11. Calcium Regulation Flashcards
Where is calcium found in the body?
Calcium found in functionally distinct pools in body;
- Bony skeleton: 99%
- Intracellular pool: ~1%
- Extracellular pool: ~0.1%
Describe calcium uptake
Only in = dietary calcium;
- dietary habits
- supplements
Intestinal absorption;
- absorbed across int epith cell’s brush border membrane
- TRPV6 channel proposed: k/o didn’t change uptake
- TRPM7 linked: k/o = strongly reduced Ca in serum/bones (prob for bulk intest uptake)
After cell uptake Ca bound to calbindin (vit D dependent Ca binding protein);
- transfers to cells ER
- transports to basal membrane on opposite side of cell (doesn’t enter cytosol/ICF)
Ca pumps (PMCA1) actively TP Ca into body;
- occurs primarily in duodenum when Ca intake low
- passive paracellular TP in jejunum + ileum when Ca intake high (independent of vit D level)
Active absorption from gut regulated by calcitriol in blood;
- increases rate of absorption
Why does Ca need to be regulated?
Excitable cells, e.g. neurons, v sensitive to changes in Ca+ conc;
Hypercalcemia = progressive depression of nervous system
Hypocalcemia = progressive excitation of nervous system
Where is phosphate found in the body?
- Bones: 85%
- ICF: 15%
- ECF: <1%
What role do the bones play in calcium homeostasis?
Bones serve as large reservoirs;
- release Ca when ECF conc decreases
- store excess Ca
Describe serum Ca
Ref range: 2.2-2.6mmol/L
Required for normal neuromuscular function
Only ~50% exists as free ions - rest mostly bound to albumin (bio-inactive)
- physiological functions depend on ionised Ca not the total Ca (inc bound)
List the circulating calcium fractions in the serum
50% ionised (free) - biologically active
40% protein-bound, non-diffusable - biologically inactive + not excreted
10% complexed with phosphate, bicarbonate + citrate
Describe Dr Sydney Ringers experiments on calcium
Experiments on calcium + frog heart contraction 1883
Recorded frog heart contractions when perfused with blood mixture;
- substituted blood for saline: amplitude of contractions declined
- when calcium chloride was added to the solution the contractions recovered to almost normal amplitude
Describe the findings in early publications on calcium ion concentration by McClean + Hastings 1934
“frog’s heart sensitive to changes in conc of Ca2+ but not to changes in conc of Ca2+ in non-ionised form”
Showed ionised/free Ca2+ correlated with heart contraction;
- protein/citrate-bound Ca2+ had no effect
Developed first assay for ionised/free Ca2+
Showed in blood this was closely regulated (humans: 1.18mmol/L)
What are the components of separated whole blood?
Plasma;
- if anticoagulant present: contains fibrinogen
Serum;
- no anticoagulant present
Clot;
- formed encapsulating cells
How is blood correctly collected + stored for calcium testing?
Preferred specimen for total Ca2+ = free flowing venous sample;
- serum
- lithium heparin-plasma
Avoid anti-coagulants, e.g. EDTA/oxalate, as bind Ca2+ tightly + interfere with measurement
Loss of CO2 increases pH = samples need to be collected anaerobically
Serum from sealed evacuated tubes can be used if clotting + centrifugation are done quickly (<30min) at room temperature
Describe the lab methods for Ca2+ measurement
Colorimetric;
- Ca2+ released from protein carrier by acidification if sample before dye-binding reaction
- ortho-cresophthaleine complexone (CPC) or arseno III dye forms complex with Ca2+
- CPC method uses 8-hydroxyquinoline to prevent Mg2+ interference
AAS (atomic absorption spectroscopy) is reference method for total Ca2+ but rarely used clinically
Commercial analysers use ISEs to measure free Ca2+;
- use membranes with molecules that selectively + reversibly bind Ca2+ ions
- as Ca2+ binds membranes, electric potential develops proportional to ionised Ca2+ conc
When measuring Ca2+ why and how do we adjust for albumin?
