Flashcards in Exam 4 Deck (23)
What are the 3 functions ADH has on the renal tubule?
- It increases the water permeability of the principal cells of the late distal tubule and collecting ducts
- It increases the activity of the Na+-K+-2Cl− cotransporter of the thick ascending limb
- this enhances countercurrent multiplication and the size of the corticopapillary osmotic gradient
- It increases urea permeability in the inner medullary collecting ducts
- enhancing urea recycling and the size of the corticopapillary osmotic gradient
Of the 3 functions which is the most important?
the effect on water permeability
In the absence of ADH, the principal cells are ______ to water
What happens to the principal cells when ADH is present?
Water channels, or aquaporins, are inserted in the luminal membrane, making them permeable to water
Describe the steps involved in the action of ADH on the principal cells
1) ADH binds to V2 receptors in the basolateral membrane
2) this activates adenylyl cyclase which catalyzes the conversion of ATP to cAMP
3) cAMP activates protein kinase A
4) this causes phosphorylation of intracellular structures
5) vesicles containing water channels are shuttled to and inserted into the luminal membrane of the permeability, thus increasing its water permeability
What is the specific water channel that is controlled by ADH?
aquaporin 2 (AQP2)
Hyperosmotic urine has an osmolarity that is _____ than blood osmolarity
Hyperosmotic urine is produced when the circulating levels of ADH are ____.
What are the 2 mechanisms by which urine becomes hyperosmotic?
- in the presence of ADH
- by equilibration of tubular fluid in the collecting ducts with the high osmolarity of the corticopapilary gradient
What are the 2 ways by which the corticopapillary gradient is established?
- countercurrent multiplication
- urea cycling
Final urine osmolarity, in the presence of ADH, will be ____ the osmolarity at the bend of the loop of Henle
Hyposmotic urine has an osmolarity that is _____ than blood osmolarity
Hyposmotic urine is produced when the circulating levels of ADH are ____.
low (or ineffective)
How is hyposomotic urine produced?
The tubular fluid is diluted in the "diluting segments" because NaCl is reabsorbed without water, because there is no ADH present for osmotic equilibrium to occur
What does hyposmotic urine osmolarity reflect?
A combination of all of the diluting segments and the remainder of the distal tubule and collecting ducts
Hyposomotic urine is produced as the normal response to what?
What are 2 abnormal conditions in which dilute urine is produced?
- central diabetes insipidus
- nephrogenic diabetes insipidus
Central diabetes insipidus follows what?
head injury, in which trauma depletes the posterior pituitary gland of ADH stores
What does central diabetes insipidus result in?
Large volumes of very dilute urine is excreted because the entire distal tubule and collecting ducts are impermeable to water in the absence of ADH
How is central diabetes insipidus treated?
Administration of dDAVP
Nephrogenic diabetes insipidus is the result of what?
A defect of the receptors for ADH in the principal cells, which makes them unresponsive to ADH
What does nephrogenic diabetes insipidus result in?
Water cannot be reabsorbed in the later distal tubule and collecting ducts which means large volumes of dilute urine are excreted