Lecture 5: Ca2+ Homeostasis

Stalagmites and stalactites

• Form in caves when water containing dissolved calcium carbonate (CaCO3) drips from the ceiling.

• As the water evaporates, the calcium carbonate precipitates out of solution and forms crystals.

• Over time, these crystals can build up to form stalagmites and stalactites.

Need for Ca2+

• 5th most abundant element in the body and element

  • Vital Organ protection                 

    • Skull, ribcage

  • Neurotransmitter release – triggers vesicle fusion to membrane

  • Cardiovascular System - Cardiac and smooth muscle contraction – shortening of myocytes

  • Movement

    • Long bones and skeletal muscle contraction

      • Needed for all muscle contraction

  • Hormone secretion

  • Blood clotting

  • Intracellular Signalling: Ca2+, oscillations, apoptosis

Basic Ca2+ Homesotatic System

• Eat, Absorb, Excrete

• Equilibrium between gain and loss of Ca2+

  • Net intake/output: 200mg

• Bone formation

• Bone resorption

• System involves the integration of 3 calciotropic hormones

Need for Mineralised Bone

• Required to resist gravity when humans became terrestrial organsims and to store Ca2+

  • low Ca2+ in the air - must be stored in the body

• Provide stregnth

Bone Formation

• Ca²⁺ enters the bone

  • Larger demand for mineralised Ca²⁺ in pregnancy and puberty - bones get longer and stronger → biologically efficent

Bone Resorption

• Ca²⁺ leaves the bone

Output of Ca²⁺

• Lost in the form of bile salts or epithelial cells sloughed off of intestine

• Prevents hypercalcemia  

• Excreted in the urine  

• Large volumes of plasma filtered by kidney and selectively reabsorb glucose, AA and 99% of Filtered Ca2+

   

2 Measures of Ca2+

• % bound to macromolecular proteins

• % complexed as salts

Total Plasma Ca²⁺ Levels

• ~2.5 mM (10mg/dl)

  • ~40% bound to macromolecular proteins, e.g. albumin.

  • ~5% complexed as Ca²⁺ salts e.g. bicarbonate, phosphate

  • free ionised Ca²⁺ concentration is ~ 1.2 mM.

    unbound Ca2

Calcium Sensting Receptors

• Located in the neck

• Can only detect Ca²⁺ in solution and can measure free ionised Ca²⁺ in the blood

  • can’t detect ion bound to protein

Calcitropic Hormones

• Parathyroid Hormone

• Vitamin D (1,25[OH]2D3)

• Calcitonin

  • Extracellular Calcium

Parathyroid Gland

• Discoverd by Ivar Sandstrom in 1880

• the last major organ to be recognized in humans.

• Linked to regulation of Ca2+ levels in the blood in ~1920s

  • 4 glands present in the neck

  • 5% of people > four glands

    • Have the wrong number of glands

Origin of Paratyhroid Gland

• Derived from the pharyngeal pouch of the endoderm - driven by GCM-2

• PTH 1/2G genes expressed in the neck

  • Derived from gills - internalised in the neck as the PT gland

GCM-2

• Gene that drives the formation of the parathyroid gland

• Gene only expressed in the parathyroid gland

Gill Bud Formation In Fish

• Gcm-2 expressed in the pharyngeal pouches of zebrafish and dogfish to drive the formation of gill → involved in breathing and ion regulation

  • gills also express PTH 1/2-encoding genes in fish and CaR

Parathyroid Hormone (PTH)

• aka parathormone

• Produced in Chief cells of the Parathyroid gland.

