Plasma and red cell ionic composition in rainbow trout exposed to progressive temperature increases

Yearling rainbow trout, Salmo gairdneri Richardson, were exposed to progressive increases in temperature from 10 to 26.1 degrees C, and variations in haemoglobin, haematocrit and plasma and erythrocytic concentrations of Na+, K+, Ca2+, Mg2+ and Cl- were compared with those of animals maintained at 10 degrees C. Despite the effects which increases in temperature are known to have on branchial ventilation, perfusion and effective exchange area, and consequently upon passive water and electrolyte fluxes, plasma ion concentrations were little affected at other than acutely stressful temperatures. Presumably this reflects the consequences of previously demonstrated changes in branchial, renal and erythrocytic (Na+/K+)- and (HCO3-)-activated ATPase and carbonic anhydrase activities. Haemoglobin and haematocrit were also little influenced by increases in temperature between 10 degrees C and about 25 degrees C. It is unclear whether the decreases seen at higher temperatures reflected accelerated ageing and breakdown of circulating red cells or were a consequence of changes in tissue water content and distribution with resulting haemodilution. Red cell levels of Cl- and K+ increased more or less steadily at temperatures exceeding 16–18 degrees C. So also did [Cl-]: [Hb] and [K+]: [Hb]. Sodium and [Na+]: [Hb] were essentially constant up to about 25 degrees C but thereafter increased sharply. Much the same was true of Ca2+. However, red cell Ca2+ concentrations were normally less than 0.15 mmol l-1 cell water, and may be physiologically insignificant in the context of the present study. A more complex pattern of change was encountered in the case of Mg2+. Concentrations of this ion and the [Mg2+]: [Hb] ratio declined between 10 degrees C and 20–22 degrees C and thereafter increased. A significant positive correlation existed between red cell levels of Cl- and K+; significant negative correlations between Cl- and Mg2+ and K+ and Na+. Negative, but insignificant correlations were also seen between Ca2+ and both Cl- and K+. Given the known direct and indirect effects of inorganic ions upon haemoglobin-oxygen affinity, the reductions in intraerythrocytic pH which accompany increases in temperature and the effects of temperature per se on affinity, the compositional changes observed in this study would be expected to prompt reductions in haemoglobin-oxygen affinity and increases in P50 values. Previous studies have, however, revealed little thermo-acclimatory variation in the P50 of this species. Thus, some as yet unidentified factor or factors may operate in opposition to these influences.