Aqueous Humor Dynamics, Composition, and Secretion

What are the major ions of the Aqueous Humor? (In order from most to least abundant)

Soduium, Chloride, Bicarbonate, Lactate (and other additional minor ions)

What are the most abundant ions of the AH?

Sodium and Chloride

What are the organic molecules present in the AH?

Ascorbate, Urea, Glucose, Lactate

What does Ascorbate do in the Aqueous Humor?

UV absorption in the Aqueous Humor

What does the Glucose do in the Aqueous Humor?

AH is the source of glucose for the cornea and lens

How does the pH of the Aqueous Humor compare to the blood plasma?

Lower pH than the blood plasma (7.21 as compared to 7.4)

Why does the Aqueous humor have more Lactate than the blood plasma?

Because metabolically active tissues are dumping lactate directly into AH (ex. corneal endothelium)

What allows the aqueous humor to have a different composition than the blood plasma?

The Blood-aqueous barrier

What are the notable differences between the aqueous humor and the plasma?

1. Protein content

2. Ascorbic Acid content

3. Lactate content

How is the protein content different between AH and plasma?

Blood plasma protein concentration is greater than in the AH

How is the Ascorbic acid/Ascorbate concentration different in AH from plasma?

Concentration is greater in the AH than the plasma

How is the Lactate concentration different in AH from the plasma? Why?

Concentration is higher in AH than plasma due to metabolic activity of Cornea and Lens

What type of UV light does Ascorbate absorb?

UV-B and UV-A light

How many ciliary processes are in the average human eye?

~80 ciliary processes

How many steps is the process of AH production?

2 Step process

What are the two steps of AH production?

1. Ultrafiltration

2. Ion/Water Secretion

What does ultrafiltration do for AH production?

Produces a reservoir of plasma ultrafiltrate in the ciliary process stroma

What is Ultrafiltration sensitive to?

IOP

What happens during the second step of AH production?

Movement of ions from plasma ultra-filtrate to AH

Where does the transport of ions/water occur during AH production?

Transport of ions occurs across the bilayered ciliary epithelium (pigmented and non-pigmented epithelium)

Ion/Water secretion during AH production is sensitive to what?

IOP

The movement of Ion/Water secretion during AH production is entirely (Paracellular/Transcellular)?

Entirely Transcellular

How does Capillary hydrostatic pressure aid in AH production?

Pushes fluid from capillaries through fenestrations into the stromal space in a process called ultrafiltration

What is the capillary derived solution from the capillary hydrostatic pressure that enters the AH referred to as?

Ultrafiltrate

Capillaries of the ciliary body stroma are blank.

Fenestrated

What is the definition of the process of ultrafiltration in general?

Solution movement across a semi-permeable barrier due to a pressure differential

What forces is ultrafiltration a balance between?

Oncotic and hydrostatic pressures

What two forces oppose ultrafiltration for AH prodution?

IOP and Oncotic pressure

What drives the oncotic pressure that opposes ultrafiltration in the ciliary process?

Blood serum Albumin

What must happen for ultrafiltration to occur in terms of forces?

Hydrostatic pressure must be larger than the sum of IOP + Oncotic pressure

As IOP rises, ultrafiltration in the ciliary processes (increases/decreases).

Decreases

What is the Starling Equation for Ultrafiltration?

Ultrafiltration = Hydrostatic pressure - Oncotic pressure

What is the starling equation for Hydrostatic pressure?

Hydrostatic pressure = Capillary pressure (P_c) - Stromal Pressure (P_s)

What is Capillary Pressure (P_c) driven by?

The heart/blood itself through vasculature

What is the starling equation for Oncotic Pressure?

(sigma)Oncotic Pressure = Plasma oncotic pressure (O_p) - Stromal oncotic pressure (O_s)

What is the approximate ultrafiltration value of the eye using the starling equation? [(Pc- IOP) – σ(Op-Os)]

~1mmHg ([(30-15)-14])

What happens to ultrafiltration if capillary pressure drops below a critical level?

Ultrafiltration will not occur because ultrafiltration cannot overcome both oncotic pressure and IOP

What happens to ultrafiltration if IOP increases to a critical level?

