Endocrine control of calcium metabolism Flashcards
What is the most abundant metal in the body and the 5th most abundant element?
Calcium is the most abundant metal in the human body (fifth most abundant element).
List 7 roles of calcium in the body?
- Neuromuscular excitability
- Muscle contraction
- Strength in bones
- Intracellular second messenger
- Intracellular co-enzyme
- Hormone/neurotransmitter stimulus-secretion coupling
- Blood coagulation (factor IV)
Where is most of the calcium in the body found and what form is it found in?
- Most calcium is present in the body as calcium salts
- It is mainly found in bone (99%, approx. 1kg) as complex hydrated calcium salt (hydroxyapatite crystals)
- In blood, some is present as ionized calcium (Ca2+), some bound to protein and the tiny bit left as soluble salts
- Only the free (unbound) Ca2+ is bioactive
What is the total amount of calcium in the blood?
approximately 2.5mM
What proportion of calcium in the blood in unbound (ionized), bound to plasma proteins and as diffusable salts?
Most of this calcium is present in the unbound ionised form (1.25 mM/L)
Calcium is present in the blood in DYNAMIC EQUILIBRIUM
The important component that needs to be controlled is the unbound ionised form (bioactive component)
What 4 parts of the body are involved in the calcium handling in the body and why?
GI tract - most calcium is absorbed in the GI tract
It is absorbed into the blood and some of it will get excreted in faece
Kidney- Once in the blood, the calcium can pass to the kidneys which regulates the content of the blood
A lot of the calcium passing into the kidneys will return to the blood - excretion and absorption of Ca2+into the blood
About 150 mg/24h is excreted from the kidneys per day - this maintains equilibrium
Bone- The hydroxyapatite crystals in the bone can be broken down to increase blood calcium levels - depository for calcium
Calcium is also needed for bone strength
Blood- Transport of calcium
Name two main hormones involved in increasing Ca2+ concentration?
Name the main hormone involved in decreasing calcium concentration?
Calcium Ion Regulation
TWO main hormones involved in RAISING blood calcium concentration:
PARATHYROID HORMONE (PTH)
1,25-dihydroxycholecalciferol (or CALCITRIOL)
NOTE: Calcitriol is a steroid and is also called 1,25-dihydroxy vitamin D3 Main
CALCITONIN
This doesn’t seem to have a major effect in the long run
No one really knows the importance of calcitonin
Calcitonin is NOT the main controlling influence on calcium ions
Draw a diagram showing wher PTH and Calcitonin is produced?
Parathyriod glands- Back of thyriod
How are calcium levels sensed in the body?
Where are these receptors primarily found?
The calcium-sensing receptor (CaSR) is a Class C G-protein coupled receptor which senses extracellular levels of calcium ion. It is primarily expressed in the parathyroid gland and the renal tubules of the kidney. In the parathyroid gland, the calcium-sensing receptor controls calcium homeostasis by regulating the release of parathyroid hormone (PTH).[5] In the kidney it has an inhibitory effect on the reabsorption of calcium, potassium, sodium, and water depending on which segment of the tubule is being activated.
Explain how parathormone (PTH), 1,25-dihydroxycholecaciferol (calcitriol) and calcitonin are synthesised.
Initially synthesised as protein pre-proPTH
What is PTH’s mechanism of action once it has binded onto a receptor?
Binds to transmembrane G-protein linked receptors
Binding of parathormone to the G-protein linked receptor leads to activation of adenylate cyclase (with phospholipase C acting as a second messenger)
PTH is a polypeptide with 84 amino acid
Draw a diagram showing the effects of PTH in the body?
