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College of Pharmacy, University of Houston, Houston, Texas
77204-5515 (C.H.P., M.F.L.),
Department of Physiology, Texas Tech
University, Lubbock, Texas 79430 (T.A.P.), and
Department of Anatomy
and Cell Biology, State University of New York at Brooklyn, New York
11203 (A.R.C.)
Na+ reabsorption is regulated in proximal tubules by
hormones that stimulate protein kinase C (PKC). To determine whether
stimulation of PKC causes a reduction in intracellular Na+
concentration ([Na+]i) that might link
Na+ pump activation to increased Na+
reabsorption, [Na+]i was measured in kidney
cells loaded with the Na+-sensitive fluorescent indicator
SBFI. Rapid digital imaging fluorescence microscopy determinations were
performed in epithelial kidney cells transfected with the rodent
Na+ pump 
1 cDNA. In 42 determinations, the basal
[Na+]i was 19.7 ± 2.4 mM.
Stimulation of PKC reduced the [Na+]i to
5.6 ± 0.6 mM in ~10 sec. This drastic change in
[Na+]i requires a transient 74-120-fold
increase in Na+ pump activity. After the new steady state
[Na+]i is reached, the Na+ pump
is 58% activated. The entry of Na+ into the cells is not
affected by stimulation of PKC; therefore, the reduction in
[Na+]i is exclusively dependent on activation
of the Na+ pump. Accordingly, PKC stimulation does not
affect the [Na+]i of cells expressing a
mutant Na+ pump that is not stimulated by PKC. The decrease
in [Na+]i observed in cells transfected with
the rodent Na+ pump 1 cDNA is large and sufficiently
fast that it is expected to stimulate rapidly passive
Na+-influx into the cells, thereby accounting for the
observed PKC-induced stimulation of Na+ reabsorption.
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