Calcium and the Bone

• Major component of the bones, teeth and connective tissue
• Blood clotting
• Second messenger
• Muscle contraction
• Required for nerve transmission at neuromuscular junctions

A lack of calcium can cause muscle paralysis, difficulty breathing due to the inability to contract the muscles of the diaphragm, heart will stop beating – there will be no neurotransmission.

Sources of calcium:

• Diet – dairy
• There is a “bank” of calcium within bones. Therefore, when blood calcium levels are low if there is not much intake from the diet, bone will be destroyed to release calcium into the blood. Note: this is release as bound calcium and phosphate (hydroxyapatite crystals)

PTH is secreted from the four parathyroid glands in response to low ionised calcium and high phosphate in the blood

PTH is synthesised as a large precursor molecule and has a short half-life of 5 minutes – it is cleaved to its active form and pre-stored for rapid release

Chief cells respond to low ECF calcium concentrations

Calcium-sensing receptors of the chief cells are G-protein coupled receptors that are switched off during high levels of calcium. They only become active when intracellular calcium levels drop (i.e. there is less calcium interacting with the receptors); this then causes the cell to secrete pre-stored PTH into the local blood system

1. Rapid mobilisation of Ca2+

There are two forms of calcium around the bone: bound to phosphates within the bone and bound to phosphates in the interstitial fluid around the bone.

PTH acts on the osteocyte membrane, making it more leaky towards calcium phosphate so that more can be released into the blood. Therefore initially there is this rapid release of Ca2+.

2. Slow mobilisation of Ca2+ (chronic low concentration of Ca2+)

• PTH receptors are located on osteoblasts
• The osteoblasts then release RANK-L which stimulates the activity (differentiation) of osteoclasts
• Osteoclasts destroy bone collagen (bone collagen = mineralised (calcium phosphate matrix) collagen)
• Hydroxyproline (a modified amino acid that has calcium phosphate deposits) is released from the collagen into the blood. It becomes a “bio-marker” for bone destruction

Hydroxyproline does not contain free/ionised calcium, so how does calcium become unbound from phosphate in order to work?

At the kidney:

• In the proximal convoluted tubule, phosphate is absorbed
• In the distal convoluted tubule, calcium is absorbed
• PTH blocks the sodium-phosphate co-transporter. This means that is stops phosphate from being reabsorbed back into the kidney.
• PTH increases the Ca2+ ATPase activity which “pushes” calcium into the interstitial fluid
• We therefore start to excrete phosphate into the urine whilst also retaining calcium 

• Binds to cell surface receptors in osteoblasts and kidney
• Direct: increases calcium release from bone (stimulates bone resorption)
• Direct: increases calcium reabsorption from urine in renal distal tubule
• Increases activation of vitamin D in the kidney (via ­ transcription of 1a hydroxylase)
• Therefore indirectly increases intestinal calcium absorption (via activation of vitamin D)
• Decrease phosphate reabsorption in kidney (phosphaturia)

Source of vitamin D:

• Over 90% of vitamin D is produced when skin is exposed to sunlight. Sunlight converts 7-dehydrocholesterol to cholecalciferol (D3 - this is inactive vitamin D)
  • Vitamin D is derived from cholesterol so acts as a steroid hormone
• Few foods contain vitamin D naturally but can be fortified
  • Dairy, oily fish, liver

Actions of active vitamin D:

• Long-term regulation of calcium absorption
• Acts on nuclear receptors (VDR) in intestinal mucosa
• Increases synthesis of calcium binding protein (calbindin) in intestinal cells
• Increases calcium (and phosphate) absorption - so the kidney must then excrete phosphate whilst retaining calcium
• Facilitates remodelling of bone
• Negative feedback on PTH

PTH increases the formation of active vitamin D (calcitriol) in the kidney. This is in 3 steps:

• There is UV-dependent photolysis of 7-dehydrocholesterol to cholecalciferol on the skin
• Cholecalciferol is bound by a specific globulin and transported in the blood to the liver where it is converted to 25(OH)2D3
• This is carried to the proximal tubule of the kidney where it is converted to 1,25(OH)2D3 (active vitamin D) by the enzyme 1α-hydroxylase

Transcription of 1α-hydroxylase is increased by PTH

There are two types of cell in the thyroid:

• Follicular cells - release and store thyroid hormone
• Parafollicular cells (C cells) - secrete calcitonin

Calcitonin release is stimulated by raised plasma calcium levels. They bind to receptors on osteoblasts to increase bone formation.

