Vitamin A Flashcards
The form of vit A found in animal tissue is….., while ……is the non-reducable form.
Retinyl esters
Retinoic acid
The vitamin A activity of b-carotene is ….that of retinol
1/12
Describe absorption and transport of vit A
Retinyl ester is hydrolysed to retinol, then retinol and carotenoids are transported in micelles inside intestinal mucosal cells where;
1. Retinol is esterified
2. B-carotene is cleaved into 2 molecules of retinal which is reduced to retinol and then esterified
Retinyl ester amd b-carotene are taken up by liver in CM remnant where they are stored as retinyl palmitate.
Retinol is hydrolysed from its ester and is bound to RBP, the complex attaches to specific cell surface receptors on peripheral cells. Inside cells, retinol is attached to cellular RBP which carries it to the nucleus to act in a manner analogous to steroids.
The requirement for vit A in males is…. and in females is…..
900 RAE
700 RAE
Describe role of Vitamin A in vision
Retinaldehyde functions as prosthetic group for light-sensitive opsin protien forming rhodopsin in rods and iodopsin in cones
Visual cycle: 11-cis retinal is specifically bound to opsin protein, when rhodopsin is expsed to light a series of photochemical isomerization, leads to results in bleaching of all-trans retinal and opsin. This process triggers nerve impulse. All trans-retinal after being released from rhodopsin is reduced to all-trans retinol, esterified and isomerized to 11-cis retinol that is oxidized to 11-cis retinal. The latter combines with opsin to form rhodopsin.
Describe non-visual role of vitamin A
- Role in cell growth and differentiation. Retinoic acid binds to nuclear receptors that bind to response elements of DNA and regulate the transcription of specific genes.
- Role in maintenance of healthy epithelial cells, retinol in converted to retinyl phosphate which serves as glycosyl carrier in synthesis of glycoproteins and mucopolysaccharodes needed for normal mucus formation
- Role in reproduction, retinol & retinal but not retinoic acid support spermatogenesis and prevent fetal resorption in female rates
- Role as anti-oxidant, b-carotene trap free radicals at low O2 pressures. It may protect against cancer amd heart attack.
Causes of vit A def
Decrease dietary intake
Malabsorption (biliary obstruction)
Decreased fat intake
Advanced liver disease (dec retinol-binding proteins)
Describe symptoms of vit A deficiency
- Blindness, earliest night blindness then xerophthalmia, then corneal degeneration & ulceration (keratomalacia), eventually blindness.
- Dermatological problems: xeroderma
- Increased susceptibility to infection bec of keratinizing metaplasia of epithelial & mucosal cells, and impaired immune system as vit A is imp for immune cell differentiation
Describe symptoms of hypervitaminosis A
Causes cell membrane lysis & tissue damage
CNS symptoms related to inc ICT
Liver hepatomegaly with histological changes & hyperlipidemia
Skin (excessive dryness, desquamation & alopecia
Teratogenic for fetus & pregnant women
The onset of b-adrenergic antagonists is…..and its duration is…..
30 min
12-24 hrs
With respect b-adrenergic antagonist drugs usage in glaucoma state the following:
1. Mechansim of action
2. Advantages
3. Uses
- Decreasing secretion of aqueous humor by ciliry body through b2-receptor blockade
- Unlike cholinergics, it does not affect ability of eye to focus for near vision nor change pupil size
- Chronic management of glaucoma
Enumerate adverse effects of timolol/betaxolol in glaucoma
- Ocular irritation
- Bronchospasm (CI asthma)
- Bradycardia, CHF (CI in cardiac patients)
Describe mechanism of action & adverse effects of apraclonidine in glaucoma
MOA: a2 agonist causing dec secretion of aqueous humor
AE: red eye, ocular irritation, allergic reactions, malaise, headache.
MOA & usage of pilocarpine
-Acts on M3 leading to miosis (open the angle) and contraction of longitudinal fibers of ciliary muscles thus open trabecular meshwork around the Schlemm canal and cause immediate inc in drainage of aqueous humor and dec IOP
-Drug of choice in glaucoma emergencies
Pilocarpine onset is within….., duration is…..
Few minutes
4 to 8 hrs
