The role of intracellular Mg^<2+> and its mechanism in the regulation of cardiac Ca channels

本研究揭示了细胞内镁离子对青蛙心室肌细胞L型钙通道的作用，揭示了镁离子在β-肾上腺素能信号转导通路最后一步的生理作用如下：1.细胞内镁离子浓度降低到1um时，L型钙电流(i&lt；Ca&gt；)增加10倍。这不是通过磷酸化或GTP结合蛋白介导的，而可能是通过直接与通道结合来实现的。I&lt；Ca&gt；-[mg^&lt；2+&gt；]_i关系表明，在生理条件下，80-90%的通道失活。这种现象是通过解除对镁离子的阻断而实现的，因为钙离子与镁离子竞争结合部位，并与镁离子解离。3.细胞内GTP的存在抑制了细胞内钙离子浓度的升高。4.L型钙通道的磷酸化改变了[mg^&lt；2+&gt；]_i与[mg^&lt；2+&gt；]_i之间的关系，使[mg^&lt；2+&gt；]_i在1 mM到1 mM之间的变化不会引起I_&lt；Ca&gt；的变化，而I_&lt；Ca&gt；保持在最大值。这一结果引发了一种观点，认为磷酸化通过降低钙通道对镁离子阻断作用的敏感性而增加钙离子通道的敏感性，因此，通道磷酸化和[镁离子]_i的还原可能共同参与了通道调控的最后一步。

This project revealed a great detail about action of intracellular Mg^<2+> on the L-type Ca channels in frog ventricular myocytes and indicated a physiological role of Mg^<2+> in the final step of the beta-adrenergic signal transduction pathway as follows.1.Decrease in the concentration of intracelluar Mg^<2+> ([Mg^<2+>]_i) down to 1 muM resulted in increase in L-type Ca current (I_<Ca>) up to 10 times of the control. This was not mediated through phosphorylation nor GTP-binding protein but possibly by direct biding of Mg^<2+> to the channel. I_<Ca>- [Mg^<2+>]_i relationship was obtained and this relationship indicated that 80-90% of channels are inactivated under phsyiological [Mg^<2+>]_i.2.Increase in the intracellular concentration of Ca^<2+> ([Ca^<2+>]_i) induce increase in I_<Ca>. This phenomenon is mediated through unblock of Mg^<2+> because Ca^<2+> competes for the site with Mg^<2+> and unbinds Mg^<2+>.3.Increase in I_<Ca> by reducing [Mg^<2+>]_i was inhibited in the presence of intracellular GTP.This phenomenon has been shown for the first time. The inhibition by GTP was not mediated through GTP-binding protein but by its direct binding to the channel competetively with Mg^<2+>.4.Phosphorylation of the L-type Ca channel altered the relationship between I_<Ca> and [Mg^<2+>]_i so that change in [Mg^<2+>]_i between 1muM and 1 mM did not induce change in I_<Ca> while I_<Ca> remained in the maximum value. This result provoked the idea that phosphorylation increases I_<Ca> by means of reducing the sensitivity of Ca channels to the blocking action of Mg^<2+> and, therefore, both channel phosphorylation and reduction of [Mg^<2+>]_i may share a common final step in modulation of the channel.