Ca2+ = metabolically active
40-50% of measured total Ca bound to albumin (inactive);
- serum albumin must be considered when assesing Ca2+
Labs use formula to adjust measured Ca2+ relative to albumin levels;
- “adjusted Ca” = approximation of metabolically active Ca2+
Adjusted calcium = measured calcium + 0.02 (40 - albumin conc)
Errors are greatest at extremes of albumin concentration + when there is a suspicion/evidence of acidosis/alkalosis
How does the reference range for calcium vary?
Total Ca2+ varies with age;
- higher in adolescence when bone growth most active
Ionised Ca2+ changes from day 1-3 of life then stabilises at relatively high levels with gradual decline through adolescence
Describe calcium balance + its consequences
Calcium balance: intake = output
Negative balance: output>intake = osteoporosis
Positive balance: intake>output = occurs during growth
Why is calcium considered essential?
We can’t synthesis it, must acquire through diet
Which organ systems are involved in calcium metabolism?
Skeleton
GI tract
Kidneys
Define calciotrophic hormone
Any hormone with a major role in bone growth + remodelling
List the main calciotrophic hormones
Parathyroid hormone (PTH)
Vitamin D (1, 25 dihydroxycholecalciferol)
Calcitonin (CT)
Parathyroid hormone related protein (PTHrP)
Describe the general pathway of calcium in the body beginning at ingestion
Ca2+ ingested
PTH promotes active vit D formation in kidney;
- vit D promotes absorption in small intestine
- unabsorbed Ca2+ is lost in faeces
Ca2+ lost in the urine at kidney;
- PTH promotes Ca2+ reabsorption from urine
Ca2+ removed from blood by osteoblasts in bone;
- PTH promotes Ca2+ release into blood by osteoclasts
- calcitonin inhibits Ca2+ release into blood by osteoclasts
Discuss PTH + its role in calcium homeostasis
Parathyroid hormone;
- peptide hormone
- manufactured in parathyroid glands behind thyroid
Normally tight feedback loop b/n PTH release + serum Ca2+;
- PTH release stimulated by fall in Ca2+ = acts to restore Ca2+
- preserve serum Ca2+ through actions on bone + kidney
Example of negative feedback control
How many parathyroid glands are there + what are the consequences of thyroid surgery?
4 parathyroid glands
Difficult to locate during thyroid surgery;
- total/subtotal thyroidectomy usually = parathyroid removal
Loss of 2 = usually no major physiologic impact
Loss of 3 = transient hypoparathyroidism
Any remaining parathyroid tissue: hypertrophy
How is PTH biosynthesis + secretion regulated by Ca2+?
Secretion determined chiefly by serum ionised calcium concentration through negative feedback inhibiting PTH gene transcription
Parathyroid cell CaSR (G-protein couple receptor) on surface bind extracell Ca
- high conc Ca initiates phospholipase C pathway via Gq G-protein
- hydrolyses PIP2 > liberates intracell messengers IP3 + DAG
IP3 + DAG (diacylglycerol) result in release of extracell Ca from intracell stores into cytoplasmic space
High extracell Ca -> increase in cyto Ca in parathyroid cells;
- inhibits fusion of vesicles containing granules of preformed PTH with cell membrane = inhibits release of PTH
What are the 4 main actions of PTH during low serum Ca2+ (ie when it is actively secreted)?