Ultrafiltration will also be inhibited

How is blood flow in the capillaries of the ciliary processes regulated?

Autonomically regulated in response to changes in blood pressure and IOP

When is Aqueous humor production independent of bloodflow?

When bloodflow is flowing within a normal range (75-100% of normal)

When is AH production dependent on bloodflow?

When bloodflow falls below the normal range (Range < 75%)

What happens to AH production under physiological conditions where blood flow is reduced <75% of normal?

Aqueous humor production is also reduced

Under normal physiological conditions (in awake individuals) aqueous humor production remains approximately blank (value).

2.5ul/min

How is aqueous humor production affected by normal levels of IOP?

Unaffected

How is AH production affected by IOP when it is very high (around 40)?

AH production decreases

What drives the relationship between IOP and AH production?

The ability of blood flow to be autoregulated (IOP and blood pressure have an affect on blood flow and therefore AH production)

What does the basolateral surface of the pigmented ciliary epithelium face?

Ciliary process stroma

What does the apical domain of the pigmented ciliary epithelium face?

The apical domain of the non-pigmented ciliary epithelium

What does the basolateral domain of the non-pigmented ciliary epithelium face?

The aqueous humor

Where are non-leaky tight junctions found in the ciliary processes?

Only found in the non-pigmented epithelium

Where are gap junctions found in the ciliary process epithelia?

Between pigmented ciliary epithelial cells, between non-pigmented ciliary epithelial cells and between pigmented and non-pigmented ciliary epithelial cells (apical-apical junction).

What do the tight junctions of the ciliary process epithelia help maintain?

Blood-Aqueous Barrier

How do gap junctions aid in Aqueous humor secretion?

Gap junctions allow free diffusion of ions and water between epithelial layers

How does the sodium content of the stroma ultrafiltrate compare to the intracellular concentration of sodium?

Stroma ultrafiltrate has a higher sodium concentration relative to intracellular concentrations

What do the tight junctions in the ciliary processes do?

Prevent paracellular transport

What is step 1 of the Aqueous humor secretion mechanism? (Three parts to this step)

1. High ciliary body stromal sodium drives NKCC by utilizing the Na+ concentration gradient

   a. NKCC loads the pigmented ciliary epithelium cell with Cl^-

   b. Transport occurs across the basolateral membrane of the pigmented ciliary epithelium

What is step 2 of the Aqueous humor secretion mechanism?

2. The sodium hydrogen exchanger (NHE) utilizes sodium gradient to transport H+ out of ciliary pigmented epithelial cells basolaterally

What is step 3 of the Aqueous humor secretion mechanism?

3. Carbonic anhydrase (CA) converts available CO2 into hydrogen ions and bicarbonate

Why is the carbonic anhydrase reaction favored in the direction of converting CO2 to hydrogen ions and bicarbonate?

Due to the constant removal of H+ via NHE

What is step 4 of the Aqueous humor secretion?

4. Anion Exchanger 2 (AE2) utilizes the build up of HCO3- in pigmented epithelial cells, due to CA, to transport Cl- into pigmented epithelial cells

What is step 5 of the Aqueous humor secretion mechanism?

5. Build up of ions in the pigmented epithelium will draw water through aquaporin water channels

What is step 6 of the Aqueous humor secretion mechanism?

6. Na+ and Cl-, and water all can freely diffuse through the gap junctions and aquaporins between epithelia

What is step 7 of the Aqueous humor secretion mechanism?

7. The basolateral NKA pump in non-pigmented epithelial cells uses ATP and available intracellular sodium provided by the transporters in the pigmented epithelium and transports Na+ out of NPE cells into the aqueous humor

What is step 8 of the Aqueous humor secretion mechanism?

8. The positive charge of Na+ in aqueous humor AND available intracellular Cl-, provided by the PE transporters, facilitates basolateral transport of Cl- through chloride channels on NPE into the aqueous humor

What is step 9 of the Aqueous humor secretion mechanism?

Water is drawn basolaterally and transcellularly into the aqueous humor, due to Na+ AND Cl- transport, through aquaporin channels via osmosis

What transporter is the source of the HCO3- in the Aqueous humor, and where is it found?

