title: Metabolic responses to light in monkey photoreceptors.
abstract: OBJECTIVE
  Transient changes in intraretinal oxygen tension (PO(2)) in response to light stimuli were studied in order to understand the dynamics of light-evoked changes in photoreceptor oxidative metabolism.
  
  
  METHODS
  PO(2) changes during illumination were recorded by double-barreled microelectrodes in the outer part of the perifoveal retina in five macaques (Rhesus and Cynomolgus) and were fitted to a single exponential equation to obtain the time constant (tau) and maximum PO(2) change.
  
  
  RESULTS
  At the onset of light, PO(2) increased at all illuminations in all animals. The magnitude of the light-evoked PO(2) change increased with increasing illumination over 3-4 log units but decreased in all animals at the maximum illumination. The median time constant of the PO(2) change (tau) was 26 sec and was not correlated with illumination. The time constant for the return to darkness was similar for illuminations below rod saturation. Since O(2) diffusion is fast over the short distance from the choroid to the inner segments, tau reflects the time course of the underlying change in oxidative metabolism.
  
  
  CONCLUSIONS
  Previous results suggested that two competing processes influence the change in photoreceptor oxidative metabolism with light, Na(+)/K(+) pumping and cyclic guanosine monophosphate (cGMP) turnover. Because a single exponential fitted the PO(2) data, it appears that these processes have time constants that differ by no more than a few seconds in primate. In monkeys, tau is longer than previously reported values for cat and rat. Longer time constants are related to larger photoreceptor volume, possibly because metabolic rate is controlled by intracellular Na(+), and a change in intracellular Na(+) after the onset of illumination occurs more slowly in larger photoreceptors. The "metabolic threshold" illumination that reduced oxygen consumption by about 10% is approximately the same as the illumination that closes 10% of the light-dependent cation channels that are open in the dark.
authors.affiliation: Department of Biomedical Engineering, Northwestern University, Evanston, Illinois 60208-3107, USA.
authors.forename: Shufan
authors.initials: S
authors.lastname: Wang
authors.forename: Gülnur
authors.initials: G
authors.lastname: Birol
authors.forename: Ewa
authors.initials: E
authors.lastname: Budzynski
authors.forename: Robert
authors.initials: R
authors.lastname: Flynn
authors.forename: Robert A
authors.initials: RA
authors.lastname: Linsenmeier
mesh_terms: D000818:Animals; D008027:Light; D016014:Linear Models; D008251:Macaca / Q000378:metabolism*; D008252:Macaca fascicularis; D008253:Macaca mulatta; D008839:Microelectrodes; D008954:Models, Biological; D010100:Oxygen / Q000378:metabolism*; D010101:Oxygen Consumption / Q000528:radiation effects; D010313:Partial Pressure; D020419:Photoreceptor Cells, Vertebrate / Q000378:metabolism* / Q000528:radiation effects*; D013997:Time Factors
chemical_list: D010100:Oxygen
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