response to parathyroid hormone

Definition

Target type: biologicalprocess

Any process that results in a change in state or activity of a cell or an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of a parathyroid hormone stimulus. [GOC:mah, GOC:yaf]

Parathyroid hormone (PTH) is a crucial regulator of calcium homeostasis in the body. It is secreted by the parathyroid glands in response to low blood calcium levels. Upon secretion, PTH travels through the bloodstream and binds to its receptor, the PTH receptor (PTHR), located on the surface of target cells in various tissues, primarily bones, kidneys, and the intestines. This binding activates a signaling cascade within the cells, leading to a coordinated response to raise blood calcium levels.

Here's a detailed breakdown of the process:

**1. PTH Binding and Signaling:**

* PTH binds to its receptor (PTHR) on target cells.
* This binding activates a G protein-coupled receptor signaling pathway, which involves:
* Activation of adenylate cyclase.
* Production of cyclic AMP (cAMP).
* Activation of protein kinase A (PKA).

**2. Bone Remodeling:**

* PTH stimulates bone resorption, the breakdown of bone tissue, by:
* Promoting the activity of osteoclasts, the bone-resorbing cells.
* Increasing the release of calcium and phosphate from bone into the bloodstream.

**3. Kidney Function:**

* PTH enhances calcium reabsorption in the kidneys, reducing calcium excretion in urine.
* PTH inhibits phosphate reabsorption in the kidneys, promoting phosphate excretion in urine.
* PTH also promotes the synthesis of 1,25-dihydroxyvitamin D (calcitriol), the active form of vitamin D, which further enhances calcium absorption in the intestines.

**4. Intestinal Calcium Absorption:**

* Calcitriol, stimulated by PTH, increases the absorption of calcium from the intestines.
* This process is crucial for maintaining adequate calcium levels in the blood.

**5. Feedback Regulation:**

* As blood calcium levels rise due to PTH actions, the secretion of PTH is suppressed, preventing excessive calcium levels.
* This negative feedback loop ensures that blood calcium levels remain within a narrow and tightly controlled range.

**6. Disorders of PTH Regulation:**

* Hyperparathyroidism: Excess PTH production leads to high blood calcium levels and bone loss.
* Hypoparathyroidism: Deficient PTH production results in low blood calcium levels and associated symptoms such as muscle cramps and seizures.'
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