Lecture 12 - Retinal Physiology

What is one function of the RPE in relation to the blood-retinal barrier?

It forms part of the blood-retinal barrier

What role does the RPE play in nutrient absorption?

Selective absorption of nutrients from the choriocapillaris

What role does the RPE play in waste removal?

Excretion of metabolic waste through the choriocapillaris

Where does exchange between RPE and retina occur?

Through the interphotoreceptor matrix (IPM)

What is one phagocytic function of the RPE?

Phagocytosis of photoreceptor discs

What optical function does the RPE perform?

Absorption of light

What pigment in the RPE absorbs light?

Melanin

What wavelengths does melanin absorb?

UV and visible light

What structural role does the RPE have in the retina?

Formation of the interphotoreceptor matrix (IPM)

What is the role of the RPE in vitamin A metabolism?

Storage and metabolism of vitamin A

Why is vitamin A important in the retina?

It is a component of photopigments

What growth factors are produced by the RPE?

Vascular endothelial growth factor (VEGF) and pigment epithelial derived factor (PEDF)

What type of fluid transport occurs in the RPE?

Isotonic fluid transport

What does isotonic transport mean in the RPE?

No difference in osmolarity between cell and extracellular space

Does isotonic mean equal charge?

No, isotonic does not mean equal charge

What electrical potential exists across the RPE?

~30 mV transepithelial potential difference

Which side of the RPE is more positive?

Apical side is more positive than basal side

What determines ion permeability in the RPE membrane?

Location of transporters

What pump is involved in RPE ion transport?

Na+/K+ ATPase pump

What cotransporter moves Na+, K+, and Cl-?

Na+/K+/Cl- cotransporter

What cotransporter moves Na+ and HCO3-?

Na+/2HCO3- cotransporter

What cotransporter moves H+ and lactate?

H+/lactate cotransporter

What exchanger moves Na+ and H+?

Na+/H+ exchanger

What exchanger moves Cl- and HCO3-?

Cl-/HCO3- exchanger

What channels assist in ion movement in the RPE?

Cl- channels

What proteins facilitate water transport in the RPE?

Aquaporins

What transports glucose in the RPE?

Glucose transporters

What is the direction of net Na+ transport in the RPE?

Toward the choroid

What determines the movement of other ions in the RPE?

Na+ gradient

Which ions move into the RPE due to the Na+ gradient?

HCO3-, K+, Cl-

Can the RPE use lactate metabolically?

Yes

What is the direction of net Cl- transport in the RPE?

Toward the choroid

Which ion is more abundant in RPE transport: Cl- or HCO3-?

Cl-

What is the main driver of water transport in the RPE?

Cl- transport

What enzyme in the RPE catalyzes CO2 + H2O → HCO3- + H+?

Carbonic anhydrase

What function does this reaction serve in the RPE?

Buffer function

What is continuously produced in this reaction?

HCO3-

What does the HCO3- exchanger do in the RPE?

Brings Cl- into the cell and secretes HCO3- toward the choroid

What does the Na+/HCO3- cotransporter do?

Moves Na+ and HCO3- toward the choroid

What is the direction of net HCO3- transport?

Toward the choroid

What role does HCO3- play in fluid movement?

Helps determine direction of water movement

What is the direction of net glucose transport in the RPE?

Toward the retina

Why is glucose transported toward the retina?

Retina consumes more glucose than the RPE

What is the direction of net lactate transport in the RPE?

Toward the choroid

Why is lactate transported toward the choroid?

Retina releases more lactate than the RPE

Which tissue consumes more lactate?

RPE consumes more lactate than the retina

Why does the RPE use lactate metabolically?

To conserve glucose for the retina

Which retinal cells can also use lactate as an energy source?

Müller cells and ganglion cells

What structure of the photoreceptor contains stacked membranous discs?

Outer segment

What is the function of the outer segment discs?

They house visual pigments

What surrounds the outer segment of photoreceptors?

Interphotoreceptor matrix (IPM)

What connects the outer segment to the inner segment in photoreceptors?

Cilium

What is the function of the inner segment of photoreceptors?

Produces photopigments

What are the two zones of the inner segment?

Ellipsoid zone and myoid zone

What is contained in the ellipsoid zone of the inner segment?

Mitochondria

What is contained in the myoid zone of the inner segment?

Golgi apparatus and endoplasmic reticulum

What connects the inner segment to the cell body in photoreceptors?

Outer fiber

What is located in the photoreceptor cell body?

Nucleus

What connects the photoreceptor cell body to other retinal cells?

Inner fiber (axon)

What is found at the end of the inner fiber?

Synaptic terminals with synaptic vesicles

What neurotransmitter do both rods and cones release?

Glutamate

Which cells are photoreceptors connected to?

RPE and bipolar cells

What type of vision are rods responsible for?

Scotopic (low-light) vision

What type of vision are cones responsible for?

Photopic (bright-light) and color vision

Which photoreceptors are more numerous in the retina?

Rods (except at the fovea)

How do outer segments differ between rods and cones?

Outer segments are shorter in cones

How does the plasmalemma differ between rods and cones?

Separate from discs in rods and continuous in cones

How does convergence differ between rods and cones?

More rods connect to a single ganglion cell compared to cones

What differs in rods and cones regarding wavelength sensitivity?

Peak wavelength absorption varies

What is the approximate lifespan of photoreceptor discs?

~10 days

What happens to older photoreceptor discs over time?

They are displaced and shed

Where are new photoreceptor discs formed?

Inner segment

How do discs move to the outer segment?

Through the cilium

What happens to older discs as new discs form?

They are displaced

Where are old discs shed from?

Tip of the outer segment

How are discs typically shed?

As a group, not individually

Which cells assist in removing shed photoreceptor discs?

RPE cells

What happens to shed discs after removal?

Taken up by RPE cells

How do RPE cells degrade photoreceptor discs?

Phagocytosis

What are the steps of RPE phagocytosis?

Recognition/binding, internalization, digestion

When does photoreceptor disc shedding occur?

When photoreceptor is less active

When do cones shed their discs?

At night

When do rods shed their discs?

Early morning

What is the purpose of the photoreceptor renewal system?

To combat accumulation of damage from light absorption

Why is the retina susceptible to damage?

High levels of light exposure

What does excess light exposure produce?

Oxidative species such as free radicals and oxidized membrane lipids

What are photopigments?

Photosensitive pigments that undergo chemical changes when absorbing photons of light

What are the two components of photopigments?

Opsin and chromophore

What is opsin?

A membrane protein interlaced into disc membranes

What determines the absorption profile of a photopigment?

Opsin

How many times does opsin cross the membrane bilayer?

Seven times

What is the chromophore in photopigments?

Molecule that absorbs photons of light

What is the typical chromophore in photoreceptors?

11-cis-retinal

What is 11-cis-retinal derived from?

Vitamin A

Why is RPE vitamin A storage important?

It supports photopigment function

What is the photopigment in rods?

Rhodopsin (~498 nm)

What are the photopigments in cones?

Erythrolabe (~588 nm), Chlorolabe (~532 nm), Cyanolabe (~420 nm)

Which cones contain erythrolabe?

L-cones

Which cones contain chlorolabe?

M-cones

Which cones contain cyanolabe?

S-cones