Chronic kidney disease mineral and bone disorders can be manifested in one of three ways. Name them.
99% of the calcium in the body is stored in the bone in what form?
Hydroxyapatite
What percent of the plasma calcium is in a free, ionized form under normal conditions?
55%
In the presence of acidemia, what happens to the protein bound and ionized calcium concentrations in the blood?
Calcium and hydrogen ions compete for the negatively-charged protein binding sites
Therefore, protein bound calcium decreases and ionized calcium increases with acidemia
What are the major functions of PTH?
What is the effect of FGF23 on PTH synthesis?
FGF23 inhibits PTH synthesis and secretion - negative feed back loop
What are the major functions of FGF23?
What are the major functions of calcitriol?
Under normal conditions, what percent of filtered calcium is reabsorbed in the renal tubules?
98%
- Proximal tubule 60-70%
What is the paracellular pathway of calcium reabsorption in the renal tubules?
Movement of calcium through tight junctions between epithelial cells
- Primary method of calcium reabsorption in the proximal tubule and thick ascending limb
- Driven by passive diffusion or solvent drag down an electrochemical gradient
What is the transcellular pathway of calcium reabsorption in the renal tubules
Calcium transport through tubular epithelial cells
- Primary method of calcium reabsorption in the distal convoluted tubule (5-10% of calcium reabsorption)
- Crucial to the fine regulation of calcium reabsorption, where hormones act
When dietary calcium intake is low, what absorption method predominates? How does it occur?
Active, hormonal-dependent, transcellular absorption
- Occurs in the duodenum
- Via vitamin D receptors that are highly expressed under the influence of calcitriol
Passive paracellular calcium diffusion occurs when the dietary intake is higher. Where does it occur?
Throughout the small intestine, cecum, and proximal colon
How does calcitriol enhance calcium absorption in the intestines?
Calcitriol binds to the vitamin D receptor (transcription factor) => transcription of genes encoding for:
- The transient receptor potential vanilloid subtype 6: absorbs calcium on the intraluminal surface of the cell
- Calcium ATPase and sodium-calcium exchangers - on the basolateral membrane to move calcium into the blood
What cell in the parathyroid gland synthesizes PTH?
Parathyroid chief cells
What stimulates PTH release?
How does PTH promote bone resorption?
Indirectly activates osteoclasts through increased expression of RANKL on the osteoblasts and osteocytes
What factors increase the activity of 1-alpha-hydroxylase activity in the kidneys, leading to increased calcitriol production?
PTH, calcitonin, low extracellular calcium and phosphate
Where is FGF23 produced?
Osteocytes and osteoblasts in the bone
What stimulates FGF23 production?
Calcitriol, PTH, increased phosphorus
How does FGF23 lead to increased renal excretion of phosphorus?
Decreases expression of the renal sodium phosphate cotransporters
FGF23 exerts its biological function by interacting with what receptor complex?
alpha-Klotho-FGF receptor complex
How does membrane-bound alpha-Klotho affect FGF23?
Acts as a co-factor to enhance ligand-receptor affinity
Where is calcitonin produced?
Parafollicular cells of the thyroid gland