Tubuloglomerular feedback

Macula densa cells

• At the junction of ascending limb of loop of Henle and distal convoluted tubules
• Have Na-K-2Cl symporter

When NaCl content is high at macula densa cells

--> Increased NaCl uptake

--> Swelling of macula densa cells

--> Release of ATP

--> Stimulation of purinergic P2 receptors on mesangial cells and afferent arteriole smooth muscles

NB:

• ATP may be metabolised to adenosine, which also causes vasoconstriction here
• Adenosine normally causes vasodilation in other tissues
• [WG21:p715] Thromboxane A2 may be the mediator
• NO plays a role in sustaining tubuloglomerular feedback, but not initiating it

Effect of increased NaCl content

Contraction of mesangial cells
--> Reduction in effective filtration area
--> Decreases GFR

Contraction of afferent arteriole
--> Decreases GFR and RBF

Thus,

• Pressure natriuretic and diuretic responses blunted

Other notes

NaCl content at macula densa also affect renin release

Thus,

Increased NaCl content at macula densa leads to

• Decreased GFR (tubuloglomerular feedback)
• Decreased RBF (tubuloglomerular feedback)
• Decreased renin