Session 11 - incomplete Flashcards
In what form, where and within what ranges is calcium carefully regulated?
- Plasma concentration of free ionised calcium (Ca2+)
- 1.0 - 1.3 mmol/L
In what cellular processes does calcium play a critical role?
- Hormone secretion
- Muscle contraction
- Nerve conduction
- Exocytosis
- Activation/inactivation of many enzymes
- Intracellular second messenger (carries information from cell membrane to cell interior)
Where is most calcium in the body located?
- Bone
What is the physiologically active form of calcium?
- Ca2+ (free ionised calcium in plasma)
In what form does calcium exist in the plasma?
- Free ionised species (Ca2+) (45%)
- Bound to/associated with a anionic sites on serum proteins, especially albumin (45%)
- Complexed with low-molecular-weight organic anions (eg citrate and oxaloate (10%)
What is the total normal concentration of all forms of calcium in the plasma?
- 2.2-2.7 mmol/L
What is the importance of phosphate?
- Part of the adenosine triphosphate molecule
- Therefore plays a crucial role in: cellular energy metabolism; activation/inactivation of enzymes
Is phosphate strictly regulated?
- No
- Fluctuates during the day eg after meals
How are calcium and phosphate linked?
- Are both the major components of hydroxyapatite crystals [Ca10(PO4)6(OH)2], which constitute the major portion of the mineral in bone
- Regulated by the same hormones: parathyroid hormone (PTH); 1,25-dihydroxyvitamin D (calcitriol) and to a lesser extent calcitonin. Hormones act on the bone, kidneys and the gastrointestinal tract to control levels of these two ions in the plasma. Actions of these hormones on calcium and phosphate are opposed ie a particular hormone may raise levels of one ion while lowering the other
What are the key hormones involved in the control of calcium levels in serum?
- Parathyroid hormone (PTH)
- Vitamin D (active form calcitriol)
What effect do certain hormones have on serum calcium levels?
- Parathyroid hormone and vitamin D (calcitriol): raise serum calcium concentrations
- Calcitonin: lowers serum calcium in animals during pregnancy to protect maternal skeleton (lower serum calcium -> lower osteoclast activity); released from thyroid gland (if thyroid gland is removed there is no effect of calcium homeostasis)
How is serum calcium regulated by hormones?
- Changes in Ca2+ concentration alters PTH by negative feedback
- Detected by unique G-protein receptors in chief cell surface
What happens when [Ca2+] is high?
- Increased Ca2+ -> binds to G-protein receptor -> stimulates Phospholipase C (PLC) -> inhibits adenylate cyclase -> reduced cAMP -> reduced PTH release
- Reverse happens when Ca2+ is low
Which hormone controls the short term regulation of serum calcium?
- PTH
What hormone controls the long term regulation of serum calcium?
- Calcitriol (active form of Vitamin D)
What is vitamin D and what types are there?
- Group of biologically inert prohormones that must undergo 2 hydroxylation reactions to be activated
- Vitamin D2 (ergocalciferol) and D3 (cholecalciferol)
Where does Vitamin D come from?
- Formed in the skin
- Absorbed in the gut from the diet
What happens to vitamin D in the body and why?
- Converted to calciferol (25-hydroxyvitamin D) in the liver
- Vitamin D has a shorter half life than calciferol (~2 weeks)
- Is then converted to its final from calcitriol in the kidney
Is vitamin D regulated?
- No
- But final conversion to calcitriol in the kidneys is regulated by PTH (hormones are linked)
How is Vitamin D2 produced and what is its action?
- Absorbed by gut
- No action (is a prohormone)
Where is Vitamin D3 produced and what is its action?
- Skin in the presence of UV light
- No action (is a prohormone)
Where is Calciferol produced and what is its action?
- Liver (1st hydroxylation of Vitamin D)
- No action (prohormone)
Where is Calcitriol produced and what is its action?
- Kidney (2nd hydroxylation of Vitamin D)
- ⬆️ Ca2+ absorption (binds to Ca2+ in the gut)
Where is parathyroid hormone produced and what is its function?
- Parathyroid gland
- ⬆️ conversation of calciferol -> calcitriol
- ⬆️ Ca2+ release from bone
- ⬆️ Ca2+ reabsorption in kidney
