Other Endocrine Conditions Flashcards
What is the constant level conc that free calcium needs to be kept in and where?
1.25nm in the extracellular fluid and plasma
What are calcium levels controlled by?
*Parathyroid hormone (PTH)
-Calcitonin
-Vitamin D
What is calciums major role and give examples:
Ca2+ is important in controlling the permeability of the cell membrane to Na+
Low Ca2+, increase Na+ permeability so depolarisation
High Ca2+, decrease in Na+ permeability so hyperpolarization
What are the symptoms of hypocalcaemia and why?
Increase in muscle/ nerve excitability (as closer to threshold)
Muscle spasm (death via spasm of respiratory muscles)
What are the symptoms of hypercalcaemia?
Decrease in muscle/ nerve excitability
Causes cardiac arrhythmias
What are the main factors which controls extracellular calcium?
Ca2+ reservoir (bones and teeth) = 99% of all Ca2+ in body
Ca2+ kidney excretion
Dietary calcium
Describe the structure of the cortical (compact) bone:
Found on the outer part of bone
Have a central canal which has blood vessels
Made of osteocytes (network of these called lamella), which sit in microscopic channels called canaliculi- which contains the extracellular fluid of these cells (bone fluid)
Form a network with osteoblasts which forms connections called osteocytic-osteoblastic bone membrane
Osteoclasts sit on outside of bone and mulinucleate
Describe the mineralised bone:
Most of the bone
The majority of the bone compromises of hydroxyapatite crystals which precipitates around the collagen extracellular matrix (the osteoid) in the bone
The osteoblasts secrete the osteoid , causes the high levels of Ca2+ and PO43- in the fluid of the bone to precipitate out and form the hydroxyapatite crystals
Describe bone turnover:
Osteoclasts dissolve the bone, produces HCl (dissolves the mineralised bone)
Enzymes ‘cathepsin k’ breaks down collagen matrix
OCs die or migrates
OBs secrete extracellular matrix and fills the cavity
Describe cross talk for bone deposition and resorption:
Recruitment of OC precursors and differentiate and attach to surface of the bone
Within the bone as the OC breaks down the bone, it releases IGF and TGF-B from the bone to the bone fluid which causes more differentiation of OB from OB precursor cells and attach T-bone and increase bone deposition
Describe the regulation of OCs using RANKL and OPG:
Numbers of OC are controlled by factors from the OB
RANKL (RANK ligand (made by OB), RANK= Receptor Activator of NFkB) increases OC differentiation and decreases OC apoptosis, increasing the number and promoting bone resorption over the long-term
Osteoprotegerin (OPG) is a decoy receptor for the RANKL, decreases OC numbers and promotes bone deposition over long-term
The balance of these two factors is an important determinate of bone density
How is oestrogen involved in bone density?
Oestrogen stimulates production of OPG and promote apoptosis of OC and increases a number of OB
Describe the parathyroid hormone in Ca2+ homeostasis:
Peptide hormone (84 a.a)
Secreted from the parathyroid gland
Secretion increases with plasma Ca2+ conc falls
Acts to increase plasma Ca2+
How does the parathyroid cells monitor calcium concentration?
Monitor directly by the parathyroid cells via Ca-sensing receptors (GPCRs), negative feedback loop, as Ca2+ decrease causes signalling, therefore signalling of PTH
Acts on *bone, kidneys and GIT
What is the effect of PTH on the bone?
PTH leads to a release of Ca2+ from the bone to increase the plasma Ca2+ levels
This occurs in 2 phases:
-fast exchange
-slow exchange
Describe the fast exchange due to hypocalcaemia:
PTH induces a rapid efflux of Ca2+ within the canaliculi of the bone
There are PTH receptors on OBs and OCs
PTH receptors are GPCRs coupled to Gas, when stimulated there is an increase in cAMP and movement of Ca2+ into the cells from the bone fluid
Gap junctions allowing the movement of Ca2+ via the OCs out through the OBs and into the plasma to the BVs
Describe the slow exchange due to hypocalcaemia:
Activated under conditions of prolonged hypocalcaemia
PTH activates OBs to increase RANKL expression, so increases OCS
The OCs increase bone resorption which increases plasma Ca2+
OB building activity is inhibited
Plasma PO43- levels are also increases
Balance is restored when Ca2+ absorption in the GIT is increased
What is the effect of PTH on the kidneys?
PTH promotes Ca2+ retention (Ca2+ reabsorption occurs)
Promotes PO43- excretion (decreases PO43- reabsorption)
Causes an activation of Vitamin D (occurs in the kidney)
Activated Vitamin D causes an increase in the Ca2+ absorption of the GIT, so PTH indirectly causes an increase of uptake form the diet
Describe Calcitonin in Ca2+ homeostasis:
The antagonistic hormone to PTH
It is secreted by the cells of the thyroid hormone
Secretion is increased when there is an increase in plasma Ca2+
Peptide hormone (32 a.a)
How does Calcitonin monitor calcium concentration?
