Osmoregulation of gpc:choline phosphodiesterase in mdck cells; different
effects of urea and nacl.
Kwon, Eugene D., Krzysztof Zablocki, Kyu Yong Jung, Eugenia M. Peters, Arlyn
Garcia-Perez, and Maurice B. Burg.
Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung and
Blood Institute
APStracts 2:0055C, 1995.
The organic osmolyte, glycerophosphocholine (GPC), accumulates in renal cells
in response to high concentrations of either NaCl or urea despite the very
different effects of these solutes on cell function and volume. We previously
concluded that, when combined, high levels of these solutes increase the
amount of GPC in MDCK cells by inhibiting its enzymatic degradation. The
present studies were undertaken to test the effects of NaCl and urea
individually on GPC accumulation and on its degradation. A technique was
developed to determine the absolute rate of GPC degradation by measuring the
initial rate of disappearance of [3H]GPC (pulsed into the cells by
hypotonic shock) and the specific activity of GPC in the cells. The mass of
GPC in the cells was measured by another newly developed method, a sensitive
chemiluminescent assay. We find that the [3H]GPC that disappears from
the cells during a 16 hour chase is degraded; the 3H lost from GPC appears in
cell lipids, and no significant amount of 3H appears in cellular water
-soluble, choline-containing compounds or in the medium. Exposure to high NaCl
or urea decreases the absolute rate of cellular GPC degradation by
approximately half during the first 20.5 hours. Reductions in GPC degradation
are accompanied by commensurate decreases in the activity of GPC:choline
phosphodiesterase (GPC:PDE; EC 3.1.4.2), an enzyme that catalyzes degradation
of GPC. Activity of GPC:PDE falls more than 50% in cells exposed for 2 h to
high osmolality. Inhibition is maximal (70%) at 2 days and is sustained for 7
days with high urea alone. With high NaCl alone, GPC:PDE activity is also
inhibited at 2 days but reverts to control values by 7 days by which time
synthesis of GPC is increased, accounting for sustained GPC accumulation.
Collectively, these data suggest that GPC accumulation in response to either
high NaCl or urea occurs initially by inhibition of its degradation, but that
the effect of NaCl differs in that it is transient, while that of urea is
sustained.
Received 2 September 1994; accepted in final form 19 December 1994
APS Manuscript Number C0499-4.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1994 The American Physiological Society.
Published in APStracts on 27 February 1995.