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Department of Medicine, University of Cincinnati School of Medicine, and Veterans Affairs Medical Center, Cincinnati, Ohio 45267-0585
The effect of
hypotonicity on H+-ATPase activity
was examined in cultured inner medullary collecting duct (mIMCD-3)
cells. mIMCD-3 cells were grown to confluence, loaded with
2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF),
and assayed for H+-ATPase activity
measured as the Na+-
and K+-independent intracellular
pH (pHi) recovery following an
acid load. Exposure of mIMCD-3 cells to a hypotonic solution (150 mosmol/kgH2O) increased
pHi recovery by ~350%
(P < 0.0001). This effect was inhibited by diethylstilbestrol (an inhibitor of
H+-ATPase) and was not dependent
on external K+, indicating lack of
involvement of
H+-K+-ATPase.
H+-ATPase activation was acute,
independent of cell calcium, and was not secondary to
Cl
channel activation. The
magnitude of H+-ATPase
upregulation was dependent on the osmolarity of the media, with maximum
stimulation at 150 mosmol/kgH2O.
H+-ATPase upregulation in
hypotonicity was significantly blocked in the presence of staurosporine
or calphostin C or in cells pretreated with phorbol 12-myristate
13-acetate (PMA), indicating involvement of protein kinase C. Hypotonicity inhibited the
Na+/H+
exchanger activity in mIMCD-3 cells, indicating that its stimulatory effect is specific to H+-ATPase.
In conclusion, a novel regulatory mechanism of
H+-ATPase by hypotonicity is
described. The increased H+-ATPase
activity in hypotonicity may be responsible for increased HCO3 reabsorption and maintained
acid-base homeostasis in hyposmolar states.
proton-adenosinetriphosphatase; inner medulla; hypotonicity; sodium/proton exchange; acid-base balance
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