Parathormone increases calcium absorption by acting at ?
**Core Concept**
Parathormone (PTH), also known as parathyroid hormone, is a hormone secreted by the parathyroid glands that plays a crucial role in regulating calcium levels in the blood. It acts on various tissues to increase calcium absorption, excretion, and mobilization from bone.
**Why the Correct Answer is Right**
PTH increases calcium absorption in the intestine by acting on the small intestine's epithelial cells, specifically by binding to the parathyroid hormone receptors. This binding activates the adenylate cyclase pathway, which increases the intracellular concentration of cyclic AMP (cAMP). Elevated cAMP levels activate protein kinase A (PKA), which in turn phosphorylates and activates the calcium-binding protein calmodulin. Calmodulin regulates the activity of the calcium pump in intestinal epithelial cells, increasing the absorption of calcium from the lumen into the cell. This process is essential for maintaining normal blood calcium levels.
**Why Each Wrong Option is Incorrect**
**Option A:** This option seems to be a placeholder. Since it's not provided, I'll move on to the next options.
**Option B:** This option might refer to the kidneys, where PTH acts to increase calcium reabsorption. However, the question asks about increasing calcium absorption, which is primarily achieved in the intestine.
**Option C:** This option might refer to the bones, where PTH acts to increase calcium release. While PTH does increase calcium mobilization from bone, the question asks about increasing calcium absorption, not mobilization.
**Clinical Pearl / High-Yield Fact**
Remember that PTH has a dual effect on calcium levels: it increases calcium absorption in the intestine and increases calcium mobilization from bone. This dual effect helps maintain normal blood calcium levels in response to changes in dietary calcium intake or other physiological demands.
**Correct Answer: D. Intestine**