Thermoacclimatory changes in the ionic microenvironment of haemoglobin in the stenothermal rainbow trout (Salmo gairdneri) and eurythermal carp (Cyprinus carpio).

1. Haematological characteristics (erythrocyte number, haematocrit, haemoglobin, mean erythrocyte volume and haemoglobin content) and plasma and packed red blood cell water and electrolyte (Na+, K+, Ca2+, Mg2+, Cl-) levels were determined in summer and winter populations of rainbow trout acclimated to 2, 10 and 18 degrees C, and for carp held at 2, 16 and 30 degrees C. Erythrocyte electrolyte concentrations and ion:haemoglobin ratios were calculated from these data. 2. Modest increases in red cell abundance and reductions in mean erythrocytic volume were the most obvious haematological changes accompanying acclimation to higher temperatures. Haemoglobin levels in carp also tended to increase with temperature. 3. In winter trout plasma sodium and potassium were elevated following acclimation to increased temperature. No significant cges in plasma composition were observed in summer fish. Carp held at higher temperatures were characterized by increases in plasma chloride and calcium and reductions in sodium and magnesium levels. 4. Red cell potassium and magnesium and K+:Hb and Mg2+:Hb ratios tended to be higher in winter than in summer trout, with the converse being true of chloride and calcium and Cl-:Hb and Ca2+:Hb. Only potassium and K+:Hb were significantly altered following acclimation; rising at higher temperatures. In carp, potassium and K+:Hb were relatively thermostable, but sodium and chloride and Na+:Hb and Cl-:Hb increased with temperature while magnesium and Mg2+:Hb decreased. Changes in the ionic composition of carp red cells support the suggestion that cellular pH is reduced in the warm-acclimated animal. 5. These variations may be of adaptive value. Increases in chloride and hydrogen ion commonly reduce haemoglobin=oxygen affinity, and should facilitate oxygen unloading at the tissue level. Reductions in cellular magnesium, by maximizing organophosphate modulator levels, should produce much the same effect. 6. In both species reductions in mean erythrocytic volume took place at higher temperatures despite increases in cellular ion content which exceeded those of plasma. It is probable that reductions in cellular volume, which should favour branchial oxygen loading, were achieved by export of some as yet unidentified solute or solutes.