Hyperosmolarity inhibits the cloned na+/h+ exchanger isoforms, nhe2
and nhe3 : an effect opposite to that on the housekeeping isoform
nhe1.
Nath, Samir K, Cynthia Yue Hang, Susan A Levine, C. H. Chris Yun,
Marshall H Montrose, Mark Donowitz, Chung-Ming Tse.
Departments of Medicine and Physiology, GI Unit, The Johns Hopkins
University School of Medicine, Baltimore, MD
APStracts 2:0190G, 1995.
The effect of hyperosmolarity on cloned Na+/H+ exchanger isoforms -
NHE2 and NHE3, was studied in stably transfected PS120 fibroblasts.
Na+/H+ exchanger activity was determined spectrofluorometrically in
acidified cells which were exposed to either isoosmolar (300
mOsmol/kg) or hyperosmolar (450 mOsmol/kg) media, differing only in
150 mM mannitol. Hyperosmolar solution reversibly inhibited NHE2 and
NHE3 with a delay of _ 15 seconds. Hyperosmolarity significantly
reduced their Vmax compared to isoosmolar medium but did not alter
their Km for intracellular H+. The Km of the exchangers for
extracellular Na+ was not different in hyperosmolar medium compared
to isoosmolar medium. Pretreatment of PS120/NHE3 cells with the
protein kinase C inhibitor H7, the tyrosine kinase inhibitor
genistein, and the serine-threonine protein phosphatase inhibitor
okadaic acid did not affect the hyperosmolar inhibition of NHE3.
Hyperosmolar inhibition of Na+/H+ exchanger activity was also
observed in PS120 cells transfected with truncated NHE3 cDNAs:
E3/585, E3/543, E3/509 and E3/475 and NHE2 cDNA E2/499. We conclude
that 1) hyperosmolarity inhibits NHE2 and NHE3, in contrast to the
stimulatory effect on the housekeeping isoform NHE1; 2) this
inhibition is reversible; and 3) the C-termini of NHE2 and NHE3 are
not necessary for hyperosmolar inhibition of both NHE2 and NHE3.
Received 8 May 1995; accepted in final form 15 September 1995.
APS Manuscript Number G190-5.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 31 October 95