Lens Metabolism

Properties of the lens similar to cornea to maintain clarity? (4)

No blood vessels

Few cellular organelles

Orderly arrangement of fibers and proteins (crystallin's)

Minimized space between cells and fibers

Sodium concentration is (low/high) in the lens.

Low

Calcium concentration is (low/high) in the lens.

Low

Chloride concentration is (low/high) in the lens.

Low

Potassium concentration is (low/high) in the lens.

High

Lens has an active process to maintain what two things?

Water content

Ionic/molecular content

Is the lens membrane permeable?

Yes

A pump/leak system is a combination of ______ ________ and __________ _________

Active transport

Membrane permeability

Anterior surface of lens epithelium contains what ion transporter?

Na/K ATPase

Na/K ATPase pumps K (in/out) and Na (in/out)

K in

Na out

Na/K ATPase generates an _______________ ________ to circulate ions

Electrochemical gradient

Which region of the lens has the most sodium being pumped out?

Equator

How does the lens control calcium content?

Ca driven into lens fibers, and pumped out by surface cells via Na/Ca exchange and Ca ATPase

What can happen if our lens has too much calcium?

Aggregation of high MW proteins

Cytotoxicity

Diffusion of water into the lens is kept in check by ___ __________

Ion homeostasis

Other things that require transport in the lens: (3)

Amino acids

Ascorbate

Glutathione

Proteins in the lens need to be synthesized before what?

Separation from basement membrane

High concentrations of what in the cytoplasm increases the index of refraction of the lens fibers?

crystallin's

What percent of the lens is made of proteins?

30-35%, mostly crystallins

Two families of crystallins?

Alpha

Beta/Gamma

Subtypes of alpha crystallins?

Alpha A and alpha B

What kind of crystallins are considered the "chaperone" crystallins?

Alpha

_____ crystallins associate in the cytoplasm to make high MW complexes.

Alpha

Function of alpha crystallins?

Stabilize other crystallins so they do not aggregate

How many beta/gamma crystallins does our lens have?

6 types of beta

3 types of gamma

What is the role of beta/gamma crystallins?

buffering calcium by binding it

What is glutathione?

A tripeptide made by lens epithelial cells

Function of glutathione?

Detoxify peroxides

Two forms of glutathione?

Reduced

Oxidized

Can reduced glutathione be regenrated?

Yes, from oxidized glutathione

Glutathione can form _________ bonds with oxidized sulfahydril groups of _______

Disulfide

Proteins

A glutathione-protein mixed disulfide can be reduced by a...

Second molecule of glutathione

Where is GSSG reduced?

in more superficial layers

Ascorbate is actively transported from _____ to _______

Blood to aqueous

Ascorbate is readily __________ into dehydroascorbic acid

Oxidized

Oxidized ascorbate can be reduced using a ___________-dependent process

Glutathione

Main function of ascorbate?

React with free radicals and other oxidants

Glucose supply for the lens?

Aqueous humor

Lens mostly depends on (aerobic/anaerobic) metabolism of glucose

Anaerobic (glycolysis)

How many ATP does glycolysis provide from one molecule of glucose?

2

The lens gets 80% of its energy from ________ and the other 20% comes from ________.

glycolysis, kreb's cycle

Lens has a (high/low) concentration of oxygen

Low

Other sources of energy for the lens? (2)

Hexose monophosphate shunt

Sorbitol (polyol) pathway

What happens in the polyol pathway?

glucos is reduced to sorbitol and then to fructose

When there is a high amount of glucose in the aqueous, it is pushed toward the polyol pathway leading to what?

an excess of sorbitol which draws in excess water causing swelling

A ________ is an opacification of the lens causing light scattering or absorption.

Cataract

2 different locations for cataracts?

On-axis

Off-axis

(On/Off) axis cataracts generally have a worse visual effect.

On

Three major types of cataracts?

Nuclear

Cortical

Subcapsular (posterior)

Risk factors for cataracts (11 sorry)

Age

Lower socioeconomic status

Women

Smoking/alcohol

Dark iris

Steriods

Diseases

Radiation

Trauma

Thin lens

Thick lens

Most common type of cataract?

Nuclear

A nuclear cataract affects the (newest/oldest) fibers

Oldest (center of lens)

Nuclear cataracts cause a ______ shift

Myopic

In a nuclear cataract, the nucleus _______.

Hardens

Nuclear cataracts are associated with _________ _______

Oxidative damage

What changes in the eye does oxidative damage cause in nuclear cataracts? (2)

Aggregation of proteins as disulfide bonds form

Crystallins get larger

Two pieces of supporting evidence for nuclear cataracts?

Hyperbaric oxygen patients

Removal of vitreous body

Both see increase nuclear cataract rates

Which cells are affected in cortical cataracts?

Mature fibers close to surface

What region of lens do cortical cataracts usually start in?

Periphery

Cortical cataracts are associated with _____ or _____ imbalance

Ionic or Water

Cortical cataracts usually form a _____ pattern

Spoke

Are early cortical cataracts usually visually significant?

No

What causes opacification in cortical cataracts?

Plasma membranes rupture and proteins aggregate

What causes the spread of cortical cataracts?

Calcium imbalance

Where are posterior subcapsular cataracts loacted?

Posterior pole, just beneath the capsule

Posterior subcapsular cataracts are a cluster of what?

Swollen epithelial like cells

Are posterior subcapsular cataracts visually significant?

Yes

Posterior subcapsular cataracts are associated with what?

Disorganization in a region of the lens equator

What can cause posterior subcapsular cataracts?

Steroids

Radiation

Are posterior subcapsular cataracts common?

No, not on their own.

What is the name for a rare cataract in diabetic patients?

Snowflake cataracts

Features of snowflake cataracts? (3)

Posterior subcapsular

Rapid onset

Younger patients

___________ is a congenital condition with a deficiency in GPUT enzyme

Galactosemia

What does galactosemia cause in the lens?

Central oil droplet cataract since the body cannot metabolize galactose

What kind of cataract surgery is the most common today?

Extracapsular extraction

If epithelial cells migrate posteriorly after cataract surgery, we can get a _________ ________ _______

Posterior capsular opacity

How is a posterior capsular opacity treated?

Laser

WHat is a hypermature/Morgagnian cataract?

mature cataract with liquefaction of the cortex and sinking of the nucleus to the bottom of the lens capsule

What kind of cataract is usually congenital and is blueish in color?

Cerulean cataract

What kind of cataract is associated with myotonic dystrophy?

Christmas tree cataract