NEPHRO-News
In the juxtaglomerular apparatus (JGA) of the kidney the loop of Henle
gets into close contact to its parent glomerulus. This anatomical link
between the tubular system and the vasculature of the afferent and
efferent arteriole enables specialized tubular cells, the macula densa
(MD) cells, to establish an intra-nephron feedback loop designed to
control both pre-glomerular resistance (and by that single nephron GFR)
and renin secretion from granular cells of the afferent arteriole. This
review focuses on the signaling mechanisms, which link salt sensing MD
cells and the regulation of pre-glomerular resistance, a feedback loop,
known as tubuloglomerular feedback (TGF).
The initial step of the TGF is the detection of the NaCl concentration
in the tubular lumen by MD cells. As tubular NaCl concentration
increases MD cells initiate a signaling process, which eventually leads
to a vasoconstriction of the afferent arteriole and by that to a
decrease in single nephron GFR, a protective mechanism against the
threat of renal salt loss. In pathologic situations, like acute tubular
necrosis, with reduced proximal reabsorption and increased salt delivery
to the MD segment this feedback loop was suggested to contribute to the
fall in total GFR.
The role of TGF in the physiology and patho-physiology of the kidney
nourished the interest in identifying the factor(s), which mediate the
constriction of the afferent arteriole upon detection of increased
tubular salt concentration by MD cells. A possible mediator of the TGF
response is supposed to fulfill two key characteristics; (1) it needs to
be generated in a regulated fashion in the JGA and (2) it needs to
cause vasoconstriction of the afferent arteriole. Both adenosine and ATP
fulfill these prerequisites, and a bunch of experimental evidence has
accumulated over the years supporting a role of either one as the
mediating factor of TGF.
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