Calcium homeostasis Flashcards
(39 cards)
What are the different roles of calcium in the body?
- Skeleton and bone growth/maintenance
- Metabolic - Action potentials and IC signalling
Describe calcium handling in the body
99% of the body’s calcium is in the bones/skeleton. We can get calcium from our diet, where it is absorbed in the GI tract. However, we cannot fully absorb calcium without adquate vitamin D levels, which is dependent on sunlight exposure and diet, as well as renal function.
When Ca is low, PTH is produced by the parathyroid glands which acts to activate osteoblasts and suppress osteoclast activity, to increase break down of bone to release calcium. PTH also increases renal reabsorption of calcium and alpha 1 hydroxylase activation, and causes increased excretion of phosphate.
99% of calcium is in the bones, only 1% is found in the serum.
What are the 3 forms that calcium that can be found in the serum, and there approx amounts?
- Free “ionized” - the only one that is biologically active (50%)
- Protein-bound - bound to albumin (40%)
- Complexed - with citrate/phosphate (10%)
- What is the reference range for total serum calcium?
- Why do we correct serum calcium levels, and whta is the formula?
- 2.2-2.6mmol/L
- Corrected calcium is reported to correct for albumin levels. If albumin levels are incorrect, can cause a false reading for calcium levels
corrected calcium = serum Ca + 0.02 *(40 - serum albumin in g/L)
What will happen to calcium levels if you have a low albumin?
If you have a low albumin, the bound calcium wil be low, but the free calcium will be normal. Therefore, corrected calcium can show you whether the problem is the low albumin or whether the ionized calcium level really is too low.
Why is calcium important?
- Important for normal nerve and muscle function
- Chronic calcium deficiency can result in loss of calcium from bone in order to maintain calcium.
PTH ‘obtains’ Ca from what 3 sources in the body?
- Bone - stimulation of osteoblasts
- Gut - absorption
- Kidney - resorption and renal 1 alpha hydroxylase activation
What two hormones are involved in calcium homeostasis?
- PTH
- Vitamin D
- How many amino acids make up PTH?
- What are the roles of PTH in the body?
- 84 amino acids long
- Bone and renal Ca resorption, stimulates 1,25(OH)2 vitamin D synthesis (1 alpha hydroxylation) and stimulates phosphate wasting
Describe the process of vitamin D synthesis
- 7-dehydrocholesterol in the skin, reacts with sunlight to produce
- cholecalciferol D3 which is converted by 25 hydroxylase from the liver to produce
- 25-hydroxycholecalciferol (25-OH D3) which is converted by 1 alpha hydroxylation in the kidney and the presence of PTH to produce
- 1, 25-dihydroxycholecalciferol (1,25(OH)2 D3 which is the physiologically active form
Which of these is a PLANT product?
- Ergocalciferol
- Cholecalciferol
- Ergocalciferol
Where is vitamin D3 synthesized?
The skin
Where the following enzymes found and what do they do?
- 25 hydroxylase
- 1 alpha hydroxylase
- 25 hydroxylase - liver - 100% of absorbed vitamin D is hydroxylated at the 25 position by this enzyme. 25 hydroxy vitamin D is inactive and stored in the liver and measured form of vitamin D
- 1 alpha hydroxylase - kidney - next step after the liver, 25 hydroxy vitamin D is actiavted to its active form (1,25(OH)2D3) by 1 alpha hydroxylase. The kidney is the rate limiting step
What cells/conditions can ectopically secrete 1 alpha hydroxylase?
- Lung cells
- Sarcoid tissue
What is the role of 1,25(OH)2 D3?
- Activated form of vitamin D
- Intestinal Ca absorption
- Also intestinal Phosphate absorption
- Bone formation
Vitamin D deficiency
- What is it in adults and in children?
- What are the risk factors?
- Defective bone mineralization. In children vit D deficiency is called Rickets, and in adulthood it is Osteomalacia
- Lack of sunlight exposure, dark skin, dietary and malabsorption
What are the clinical features of osteomalacia?
- Bone and muscle pain
- Increased risk of fracture
- Low Ca and phosphate levels, raised ALP
- Looser’s zones - pseudo fractures
What are the clinical features of Rickets?
- Bowed legs
- Costochondral swelling
- Widened epiphyses at the wrists
- Myopathy
What are the different causes of osteomalacia?
- Bone is demineralised, most commonly caused by vitamin D deficiency
- Renal failure
- Anticonvulsants induce breakdown of vitamin D
- Lackf of sunlight
- Chappatis (phytic acid)
What is osteoporosis?
- Cause of pathological fracture
- Loss of bone mass with increasing age, especially in post-menopausal women. Residual bone normal in structure but not in mass
- Has a normal Ca/biochemistry
- Aysmptomatic until a fracture, most commonly NOF
How is osteoporosis diagnosed?
- DEXA scan
- A T score of <-2.5 is osteoporosis
- A T scpre of between -1 & -2.5 is osteopenia
What are the causes of osteoporosis?
- Childhood illness
- Early menopause
- Lifestyle: sedentary, alcohol, smoking, low BMI/nutritional
- Endocrine - hyperprolactinemia, thyrotoxicosis, Cushing’s
- Drugs - steroids
How is osteoporosis treated?
- Lifestyle changes - increased weight bearing exercise, stop smoking and reduce alcohol intake
- Drug therapy - vitamin D/Ca, bisphosphonates e.g. alendronate that reduces bone resporption, SERMs, Teriparatide (PTH derivative)
What are the symptoms of hypercalcemia?
- Polyuria/polydipsia
- Constipation
- Neurological - confusion/seizures/coma
