06. Role of vitamin D and calcitonin in calcium and phosphate metabolism Flashcards
(44 cards)
what is VITAMIN D IMPORTANT FOR
ENHANCES Ca2+ and INORGANIC PHOSPHATE Pi ABSORPTION FROM the GUT/INTENTINE
so ENHANCES BONE MINERALISATION
- important for normal SKELETAL MUSCLE FUNCTION
VITAMIN D also INCREASES BONE…
RESORPTION
so RELEASING Ca2+ and Pi from BONE
what does VITAMIN D METABOLISM START off with (2)
provitamin D3 : 7-DHC from SKIN (activated by UVB from SUNLIGHT)
provitamin D2: VIT D from DIET
Where does provitamin D3 : 7-DHC come from for VITAMIN D METABOLISM
SKIN
- activated by UVB from SUNLIGHT
where does provitamin D2: VIT D come from for VITAMIN D METABOLISM
DIET
VITAMIN D METABOLSIM steps (6)
- 7-DHC (provitamin D3) from SKIN (activated by UVB from sunlight)
VITAMIN D (provitamin D2) from DIET
are ABSORBED and enter CIRCULATION - ENTER LIVER where 25(OH)ASE is ADDED
- 25(OH)D / CALCIDIOL formed - in KIDNEY 1-ALPHA-(OH)ASE ADDED
- form ACTIVE FORM 1,25(OH)2D CALCITRIOL - CALCITRIOL 1,25(OH)2D BINDS to VIT D RECEPTORS in various tissues
- VIT D RECEPTORS with bound calcitriol BIND to RXR - RETANOIC ACID RECEPTORS to ACTIVATE VITAMIN D RESPONSE ELEMENTS
leading to various GENE consequences
what does VITAMIN D UNDERGO in VITAMIN D METABOLISM to form the active form
2 HYDROXYLATION STEPS
– 25(OH) Vitamin D reflects vitamin D status
– 1,25(OH)2 Vitamin D is metabolically active
where do 7-DHC and VIT D ENTER
LIVER
what is ADDED to 7-DHC & VIT D in LIVER and what is FORMED
- 25(OH)ASE ADDED
FORM: 25(OH)D
CALCIDRIOL
( 2 OH groups)
what is ADDED to CALCIDRIOL 25(OH)D in KIDNEY and what is FORMED
- 1-ALPHA-(OH)ASE added
FORM: 1,25(OH)2D
CALCITRIOL
- ACTIVE
what does VITAMIN D INHIBIT or increase as NEGATIVE FEEDBACK
(to prevent formation of more active)
- INHIBITS 1-OH-ASE in KIDNEY
- INHIBITS PTH from PARATHYROID GLANDS
INCREASES 25(OH)ASE - inactive form
VITAMIN D in CIRCULATION is BOUND to..
DBP (VITAMIN D BINDING PROTEIN)
what is CaBP and what does it increase
Calcium Binding Protein
Increases expression of TRPV6 - CALCIUM CHANNEL
for CALCIUM ABSORPTION
in VITAMIN D METABOLISM what does ACTIVE VITAMIN D (CALCITRIOL) BIND to and what does this lead to (2)
binds to VITAMIN D RECEPTORS - VDR
which BIND to RETANOIC ACID RECEPTORS - RXR
ACTIVATION of VITAMIN D RESPONSE ELEMENTS - VDRE
7 CAUSES of VITAMIN D DEFICIENCY
- REDUCED SKIN SYNTHESIS
- DECREASED BIOAVAILABILITY
- INCREASED CATABOLISM
- DECREASED SYNTHESIS of 25(OH)D
- INCREASED URINARY LOSS OF 25(OH)D
- DECREASED SYNTHESIS OF 1,23(OH)2D
- BREAST FEEDING
7 CAUSES of VITAMIN D DEFICIENCY
REDUCED SKIN SYNTHESIS due to:
- SUNSCREEN use: absorption of UVB radiation by sunscreen
- Skin PIGMENT : absorption of UVB radiation by melanin
- AGEING : reduced D3 synthesis by ~75% at age 70 y
- SEASON, latitude and TIME of day: no D3 synthesis between Nov to Feb in UK
- Skin GRAFTS for burns (skin taken from the person burned, which is used to cover wounds): reduced D3 synthesis
7 CAUSES of VITAMIN D DEFICIENCY
DECREASED BIOAVAILABILITY due to:
- Malabsorption: reduced fat absorption in cystic fibrosis, coeliac disease, etc
- Obesity: sequestration of Vit D in body fat
(vitamin D is FAT SOLUBLE)
7 CAUSES of VITAMIN D DEFICIENCY
INCREASED CATABOLISM due to:
Anticonvulsants, steroids: induce deactivation of 25(OH)D and 1,25(OH)2D
7 CAUSES of VITAMIN D DEFICIENCY
DECREASED SYNTHESIS OF 25(OH)D due to:
LIVER FAILURE: malabsorption contributes to D deficiency
7 CAUSES of VITAMIN D DEFICIENCY
INCREASED URINARY LOSS OF 25(OH)D due to:
NEPHROTIC SYNDROME: loss of 25(OH)D bound to Vit D binding protein
7 CAUSES of VITAMIN D DEFICIENCY
DECREASED SYNTHESIS OF 1,25(OH)2D due to:
CHRONIC KIDNEY DISEASE
7 CAUSES of VITAMIN D DEFICIENCY
BREAST FEEDING when..
breast milk only source - poor D content in human milk
how are VITAMIN D LEVELS ASSESSED
by CIRCULATING SERUM CONTENT of 25(OH)D (calcidiol)
SUFFICIENT VIT D LEVELS:
OPTIMAL FIT D LEVELS:
sufficient: 50-74 mmol/L
optimal: 75-100 mmol/L
> 370 nmol/L = Intoxication
