Ion regulation in ion-poor acidic water by
the blackskirt tetra, a fish native to the
Physiological Zoology, Volume 70, Issue 4,
Gonzalez, R J;
Summary of
Points:
·
The ionoregulatory capabilities of the blackskirt
tetra (Gymnocorymbus ternetzi)
were examined, which is native to ion-poor acidic waters of the
·
Examination
of Na+ uptake, which was only slightly sensitive to the uptake blocker amiloride, revealed several specializations for uptake in
these waters. Kinetic analysis of Na+ uptake (at pH 6.5) revealed a high
maximum rate of uptake and a low Michaelis-Menten
constant, which allows the tetras to take up Na+ at high rates even at very low
water levels.
·
At pH
4.5, a pH where they experience sizable ion disturbances, they displayed
several mechanisms to restore balance. Kinetic analysis at pH 4.5 revealed that
the maximum uptake rate rose 67% while the Michaelis-Menten
constant remained unchanged.
·
Further
tests showed that the upregulation of Na+ uptake
occurred within 12 h in response to a doubling of Na+ efflux. Despite these
specializations of the Na+ uptake mechanism, blackskirt
tetras were not especially tolerant of low pH. Upon exposure to pH 4.0, they
experienced a massive loss of Na+ due to a fourfold increase of Na+ efflux
(relative to pH 5.0) and an 80% inhibition of uptake.
·
Measurement
of Na+ efflux in waters with different Ca2+ levels and in the presence of LaCl, a strong Ca2+ competitor, correlated the stimulation
of Na+ efflux at low pH with a low branchial affinity
for Ca2+. These tests indicate that blackskirt tetras
possess abilities to resist the disruptive effects of moderately low pH but
cannot survive in waters with a pH of 4.0 or less because of leaching of Ca2+
from branchial tight junctions, which stimulates ion
losses.