The abrupt decrease in V-ATPase specific activity in D. pagei
posterior gills on transference from fresh water to 21‰ salinity,
attaining basal values just after 1 h exposure , reveals the
action of a very efficient short-term regulatory mechanism, resulting
in early inhibition of ion uptake processes. In 21‰ salinity, external
Na+ (≈300 mmol L−1
) and Cl− (≈340 mmol L−1
) concentrations
are much higher than those in D. pagei hemolymph in fresh water (200
and 215 mmol L−1
, respectively), generating passive influx to the
hemolymph in addition to the active uptake driven by the gill (Na+,
K+)- and V-ATPases. When D. pagei is acclimated to 25‰, hemolymph
Na+ and Cl− concentrations increase rapidly, reach values similar to
those of the external medium after just 48-h exposure, remaining
unchanged for 10 days.