Functional characterization of the rat na/h exchanger isoform, nhe
-4, in cultured nhe-deficient fibroblasts and localization in rat
hippocampus.
Bookstein, Crescence, Mark W. Musch, Alex Depaoli, Yue Xie, Karen
Rabenau, Mitchel Villereal, Mrinalini C. Rao, and Eugene B. Chang.
Departments of Medicine and Pharmacological and Physiological
Sciences, University of Chicago, 5841 S. Maryland Ave., Chicago, IL.
60637, Department of Physiology and Biophysics, University of
Illinois at Chicago, Chicago, IL. 60612
APStracts 3:0171C, 1996.
The kinetics of the Na/H exchanger isoform NHE-4 were studied by
measuring 22Na+ fluxes in stably transfected NHE-deficient
fibroblasts. Unlike NHE-1, 2 and 3, activation of this isoform is
dependent on hyperosmolarity-induced cell shrinkage. It is virtually
inactive at isoosmolarity and most active at 490 mosM. When induced
by cell shrinkage, NHE-4 exhibits a sigmoidal response to increasing
extracellular sodium concentrations, suggesting allosteric or
cooperative binding kinetics. By comparison, NHE-1 and 3 exhibit
hyperbolic velocity vs [Na]o responses at both iso- and hyperosmolar
conditions. Unlike NHE-1 and NHE-4, hyperosmolarity-induced cell
shrinkage inhibits NHE-3 activity in transfected fibroblasts,
reducing Vmax by 40%, with no effect on KNa. NHE-4 is relatively
insensitive to inhibition by amiloride analogs in the order:
DMA>HMA>Amiloride>EIPA. Time dependent inhibition of
activity by cytochalasin D suggests a relationship between the actin
cytoskeleton and regulation by cell-shrinkage. By in situ
hybridization of fixed tissues, NHE-4 mRNA was found to be highly
expressed in the cavi amnoni (CA) fields of rat hippocampus. The
kinetics of this exchanger, when considered with its unusual tissue
distribution in renal inner medullary collecting tubules and
hippocampus, are consistent with NHE-4 having a specialized role in
cell functions.
Received 2 October 1995; accepted in final form 15 May 1996.
APS Manuscript Number C602-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 June 96