- BONE: INCREASED OSTEOCLAST ACTIVITY = MORE BONE RESORPTION/ REDUCED OSTEOBLAST ACTIVITY = LESS BONE DEPOSITION
- stimulates Ca2+ release;
- initial osteocyte mobilisation of Ca2+ from bone > ECF
- continued slow release of bone Ca2+ + phosphate from osteoblast breakdown of bone matrix
- also reduces osteoblast activity - KIDNEY: LESS URINARY CALCIUM EXCRETION = CONSERVATION OF Ca
- increases rate of Ca2+ reabsorption from renal tubules
- less excreted in urine - KIDNEY: MORE URINARY PHOSPHATE EXCRETION = PREVENTS HYDROXYAPATITE FORMATION = LESS BONE DEPOSITION
- increases urinary phosphate excretion
- lowers plasma phosphate
- reduces tendency for Ca2+ + phosphate to react (ppts) - KIDNEY
- upregulates transcription of 1-alpha hydroxylase for vit D activation in kidney
- increases rate of vit D conversion to active form calcitriol (1, 25-dihydroxycholecalciferol)
Describe vitamin D + its role in calcium homeostasis
Vit D;
- fat soluble vitamin = can be stored
- found in food (vit D2)
Sources;
- eggs, fort cereals, oily fish, liver (10% from diet)
- made in skin under influence of UVB rays in sunlight (vit D3)
Function in Ca homeostasis;
- promotes Ca2+ absorption in GI tract + deposition in bone
- downregulates transcription of PTH gene (neg feedback)
Describe influence of pH on location of Ca absorption
Absorption of Ca: duodenum > jejunum > ileum
Absorption is better at low pH;
- stomach pH2 = highest absorption at beginning of duodenum
Describe vit D synthesis + metabolism in tissues
Skin: 7-dehydrocholesterol -UV-> vitamin D3
Liver: vitamin D3 -25-hydroxylase-> 25(OH)vit D [hepatic 25-hydroxylation]
Kidney: 25(OH)vit D -1alpha-hydroxylase-> 1,25 dihydroxy vit D (active metabolite) [renal alpha1 hydroxylation]
Describe vitamin D activation + its importance in disease
Reqs;
- 25 hydroxylase (liver)
- 1alpha-hydroxylase (kidney)
Conversion of 25-hydroxy cholecalciferol > 1,25 dihydroxycholecalciferol;
- occurs in prox renal tubules
- most active form of vit D
- Ca ion itself slightly prevents conversion
Conversion in prox tubule requires PTH from parathyroid;
- PTH promotes conversion if Ca conc <9mg/100mL
At higher conc Ca = PTH suppressed;
- 25-hydroxy vitD in prox tubule converted to 24, 25-dihydroxy vitD (almost no vit D effect)
Patients with CKD fail to activate vitamin D = bone disease
List the actions of vit D in calcium homeostasis
Increase rate of Ca2+ uptake in gut;
- replaces Ca2+ lost in urine
Stimulates phosphate absorption in gut
Stimulates Ca2+ + phosphate reabsorption in kidney
In very high levels of activated Vit D;
- increases osteoclastic bone resorption + release of Ca2+ + phosphate to ECF
What 2 factors stimulate Vit D formation?
- PTH (in response to low Ca2+)
2. low plasma phosphate
Describe the mechanisms of action of vitamin D
Vitamin D binds the VD receptor (VDR) + translocates to the nucleus to upregulate transcription of the vit D-response gene
Vit D inhibits transcription of PTH gene + stimulates transcription of the Ca2+ transporter in the intestinal brush border epithelium
Binding of active vit D to VDR in the intestine affects transcription of genes inc calbindin + may also permit Ca2+ entry through channels in brush border
- calbindin proposed to ferry Ca2+ through cell
- exit step through basolateral membrane may involve a Ca2+ pump and/or Na/Ca exchange
Describe vitamin D deficiency + associated disorders
Cause: inadequate intake + absence of sunlight
Lesser degrees of vit D deficiency common;
- 1 in 7 adults
- esp in winter
- contributes to osteoporosis
Most prominant clinical effect = osteomalacia;
- defective mineralisation of bone matrix (rare)
Vit D deficiency in children = rickets (rare)
Deficiency of renal 1alpha-hydroxylase = vit D-resistant rickets;
- sex linked gene on x-cr
- renal tubular defect of phosphate resorption
- teeth may be hypoplastic + eruption affected
Describe the role of calcitonin in Ca homeostasis
Source: released from parafollicular C cells in thyroid gland
Secretion: during hypercalcemia, may protect against abn rises
Bone: reduces rate of Ca2+ release to ECF (lowers plasma Ca2+)
Normal physiology: minimal if any role in control of Ca2+
Discuss the feedback loops in calcium homeostasis
HIGH CA + VIT D INTAKE;
