Which of the following aquaporin water channels is located on apical membrane of principal cells of the collecting duct
**Core Concept**
Aquaporin water channels are a family of proteins that facilitate the transport of water across cell membranes in various tissues, including the kidney. In the collecting duct, aquaporins play a crucial role in regulating water reabsorption, which is essential for maintaining proper electrolyte and water balance in the body.
**Why the Correct Answer is Right**
The aquaporin water channel located on the apical membrane of principal cells in the collecting duct is Aquaporin-2 (AQP2). AQP2 is regulated by vasopressin (ADH), a hormone released by the posterior pituitary gland in response to increased plasma osmolality. When vasopressin binds to its receptor on the basolateral membrane of principal cells, it triggers a signaling cascade that phosphorylates AQP2, allowing it to translocate to the apical membrane and increase water reabsorption. This process helps to concentrate the urine and conserve water in the body.
**Why Each Wrong Option is Incorrect**
**Option A:** Aquaporin-1 (AQP1) is primarily located on the apical membrane of proximal convoluted tubule cells and is involved in water reabsorption in this segment, but not in the collecting duct.
**Option B:** Aquaporin-4 (AQP4) is mainly expressed in the brain and is involved in water transport across the blood-brain barrier, but not in the kidney.
**Option C:** Aquaporin-6 (AQP6) and Aquaporin-8 (AQP8) are less well characterized and not primarily involved in water reabsorption in the collecting duct.
**Clinical Pearl / High-Yield Fact**
Vasopressin (ADH) is essential for regulating water reabsorption in the collecting duct, and its deficiency can lead to central diabetes insipidus, characterized by polyuria and polydipsia.
**Correct Answer:** C.