The tubuloglomerular feedback (TGF) mechanism senses NaCl concentration at the macula densa and adjusts GFR. Which mediator is released by macula densa cells to constrict the afferent arteriole when NaCl delivery is high?
- A Adenosine (via ATP → AMP → adenosine pathway) ✓
- B Renin
- C Nitric oxide
- D Angiotensin II
Correct answer: A. Adenosine (via ATP → AMP → adenosine pathway)
Explanation
High NaCl at the macula densa triggers ATP release, which is rapidly converted to adenosine in the interstitium. Adenosine acts on A1 receptors on afferent arteriolar smooth muscle, causing vasoconstriction and reducing GFR (negative feedback). Renin is released when NaCl delivery is LOW. Nitric oxide from macula densa actually promotes vasodilation and countermodulates TGF. This adenosine-mediated mechanism explains why caffeine (an adenosine antagonist) can transiently increase GFR.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.