Okawa Haruhisa, Sampath Alapakkam P, Laughlin Simon B, Fain Gordon L
Neuroscience Graduate Program, Zilkha Neurogenetic Institute, USC Keck School of Medicine, Los Angeles, CA 90089, USA.
Curr Biol. 2008 Dec 23;18(24):1917-21. doi: 10.1016/j.cub.2008.10.029. Epub 2008 Dec 11.
Why do vertebrates use rods and cones that hyperpolarize, when in insect eyes a single depolarizing photoreceptor can function at all light levels? We answer this question at least in part with a comprehensive assessment of ATP consumption for mammalian rods from voltages and currents and recently published physiological and biochemical data. In darkness, rods consume 10(8) ATP s(-1), about the same as Drosophila photoreceptors. Ion fluxes associated with phototransduction and synaptic transmission dominate; as in CNS, the contribution of enzymes of the second-messenger cascade is surprisingly small. Suppression of rod responses in daylight closes light-gated channels and reduces total energy consumption by >75%, but in Drosophila light opens channels and increases consumption 5-fold. Rods therefore provide an energy-efficient mechanism not present in rhabdomeric photoreceptors. Rods are metabolically less "costly" than cones, because cones do not saturate in bright light and use more ATP s(-1) for transducin activation and rhodopsin phosphorylation. This helps to explain why the vertebrate retina is duplex, and why some diurnal animals like primates have a small number of cones, concentrated in a region of high acuity.
当昆虫眼睛中的单个去极化光感受器在所有光照水平下都能发挥作用时，为什么脊椎动物使用超极化的视杆细胞和视锥细胞呢？我们至少部分地通过根据电压和电流以及最近发表的生理和生化数据对哺乳动物视杆细胞的ATP消耗进行全面评估来回答这个问题。在黑暗中，视杆细胞消耗10⁸个ATP·s⁻¹，与果蝇光感受器消耗的量大致相同。与光转导和突触传递相关的离子通量占主导；与中枢神经系统一样，第二信使级联反应的酶的贡献小得出奇。在白天对视杆细胞反应的抑制会关闭光门控通道，并使总能量消耗降低超过75%，但在果蝇中，光会打开通道并使消耗增加5倍。因此，视杆细胞提供了一种横纹肌光感受器中不存在的节能机制。视杆细胞在代谢上比视锥细胞“成本”更低，因为视锥细胞在强光下不会饱和，并且在转导素激活和视紫红质磷酸化过程中消耗更多的ATP·s⁻¹。这有助于解释为什么脊椎动物的视网膜是双功能的，以及为什么一些昼行性动物，如灵长类动物，视锥细胞数量较少，且集中在高敏锐度区域。
