Role of protein kinase C in light adaptation of molluscan microvillar photoreceptors.
The mechanisms by which Ca2+ regulates light adaptation in microvillar photoreceptors remain poorly understood. Protein kinase C (PKC) is a likely candidate, both because some sub-types are activated by Ca2+ and because of its association with the macromolecular 'light-transduction complex' in Drosophila. We investigated the possible role of PKC in the modulation of the light response in molluscan photoreceptors. Western blot analysis with isoform-specific antibodies revealed the presence of PKCalpha in retinal homogenates. Immunocytochemistry in isolated cell preparations confirmed PKCalpha localization in microvillar photoreceptors, preferentially confined to the light-sensing lobe. Light stimulation induced translocation of PKCalpha immunofluorescence to the photosensitive membrane, an effect that provides independent evidence for PKC activation by illumination; a similar outcome was observed after incubation with the phorbol ester PMA. Several chemically distinct activators of PKC, such as phorbol-12-myristate-13-acetate (PMA), (-)indolactam V and 1,2,-dioctanoyl-sn-glycerol (DOG) inhibited the light response of voltage-clamped microvillar photoreceptors, but were ineffective in ciliary photoreceptors, in which light does not activate the G(q)/PLC cascade, nor elevates intracellular Ca2+. Pharmacological inhibition of PKC antagonized the desensitization produced by adapting lights and also caused a small, but consistent enhancement of basal sensitivity. These results strongly support the involvement of PKC activation in the light-dependent regulation of response sensitivity. However, unlike adapting background light or elevation of [Ca2+]i, PKC activators did not speed up the photoresponse, nor did PKC inhibitors antagonize the accelerating effects of background adaptation, suggesting that modulation of photoresponse time course may involve a separate Ca2+-dependent signal.
Department of Physiology and Biophysics, Boston University School of Medicine, Boston, MA 02118, USA.
Giuseppe
G
Piccoli
Maria
M
Del Pilar Gomez
Enrico
E
Nasi
D000221:Adaptation, Ocular / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007211:Indoles / Q000494:pharmacology; D007527:Isoenzymes / Q000032:analysis / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology; D008974:Mollusca; D010775:Photic Stimulation; D017956:Photoreceptor Cells, Invertebrate / Q000201:enzymology*; D011493:Protein Kinase C / Q000032:analysis / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051571:Protein Kinase C-alpha; D011758:Pyrroles / Q000494:pharmacology
D004791:Enzyme Inhibitors; D007211:Indoles; D007527:Isoenzymes; D011758:Pyrroles; C085989:Ro 32-0432; D011493:Protein Kinase C; D051571:Protein Kinase C-alpha; D002118:Calcium