The late photoproducts that result from the isomerization of rhodopsin have been identified in the isolated all-rod retina of the skate by means of rapid spectrophotometry. The sequence in which these intermediates form and decay could be described by a scheme that incorporates two pathways for the degradation of metarhodopsin II (MII) to retinol: one via metarhodopsin III (MIII) and the other (which bypasses MIII) through retinal. Computer simulation of the model yielded rate constants and spectral absorbance coefficients for the late photoproducts which fit experimental data obtained at temperatures ranging from 7 degrees C to 27 degrees C. Comparing the kinetics of the thermal reactions with the changes in rod threshold that occur during dark adaptation indicated that the decay of MII and the fall in receptor thresholds exhibit similarities with regard to their temperature dependence. However, the addition of 2 mM hydroxylamine to a perfusate bathing the retina greatly accelerated the photochemical reactions, but had no significant effect on the rate of recovery of rod sensitivity. It appears, therefore, that the late bleaching intermediates do not control the sensitivities of skate rods during dark adaptation.

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