Transport and regulatory properties of the apical na:k:2cl
cotransporter of macula densa cells.
Darwin, M. Anuar Laamarti P., Bell_ & Jean-Yves Lapointe.
Groupe de recherche en transport membranaire, Universit[acute]e de
Montr[acute]eal, Montr[acute]eal, Qu[acute]ebec, Canada, H3C 3J7 and,
_ Nephrology Research and Training Center, Division of Nephrology and
Departments of Medicine and Physiology, University of Alabama at
Birmingham, Birmingham, Alabama, 35294.
APStracts 5:0140F, 1998.
NH4+/NH3 fluxes were used to probe apical Na+:K:2Cl transport activity
of macula densa cells from rabbit kidney. In the presence of 25 mM
NaCl and 5 mM Ba2+, addition of 20 mM NH4+ to the lumen produced a
profound intracellular acidification and 80% of the initial
acidification rate was bumetanide sensitive. The NH4+-induced
acidification rate was dependent on luminal Cl- and Na+ with apparent
affinities of 17 +/- 4 mM (Hill number 1.45) and 1.0 +/- 0.3 mM,
respectively. In the presence of saturating luminal [NaCl], blockade
of basolateral Cl- efflux with 10 (M nitro-phenylpropyl-amino-benzoic
acid (NPPB) reduced the NH4+-induced acidification rate by 51 +/- 6 %
(p> 0.01, n=5). Under similar conditions, dbcAMP + forskolin
increased the NH4+-induced acidification rate by 27% while it
produced no detectable effect at low luminal NaCl concentration. Most
of the observed dbcAMP + forskolin effect was probably due to the
stimulation of the basolateral Cl- conductance since, in the presence
of basolateral NPPB, this activation was changed to a 17.1% and 16.6%
inhibition of the NH4+-induced acidification rate observed at high or
low luminal [NaCl], respectively. We conclude that the cotransporter
found in MD cells displays, with respect to other Na:K:2Cl
cotransporters, a relatively high affinity for luminal Na+ and
luminal Cl- and can be specifically inhibited by increases in
intracellular Cl- and cAMP concentrations.
Received 3 November 1997; accepted in final form 13 August 1998.
APS Manuscript Number F346-7.
Article publication pending Am. J. Physiol. (Renal Physiology).
ISSN 1080-4757 Copyright 1998 The American Physiological Society.
Published in APStracts on 21 September 1998