Rushton's paradox: rod dark adaptation after flash photolysis

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Rod dark adaptations after a photoregenerating flash and quantum equivalent 30 sec bleach are found to be in exact agreement, while the measured rhodopsin regenerations are grossly different. This finding confirms and clarifies 'Rushton's paradox', the failure of the Dowling Rushton equation (linking log sensitivity linearly with unregenerated rhodopsin) to account for human rod dark adaptation after flash photolysis. The hypothesis that the agreement between rod dark adaptation curves after a photoregenerating flash and after a quantum equivalent 30 sec bleach is coincidental is rejected on the basis of two classes of experiments. Rod 'bleaching' adaptation is demonstrated to be entirely determined by the number of rhodopsin molecules which absorb at least one quantum in a temporal period T, whose range includes the time interval 600 μsec ≤ T ≤ 30 sec. This generalization obtains over the entire scotopic energy range ( 3 log units) where rod dark adaptation has been studied. Thus, the state of 'bleaching' adaptation is determined by some by product of the normal chain of events in scotopic excitation. About this by product three important deductions are made: its production is a monotonic function of the initial effective quantum absorptions; its production occurs before the metarhodopsin I → to metarhodopsin II dark reaction; it cannot be any photoproduct of the rhodopsin cycle.

Original languageEnglish (US)
Pages (from-to)413-431
Number of pages19
JournalJournal of Physiology
Issue number2
StatePublished - 1975
Externally publishedYes

ASJC Scopus subject areas

  • Physiology


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