CS 2014

Calcium and Phosphate Homeostasis

Christian StrickerAssociate Professor for Systems Physiology

ANUMS/JCSMR - ANU

Christian.Stricker@anu.edu.au http://stricker.jcsmr.anu.edu.au/Ca&Pi.pptx

THE AUSTRALIAN NATIONAL UNIVERSITY

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AimsThe students should

• appreciate the physiological role of extracellular Ca2+;• know the principles of storage of Ca2+ in different

compartments;• be able to explain the major factors determining

extracellular Ca2+;• recognize principles involved in Ca2+ and Pi

homeostasis via hormones;• understand how extracellular Ca2+ in sensed and

signalled; and• be able to distinguish differences between regulation

of Ca2+ and Pi.

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Contents

• Calcium (Ca2+)– Notion of free and bound Ca2+

– Extracellular versus intracellular Ca2+

– Ca2+ exchanges (uptake → storage → excretion)– Regulation of Ca2+

• Short-term: pH and its consequences• Long-term

– Nature of the Ca2+-sensor– Hormonal control of Ca2+ homeostasis

• Phosphate (Pi)– Free and bound– Regulation of Pi

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Total & “Free” Extracellular Ca2+

• Non-filterable = protein bound (“albumin”) = cannot permeate through filter pores = non-diffusible = biologically not available (on normal time scale).

• Filterable = can permeate filter pores = diffusible; only some Ca2+ can be excreted.

• The absolute numbers might vary by a few percentages depending on source.

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Extra- and Intracellular Ca2+

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Ca2+ Balance• Homeostasis is result of

– uptake (absorption),– deposition and resorption

(bone),– secretion and excretion, and– exchange between ICF ↔ ISF.

• Ca2+ taken up in proximal part of small bowel (duodenum > jejunum).

• Excretion primarily via urine (kidney).

• Constant exchange between ICF and ISF as well as ISF and bone (no net change).

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Exchange between ECF and ICF• Influx into cell via

– Ion channels• Voltage-dependent, non-selective

cation, store-operated and ligand-gated ion channels

– Transporters (reversed transport in pathology)

• Outflow via transporters– Ca-ATPase– Na/Ca-exchanger (3 Na+

against 1 Ca2+)

• Sequestration within cell– Smooth endoplasmic

reticulum– Mitochondria– Calcicosomes– Fixed and soluble buffers

(Ca2+-binding proteins)

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Ca2+ Deposition and Resorption• Deposition/accretion via

osteoblasts into bone• Resorption via osteoclasts

from bone• Tightly controlled by

hormones• Guarantees a rapidly

exchangeable pool (4 g out of recently made-up bone)

• Bone– Inorganic minerals

70%• Hydroxyapatite

– Organic part (matrix)

30%• Collagen

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Dietary Uptake• Demand:

1 g / d

25 mmol / d• Demand dependent on age and

gravidity– Children:

1.2 g / d

– Adolescent:

1.0 g / d

– Pregnancy:

1.5 - 2.5 g / d

– Lactation:

1.5 - 2.5 g / d

• Resorption:

25 - 40%**demand limited;

– also depends on Ca2+-complexes formed/presented in GI tract (oxalic acid, phytin)

– pH

• Source:– Predominantly dairy products (milk,

cheese, etc.)

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Mechanism of Dietary Uptake

• Two pathways: Paracellular and transcellular.• Paracellular: Uptake from ISF into blood via diffusion through vessel fenestrations.• Transcellular:

– Requires a luminal Ca2+ channel and intracellular binding to a Ca2+-binding protein.– Rate limiting step is into ISF; Ca-ATPase and/or Na/Ca-exchanger.– Calcitriol (“vitamin D”) can speed up resorption rate (via protein expression).

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Excretion of Ca2+

• Kidney is organ for Ca2+ net-excretion.

• Only 65% of Ca2+ can be filtered (“protein free” fraction).

• 98% of filtered Ca2+ resorbed.• 2% of Ca2+ excreted.• Mechanism of resorption is

– paracellular• TAL (driven by voltage).

