Retinal Metabolism

Question 1

Sources of metabolites for retina

Answer 1

Glycogen stores (retina)
Glucose (aqueous)
Amino acids (aqueous)
A variety of saturated and unsaturated fatty acids (aqueous)

Question 2

Delivery of glucose into the retina

Answer 2

Derived from the serum

Simple diffusion into aqueous

Question 3

Glucose in the aqueous

Answer 3

Due to the open system nature of glucose metabolism, there is a constant flux of glucose from the plasma into the aqueous, and utilization by the retinal tissues
-aqueous glucose levels is ~80% of serum glucose level

Question 4

Glucose metabolism of the retina, ciliary epithelium, lens and cornea contribute to the production of _________ even with aerobic conditions

Answer 4

Lactic acid

Question 5

Trafficking of lactic acid in the eye

Answer 5

Lactic acid, although produced in significant quantities, does not accumulate in the aqueous of the posterior chamber to levels much above serum levels. Suggesting a mechanism for trafficking of lactic acid away from the aqueous

Question 6

Accumulation of lactic acid in the aqueous

Answer 6

Does not accumulate, has a mechanisms for quick trafficking of lactic acid away from the aqueous

Question 7

Although lactic acid is always produced by the tissues listed above (lens, cornea, ciliary epithelium, retina), when is lactic acid produced in the largest quantities?

Answer 7

During sleep or conditions that block atmospheric O2 from being absorbed by the eye. Aqueous O2 levels decreases significantly during sleep, resulting in less aerobic metabolism, thus production of more lactic acid

Question 8

What effect of topical epinephrine to the eye would you expect on the metabolism?

Answer 8

Vasoconstriction, reduction in oxygen flow, increase in lactic acid

Question 9

What supports the retinal tissue metabolism

Answer 9

Vitreous

Question 10

Where does the retina get O2 from

Answer 10

In addition to the aqueous providing O2 derived directly from the atmosphere, the retina also abstains O2 from the vitreous.

Question 11

O2 concentrations across the vitreous increase int he ______ direction in areas furthest away from the _____ of the eye

Answer 11

Posterior

Anterior

Question 12

Evidence for O2 support by the vitreous for the retina

Answer 12

Retinal tears dramatically increase the level of O2 int he vitreous near the tear site

Question 13

Other than O2, what does the vitreous do for the retina

Answer 13

Serves as a metabolic reservoir for the retina providing short-term nutrients during emergencies

Question 14

Vitreous’ role in retinal ischemia

Answer 14

After 1 hour:

• retinal glycogen decreased 35%
• vitreous glucose decreased to 60% and lactic acid increased slightly
• vitreous glycogen decreased

Question 15

Ascorbic acid in the vitreous

Answer 15

• High levels
• Vitreous to serum ratio 9:1
• Concentrated by active transport in ciliary body epithelium
• supplied by the diet

Question 16

Reason for high ascorbic acid in the vitreous

Answer 16

Can absorb UV light

Serves as a free radical scavenger

Question 17

Benefits of ascorbic acid in the vitreous

Answer 17

To protect the retina (and lens) from metabolic and light induced production of singlet oxygen

Question 18

Evidence of the benefits of ascorbic acid in the vitreous

Answer 18

Supplements are shown to reduce the irreversible type I form of light damage to dark-reared rats and shift light damage to the reversible type II form of damage in cycle-light-reared animals

Question 19

What can protect against oxidative damage due to inflammation

Answer 19

Ascorbic acid

Question 20

Evidence that ascorbic acid protects against inflammation

Answer 20

Endotoxins induced ocular inflammation in rabbit resulted in an increased ascorbic acid concentration in the vitreous that was not derived from the aqueous
-it was likely that ascorbic acid synthesis was up regulated in response to inflammatory mediators such as histamine

Question 21

What would be the significance of inflammation and ascorbic acid in humans

Answer 21

If you are having an inflammatory response, you should supplement with vitamin C to get more ascorbic acid since we don’t synthesize our own

Question 22

What transporter used for glucose in retina?

Answer 22

GLUT3

Question 23

Which of the following is insulin depending and would not be found in the retina?

Answer 23

GLUT 4

Question 24

What si the major energy metabolism pathways in the retina?

Answer 24

Glycolysis

TCA and electron transport

Question 25

When is lactic acid produced int he largest quantities in the retina

Answer 25

When eyes are closed, sleeping

Question 26

What are the important things that ascorbic acid do to protect the lens and the retina?

Answer 26

Protects against UV light damage and protects against radical damage and inflammatory damage

Question 27

When the eye is subject to endotoxins that cause an inflammatory response, which of the following would likely protect the eye?

Answer 27

Ascorbic acid