Gallaugher P, Thorarensen H, Farrell A P
Department of Biological Sciences, Simon Fraser University, Burnaby, B.C., Canada.
Respir Physiol. 1995 Dec;102(2-3):279-92. doi: 10.1016/0034-5687(95)00065-8.
The optimal hematocrit (Hctopt) hypothesis was tested by altering Hct (and arterial blood oxygen content, CaO2) between extreme states of anemia and polycythemia (Hct = 8-55%) in the rainbow trout. Since blood viscosity (eta) effects on cardiac output (Q) and O2 transport (TO2) are likely to be greatest when O2 demand and Q are maximal, we challenged fish to swim to their critical swimming velocity (Ucrit) in a swim-tunnel respirometer at 13 degrees C and measured maximal oxygen uptake (VO2max), maximum Q(Qmax), and other cardiovascular variables. In addition, experimental temperature was lowered to 5 degrees C to increase eta. Consistent with the Hctopt hypothesis, the decreased CaO2 in anemic (Hct < 22%) fish caused significant reductions in Ucrit and VO2max. In contradiction to the Hctopt hypothesis, and despite an exponential relationship between eta and Hct, maximal TO2 (TO2max) and Ucrit increased with polycythemia up to Hct 55%. Although there was a peak for VO2max, it occurred at an Hct (42%) well above the normocythemic range (23-33%). These results clearly demonstrate that eta 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.
通过在虹鳟鱼的贫血和红细胞增多症极端状态（血细胞比容 = 8 - 55%）之间改变血细胞比容（Hct）（以及动脉血氧含量，CaO2）来检验最佳血细胞比容（Hctopt）假说。由于当氧气需求和心输出量（Q）最大时，血液粘度（η）对心输出量（Q）和氧气运输（TO2）的影响可能最大，我们让鱼在13摄氏度的游泳隧道呼吸计中游至其临界游泳速度（Ucrit），并测量最大摄氧量（VO2max）、最大心输出量（Qmax）和其他心血管变量。此外，将实验温度降至5摄氏度以增加η。与Hctopt假说一致，贫血（Hct < 22%）鱼体内CaO2的降低导致Ucrit和VO2max显著降低。与Hctopt假说相反，尽管η与Hct呈指数关系，但直至血细胞比容达到55%，最大氧气运输量（TO2max）和Ucrit随红细胞增多症而增加。尽管VO2max存在一个峰值，但它出现在远高于正常血细胞比容范围（23 - 33%）的血细胞比容（42%）处。这些结果清楚地表明，η在虹鳟鱼正常血细胞比容的设定中并不重要。运动诱导的动脉低氧血症与血细胞比容相关关系这一新发现可能表明对正常血细胞比容存在扩散限制。我们认为，除了血液粘度和氧气运输限制外，参与脊椎动物正常血细胞比容设定的因素应包括氧气转移的扩散限制。