– transcellular• Ca-ATPase• Na/Ca-exchanger

• Resorption is hormonally controlled (see later).

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Calcium Regulation

• Short-term: pH (faster than regulatory hormonal changes can act…) and its consequences.

• Long-term– Ca2+ sensing on cell membrane. – hormonal feedback-loops for Ca2+ homeostasis.

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Short-Term Regulation

pH and Its Consequences

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pH and [Ca2+]: Competition

• Acidosis → total Ca2+↑: Ca2+ unbinds not only from plasma, but also from protein on endothelial membrane/sub-endothelial space (significant volume).

• pH also affects solubility products (Ca-phosphates, -carbonates, etc).

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Effects of [Ca2+] Change• György’s formula for serum electrolytes:

• Describes qualitatively excitability of neuromuscular system; i.e. tendency for tetanic reactions (cramps).(tetanus = steady muscle contraction without distinct twitching)

• Examples:– [K+]↑ (hyperkalaemia): tendency for tetanus since excitability increases

(depolarisation of membrane potential).

– [Ca2+]↓ (hypocalcaemia): tendency for tetanus since excitability increases.

– [H+]↓ (alkalosis): tendency for tetanus since excitability increased (respiratory; i.e. during hyperventilation; double whammy since [Ca2+] also drops…).

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Long-Term Regulation

Sensor of Extracellular [Ca2+]

Hormonal Ca2+ Homeostasis

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http://www.liv.ac.uk/physiology/ncs/index.html

Ca2+- Sensor/Receptor (FREQ)• Gene on chromosome 9: Frequenin-like

protein (FREQ) or NCS-1; GPCR – dimer.• Binds to a host of adaptor proteins.• Transduction pathways (incomplete)

– Phospholipases (C, A2, D): DAG, IP3

– Adenylyl cyclase (inhibits)– MAPKinase

• Regulated processes– Secretion (hormones - relevant here…)– Synaptic plasticity, memory formation– Proliferation, differentiation, apoptosis– Gene expression

• Diseases– Rod and cone diseases (phosphorylation of

rhodopsin)– Up-regulated in bipolar disease and

schizophrenia/autism

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Parathyroid Hormone: [Ca2+]↑• Peptide (84 AA) from parathyroid gland• Secretion “threshold”: < 1.5 mM

(i.e. normally, very little PTH is secreted).• Ca-sensor transduction: via phospholipase

C → IP3 production AND adenylyl cyclase

• Target organs:– Bone: osteoclasts (resorption)– Kidney: accelerates vitamin D synthesis– Indirectly: Gut, kidney (reabsorption, loss of

phosphate…)

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• Lipophilic, steroid-like hormone• Synthesis involves 3 steps:

– Skin: Calciol– Liver: 25-OH-cholecalciferol– Kidney: Calcitriol

• Target organs:– Primary: gut (uptake)– Secondary: bone, kidney, placenta,

breasts, hair follicles, skin, etc.• Dosage: 5 µg/d adults; 10 µg/d children.• Deficiencies:

– Inadequate dietary intake.– Reduced absorption.– Insufficient sun light exposure (England 19th

century; kids/adults in front of computers, hospital beds, etc…).

– Reduced 1α-OH-ation (kidney disease).

Calcitriol: [Ca2+]↑

Despopoulos & Silbernagl 2003

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Calcitonin: [Ca2+]↓• Peptide hormone (32 AA)• Secreted from C-cells in the thyroid gland.• Secretion directly proportional to [Ca2+]

(secreted when [Ca2+] normal).• Ca-sensor transduction: via AC → cAMP.• Target organs:

– Bone: inhibits osteoclast activity.– Kidney: converts vit. D precursor to 24,25-(OH)2-

cholecalciferol (inactive) → Ca2+ absorption↓.

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Ca2+- Regulation Overview

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Phosphate (HPO42-; Pi)

• Largest amount intracellularly: ~ 75 - 90 mM; ~ 200 g.• Extracellularly, mostly as HPO4

2-; H2PO4- (Pi; 25%; pH!)

