Szikra T, Krizaj D
Department of Ophthalmology, UCSF School of Medicine, 10 Koret Way, San Francisco, CA 94143-0730, USA.
Neuroscience. 2006 Aug 11;141(1):143-55. doi: 10.1016/j.neuroscience.2006.03.054. Epub 2006 May 6.
Vertebrate photoreceptors consist of strictly delimited subcellular domains: the outer segment, ellipsoid, cell body and synaptic terminal, each hosting crucial cellular functions, including phototransduction, oxidative metabolism, gene expression and transmitter release. We used optical imaging to explore the spatiotemporal dynamics of Ca(2+) signaling in non-outer segment regions of rods and cones. Sustained depolarization, designed to emulate photoreceptor activation in the darkness, evoked a standing Ca(2+) gradient in tiger salamander photoreceptors with spatially-averaged intracellular Ca(2+) concentration within synaptic terminals of approximately 2 microM and lower (approximately 750 nM) intracellular calcium concentration in the ellipsoid. Measurements from axotomized cell bodies and isolated ellipsoids showed that Ca(2+) enters the two compartments via both local L-type Ca(2+) channels and diffusion. The results from optical imaging studies were supported by immunostaining analysis. L-type voltage-operated Ca(2+) channels and plasma membrane Ca(2+) ATPases were highly expressed in synaptic terminals with progressively lower expression levels in the cell body and ellipsoid. These results show photoreceptor Ca(2+) homeostasis is controlled in a region-specific manner by direct Ca(2+) entry and diffusion as well as Ca(2+) extrusion. Moreover, quantitative measurement of intracellular calcium concentration levels in different photoreceptor compartments indicates that the dynamic range of Ca(2+) signaling in photoreceptors is approximately 40-fold, from approximately 50 nM in the light to approximately 2 microM in darkness.
外段、椭球体、细胞体和突触终末，每个结构域都承担着关键的细胞功能，包括光转导、氧化代谢、基因表达和神经递质释放。我们利用光学成像技术探究了视杆细胞和视锥细胞非外段区域Ca(2+)信号的时空动态变化。持续去极化旨在模拟光感受器在黑暗中的激活状态，在虎螈光感受器中诱发了一个稳定的Ca(2+)梯度，突触终末内的空间平均细胞内Ca(2+)浓度约为2微摩尔，而椭球体内的细胞内钙浓度较低（约750纳摩尔）。对轴突切断的细胞体和分离的椭球体的测量结果表明，Ca(2+)通过局部L型Ca(2+)通道和扩散进入这两个区室。光学成像研究的结果得到了免疫染色分析的支持。L型电压门控Ca(2+)通道和质膜Ca(2+)ATP酶在突触终末高度表达，在细胞体和椭球体内的表达水平逐渐降低。这些结果表明，光感受器Ca(2+)稳态通过Ca(2+)的直接进入、扩散以及Ca(2+)的外流以区域特异性方式受到控制。此外，对不同光感受器区室中细胞内钙浓度水平的定量测量表明，光感受器中Ca(2+)信号的动态范围约为40倍，从光照下的约50纳摩尔到黑暗中的约2微摩尔。
