Calcium, Phosphate and Magnesium

8/14/2019 Calcium, Phosphate and Magnesium

1/46

Calcium, Phosphate andMagnesium

Mineral Metabolism


2/46

Calcium, Phosphate and Magnesium

Lecture Outline

Calcium

Intake and excretion

Distribution

Function

Control

Abnormality

Assessment


3/46


4/46

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


5/46

Calcium

~ 25 mmol (1g)

ingested per day

Net absorption =

6 12 mmol


6/46

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


7/46

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


8/46


9/46

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


10/46

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


11/46

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


12/46

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?


13/46

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


14/46

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


15/46


Parathyroid hormone (PTH)

Most important controlling factor

Secreted by parathyroid glands

Increases circulating free-ionised calcium


16/46


Calcitonin

Produced in C-cells of thyroid glands

Slows calcium release from bones by decreasingosteoclastic activity

Decreases circulating calcium


17/46


18/46

Parathyroid hormone

Biological actions

Stimulate osteoclastic bone resorption release free-ionised calcium and

phosphate into extracellular fluid

Plasma [calcium and phosphate ]


19/46

Parathyroid hormone

Biological actions

Decrease renal tubular reabsorption ofphosphate phosphaturia and

increased reabsorption of calcium

Plasma [calcium ] ;

Plasma [ phosphate ]


20/46

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


21/46


22/46

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


23/46

Low blood calcium High blood calcium

Normal Range


24/46


25/46


26/46


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


27/46


Impairment of control loss of calcium

from bone: plasma [calcium] maintained at

the expense of bone calcification


28/46



29/46

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


30/46

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


31/46

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 ]


32/46


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 ]


33/46

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


34/46

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.


35/46

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


36/46

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


37/46

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


38/46

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


39/46

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


40/46

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


41/46

Assays for hormones

Parathyroid Hormone Radioimmunoassay

Calcitonin Radioimmunoassay

Serum samples yield higher calcitonin levels thanplasma samples

Haemolysis: false elevated values


42/46

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


43/46

Diagnosis of disorders ofcalcium metabolism

Plasma parathyroid hormone assay Urinary calcium and phosphate

estimation

Plasma Vitamin D assay


44/46


There are bone disorders that do not affectplasma [ Ca ]

Osteoporosis Pagets disease of bone Osteomalacia caused by renal tubular

disorders of phosphate reabsorption Rickets


45/46


46/46

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

Date post:	30-May-2018
Category:	Documents
Upload:	kiedd04
View:	224 times
Download:	0 times

Calcium, Phosphate and Magnesium

Documents