– Largest buffer in urine (some Pi always excreted).

• Reference range (plasma): 0.8 – 1.5 mM– 85 – 90% filterable (Pi): 50% ionised; 50% complexed (Ca, Mg).

– 10 – 15% protein bound (as phosphorylation products).

• Less well regulated than Ca2+ (concentrations vary largely after food intake).

• Despite Ca/Pi supersaturation, no precipitation in tissue (pyrophosphate is one of many inhibitors of precipitation).

• Regulation tightly linked to Ca2+ because– of bone (large deposit): hydroxyapatite, and– Pi regulation is linked to PTH, calcitriol and calcitonin (little!).

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Pi Balance• Pi homeostasis is result of

– amount of Pi in the body (bones); and

– distribution between ICF and ECF.

– Under normal conditions, Pi absorption = excretion (ss).

– Constant exchange between ICF and ECF as well as ECF and bone (no net change).

• Mostly from dairy products.– Absorbed in proximal small

bowel (duodenum > jejunum) by Na/Pi symporter.

• Excretion primarily via urine.

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Targets of Pi Regulation• Regulation by maximal renal reabsorption capacity.

– Pi excess: rate of renal excretion↑; and

– Pi shortage: rate of renal excretion↓.

– Volume expansion response renal excretion↑ and vice versa.

• Primary targets (very similar to Ca2+)– Kidney (80% reabsorbed in PT)

• Under normal conditions, Pi transport is saturated and matched to absorption: if Pi↑, more is lost than absorbed; if Pi↓, more Pi is retained.

• Low levels of Pi, alkalosis and hypercalcaemia cause insertion of Na/P i symporter into apical membrane → reabsorption↑.

• High levels of Pi, acidosis and hypocalcaemia cause removal of Na/P i symporter from apical membrane → reabsorption↓.

– Bone• Resorption at the level of osteoclast (PTH, calcitriol)• Formation at the level of osteoblast (calcitonin)

– Gut• Enterocyte in duodenum/jejunum (calcitriol)

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Pi Homeostasis• PTH (most important): [Pi]↓

– increases bone resorption– increases renal filtration

• Calcitriol: [Pi]↑

– increases gut absorption– increases bone resorption– increases renal reabsorption

• Calcitonin (transient; least important): [Pi]↓– increases bone formation– increases renal filtration

• Other hormones:– growth hormone: Pi↑ in children

– glucocorticoids: Pi excretion↑

• Not in line with Ca2+ homeostasis (more complex).

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Take-Home Messages

• Ca2+ is central to extra- and intracellular signalling.

• [Ca2+]e determines excitability.

• [Ca2+] is sensed via FREQ (NCS-1), which requires adaptor protein(s) for transduction.

• Short-term, pH determines [Ca2+] and [Pi].

• Hormones regulate [Ca2+] and [Pi]:

– PTH: [Ca2+]↑ and [Pi]↓.

– Calcitriol (vit. D): [Ca2+]↑ and [Pi]↑ via uptake↑ and also [Pi]↑ via renal reabsorption↑.

– Calcitonin: [Ca2+]↓ and [Pi]↓.

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MCQ

John Mak, a 58 year-old male, was diagnosed with

prostate cancer and multiple osteolytic lesions causing

markedly increased serum calcium. Which of the following

statements best describes the accompanying blood Pi

concentration?

A. Normal independent of calcium concentration

B. Elevated dependent on calcium concentration

C. Lowered dependent on calcium concentration

D. Lowered independent of calcium concentration

E. Elevated independent of calcium concentration

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That’s it folks…

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MCQ

John Mak, a 58 year-old male, was diagnosed with

prostate cancer and multiple osteolytic lesions causing

markedly increased serum calcium. Which of the following

statements best describes the accompanying blood Pi

concentration?

A. Normal independent of calcium concentration

B. Elevated dependent on calcium concentration

C. Lowered dependent on calcium concentration

D. Lowered independent of calcium concentration

E. Elevated independent of calcium concentration