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Physiology 3120 Lecture Notes - Agranulocyte, Distal Convoluted Tubule, Trpv5

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Tom Stavraky

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Human Physiology
Wednesday, March 3, 2010
“Endocrine VI”
Parathyroid Physiology & Calcium Metabolism
Regulation of Ca mostly by parathyroid hormone (PTH); more tightly regulated than any other ion in the
Processes requiring calcium
Signal transduction (secretory vesicle release, transcriptional activation [coactivator])
Maintenance of membrane potential
Muscle contraction
Enzyme cofactor (troponin C, calcium-calmodulin)
Human skeleton
Important for. . .
Support & protection
Hematopoiesis (formation of blood cells)
Calcium regulation
Calcium distribution
1% of Ca is non-skeletal (blood, in ionized, protein-bound, or complexed forms); only about half
of this non-skeletal Ca is “free” and diffusible
99% of Ca found in bone (calcium phosphate; accounts for 50% of skeletal weight)
3 organs involved in Ca balance
1. Intestine (about 1000mg)
Site for Ca absorption from diet
Apical surface = faces gut lumen; basolateral surface = faces circulation
Contain Ca transporters that open up in presence of gut Ca, allowing Ca into cells; carried
across cytoplasm to basolateral membrane, where a Ca ATPase pumps it into
bloodstream; maintains gradient across cell
Some excreted in feces (~800mg); can be from (I) unabsorbed Ca, and (II) normal gut
secretion of fluids and enzymes (many enzymes require Ca)
2. Kidney
Some excreted in urine
Most (98%) is reabsorbed, 25-30% of that occurs in distal tubule (here, it is an active
process that is saturable and regulated by PTH)
Ca reabsorbed passively in Loop of Henle, but actively in distal tubule
Reabsorption occurs through channel TRPV5
Production and activity of it can be stimulated
Have binding proteins (calbindin) to carry Ca across cell; there is a Na/Ca
exchanger on basolateral membrane and a Ca ATPase that pumps it into
All these actions stimulated by PH; its receptors are on the basolateral surface
3. Bone
Large daily movement of Ca both into and out of
In the long, narrow shaft of a long bone, the marrow is primarily involved in Ca
Two types of bone
Compact bone protection
Spongy/trabecular bone inner scaffold to support and give flexibility to outer
layer; also highly metabolically active; involved in providing Ca
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