The parathormone (PTH) exerts its rapid calcium resorptive action on by acting on on PTH receptors present on?
**Core Concept**
Parathormone (PTH) is a hormone secreted by the parathyroid glands, playing a crucial role in maintaining calcium homeostasis in the body. Its rapid action on bone resorption is mediated through specific receptors on osteoblasts, which are the bone-forming cells.
**Why the Correct Answer is Right**
PTH exerts its rapid calcium resorptive action on osteoblasts by acting on PTH receptors present on these cells. This interaction activates the intracellular signaling pathways, leading to the expression of genes involved in bone resorption. The PTH receptors on osteoblasts are part of the G protein-coupled receptor family, specifically the PTH1 receptor. The binding of PTH to its receptor triggers the exchange of GDP for GTP on the Gαs subunit, activating adenylate cyclase and increasing cyclic AMP (cAMP) levels. Elevated cAMP levels stimulate the activity of protein kinase A (PKA), which in turn phosphorylates and activates various downstream effectors, ultimately leading to the expression of genes involved in bone resorption.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option might be tempting, but PTH does not act directly on osteoclasts, which are the bone-resorbing cells. While osteoclasts are involved in bone resorption, PTH's rapid action is mediated through osteoblasts.
* **Option B:** This option is incorrect because PTH receptors are not primarily located on osteocytes, which are mature bone cells embedded within the bone matrix. Osteocytes play a role in sensing mechanical stress and regulating bone remodeling, but they are not the primary target of PTH's rapid action.
* **Option D:** This option is incorrect because while PTH receptors are present on kidney cells, the rapid calcium resorptive action of PTH is primarily mediated through its effects on bone, not the kidney.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that PTH's rapid action on bone resorption is mediated through osteoblasts, which are the bone-forming cells. This is a critical distinction, as osteoblasts and osteoclasts have different roles in bone remodeling, and PTH's effects on these cells are crucial for maintaining calcium homeostasis.
**Correct Answer:** C. osteoblasts