Hughes B P, Barritt G J
Department of Medical Biochemistry, Flinders University School of Medicine, Bedford Park, Australia.
Biochim Biophys Acta. 1989 Oct 9;1013(3):197-205. doi: 10.1016/0167-4889(89)90135-3.
The properties of the receptor-activated Ca2+ inflow system in the liver cell plasma membrane were compared with those of voltage-operated Ca2+ channels and receptor-operated Ca2+ channels present in other cell types by testing the susceptibility of the Ca2+ inflow system to inhibition by other metal ions and known inhibitors of Ca2+ movement across membranes. Co2+ inhibited Ca2+ inflow through the receptor-activated Ca2+ inflow system, as assessed by measurement of (a) the activation by extracellular Ca2+ (Cao2+) of glycogen phosphorylase in the presence of vasopressin and (b) 45Ca2+ exchange in the presence of the hormone. The concentration of Co2+ which gave half-maximal inhibition was 280 microM. The inhibition by Co2+ was reversed by high Cao2+. Co2+ did not inhibit basal Ca2+ inflow as measured by 45Ca2+ exchange in the absence of vasopressin. Zn2+, Cd2+, Ni2+ and Mn2+ each inhibited Ca2+ inflow through the receptor-activated Ca2+ inflow system. The concentrations of these ions which gave half-maximal inhibition were 10, 50, 220 and 400 microM, respectively. Little inhibition of receptor-activated Ca2+ inflow was observed in the presence of Sr2+ or Ba2+. However, substantial amounts of 90Sr2+ were taken up by hepatocytes. Rates of 90Sr2+ uptake increased from 0.5-8 nmol per min per mg wet wt. when the extracellular concentration of Sr2+ was varied from 0.25 to 2.5 mM. Sr2+ uptake was inhibited 50% by Cao2+ with half-maximal inhibition at 100 microM Cao2+, but was not inhibited by verapamil and was not stimulated by vasopressin. The movement of Ca2+ through the receptor-activated Ca2+ inflow system was not inhibited by high concentrations of each of a number of inhibitors of voltage-operated and receptor-operated Ca2+ channels and intracellular Ca2+ movement. It is concluded that while the susceptibility to inhibition by metal ions of the receptor-activated Ca2+ inflow system in the liver cell plasma membrane is similar to that of voltage-operated Ca2+ channels, there are significant differences between the liver cell receptor-activated Ca2+ inflow system and both voltage-operated Ca2+ channels and some other receptor-operated Ca2+ channels with respect to inhibition by organic compounds.
通过检测钙离子内流系统对其他金属离子及已知的跨膜钙离子移动抑制剂的敏感性，将肝细胞质膜中受体激活的钙离子内流系统的特性，与其他细胞类型中存在的电压门控钙离子通道和受体门控钙离子通道的特性进行了比较。通过测量以下指标评估，钴离子抑制了通过受体激活的钙离子内流系统的钙离子内流：（a）在血管加压素存在下，细胞外钙离子（Cao2+）对糖原磷酸化酶的激活作用；（b）在激素存在下的45Ca2+交换。产生半数最大抑制作用的钴离子浓度为280微摩尔。高浓度的细胞外钙离子可逆转钴离子的抑制作用。在没有血管加压素的情况下，通过45Ca2+交换测量，钴离子不抑制基础钙离子内流。锌离子、镉离子、镍离子和锰离子均抑制通过受体激活的钙离子内流系统的钙离子内流。产生半数最大抑制作用的这些离子的浓度分别为10、50、220和400微摩尔。在锶离子或钡离子存在的情况下，未观察到对受体激活的钙离子内流的显著抑制。然而，肝细胞摄取了大量的90Sr2+。当细胞外锶离子浓度从0.25毫摩尔变化到2.5毫摩尔时，90Sr2+摄取速率从每分钟每毫克湿重0.5 - 8纳摩尔增加。细胞外钙离子可抑制50%的90Sr2+摄取，在100微摩尔细胞外钙离子浓度时产生半数最大抑制作用，但维拉帕米不抑制，血管加压素也不刺激其摄取。多种电压门控和受体门控钙离子通道抑制剂以及细胞内钙离子移动抑制剂的高浓度均未抑制钙离子通过受体激活的钙离子内流系统的移动。结论是，虽然肝细胞质膜中受体激活的钙离子内流系统对金属离子抑制的敏感性与电压门控钙离子通道相似，但在有机化合物抑制方面，肝细胞受体激活的钙离子内流系统与电压门控钙离子通道以及其他一些受体门控钙离子通道之间存在显著差异。
