Abstract
Abstract: :
Purpose: Rod outer segments contain millimolar concentrations of retinaldehyde in the 11–cis form, bound to rhodopsin and released as the all–trans form following photoactivation (1). Given that a large amount of retinaldehyde could be released when retina is exposed to bright light, it was of interest to determine whether retinaldehyde could inhibit gap junctional communication between neighboring cells – between retinal neurons and between pigment epithelial cells. Methods: Gap junctional intercellular communication was measured by the scrape–loading method (2). Confluent monolayers of retinal pigment epithelial cells, and transformed ganglion cells, as well as liver and lens epithelial cells were investigated. Monolayers of cells in 35–mm dishes were incubated with retinaldehyde or vehicle for 30 min at 37oC, washed and covered with a tracer solution (Lucifer yellow or Neurobiotin). Three scrapes were made in the monolayer and dye transfer was allowed to progress for 4 min. After histological processing, the tracer–labeled cells were observed under a fluorescent microscope. Results: All–trans–retinaldehyde inhibited tracer transfer between cells of every type tested. The inhibition was stronger than that by glycyrrhetinic acid and retinoic acid, two well known inhibitors of gap junctional communication. Twenty micromolar retinaldehyde, less than 1% of the concentration in rod outer segments, was sufficient for nearly complete inhibition. Both 11–cis and all–trans forms were effective as inhibitors. Inhibition depended upon both the concentration of retinaldehyde and the length of incubation. At 50 micromolar retinaldehyde, inhibition developed rapidly, within 2 minutes, but dye transfer ability recovered rather slowly, 2–6 hours after removal of the inhibitor. Conclusions: Retinaldehyde appears capable of serving as a non–specific inhibitor of gap junctional communication. Since it is present at high concentration only in retinal photoreceptors, its likely physiological role as an inhibitor of gap junctional communication might be restricted to these cells. Retinaldehyde might serve as an inhibitor of rod–cone gap junctional communication in bright light. (1) Liebman, P. A., Biophys.J. 2, 161–178, 1962. (2) el Fouly, M. H. et al., Exp.Cell Res. 168, 422–430, 1987.
Keywords: cell–cell communication • retinoids/retinoid binding proteins • gap junctions/coupling