Calcium, Phosphate and Magnesium
Transcript of Calcium, Phosphate and Magnesium
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Calcium, Phosphate andMagnesium
Mineral Metabolism
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Calcium, Phosphate and Magnesium
Lecture Outline
Calcium
Intake and excretion
Distribution
Function
Control
Abnormality
Assessment
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Calcium, Phosphate and Magnesium
Calcium Mostly in bones
Prolonged deficiency causes bone disease
Total body calcium depends on Ca2+ absorbed
from diet and Ca2+ lost from body
1,25-dihydroxyvitamin D (1,25-
dihydroxycholecalciferol) the active metabolite of
Vitamin D needed for calcium absorption
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Calcium
~ 25 mmol (1g)
ingested per day
Net absorption =
6 12 mmol
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Calcium
Loss: in faeces and urine
Faecal calcium
Derived from diet and large amountintestinal secretions that has not been
reabsorbed
Intestinal Ca2+ + phosphate or fatty acids
insoluble, poorly absorbed Steatorrhoea: excess fatty acids in
intestinal lumen Ca2+ malabsorption
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Calcium
Loss: in faeces and urine
Urinary calcium
Amount of Ca2+ reaching glomeruli,
glomerular filtration rate, renal tubular
function amount of urinary excretion
Parathyroid hormone and 1,25-dihydroxy-vitamin D increase calcium reabsorption
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Calcium Distribution
5th most common element in the body 99% crystalline hydroxyapatite in bones and
teeth 1% extracellular fluid; contains ~25 mmol of
calcium Blood contains 7.5 mmol = virtually all in the
plasma
~125 mmol of calcium is in the exchangeablecalcium pool, located mainly on the bone formingsurfaces and in soft tissue where Ca2+ is readilyavailable for physiological functions
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Calcium: Function
Prime inorganic messenger for the regulation of cell function Activator of blood coagulation Activator of enzymes Control secretion of endocrine glands:
Parathyroid glands Thyroid cells Pancreatic beta cells
Function at the plasma membrane: Regulating membrane permeability
Regulating transmitter release Regulating neuromuscular excitability
Involved in cell to cell adhesion and possibly communication In striated muscles activates the contraction of myosin fibril
in combination with troponin
Contribute to structure of bone and teeth
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Plasma Calcium
Normal plasma concentration = 2.5 mmol/L ( 10 mg/dL)
Calcium circulates in three physiochemical states
a. Protein-bound (45%)
b. Complexed with small, diffusible ligands: citrate,lactate, phosphate, carboxylate and bicarbonate(10%)
c. Ionized states (ionized calcium)
Circulate freely in bloodstream
[ ] is affected by pH and plasma proteinconcentration
Acidosis favors dissociation
Alkalosis has the opposite effect
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Plasma Calcium
Ionized calcium is the physiologically
important fraction Form (b) and (c) can pass through porous
membrane, leaving bound form (a) behind
Which form can be found in urine?
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Control of plasma calcium levels
Control of plasma level is very important
Level must be maintained
Low levels tetanic contraction of muscles, seizures
and death
High levels depression, coma, death
3 hormones control maintenance of calcium
homeostasis
1) Parathyroid hormone
2) Calcitonin
3) Vitamin D
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Hormones controlling calcium
Nuclear
receptorSteroid-like
Plasma Ca
Plasma PO
GI tract
Bone
Vitamin D
cAMP32 aaplopeptide
Plasma CaBoneOsteoclasts
Calcitonin
cAMP84 amino acids
PolypeptideStimulated by
Ca
Inhibited by
Ca
Plasma Ca
Plasma PO
Kidney
Bones
Parathyroid
hormone(PTH)
Second
Messengers
PropertiesActionsTargetHormones
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Control of plasma calcium levels
Parathyroid hormone (PTH)
Most important controlling factor
Secreted by parathyroid glands
Increases circulating free-ionised calcium
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Control of plasma calcium levels
Calcitonin
Produced in C-cells of thyroid glands
Slows calcium release from bones by decreasingosteoclastic activity
Decreases circulating calcium
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Parathyroid hormone
Biological actions
Stimulate osteoclastic bone resorption release free-ionised calcium and
phosphate into extracellular fluid
Plasma [calcium and phosphate ]
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Parathyroid hormone
Biological actions
Decrease renal tubular reabsorption ofphosphate phosphaturia and
increased reabsorption of calcium
Plasma [calcium ] ;
Plasma [ phosphate ]
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Parathyroid hormone Control of PTH secretion depends on
concentration of free-ionised calcium
circulating through parathyroid glands
Calcium PTH secretion
Affected by extracellular [ magnesium ]
Decreased by severe, chronichypomagnesaemia
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Parathyroid hormone Control of PTH secretion depends on
concentration of free-ionised calcium
circulating through parathyroid glands
Calcium PTH secretion
Affected by extracellular [ magnesium ]
Decreased by severe, chronichypomagnesaemia
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Low blood calcium High blood calcium
Normal Range
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Control of plasma calcium levels
General rule: control of extracellular [calcium]
rather than total body content
Effective control depends on:
Normal functioning of:Parathyroid glands
Kidneys
Intestine
Adequate supply of
Calcium
