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Abstract
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.