Low plasma glucose limits glucose metabolism by RBCs and heart in some species of teleosts

Within teleosts there is a species range in plasma glucose levels from undetectable to 20 mM. At low plasma glucose levels the gradient from the extracellular to the intracellular space is decreased. The impact of this on glucose metabolism by RBCs and heart from species with different steady state levels of plasma glucose (Atlantic cod ~ 5 mM; Atlantic salmon ~ 5 mM, cunner ~ 1 mM, lumpfish < 1 mM; short-horned sculpin < 1 mM) is the subject of this review. Under normoxia, at physiological levels of extracellular glucose, RBCs and heart produce lactate although the contribution of anaerobic metabolism to ATP production is small. Sustained lactate production from extracellular glucose appears to be the primary fate of extracellular glucose. In many cases, glycogen is not mobilized and the rate of glucose metabolism = two times the rate of lactate production. As such, alternative metabolic sources are required to fuel oxidative metabolism. Under hypoxia, hearts from Atlantic cod and rainbow trout increase rates of both glucose metabolism and lactate production, partially supported by glycogen reserves. But in lumpfish and short-horned sculpin hearts there is no change in rates of glucose metabolism. The most likely explanation is that glucose uptake is compromised in lumpfish and short-horned sculpin hearts due to a low diffusion gradient. Under these conditions rates of lactate production are well below that of Atlantic cod or rainbow trout. Energy demand must be reduced under hypoxia in lumpfish and short-horned sculpin hearts in order to maintain ATP balance.