Oxygen uptake and the potentiating effects of increased hemolymph lactate on oxygen transport during exercise in the blue crab,Callinectes sapidus

Summary At the onset of moderate swimming activity,Callinectes sapidus rapidly increased branchial ventilation, heart rate, and oxygen uptake, reaching steady state values in 2–3 min, with a half-time of 30 sec. Although O₂ extraction efficiency decreased slightly (50% to 43%) upon reaching steady state, O₂ uptake was increased 2.6 fold over resting (routine) levels. HemolymphP_O2 did not change during sustained (30–60 min) exercise, but a marked decrease in pH (7.60 to 7.10), associated with a 14-fold increase in hemolymph lactate concentration, caused decreases in both pre-and postbranchial O₂ content due to a large hemocyanin Bohr shift. The effect of the Bohr shift on O₂ binding, however, was minimized by an increase in hemocyanin O₂ affinity induced by lactate ions; the influence of lactate on hemocyaninP₅₀ was shown to be the same in vivo and in vitro. As a result of the interaction between the Bohr and lactate effects, only slight increases were observed in the a-v O₂ difference (13%) and the quantitative role of hemocyanin in oxygen transport (11%) during exercise. The increase in O₂ delivery was therefore attributed primarily to a 2.3 fold increase in cardiac output (Fick estimate), resulting from increases in both heart rate (1.61 X) and stroke volume (1.42X). During exercise hemocyanin remained 21% oxygenated upon leaving the tissues, thus maintaining a substantial venous O₂ reserve which could be utilized to fuel more strenuous levels of exercise at least partly by aerobic pathways. The high hemolymph lactate levels, however, indicate that anaerobic metabolism makes a significant contribution to energy production even during moderate exercise. These results are similar to the respiratory and circulatory responses reported for other decapod crustaceans and fish during mild exercise.C. sapidus, however, appears to be highly resistant to fatigue, which correlates with its welldeveloped locomotor capabilities.