Hematocrit in oxygen transport and swimming in rainbow trout ( Oncorhynchus mykiss)

Abstract

The optimal hematocrit (Hct opt) hypothesis was tested by altering Hct (and arterial blood oxygen content, Ca O 2 ) between extreme states of anemia and polycythemia (Hct = 8–55%) in the rainbow trout. Since blood viscosity (η) effects on cardiac output (Q̇) and O 2 transport (Ṫ O 2 ) are likely to be greatest when O 2 demand and Q̇ are maximal, we challenged fish to swim to their critical swimming velocity (U crit) in a swim-tunnel respirometer at 13°C and measured maximal oxygen uptake (V̇ O 2max ), maximum Q̇(Q̇ max), and other cardiovascular variables. In addition, experimental temperature was lowered to 5°C to increase η. Consistent with the Hct opt hypothesis, the decreased Ca O 2 in anemic (Hct < 22%) fish caused significant reductions in U crit and V̇ O 2max . In contradiction to the Hct opt hypothesis, and despite an exponential relationship between η and Hct, maximal Ṫ O 2 (Ṫ O 2max ) and U crit increased with polycythemia up to Hct 55%. Although there was a peak for V̇ O 2max , it occurred at an Hct (42%) well above the normocythemic range (23–33%). These results clearly demonstrate that η is not significant in setting normocythemia in rainbow trout. The novel finding of an Hct-dependent relationship for exercise-induced arterial hypoxemia may be indicative of a diffusion limitation to normocythemia. We suggest that factors involved in setting normocythemia in vertebrates should include diffusion limitations to oxygen transfer in addition to blood viscosity and oxygen transport constraints.