5;260(16):9185-90.
Regulation of endoplasmic reticulum (ER) Ca2+ cycling by inositol 1,4,5-trisphosphate (IP3) was studied in saponin-permeabilized RINm5F insulinoma cells. Cells were incubated with mitochondrial inhibitors, and medium Ca2+ concentration established by nonmitochondrial pool(s) (presumably the ER) was monitored with a Ca2+ electrode. IP3 degradation accounted for the transience of the Ca2+ response induced by pulse additions of the molecule. To compensate for degradation, IP3 was infused into the medium. This resulted in elevation of [Ca2+] from about 0.2 microM to a new steady state between 0.3 and 1.0 microM, depending on both the rate of IP3 infusion and the ER Ca2+ content. The elevated steady state represented a bidirectional buffering of [Ca2+] by the ER, as slight displacements in [Ca2+], by small aliquots of Ca2+ or the Ca2+ chelator quin 2, resulted in net uptake or efflux of Ca2+ to restore the previous steady state. When IP3 infusion was stopped, [Ca2+] returned to its original low level. Ninety per cent of the Ca2+ accumulated by the ER was released by IP3 when the total Ca2+ content did not exceed 15 nmol/mg of cell protein. Above this high Ca2+ content, Ca2+ was accumulated in an IP3-insensitive, A23187-releasable pool. The maximal amount of Ca2+ that could be released from the ER by IP3 was 13 nmol/mg of cell protein. The data support the concept that in the physiological range of Ca2+ contents, almost all the ER is an IP3-sensitive Ca2+ store that is capable of finely regulating [Ca2+] through independent influx (Ca2+-ATPase) and efflux (IP3-modulated component) pathways of Ca2+ transport. IP3 may continuously modulate Ca2+ cycling across the ER and play an important role in determining the ER Ca2+ content and in regulating cytosolic Ca2+ under both stimulated and possibly basal conditions.
在内质网（ER）钙循环的调节方面，研究人员在皂素通透的RINm5F胰岛素瘤细胞中对肌醇1,4,5 - 三磷酸（IP3）进行了研究。细胞与线粒体抑制剂一起孵育，并用钙离子电极监测由非线粒体池（可能是内质网）建立的培养基钙离子浓度。IP3的降解解释了分子脉冲添加所诱导的钙离子反应的短暂性。为了补偿降解，将IP3注入培养基中。这导致[Ca2+]从约0.2微摩尔升高到0.3至1.0微摩尔之间的新稳态，这取决于IP3注入速率和内质网Ca2+含量。升高的稳态代表内质网对[Ca2+]的双向缓冲，因为[Ca2+]的轻微变化，通过小份的Ca2+或钙离子螯合剂喹啉2，会导致Ca2+的净摄取或流出，以恢复先前的稳态。当停止IP3注入时，[Ca2+]恢复到其原始低水平。当总Ca2+含量不超过15纳摩尔/毫克细胞蛋白时，内质网积累的Ca2+中有90%由IP3释放。超过这个高Ca2+含量时，Ca2+积累在一个对IP3不敏感、可被A23187释放的池中。IP3可从内质网释放的最大Ca2+量为13纳摩尔/毫克细胞蛋白。这些数据支持这样一个概念，即在Ca2+含量的生理范围内，几乎所有内质网都是一个对IP3敏感的Ca2+储存库，能够通过独立的Ca2+流入（Ca2+ - ATP酶）和流出（IP3调节成分）运输途径精细调节[Ca2+]。IP3可能持续调节Ca2+在内质网中的循环，并在确定内质网Ca2+含量以及在刺激和可能的基础条件下调节细胞质Ca2+方面发挥重要作用。
