Two groups of rainbow trout were acclimated to 20 , 100 ,
and 18 o C. Plasma sodium, potassium, and chloride levels were
determined for both. One group was employed in the estimation
of branchial and renal (Na+-K+)-stimulated, (HC0 3-)-stimulated,
and CMg++)-dependent ATPase activities, while the other was
used in the measurement of carbonic anhydrase activity in the
blood, gill and kidney. Assays were conducted using two
incubation temperature schemes. One provided for incubation
of all preparations at a common temperature of 2S oC, a value
equivalent to the upper incipient lethal level for this species.
In the other procedure the preparations were incubated at the
appropriate acclimation temperature of the sampled fish.
Trout were able to maintain plasma sodium and chloride
levels essentially constant over the temperature range employed.
The different incubation temperature protocols produced different
levels of activity, and, in some cases, contrary trends
with respect to acclimation temperature. This information was
discussed in relation to previous work on gill and kidney. The
standing-gradient flow hypothesis was discussed with reference
to the structure of the chloride cell, known thermallyinduced
changes in ion uptake, and the enzyme activities
obtained in this study. Modifications of the model of gill lon
uptake suggested by Maetz (1971) were proposed; high and low
temperature models resulting. In short, ion transport at the
gill at low temperatures appears to involve sodium and chloride
2
uptake by heteroionic exchange mechanisms working in association
w.lth ca.rbonlc anhydrase. G.l ll ( Na + -K + ) -ATPase and erythrocyte
carbonic anhydrase seem to provide the supplemental uptake
required at higher temperatures. It appears that the kidney
is prominent in ion transport at low temperatures while the
gill is more important at high temperatures.
3
Linear regression analyses involving weight, plasma ion
levels, and enzyme activities indicated several trends, the
most significant being the interrelationship observed between
plasma sodium and chloride. This, and other data obtained
in the study was considered in light of the theory that a
link exists between plasma sodium and chloride regulatory
mechanisms