Glucose uptake and metabolism by red blood cells from fish with different extracellular glucose levels

The aim of the present study was to assess whether mechanisms of glucose trafficking by red blood cells (RBCs) relate to species-specific extracellular glucose levels. Atlantic cod (Gadus morhua), Atlantic salmon (Salmo salar), cunner (Tautogolabrus adspersus) and short-horned sculpin (Myoxocephalus scorpius) had plasma glucose levels of 4, 4.1, 1.95 and 0.73 mmol l(-1), respectively. Glucose uptake by isolated RBCs was measured by the initial incorporation of [6-(14)C]-glucose and steady-state glucose metabolism was determined by the production of (3)H(2)O from [2-(3)H]-glucose. Saturation kinetics of glucose uptake and inhibition of both glucose uptake and metabolism by cytochalasin B and phloretin revealed that Atlantic cod, cunner and sculpin RBCs all had a facilitated transport component to glucose trafficking. RBCs from Atlantic salmon showed a linear relationship between glucose uptake and extracellular glucose level, but exhibited clear inhibition of glucose metabolism by cytochalasin B and phloretin, suggesting a component of facilitated glucose transport that is more elusive to detect. The production of (3)H(2)O was linear for at least 6 h and as such presents a rigorous approach to measuring glycolytic rate. Steady-state rates of glucose metabolism were achieved at extracellular levels of approximately 1 mmol l(-1) glucose for RBCs from all species, showing that within-species normal extracellular glucose level is not a primary determinant of the basal level of glycolysis. At physiological levels of extracellular glucose, the ratio of initial glucose uptake to glucose metabolism was 1.5 to 4 for all RBCs, suggesting that there is scope to increase metabolic rate without alteration of the basal glucose uptake capacity.