Adrenals and Parathyroid Flashcards
Parathyroid hormone
Turns 25 hydroxy vitamin D into 1,25 dihydroxy vitamin D. Works on bone and kidney to maintain serum calcium homeostasis.
PTH Secretion controlled by
Serum calcium levels. Low calcium stimulates PTH secretion. HIgh calcium inhibits via calcium sensing receptor.
PTH actions in the kidney
Stimulates reabsorption of calcium in the distal tubule. Blocks reabsorption of PO4 in proximal tubule, leading to loss of phosphate in the urine.
Preventing formation of bone in urine. PTH also promotes active 1,25 OH2 vit D in proximal tubule
How is active D formed in the proximal tubule
25OHD bound to DBP passes into epithelial cell from glomerular filtrate. Hydroxylated by a mitochondrial enzyme. Enters circulation as active D. This process is promoted by PTH.
Which substance mediates calcium effects on intestine?
Active vitamin D, NOT PTH.
How is vitamin D created
UV light creates vitamin D3, hydroxylated in liver to create 25 OH D3. goes to kidney, intestine, stimulates absorption of calcium and movement of calcium/phosphate out of bone.
What does FGF23 do?
Causes excretion of phosphate, just like PTH. Blocks reabsorption in proximal tubule.
How is calcium distributed in blood?
40% bound to albumin. 50% is ionized ca2+.
Why is free vs total calcium concentration important?
Albumin is negatively charged. If acidemia occurs, H will compete with Ca, and free calcium levels will increase. If alkalemia occurs, free ca drops and patients become symptomatic, even through total calcium measurements are normal.
Osteoblast stimulators
Estrogens, androgens, GH, IGF1, small doses of PTH, a little bit of vitamin D.
Osteoclast stimulators
Thyroid hormone, glucocorticoids, high doses of PTH, high doses of vitamin D.
RANK - RANKL-OPG system
RANK (receptor) on osteoclasts when Rank L binds, osteoclast is stimulated. Osteoblasts release osteoprotegerin, which prevents binding of RANKL to RANK.
PTHrP
PTH related protein, binds to the PTH receptor and does similar things. Frequently secreted from tumors and causes Ca resorption from bone, and Ca reabsorption in kidney.
