Flashcards in Phototransduction/Photoreceptors – Dr. Groseclose Deck (13):
Discuss the role of vitamin A in vision.
Retinal (derivative of vitamin A, which is coverted to retinol and then finally reintal) is needed for binding to rhodopsin.
Describe the phenomenon of “light adaptation” in rod cells, being sure to mention “calcium” and “cGMP”
Light activated rhodopsin (opsin + all trans retinal) is phosphorylated by rhodopsin kinase which can then bind with arrestin. This new complex is no longer able to activate transducin, and therefore the photoreceptor remains depolarized. There is a Ca2+-dependent kinase that can also phosphorylate rhodopsin.
cGMP, which is needed to keep the photoreceptor depolarized is made by the action of guanylate cyclase (GC). GC is inhibited by Ca2+.
With light, the Na+/Ca2+/K+ channel is closed, meaning less Ca2+ and therefore less inhibition of GC. Therefore cGMP concentrations can be restored more quickly to re-depolarize the photoreceptor.
What retinal cells besides rods and cones are also photosensitive? What is the light-absorbing pigment of those cells?
Light absorbing Pigment = Melanoxin
What is the “dark current” and how is it generated?
The dark current is the flow of ions from outside the rod cell to inside the rod cell and vice versa via the "sodium channels" and ATPase. It is powered by the hydrolysis of ATP.
What ions pass through the “sodium channels”?
Sodium and Calcium (and sometimes Mg2+)
What are the 3 most abundant ions of the dark current?
Sodium, Calcium, and Potassium
What keeps the sodium channels open?
Cyclic GMP and the chronic depolarization of the cell (by being in the dark) keeps the sodium channels open.
Describe how light causes hyperpolarization of the photoreceptor membrane.
The absorption of light energy causes the [cGMP] to go down, then causing the channels to close and the cell to hyperpolarize, since the concentration outside will be greater than the concentration inside. Also, Glutamate secretion stops.
Describe rhodopsin. What is the name of its protein, and what is its light-absorbing group?
Rhodopsin is the light receptor. Light changes the conformation of rhodopsin, allowing it to accomidate the g-protein.
Describe what happens to rhodopsin after it has responded to light and been phosphorylated by rhodopsin kinase.
Rhodopsin is the light receptor. Light changes the conformation of rhodopsin, allowing it to accommodate the g-protein Transducin. When Transducin binds, it causes GDP to float away and GTP bind to the site. [REPLACEMENT procedure]. This causes alpha subunit to leave (w/ GTP), where it binds to cGMP phosphodiesterase, activating it's ability to convert cGMP to GMP. This is what causes the close of Na+ channels.
Describe the role of the pigmented epithelium cells with respect to retinal cycling.
Retinal pigmented epithelium shields the retina from excessive light and supplies nutrition, support and immune functions. Serves as the limiting transport factor in the retina cycle.
True or False: The more Na+/Ca2+ channels that are open, the less sensitive the photoreceptors are.