The receptors for parathyroid hormone are present in:
**Core Concept**
The parathyroid hormone (PTH) plays a crucial role in regulating calcium and phosphate metabolism in the body. PTH exerts its effects by binding to specific receptors on the surface of target cells, primarily in the bones, kidneys, and intestines. The PTH receptors belong to the family of G protein-coupled receptors (GPCRs).
**Why the Correct Answer is Right**
The correct answer is **C. Osteoblasts and osteoclasts in bone**. PTH receptors are predominantly expressed on the surface of osteoblasts and osteoclasts in bone, where they mediate the catabolic effects of PTH on bone resorption. The binding of PTH to its receptor activates a Gq protein, leading to an increase in intracellular calcium and the activation of various signaling pathways, including the MAPK and PI3K/Akt pathways. This ultimately results in the stimulation of osteoclast activity and the suppression of osteoblast activity, leading to increased bone resorption.
**Why Each Wrong Option is Incorrect**
* **Option A:** While PTH receptors are also present in the kidneys, the primary site of action is not the renal cortex.
* **Option B:** PTH receptors are not primarily expressed on the surface of intestinal cells.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that PTH plays a critical role in maintaining calcium homeostasis, and its receptor is predominantly expressed in bone, where it regulates bone resorption.
**Correct Answer:** C. Osteoblasts and osteoclasts in bone.