Include kidneys, bone and small intestine
Remeber calcuim is in a dynamic equilibrium with calcium phosphate
PTH stimulates the kidneys to excrete more phosphates- in urine
People used to think that as PTH causes the loss of free phosphates from the kidneys, the equation is no longer in equilibrium so the phosphate salt will dissociate to make amends for the phosphate that has been excreted and so calcium concentration increases secondarily to the replenishment of phosphate levels
We have since found out that PTH actually has direct effects on the calcium in the kidneys
The OVERALL effect is an increase in calcium reabsorption
PTH has another important effect in the kidneys: stimulates the synthesis of 1a HYDROXYLASE
1a Hydroxylase is involved in the synthesis of calcitriol
Calcitriol has an important effect on the small intestines: controls the absorption of calcium and phosphate (increases absorption)
PTH also has an effect on bone:
Stimulates osteoclasts - causes resorption of the bone matrix and release of calcium from hydroxyapatite crystals into the gut
Inhibits osteoblasts
THIS ALL LEADS TO AN INCREASE IN BLOOD CALCIUM CONCENTRATION
In the intestine, via kidney, PTH enhances the absorption of calcium in the intestine by increasing the production of activated vitamin D. Vitamin D activation occurs in the kidney. PTH up-regulates 25-hydroxyvitamin D3 1-alpha-hydroxylase, the enzyme responsible for 1-alpha hydroxylation of 25-hydroxy vitamin D, converting vitamin D to its active form (1,25-dihydroxy vitamin D). This activated form of vitamin D increases the absorption of calcium (as Ca2+ ions) by the intestine via calbindin.
However, PTH enhances the uptake of phosphate from the intestine and bones into the blood. In the bone, slightly more calcium than phosphate is released from the breakdown of bone. In the intestines, absorption of both calcium and phosphate is mediated by an increase in activated vitamin D. The absorption of phosphate is not as dependent on vitamin D as is that of calcium. The end result of PTH release is a small net drop in the serum concentration of phosphate.
PTH increases the activity of 1-α-hydroxylase enzyme, which converts 25-hydroxycholecalciferol, the major circulating form of inactive vitamin D, into 1,25-dihydroxycholecalciferol, the active form of vitamin D, in the kidney.
Draw a diagram showing the effect of PTH on blood [Ca2+]?
Include in the diagram kidneys, bone and small intestine
What are the effects of PTH on bone?
Osteoclasts are activated by PTH but NOT directly
PTH works on osteoblasts and inhibits various activities
However, PTH stimulates osteoblasts to produce various OSTEOCLAST ACTIVATING FACTORS (OAFs)
OAFs move to the osteoclasts and stimulates the breakdown of bone matrix to RELEASE CALCIUM
One of the OAFs is called RANKL which links PTH via the osteoblasts to the main target which are the osteoclasts
REMEMBER: PTH binds directly to osteoblasts but has an INDIRECT effect on OSTEOCLASTS
What does PTH cause Osteoblasts to do?
However, PTH stimulates osteoblasts to produce various OSTEOCLAST ACTIVATING FACTORS (OAFs)
OAFs move to the osteoclasts and stimulates the breakdown of bone matrix to RELEASE CALCIUM
One of the OAFs is called RANKL which links PTH via the osteoblasts to the main target which are the osteoclasts
REMEMBER: PTH binds directly to osteoblasts but has an INDIRECT effect on OSTEOCLASTS
How do OSTEOCLAST ACTIVATING FACTORS (OAFs) work?
Osteoclasts are activated by PTH but NOT directly
PTH works on osteoblasts and inhibits various activities
However, PTH stimulates osteoblasts to produce various OSTEOCLAST ACTIVATING FACTORS (OAFs)
OAFs move to the osteoclasts and stimulates the breakdown of bone matrix to RELEASE CALCIUM
One of the OAFs is called RANKL which links PTH via the osteoblasts to the main target which are the osteoclasts
REMEMBER: PTH binds directly to osteoblasts but has an INDIRECT effect on OSTEOCLASTS
An osteoclast (from the Greek words for “bone” (ὀστέον), and “broken” (κλαστός)) is a type of bone cell that breaks down bone tissue.
Draw a diagram showing how PTH is regulated?
Overall effect of PTH: raise calcium ion concentration
PTH stimulates an enzyme that leads to the synthesis of calcitriol
Increased synthesis of calcitriol also leads to increased plasma calcium concentration
The cells in the parathyroid gland which produce PTH respond to changes in plasma calcium concentration - they have calcium ion receptors
These receptors are activated whenever there is a fall plasma calcium concentration
Calcitriol also has a negative feedback effect on PTH
There are Beta Receptors on the cells that produce PTH so they can be stimulated by catecholamines to secrete PTH