Ionoregulatory responses to temperature change in two species of freshwater fish

We examined the ionoregulatory responses to temperature changes in two spec ies of freshwater fish that differ in thermal preferences; the stenothermal , cold-water rainbow trout (Oncorhynchus mykiss) and the more eurythermal, warm-water common shiner (Notropis cornutus). We found that rainbow trout m aintained constant plasma Na+ levels over the entire temperature regime (5- 20 degrees C). Upon transfer from 15 degrees C (holding temperature) to 5 a nd 10 degrees C, rainbow trout experienced a significant drop in Na+ uptake (J(in)(Na)), but after two weeks J(in)(Na) had upregulated to warm tempera ture levels. Further, Na+ efflux (J(out)(Na)) fell significantly at the col der temperatures. As a result, trout at the lowest temperatures were still in ion balance. When trout were exercised to exhaustion both O-2 consumptio n (MO2) and J(out)(Na) rose significantly at all temperatures, but while MO 2 continued to be dependent upon temperature, J(out)(Na) was high and const ant. In contrast to the trout, common shiners experienced a 20% drop in pla sma Na+ at 5 degrees C. Upon exposure to cold temperatures they experienced a reduced J(in)(Na), and showed no signs of acclimation during the subsequ ent two weeks. Likewise J(out)(Na) was constant at all temperatures. These findings raise questions regarding the degree to which fish employ homeosta tic mechanisms designed to defend a set-point (i.e., steady-state) osmolari ty and ionic composition.