1. ATP Consumption by Mammalian Rod Photoreceptors in Darkness and in Light
- Author
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Okawa, Haruhisa, Sampath, Alapakkam P., Laughlin, Simon B., and Fain, Gordon L.
- Subjects
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PHOTORECEPTORS , *PHOTOBIOLOGY , *INTERPHOTORECEPTOR matrix , *DROSOPHILA , *NEURAL transmission , *ADENOSINE triphosphate - Abstract
Summary: 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 108 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. [Copyright &y& Elsevier]
- Published
- 2008
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