Calcium Metabolism Flashcards
How much Ca is in the body?
How much is in the bone?
What about the rest?
1kg approx.
99% in bone, as Hydroxyapatite crystals
The rest is extracellular (Serum)
What is the total Serum Ca concentration?
In what 3 forms does Ca travel in the plasma?
Which form is physiologically active?
2.2-2.7 mmol/L
- Free ions, Physiologically active (45%)
- Bound to plasma proteins (45%)
- Complexed with organic anions, such as citrate (10%)
What is the normal range of free Ca ions in plasma?
Describe the regulation of plasma phosphate levels
1.0 to 1.3 mmol/L
Not strictly regulated, so levels fluctuate especially after a meal
In what 2 ways are Ca and PO4 homeostasis linked to each other
- Primary comments of hydroxyapatite crystals
- Regulated by same hormones
Which 3 hormones are involved in Ca and phosphate homeostasis
Which 3 organ systems do they act on?
Compare the hormones’ actions on Ca and Phosphate
- Parathyroid Hormone (PTH)
- Calcitriol (Activated Vitamin D)
- Calcitonin (Lesser extent)
- Bone
- Kidneys
- GI tract (Indirectly)
Actions on Ca and phosphate are typically opposed
State the net Ca uptake of the Intestines per day
State the net Ca uptake of Bone per day
State the net Ca excretion of Kidney per day
Intestines: 175mg (Absorb 500, secrete 325)
Bone: 0 (280mg deposition, 280mg resorption)
Kidney: (Filter 10kg) <2% of filtered loads
In a pension who is in Ca balance, how does urinary excretion of Ca compare to net absorption of Ca?
They are the same
- Where is PTH made?
- What is 1,25(OH)2D or 1,25(OH)2D3 also called?
- Where is Calcitonin made?
- Chief cells of parathyroid gland
- Calcitriol
- Parafollicular/ C Cells of Thyroid Gland
PTH is a straight chain polypeptide hormone.
How do Low and High serum Calcium affect its synthesis and degradation?
Low serum Ca;
- Up regulates gene transcription
- Prolongs mRNA survival
High serum Ca;
- Down regulates gene transcription
- Accelerates degradation
What cells make and degrade PTH?
What PTH’s half life?
When is PTH synthesised?
Chief cells do both
Half life of 4 mins
Continuously synthesised
What are the effects of PTH on the Kidney, Intestines and Bone?
Kidney: Increased Ca reabsorption and Phopshate excretion
Bone: Increased resorption (More Ca enters blood)
Intestines: Activated Vit D-> Increased Ca absorption
How are laboratory Ca tests corrected and why?
Levels are corrected depending on amount of Albumin, as the tests measure TOTAL Calcium, not just the free ions
Outline how Sunlight is used to make Calcitriol
- Sunlight converts cholesterol to Vit D3 (Cholecalciferol)
- In liver, Vit D3 hydroxylated-> 25(OH)D
- In kidney, 25(OH)D hydroxylated-> 1,25(OH)2D (Calcitriol)
Compare the functions and time scales of PTH and Calcitriol (Activated Vit D)
PTH;
- Increases serum Ca
- Short term serum Ca regulation
Calcitriol;
- Increases serum Ca
- Long term serum Ca regulation
(Act via different mechanisms)
What are the 2 major forms of Vitamin D?
Vit D3 (Cholecalciferol) (Animal sourced foods) Vit D2 (Ergocalciferol)(Plant sourced foods)
What are the effects of Calcitriol on the Kidney, Intestines and Bone?
Kidney: Increased reabsoprtion of Ca
Intestines: Increased Ca absorption
Bone: Increased resorption
What is PTHrP? What can it lead to?
What tumours can make it?
Parathyroid Hormone related Peptide is a peptide produced by tumours
Can cause hypercalcaemia
Squamous tumours of Lung, Head and Neck
What 3 types of cancer commonly prove PTHrP
Breast
Prostate
Occasionally Myeloma
What are 3 common effects of PTH and PTHrP
What is difference?
- Increased Resorption of bone
- Reduced Ca excretion in kidney
- Reduced Phosphate reabsorption in kidney
Unlike PTH, PTHrP doesn’t increase Calcitriol (by increasing renal C-1 hydroxylase activity)
What does Calcitonin do?
How important is this hormone?
Suggest 1 possible use
Lowers serum Ca levels
Not very important, no apparent effect on Ca homeostasis
Preservation of maternal skeleton during pregnancy
Name 4 common cancers that metastasise to bone and cause hypercalcaemia
Breast
Lung
Kidney
Thyroid
What are 6 common sites for bone metastasis
- Skull
- Ribs
- Vertebrae
- Pelvis
- Proximal femur
- Proximal humerus
Explain the symptoms of Hypocalcaemia (Give 4 symptoms)
Calcium raises the threshold for nerve membrane depolarisation
Hyper excitability in nervous system;
- Parasthesia
- Tetany (muscle spasms)
- Paralysis
- Convulsions
What are 6 symptoms of chronic hypercalcaemia
- Kidney stones (renal caniculi)
- Constipation
- Dehydration
- Kidney damage
- Tiredness
- Depression