Receptors are GPCRs that increase cAMP
Acts to decrease levels of extracellular plasma Ca2+ by:
-decreasing Ca2+ movement from the canaliculi fluid into the plasma
-inhibiting OC activity
-inhibiting reabsorption of Ca2+ and PO43- in the kidney so increases excretion
Describe Vitamin D in Ca2+ homeostasis:
A pre-hormone, that following metabolism to active hormones, increases Ca2+ absorption in the GIT
It is a steroid like hormone
Where is Vitamin D obtained from?
Is synthesised in the skin in response to sunlight (precursor 7-dihydrocholesterol into Vitamin D3 in response to UV light)
It is also absorbed from the diet (especially dairy products)
Describe the process of activating vitamin D:
Activated in the liver and kidney
- 1 OH group added in the liver -> (25-OH-vitD3) Calcifediol, this is stored until required
- 2nd OH group added in the kidney -> (1,25-(OH)2-vitD3) Calcitrol, caused by PTH which is activated
How does vitamin D monitor calcium concentration?
Activated vitamin D acts at nuclear receptors- a transcription factor promoting gene expression
Genes expressed lead to increased absorption of Ca2+ and restoring Ca2+ balance
Also increases reabsorption of Ca2+ in the kidney and works with PTH to increase mobilisation of Ca2+ in the bone
What are the two main types of bone and describe them:
Cortical bone (80%)- compact bone that forms the dense outer supporting structure
Trabecular bone (20%)- spongy bone that forms the inner supporting structure, composed of a lattice or network of branching bone spicules or trabecular and spaces filled with bone marrow
When does skeletal maturing occur and what happens after that?
Happens at around 25-30 years and plateus for around 10 years
Faster deterioration in females at menopause
What is bone mass determined by?
Peak bone mass that was attained at around age 30
Rate of bone loss that commences at 40
What genetic factors can attribute to rate of bone loss?
Genetic factors (75%)
More likely to have osteoporosis if strong family history
Possible involvement of several genes:
-Vit D receptor gene
-Oestrogen receptor gene
-IL6 gene
What environmental factors can attribute to rate of bone loss?
Risk factors for osteoporosis:
- low Ca2+ intake and/ or absorption
- low vit D intake or lack of exposure to sunlight
-physical activity
-alochol
-smoking
-thin body type
What is osteoporosis?
Common metabolic bone disease characterised by a reduction in bone mass per unit volume that occurs with age
Increase in bone fragility and susceptibility to fracture
What does BMD stand for?
Bone Mineral Density
What is a T score in osteoporosis?
Number of standard deviations by which the individuals BMD differs from the mean peak for young adults of the same gender
Fracture risk doubles for every deviation below the mean
Describe the T scores and what do they mean for osteoporosis?
Normal = above -1
Osteopenia= between -1 and -2.5
Osteoporosis= -2.5 or less
Established osteoporosis= -2.5 or less and a fracture
Is an X-ray a good diagnostic factor to use in osteoporosis and why?
No, it is not capable of detecting bone loss until at least 30% of bone mass is lost
What is used to measure BMD?
Dual Energy Xray Absorptiometry (DEXA) scan is used
Enables accurate and reproducible measurements of BMD, but is expensive
Describe primary osteoporosis and give examples:
Diagnosed when pt has no other disorders known to cause osteoporosis
e.g post menopausal (women within 15-20 years after menopause)
e.g age related, or senile
Describe secondary osteoporosis and give examples:
Related to another medical condition:
-anorexia nervosa
-inflammatory bowel disease
-endocrine e.g T1D, Cushing’s, hyperthyroidism
Related to drug therapy
-most commonly steroid (glucocoritcoid) induced
-others, carbamazepine, phenytoin, heparin, furosemide , PPIS
How do corticosteroids cause secondary osteoporosis?
Decrease OB activity and active lifespan
Decrease Ca2+ absorption from the intestine and increase renal Ca2+ loss, causing abnormal PTH and vit D activity
Suppress sex hormone production
What is fragility fracture in osteoporosis?
Fracture that occurs as a result of mechanical forces that would not ordinarily cause fracture
WHO quantifies this as a force equivalent to a fall at standing height or less
What are the symptoms due to fractures of osteoporosis?
Can gradually cause the spine to collapse, resulting in height loss, pain and a deformed back
Forward curvature= kyphosis
What most common bones are fractured in osteoporosis?
Vertebra
Distal radius (wrist)
Neck of Femur (hip)
Which patients should receive osteoporosis prophylaxis when taking glucocorticoids?
Anyone with prior fragility fracture
Women 70 or above
Post menopausal women and men 50 or above prescribed high dose steroids i.e 7.5mg/day of prednisolone or equivalent over 3 months or more than 30mg day for 4 weeks
Post menopausal women and men aged 50 or above with a high risk FRAX score
What is a FRAX score?
Fracture Risk Assessment Tool
Online tool used to predict a persons 10 year risk in developing a major osteoporotic fracture
What is the treatment for low risk patients for osteoporosis?
Lifestyle + calcium/vitD
What is the treatment for intermediate risk patients of osteoporosis:
Assess BMD with DEXA scan + lifestyle+ calcium/vitD