Effects of Ca2+ on the decay of rhodopsin photoproducts and photoreceptor adaptation in the isolated bullfrog retina.

Relationships between the change in threshold of the fast PIII response and the rhodopsin photoproduct content were studied in the isolated bullfrog retina treated with Ba2+ and aspartate. A reduction in the extracellular Ca2+ concentration from 1.0 to 0.01 mM caused an increase in the decay rate of 380 nm absorbing photoproducts (metarhodopsin II and retinal) and metarhodopsin III. In both normal and low Ca2+ solutions, the threshold change after bleaching of about 50% of rhodopsin was closely related to the 380 nm absorbing photoproducts. The results suggest that the intermediate adaptation process of rods may be regulated by the 380 nm absorbing photoproducts.