Ck. Tipsmark et Ss. Madsen, Rapid modulation of Na+/K+-ATPase activity in osmoregulatory tissues of a salmonid fish, J EXP BIOL, 204(4), 2001, pp. 701-709
The effects of cyclic AMP on Na+/K+-ATPase activity were studied in the gil
l and kidney of the euryhaline brown trout Salmo trutta using two different
experimental approaches. In the first series of experiments, in situ Na+/K
+-ATPase activity was analyzed by measuring the ouabain-sensitive uptake of
non-radioactive rubidium (Rb+) into gill cells and blocks of gill and kidn
ey tissue. Rubidium uptake was linear for at least 30 min and was significa
ntly inhibited by 1 mmol l(-1) ouabain. Several agents presumed to increase
the intracellular cyclic AMP concentration inhibited ouabain-sensitive Rb uptake in both gill (0.5 and 2 mmol l(-1) dibutyryl-cyclic AMP, 1 mmol l(-
1) theophylline, 10 mu mol l(-1) forskolin and 10 mu mol l(-1) isoprotereno
l) and kidney (10Clmoll-l forskolin) tissue from freshwater-acclimated fish
. In a separate series of experiments, ATP hydrolase activity was assayed i
n a permeabilised gill membrane preparation after incubation of tissue bloc
ks with 10 mu mol l(-1) forskolin, Forskolin elevated gill cyclic AMP level
s 40-fold, inhibited maximal enzymatic Na+/K+-ATPase activity (Vmax) in gil
l tissue from both freshwater- and seawater-acclimated fish and reduced the
apparent K+ affinity in the gills of seawater-acclimated fish, demonstrati
ng that the effects are mediated through modifications of the enzyme itself
, The protein phosphatase inhibitors okadaic acid and cyclosporin A did not
affect forskolin-induced inhibition of Na+/K+d-ATPase activity, indicating
that forskolin-mediated modulation was stable for the duration of assay. W
e suggest that cyclic-AMP-mediated phosphorylation through protein kinases
may underlie the rapid modulation of Na+/K+-ATPase activity in the osmoregu
latory tissues of euryhaline teleosts.