Vitamin D
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Control of plasma calcium levels
Impairment of control loss of calcium
from bone: plasma [calcium] maintained at
the expense of bone calcification
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Control of plasma calcium levels
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Other factors
Serum Protein Concentration
Amount of protein in circulation affects calcium
concentration by altering the bound to free calcium
ratio
Low serum albumin, unbound fraction
Parathyroid hormone detects this and increases
the unbound calcium
By reducing PTH production, lowers thecalcium concentration
This may result in reduction of total calcium levels
but keeps the unbound level normal
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Other factors
Other Hormones
Thyroxine
Stimulates bone turnover
Oestrogens and androgens
Affect production of bone matrix
With menopause, oestrogens, matrix
production
Glucocorticoids Influence bone metabolism by their action on
protein catabolism affecting production of bone
matrix
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Abnormal plasma calcium
Hypercalcaemia
Raised protein-bound Ca; normal free-ionised Ca Raised free-ionised Ca high PTH Raised free-ionised Ca due to other
causes + low [ PTH ]
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Abnormal plasma calcium
Hypocalcaemia Reduced protein-bound Ca; normal
free-ionised Ca Reduced free-ionised Ca PTH
deficiency Reduced free-ionised Ca due to
other causes + high [ PTH ]
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Assays for calcium
Total Calcium Colorimetric methods
Atomic absorption spectroscopyReference method for total calcium assayVery sensitive and specific
http://www.chemsoc.org/pdf/LearnNet/rsc/AA_txt.pdf
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Atomic absorbance spectrometry (AAS)
Analytical technique that measures the concentrations of elements So sensitive, it can measure down to parts per billion of a gram (g/dm3) Makes use of the wavelengths of light specifically absorbed by an element They correspond to the energies needed to promote electrons from one
energy level to another, higher, energy level.
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Assays for calcium
Total Calcium Photometric: Simple spectrophotometry of the
colored reaction products formed by variousindicators/dyes Indicators/dyes change colour when calciumbound
easy for automation on chemistry analyzers Allow direct measurement of calcium in
serum and other biological fluid 2 dyes commonly used: O-cresolphthaleincomplexone and arsenazo III
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Assays for calcium
Total Calcium Photometric: O-cresolphthalein complexone
(CPC)
Form red chromophore with calcium in alkalinesolution
Measured at 570 580 nm Release bound / complexed calcium by diluting
with acid and buffer with organic base Interference by magnesium reduced by:
Adding 8-hydroxyquinoline Buffering near pH 12 Measuring near 580 nm
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Assays for calcium
Total Calcium Photometric: arsenazo III
High and specific affinity for calcium At pH 6 has higher affinity for calcium thanmagnesium
Must be buffered since spectral propertiesvery dependent on pH
Use is increasing
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Assays for calcium
Total Calcium Titrimetric method
Complex calcium with calcein
Form flourescent complex that is maximally stimulated
at 490 nm and emits at 520 nm Titrate with EDTA (ethylenediamine tetraacetic acid) or
EGTA (ethyleneglycol tetraacetic acid)
EDTA / EGTA binds calcium with higher affinity andtherefore replaces calcein
End point of titration: when flourescence return tobaseline
Amount of EDTA / EGTA is directly proportional toamount of calcium
Can be automated
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Assays for calcium
Total Calcium Serum: preferred specimen
Certain anti-coagulants can interfere withanalysis Total calcium stable in serum for:
Days at 4C
Months if frozen Plastic ware / glass ware adsorb calcium uponlong storage wash well if re-using ware
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Assays for calcium
Ionised Calcium
i.e. free calcium available to cells; Ca2+ bound
to proteins are not accessible to cells
Ion selective electrode: instrument for
quantitating ionised calcium in serum
Ultrafiltration + colorimetric methods: calciumnot bound to proteins is filtered and separated
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Assays for hormones
Parathyroid Hormone Radioimmunoassay
Calcitonin Radioimmunoassay
Serum samples yield higher calcitonin levels thanplasma samples
Haemolysis: false elevated values
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Assays for hormones
Vitamin D Complex process: low concentration of vitamin D
and presence of a variety of materials with verysimilar structures
Assays involve extraction of vitamin from itstransport protein purification and separation ofvitamin D components immunoassay procedure
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Diagnosis of disorders ofcalcium metabolism
Plasma parathyroid hormone assay Urinary calcium and phosphate
estimation
Plasma Vitamin D assay
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Abnormal plasma calcium
There are bone disorders that do not affectplasma [ Ca ]
Osteoporosis Pagets disease of bone Osteomalacia caused by renal tubular
disorders of phosphate reabsorption Rickets
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Diagram of Beer-Lambert absorption of a beam
of light as it travels through a cuvette of size l.
http://en.wikipedia.org/wiki/Beer-Lambert_law
A = absorbance
= absorption coefficient
l= distance
c = concentration
http://en.wikipedia.org/wiki/Image:Beer_lambert.png