Photomembrane turnover in frog retina: light intensity and spectral correlates

Light regulates membrane turnover in vertebrate rod photoreceptor cells. Rods shed membrane-filled tips immediately after light onset, with light inhibiting the dark priming phase but initiating the light induction phase. This study examines the intensities and wavelengths of light that control these two shedding requirements, and demonstrates unexpected situations where red or dim lights are simultaneously dark to the dark priming mechanism and light to the light induction process. Since shedding takes place immediately following darkness we asked if dim or red light could substitute for true darkness and dark prime the retinas: our results confirm this. White light, less than 0.7 microE m m-2 sec-1 (0.15 W m2 or 40 lx), allows dark priming, and even 15 microE m-2 sec-1 of red fluorescent light dark primes as effectively as true darkness. Conversely, bright white light and wavelengths from 480 to 560 nm inhibit dark priming, implying that dark priming inhibition is a photopic mechanism transduced by photopigment in the 502-cone. We also asked if dim or red light could induce shedding, substituting for the bright light usually employed: again, the results confirm thus. White light as dim as 0.15 microE m-2 sec-1 induces shedding and red light is an effective light trigger. This light induction is initiated at all wavelengths tested (420-640 nm), with a maximum effect between 540 and 600 nm. Finally, we find that retinas shed continuously in red or dim white light. These lights substitute both for the darkness necessary for dark priming and for the light of light induction, extending shedding from the 20 min dark-light transition period to hours or days. We also find that the dim, red light of natural dawn is as effective a shedding stimulus as the sudden onset of bright laboratory light.