Anion Exchanger found on NPE basolateral membrane

Are the aquaporin proteins found in the NPE, PE, or both?

Both

How is glucose transported into the Aqueous humor?

Transported across the ciliary epithelia through SGLT/Sodium Glucose cotransporter (sodium dependent) and GLUT (sodium independent)

What does SGLT do for the Aqueous humor?

Transports glucose into the pigmented epithelial cells from the ciliary process stroma using a sodium gradient

What does GLUT do for aqueous humor transport?

Allows glucose to exit the non-pigmented epithelial cell through facilitated diffusion

How does glucose travel from the pigmented epithelial cell to the non-pigmented epithelial cell?

Gap junctions

Where is SGLT localized?

Pigmented epithelial cell

Where is GLUT localized?

Non-pigmented epithelial cell

What are the characteristics of Ascorbate (Vitamin C) in the aqueous humor?

1. Absorbs UV light

2. Found at concentrations much greater than plasma

3. Secreted by ciliary body epithelium via an ascorbate transporter

What is an SVCT?

Sodium-Vitamin C transporter

What are the characteristics of a Sodium-Vitamin C transporter? (two things)

Sodium dependent and requires ATP

Where is the Sodium-vitamin C transporter localized?

Localized to the pigmented epithelium

What is the role of the Sodium-vitamin C transporter?

Loads ascorbate into pigmented epithelial cell

Do we know how ascorbate leaves the non-pigmented epithelial cell?

No

What is the aqueous formation (secretion) rate? (recall)

Approximately 2.5 microliters/minute

The Aqueous formation rate is blank and can blank.

Not constant and can vary hourly/daily

When is aqueous humor production reduced?

At night (up to a 50% reduction)

How does aqueous production change with age?

Decreases 3.2-3.5% per decade of life (unclear exactly why)

With a reduction in Aqueous production with age, does IOP change significantly as well?

IOP does not significantly change (in health) due to the natural reduction in AH production because we also lose some ability to drain AH with age which compensates for the loss of AH production

The physiological regulation of AH secretion is an blank process.

Energy dependent

How is the physiological regulation of AH secretion an energy dependent process?

1. Dependent on oxygen levels (aerobic metabolism)

2. Anaerobic glycolysis alone is insufficient

3. Autoregulation of the ciliary body vasculature can regulate aqueous production

   (when and why still under investigation)

What is TRPV4?

A mechanosensing Ca2+ ion transporter found in the basolateral membrane of NPE cells that allows ions into the cell upon membrane stretch

What is the potential mechanism for intrinsic cell regulation of AH production?

1. NPE cells may be able to sense osmolarity via cell volume changes

2. TRPV4 activation due to osmolarity/cell volume changes which causes membrane stretch

3. TRPV4 increases ion transport via influx of intracellular calcium into the NPE presumably via activation of chloride channels

What is the affect of circadian rhythm on AH production?

AH production is reduced by 50% at night

What is the effect of circadian rhythm on AH production mediated by?

Autonomic/adrenergic control via B-adrenergic receptors found in the ciliary epithelium

What type of drug is Timolol?

A non-selective B-adrenergic antagonist

What is Timolol used for?

Potent drug that can be used to reduce IOP

Can Timolol be used to reduce IOP at night?

It cannot (non-effective at night)

What is a suspected mechanism of the physiological regulation of AH secretion?

Autonomic regulation

Why is autonomic regulation of AH secretion suspected?

Both sympathetic and parasympathetic nerves are associated with ciliary epithelium and ciliary blood vessels

What comprises the parasympathetic component of autonomics to the ciliary epithelium and ciliary blood vessels?

Postganglionic parasympathetic fibers from the pterygopalatine ganglion

What comprises the sympathetic component of autonomics to the ciliary epithelium and ciliary blood vessels?

Postganglionic sympathetic fibers from the superior cervical ganglion

What does the binding of agonists to B-adrenergic receptors in the ciliary epithelium trigger?

The formation of cAMP which in turn indirectly activates chloride channels

What does the activation of chloride channels by cAMP on the apical NPE cause?

Increases aqueous humor formation and increases IOP

What is an example of a non-selective B-adrenergic receptor agonist?

Isoproterenol (analog of epinephrine)