Rising blood Ca2+ = PTH suppression;
- decreased bone resorption
- increased urinary loss
- decreased 1,25dihydroxyD production = decreased GI absorption
- returns to + remains normal blood Ca2+ = good quality bones formed
LOW CA + VIT D INTAKE;
Falling blood Ca2+ = PTH stimulation;
- increased bone resorption
- decreased urinary loss
- increased 1,25dihydroxyD production (if not deficient) = increased GI absorption
- returns to + remains normal blood Ca2+
Vit D deficient = no Ca intake = bones become increasingly poor quality + liable to fracture in order to maintain plasma Ca
List the causes of hypocalcemia
Parathyroid glands don’t work;
- congenital, autoimmune, surgical damage
Parathyroid glands work but PTH receptor signalling doesn’t;
- pseudohypoparathyroidism - genetic defect of G-protein Gsalpha (for activating phospholipase C pathway)
Renal failure;
- failure to excrete phosphate = precipitates Ca+
Severe vit D deficiency;
- no GI absorb of Ca
OD of Ca controlling drugs
Hypomagnesemia;
- e.g. chronic diarrhea, diuretic therapy, alcoholism
Hypercalciuric hypocalcemia;
- genetically overactive calcium sensor
Describe the S+Ss of hypocalcemia + how it is managed
>1.8mmol/L = symptoms probably mild <1.8mmol/L = likely to be increases neuromuscular excitability, tingling in hands + feet, risk of arrhythmias + seizures (PHONE WARD)
Check if result incorrect;
- artefact?
- check potassium
- look at remainder of bone profile
Oral/IV calcium + correct underlying cause
If Mg2+ not done, suggest as addition
What is the link between calcium + magnesium and how does this affect treatment?
Mg2+ required for normal function of both parathyroid glands + PTH
Impossible to restore normal calcium balance if Mg2+ persistently low
May need to give IV mg2+ first
Describe the causes of hypercalcemia
Overactive parathyroid gland
PTH-like substance released by tumour
Direct invasion of bone by tumour;
- overwhelms normal regulatory mechanisms
Benign hypocalciuric hypercalcemia;
- genetically defective calcium sensor
Severe vit D excess
Other causes rare
Describe S+Ss based on calcium levels in hypercalcemia
- 6-3mmol/L;
- likely mild symptoms
3-3.5mmol/L;
- likely nausea, weak, complaining of aches + pains
- non-specific symptoms: hyperCa may not be suspected = contact ward
> 3.5mmol/L;
- pt very unwell + risk of death = PHONE WARD ASAP
What further tests are performed for hypercalcemia once blood shows elevated Ca?
PTH immunoassay;
- to distinguish b/n 2 main causes: hyperparathyroidism + malignancy
In hypercalcemia PTH should be fully suppressible
- if still detectable likely diagnosis = hyperparathyroidism
How is hypercalcemia managed?
In emergency;
- fluids + bisphosphates to promote urinary Ca excretion + stabilise bone
Hyperparathyroidism;
- surgical removal
- can maintain serum Ca with vit D alone but reqs careful monitoring
What is familial benign hypocalciuric hypercalcemia?
Inherited condition that can cause hypercalcemia
Most cases associated with loss of function mutations in CaSR gene expressed in parathyroid + renal tissue;
- decreased receptor sensitivity to calcium = reduced receptor stimulation at normal blood levels
- doesn’t inhibit PTH until higher levels of Ca in blood
Response curve of PTH shifts to the right
Describe PTH related protein (PTHrp) + its impact on Ca homeostasis
PTHrp;
- protein of PTH family
- regulates fetal bone development/EMT in mammary gland formation
Secreted by some tumours (breast, some lung inc squamous cell lung carcinoma;
- same N-terminal end as PTH = binds same receptor, PTHR1
- mostly same effects, except on vit D conversion = no increase in Ca gut absorption
- responsible for most humoural malignant hypercalcemia (increased plasma Ca2+) - early sign PNP syndrome
PTHrp also not subject to same feedback regulation as PTH
Describe typical requests, handling + testing of serum calcium measurements in the lab
Usually requested as part of bone profile with phosphate + ALP
Serum sample, no special requirements - but beware of EDTA contamination
Must also know serum albumin to interpret;
- most analysers do automatically + give “corrected” result
- corrects to what patients Ca would be if albumin was 40g/dL = same ref range used for all