  • Blue: Pre-PTH

  • Orange: Pro-PTH  

  • Yellow: mature PTH      

• Tags present upstream to allow for the processing of the hormone      

  • 84 aa hormone

  • (t_1/2 < 20 mins)

Relationship Between Extracellular Ca²⁺ and PTH

• Hormone secretion is inversely proportional to serum Ca²⁺      

• PTH raises blood Ca2+ levels

• Steep sigmoidal relationship - plasma [Ca²⁺] tightly regulated

  • Has 1/5th order of magnitude (rather than 3)

• Inverse sigmoidal curve:

  • Low plasma [Ca²⁺] → ↑ PTH secretion  

  • High plasma [Ca²⁺] → ↓ PTH secretion

    • Secretion is suppressed

Feedback Regulation of PTH Secretion

• Parathyroid Cell is the only cell that secretes PTH     

• This increases [Ca2+] level in the body  

• This feeds back to Calcium sensing  receptors on the cell

• Increased Ca2+ stimulates the receptors to suppress the release of further PTH

  • Feedback pathway gives rise to reverse sigmoidal curve seen

Ca²⁺ Sensing Receptors (CaR)

• A GPCR responsive to Ca²⁺ (and Mg²⁺)

  • Responds to Ca²⁺ rather than modified amino acids and polypeptides

• Coupled to Gi and Gq

• Present in the kidney to limit Ca²⁺ reabsorption

• Monitors blood Ca²⁺ levels continuously and serves as the ultimate control point for calcium homeostasis

Effects of PTH

• elevates plasma Ca2+ levels by:

  o   ­­↑ Bone resorption - release of Ca2+

  o   ­↑ Renal Ca²⁺ reabsorption i.e. ↓excretion (but also ­P_i excretion)

  o   ­ ↑ Production of 1,25(OH)₂D₃ (Vit D)

Bone Turnover and Acute PTH Secretion

• Bone has a reservoir function – stores Ca2+ that can be released in resorption

  • Released using PTH signalling      

• Small amount of Ca2+ released to raise Ca levels

Effect of PTH Secretion

• Pulsatile secretion: healthy and anabolic for bone

• Sustained secretion: unhealthy and catabolic for bone – will degrade it

Requirments to Make Bone

• Space - acquired by breaking old bones down

• Minerals – acquired from the old bone

Immediate Effect of PTH Release

• Mobilises Ca2+ from the bone → bone turnover

  • Multiple pulses of PTH, along with circadian secretion break down old bone and drive new bone formation

PTH and The Handling of Renal Ca²⁺

• PTH increases renal Ca2+ reabsorption

  • 70% of filtered Ca2+ is reabsorbed in the Proximal tubule

  • Reabsorption at the thick ascending limb – enhanced by hormone 

• Automatic process – osmotically driven due to movement of water into proximal tubule – unregulated and not hormonally modulated

• No Ca2+ is absorbed at the Collecting ducts

  • 1% of Ca2+ Excreted in the urine

Production of Vitamin D (1,25(OH)₂D₃)

• A bond in pro-vitamin D3 is broken by UVB (high energy UV) to convert it into pre-vitamin D3      

• Pre-vitamin D3 auto-converts into Vitamin D3  

  • Need for Ca2+ detected in resorption - > the sun and vitamin D assist Ca²⁺ storage hormone (PTH)

• Vitamin D3 constitutively hydroxylated at the 25^th position in the liver

• Hydroxylation at the first position occurs in the kidney and is regulated under to control of PTH

• Must be hydroxylated at the one position to assist Ca²⁺

Importance of Vitamin D

• Produced in response to PTH - acts at DCT

• It is an amplification of the PTH signal to raise Ca2+ vias resorption from the bone and increase absorption from the kidney  

  • PTH then produces vitamin D to resorb more Ca2+ from the bone and kidney

Effects of 1,25(OH)₂D₃

• Increased net intestinal Ca²⁺ uptake

  • No PTH receptors in the gut

• Increases serum Ca²⁺ by increasing bone resorption and renal Ca²⁺ reabsorption

Ca²⁺ Absorption Across Dudoenal Epithelial Cell

• Movement of Ca2+ into the cell is passive, requires a hole in the membrane to allow passage; difficult to remove from cell

  • 10,000-fold increase in the electrochemical gradient following dietary Ca²⁺ intake

• Passage across cells permitted through the expression of Calbindins (Ca²⁺ binding protein)

