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Nephrology Research and Training Center, Division of Nephrology, Departments of Medicine and Physiology, University of Alabama at Birmingham, Birmingham, Alabama 35294
Previous
micropuncture studies suggested that macula densa (MD) cells might
detect variations in luminal sodium chloride concentration ([NaCl]l) through
changes in cytosolic calcium
([Ca2+]c).
To test this hypothesis, MD
[Ca2+]c
was measured with fluorescence microscopy using fura 2 in the isolated
perfused thick ascending limb with attached glomerulus preparation
dissected from rabbit kidney. Tubules were bathed and perfused with a
Ringer solution,
[NaCl]l was varied and
isosmotically replaced with
N-methyl-D-glucamine
cyclamate. Control
[Ca2+]c,
during perfusion with 25 mM NaCl and 150 mM NaCl in the bath, averaged
101.6 ± 8.2 nM (n = 21).
Increasing [NaCl]l to
150 mM elevated
[Ca2+]c
by 39.1 ± 5.2 nM (n = 21, P < 0.01). This effect was
concentration dependent between zero and 60 mM
[NaCl]l. The presence
of either luminal furosemide or basolateral nifedipine or
5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), a potent
Cl
channel blocker,
significantly reduced resting
[Ca2+]c
and abolished the increase in
[Ca2+]c
in response to increased
[NaCl]l. Nifedipine
failed to produce a similar inhibitory effect when added exclusively to
the luminal perfusate. Also, 100 nM BAY K 8644, a voltage-gated
Ca2+ channel agonist, added to the
bathing solution increased
[Ca2+]c
by 33.2 ± 8.1 nM (n = 5, P < 0.05). These
observations suggest that MD cells may detect variations in
[NaCl]l through a
signaling pathway that includes
Na+-2Cl-K+
cotransport, basolateral membrane depolarization via
Cl channels, and
Ca2+ entry through voltage-gated
Ca2+ channels.
isolated perfused tubule; fluorescence microscopy; cytosolic calcium; furosemide; voltage-gated calcium channels; tubuloglomerular feedback
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