Biochemistry and Metabolism- Aqueous Humor, Iris, and Ciliary Body Flashcards
Where is aqueous humor secreted from
nonpigmented ciliary epithelium (NPE) from a substrate of blood plasma
Composition of aqueous humour
low protein content and a high concentration of ascorbate
Function of ascorbate
antioxidant properties, and its high concentration in the aqueous protects intraocular structures by blocking ultraviolet (UV) light
What makes up blood-aqueous barrier
tight junctions of the NPE, the iris vasculature, and the inner wall endothelium of the Schlemm canal
Function of ciliary body
produces the aqueous humor, regulates its composition, and contributes to uveoscleral outflow, thereby directly influencing the ionic environment and metabolism of the cornea, lens, and trabecular meshwork.
Does ciliary body have a role in defending against oxidative stress
It has the highest concentration of redox (oxidation-reduction) enzymes in the anterior segment. The ciliary body also contains proteins of the cytochrome P450 family, though only a small number compared with the liver. These enzymes are involved in detoxification
How is Timolol metabolised
CYP2D6 is one such enzyme that metabolizes timolol. It is expressed in low levels in ocular tissues but is abundant in the liver.
What happens when blood mixes with ocular fluids
cause blood constituents to mix with ocular fluids; this mixing leads to plasmoid aqueous, retinal exudates, or retinal edema
Which epithelium is leaky and which is tight
NPE has tight junctions proximal to the apical plasma membrane that form part of the blood–aqueous barrier, thereby preventing paracellular transport from the ciliary stroma into the posterior chamber. In contrast, the PE cell layer is considered a leaky epithelium because it allows solutes to move through the space be- tween the PE cells.
Aqueous humour production rate
2–3 μL/min, but this rate varies according to our circadian rhythm, dropping to 1.0 μL/min at night.
What is a symporter
membrane proteins that mediate the cotransport of molecules in the same direction
What is an antiporter
mediate the cotransport of molecules in opposite directions
Aqueous composition of components compared to plasma and vitreous
Highest in Na+, Cl- Ascorbate, Lactate and Inositol
Lowest in HCO3- and glucose
Impact of diabetes mellitus on lens
In individuals with diabetes mellitus, glucose levels in the aqueous humor are increased.this leads to higher glucose concentrations in the lens, which has short- term refractive and longer-term cataract implications.
Function of inositol
Inositol, which is important for phospholipid synthesis in the anterior segment, is found in the aqueous at a concentration approximately 10 times higher than that in plasma.
What contains highest concentration of glutathione
Blood however, virtually all glutathione resides within the erythrocytes, and plasma has a low concentration of only 5 μmol/L or less.
Function of glutathione
stabilizes the oxidation-reduction (redox) state of the aqueous by reconverting ascorbate to its functional form after oxidation, as well as by removing excess hydrogen peroxide
Where is glutathione S transferase found
highly concentrated in the ocular ciliary epithelium
Concentration of urea in aqueous
between 80% and 90% of that in plasma
Concentration of protein in aqueous
Normal aqueous contains approximately 0.02 g of protein per 100 mL, as compared with the typical plasma level of 7 g per 100 mL
Most abundant plasma protein in aqueous
albumin and transferrin, which together may account for 50% of the total protein content.
Other proteins hypothesised to be made by ciliary body
- C4, a component of the classical complement pathway that participates in immune- mediated inflammatory responses
- α2-macroglobulin, a carrier protein that is involved in proteinase inhibition, clear- ance, and targeting, as well as the processing of foreign peptides
- apolipoprotein D, which binds and transports hydrophobic substances, including cholesterol, cholesteryl esters, and arachidonic acid (AA)
- selenoprotein P, which has antioxidant properties
Growth factors found in aqueous
- transforming growth factor βs 1 and 2 (TGF-β1 and -β2)
- acidic and basic fibroblast growth factors (aFGF and bFGF) * insulin-like growth factor I (IGF-I)
- insulin-like growth factor binding proteins (IGFBPs)
- vascular endothelial growth factors (VEGFs)
- transferrin
How many VEGF are there
VEGF-A, -B, -C, and -D,
Function of VEGF-A
VEGF-A is a crucial regulator of vasculogenesis (embryologic blood vessel development from mesodermal elements) and a potent inducer of vascular permeability and angiogenesis (neovascularization). VEGF-C and VEGF-D regulate lymphangiogenesis. VEGF receptors (VEGFR) are tyro- sine kinases.
Impact of oxygen concentration in aqueous humor
Oxygen concentration in the aqueous humor may increase with age-related vitreous degeneration or after surgical removal of vitreous. elevated oxygen concentration induces oxidative damage in the lens and trabecular meshwork and leads to an increased risk of nuclear sclerosis, cataract, and open-angle glaucoma after vitrectomy.