Not essential for life

• Hypercalcaemia 
  • Cause: hyperparathyroidism, malignant tumour producing increased PTH
  • Signs and symptoms: kidney stones, constipation, dehydration, tiredness and depression
  • Treatment: parathyroidectomy
• Hypocalcaemia
  • Cause: vitamin D deficiency, PTH deficiency, renal disease
  • Signs and symptoms: numbness, muscle cramps, wheezing
  • Treatment: vitamin D replacement therapy
• Rickets (children) and osteomalacia (adults)
  • Cause: vitamin D deficiency due to poor diet, lack of sunlight or chronic renal failure
  • Signs and symptoms: bowing of legs, muscle weakness, bone fractures
  • Treatment: vitamin D replacement

A chronic lack of calcium in the body would lead to sodium being used in muscle contraction - this would lead to hyperexcitability of the neuromuscular junction. This can cause spasms/convulsions and eventually death due to asphyxiation

Functions of osteoblasts:

• Formation of the bone matrix
• Regulation of mineralisation
• Regulation of osteoclast differentiation
• Precursors of bone lining cells and osteocytes

Osteoblasts express the transcription factors RUNX2 and Osx

Osteoblasts are made from mesenchymal cells and regulated by apoptosis

Function of osteoclasts:

• Bone resorption
• Contribute to calcium homeostasis (along with osteoblasts and parathyroid)

Differentiation of osteoclasts:

• The release of RANK-L from osteoblasts stimulates osteoclast differentiation
• Osteoclasts then "sit" on the bone and seals it off. They then secrete H+ and phosphotases to create an acidic environment and break apart the mineralised collagen - this releases the calcium phosphate from within. 

• PTH receptor is located on osteoblasts
• RANK is a receptor on the osteoclasts
• RANK-L is expressed from osteoblasts (and stromal cells)
• RANK-L binds to RANK, causing osteoclasts to differentiate and activate
• OPG is a "dummy receptor" for RANK-L that is also located on the osteoclasts and can block RANK-L signalling to osteoclasts. This blocks osteoclast differentation

Therefore, if there is more PTH secretion (due to low plasma calcium levels), there is more osteoblast activation, more RANK-L expressed, and it is more likely that RANK-L can bind to RANK causing osteoclast differentation and bone breakage to release more calcium phosphate 

These are osteoblasts that have been incorporated into the bone matrix during bone mineralisation.

Function of osteocytes: They produce an osteocyte mechanosensing network that senses microfractures within the bone; they encourage remodelling of this bone. Osteocytes also regulate local mineralisation. Osteocytes are regulated by local/shear stress and damage.

During growth:

• Formation exceeds resorption
• New bone growth on periosteal surfaces

During adulthood:

• No net change in amount of bone
• Repair of damaged bone

During ageing:

• Resorption exceeds formation
• Net loss of bone from the skeleton
• Failure of bone repair mechanisms

Osteopetrosis

• A decrease in bone resorption leading to very dense but fragile bone
• Caused by:
  • Overexpression of OPG
  • Absence of RANK-L
  • Absence of RANK

Osteoporosis 

• Massive increase in bone resorption leading to fragility due to loss of bone density
• Caused by:
  • Absence of OPG
  • Overexpression of RANK-L

In post-menopausal women, the decrease of oestrogen levels lead to an increase in RANK-L production. This overwhelms OPG dummy receptors, so there are more osteoclasts activated, causing massive bone resorption and a loss of bone density. They are more at risk of osteoporosis.