Tubuloglomerular feedback
[Ref:AV6:p110-112]
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