• TRPV 6 channels in the gut act as a pore to allow Ca²⁺ to flood in and bind to CalD9K to be transported across the cell

  • [Ca] doesn’t change

Consequence of High Intracellular [Ca²⁺]

• Acts as an apoptotic signal

  • Only want a small amount present in the cell at once

2 Systems Upregulated By Vitamin D

• Calbindins

• TRPV Channels

Vitamin D Deficiency

• Not enough TRPV channels or Calbindins expressed;

  • not enough Ca2+ reabsorbed

  • Some passive reabsorption still occurs

2 Main Methods of Dietary Absoprtion

• 90% absorbed in the small intestine via

  • passive, paracellular diffusion down its electrochemical gradient

  • by active transcellular transport under the control of 1,25(OH)₂D₃, calbindins and TRPV6 channels

Calbindins

• Ca²⁺ binding proteins

  • CalD28K in kidney

  • CD9K in gut

• Expression dependent on Vitamin D₃ - No trafficking process without

• Binds to Ca²⁺ from TRPV channels and traffics it to the other side where it is actively pumped out against the concentration gradient

  • Is recycled to pick up more calcium

Role of Vitamin D in The Kidney

• Acts on the DCT

• Simillar process in intestines occurs here as but

  • TRPV5 is more abundant in the kidney than TRPV6  

  • Calbindin(28KD)

    • both upregulated by 1,25[OH]2D3

  o   Upregulates Ca2+ absorption in the gut and RE-absorb Ca2+ in the DCT

Calcitonin

• 32 aa peptide from pro-calcitonin – large polypeptide

• Secreted from Parafollicular thyroidal “C cells”

  • t_1/25 min - short half-life

Effects of Calcitonin

• Decrease plasma Ca²⁺ following a calcium load → excretes Ca2+ - is stimulated by Ca2+ and has a suppressive effect

  • Has the opposite effect to Vit.D and PTH

• A vestigial hormone - absent in the presence of high [Ca²⁺]

• Decreases osteoclast activity

• Decreases bone resorption allowing for rapid bone deposition – allows for Ca2+ storage

• post prandal mean

Calactonin Secretions

• Rises post-prandially as blood Ca²⁺ rises, (gastrin may be involved in this secretion) and is inhibited by LOW Ca²⁺ levels in the blood

• Contribution to mammalian calcium homeostasis is very modest & << to fish

  • High [Ca2+] in ocean – required to keep levels low

Disease of Ca2+ Homeostasis

• Primary Hyperparathyroidism:       

• Secondary Hyperparathyroidism

• Osteoporosis: bone weakness

• Rickets (Vit D deficiency/1aOHase mutation)

• Nephrolithiasis (Calcium Stones): painful

  • 20M suffer from this in US

• Receptor Mutations: PTH receptor, CaR

Primary Hyperparathyroidism

• Excess release of PTH in response to a problem in the PT gland

Secondary Hyperparathyroidism

• a downstream problem causes a signal that triggers the release of PTH e.g. kidney/renal failure

Rickets

• (Vit D deficiency/1aOHase mutation)

  • Nutrient problems, Low sunlight, altitude, skin tone etc

  • Can be due to the enzyme stimulated by PTH to hydroxylate VitD

  • Mutation in the receptor

  • Long bones don’t mineralize in puberty to manage additional weight – they bow out – weak

Receptor Mutations (PTH and CaR)

• Shifts Ca2+ and PTH sensitivity

• Dependent on loss or gain of function

Problem of Space Flight

• Bone demineralisation and weakness

  • 10-20% of bone mass is lost

  • Stressing bone is healthy – puts pressure on it and keeps it well mineralized

  • Microgravity – don’t need to resist gravity; no stress on bone – demineralizes; loss of bone mass

Effects of Microgravity

• ↑ bone resorption

• ↑ hypercalciuria & hyperphosphaturia ­

• ↑ risk renal stones – mineral enters kidneys

  • Removed via ultrasound and peed out