QBank notes: 2/26/2010 [48]

Parathyroid hormone acts on bone, kidney, and intestine via kidney. PTH binds osteoblasts in bone, makes them present RANKL to RANK on osteoclast precursors, which fuse to form osteoclasts that resorb bone, increasing serum calcium. PTH causes increased reabsorption of calcium, and increased excretion of phosphate in the kidney. Finally, PTH causes increased calcium absorption in the GI via its activation of 1-alpha hydroxylation of 25-OH D3 in the kidney.

Calcitonin is less important for daily calcium homeostasis. Calcitonin decrease calcium absorption in the gut, but conserves calcium in the kidney. It causes increased excretion of phosphate in the kidney. In the bone, calcitonin decreases the activity of osteoclasts, therefore depressing serum calcium levels (opposite of PTH). Calcitonin is stimulated by an increase in serum calcium, gastrin, and pentagastrin.