- Certain statements made on the mechanisms of action of a neuro-hormone, vasopressin (VP) are given below:
A. VP acts on collecting ducts of kidney to concentrate urine by binding to V1 receptors and activating cAMP cascade
B. VP acts on collecting ducts of kidney to concentrate urine by binding to V2 receptors and activating cAMP cascade
C. VP acts on blood vessels to produce constriction by binding to V1 receptors and activating cAMP cascade
D. VP acts on blood vessels to produce constriction by binding to V1 receptors and activating
phosphatidylinositol cascade
Which one of the following options represents a combination of correct statements?
(1) A and B (2) C and D
(3) B and D (4) A and D
Vasopressin (VP), also known as antidiuretic hormone, is a neuro-hormone with dual physiological roles critical in water balance and vascular resistance. It acts through specific receptors—primarily V1 and V2—that trigger distinct intracellular signaling pathways. The actions of vasopressin are central to maintaining blood pressure and regulating urine concentration.
Vasopressin Receptor Subtypes and Their Functions
V2 Receptors: Role in Kidney Collecting Ducts
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V2 receptors are predominantly located on the principal cells of the collecting ducts in the kidney.
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Vasopressin binding to V2 receptors activates the adenylyl cyclase/cAMP/Protein kinase A (PKA) signaling cascade.
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This leads to phosphorylation and insertion of aquaporin-2 water channels into the apical membrane, increasing water reabsorption.
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The result is concentrated urine and conservation of body water.
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This signaling is effective and vital for the antidiuretic action of vasopressin.pmc.ncbi.nlm.nih+2
V1 Receptors: Role in Blood Vessels
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V1 receptors (also termed V1a) are located mainly in vascular smooth muscle cells.
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Binding of vasopressin to V1 receptors activates the phosphatidylinositol (IP3/DAG) signaling pathway, mobilizing intracellular calcium.
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Increased intracellular calcium triggers vascular smooth muscle contraction, causing vasoconstriction.
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This helps to increase systemic vascular resistance and blood pressure.pmc.ncbi.nlm.nih+2
Clarifying the Statement Evaluation
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A. VP acts on collecting ducts via V1 receptors activating cAMP cascade:
Incorrect. VP acts on collecting ducts through V2 receptors activating cAMP, not V1 receptors. -
B. VP acts on collecting ducts via V2 receptors activating cAMP cascade:
Correct. This fits the classical mechanism of antidiuretic action. -
C. VP acts on blood vessels causing constriction by binding to V1 receptors and activating cAMP cascade:
Incorrect. V1 receptor activation does not involve cAMP but the phosphatidylinositol signaling pathway, leading to calcium-mediated contraction. -
D. VP acts on blood vessels causing constriction by binding V1 receptors and activating phosphatidylinositol cascade:
Correct. This is the canonical pathway for vascular vasopressin action.
Conclusion
The correct combination of statements that accurately describe vasopressin’s mechanisms of action is:
(3) B and D
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Vasopressin binds to V2 receptors in kidney collecting ducts, activating cAMP to concentrate urine.
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Vasopressin binds to V1 receptors in vascular smooth muscle, activating phosphatidylinositol signaling, leading to vasoconstriction.
