Free magnesium concentration in salamander photoreceptor outer segments
Free magnesium concentration in salamander photoreceptor outer segments
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DOI：
10.1113/jphysiol.2003.053280
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发表时间：
2003-11-15
影响因子：
5.5
通讯作者：
Koutalos, Y
中科院分区：
文献类型：
--
作者：
Chen, CH;Nakatani, K;Koutalos, Y
镁离子（Mg2+）在生物化学功能中起着重要作用。在脊椎动物的光感受器外节中，许多反应利用MgGTP和MgATP，并且Mg 2+还调节几种光转导酶。虽然Mg2+在一定条件下可以通过光敏通道，但尚未确定明确的挤出机制，并且去除细胞外Mg2+对光敏感性或光响应动力学没有显著影响。我们已经使用荧光Mg2+染料Furaptra直接测量和监测感光细胞外节中的游离Mg2+浓度，并检查游离Mg2+浓度是否在生理条件下发生变化。漂白的蝾螈视杆细胞和视锥细胞外节的静息游离Mg 2+浓度分别为0.86 +/-0.06和0.81 +/-0.09 mm。外段游离Mg2+浓度没有显着影响细胞外pH值，Ca2+和Na+的变化，排除了各自的交换机在调节Mg2+稳态的显着作用。静息游离Mg 2+浓度也没有受到暴露于0 Mg 2+的显著影响，表明缺乏显著的基础Mg 2+通量。打开cGMP门控通道导致在不存在Na+和Ca2+的情况下Mg2+浓度显著增加，但在它们存在的情况下没有，这表明去极化仅在不存在其他渗透离子的情况下可引起显著的Mg2+流入，但在生理条件下不会。最后，光刺激并没有改变Mg2+浓度的外段的暗适应光感受器。结果表明，有没有流入和流出途径，可以显着影响外节在生理条件下的Mg 2+浓度。因此，Mg2+不太可能在光转导的动态调节中起重要作用。
Magnesium ions (Mg2+) play an important role in biochemical functions. In vertebrate photoreceptor outer segments, numerous reactions utilize MgGTP and MgATP, and Mg2+ also regulates several of the phototransduction enzymes. Although Mg2+ can pass through light-sensitive channels under certain conditions, no clear extrusion mechanism has been identified and removing extracellular Mg2+ has no significant effect on the light sensitivity or the kinetics of the photoresponse. We have used the fluorescent Mg2+ dye Furaptra to directly measure and monitor the free Mg2+ concentration in photoreceptor outer segments and examine whether the free Mg2+ concentration changes under physiological conditions. Resting free Mg2+ concentrations in bleached salamander rod and cone photoreceptor cell outer segments were 0.86+/-0.06 and 0.81+/-0.09 mm, respectively. The outer segment free Mg2+ concentration was not significantly affected by changes in extracellular pH, Ca2+ and Na+, excluding a significant role for the respective exchangers in the regulation of Mg2+ homeostasis. The resting free Mg2+ concentration was also not significantly affected by exposure to 0 Mg2+ suggesting the lack of significant basal Mg2+ flux. Opening the cGMP-gated channels led to a significant increase in the Mg2+ concentration in the absence of Na+ and Ca2+, but not in their presence, indicating that depolarization can cause a significant Mg2+ influx only in the absence of other permeant ions, but not under physiological conditions. Finally, light stimulation did not change the Mg2+ concentration in the outer segments of dark-adapted photoreceptors. The results suggest that there are no influx and efflux pathways that can significantly affect the Mg2+ concentration in the outer segment under physiological conditions. Therefore, it is unlikely that Mg2+ plays a significant role in the dynamic modulation of phototransduction.