High rates of glucose utilization in gas gland of Atlantic cod (Gadus morhua) are supported by GLUT1 and HK1b

Abstract

Gas gland of physoclistous fish utilizes glucose to generate lactic acid that leads to the off loading of oxygen from haemoglobin. This study addresses characteristics of the first two steps in glucose utilization in the gas gland of Atlantic cod (Gadus morhua). Glucose metabolism by isolated gas gland cells was 12-fold and 170-fold higher than in heart and RBCs as determined by the production of 3H2O from [2-3H]-glucose. In gas gland essentially all of the glucose consumed was converted to lactate. Glucose uptake in gas gland shows a very high dependence upon facilitated transport as evidenced by saturation of uptake of 2-deoxyglucose at a low extracellular concentration and a requirement for high levels of cytochalasin B for uptake inhibition despite high efficacy of this treatment in heart and RBCs. Glucose transport is via glucose transporter 1 (GLUT1) that is localized to the glandular cells. GLUT1 Western blot analysis from whole tissue lysates displayed a band with a relative molecular mass of 52kDa consistent with the deduced amino acid sequence. Levels of 52kDa GLUT1 in gas gland were 2.3-fold and 33-fold higher than in heart and RBCs, respectively. Glucose phosphorylation is catalyzed by hexokinase Ib (HKIb), a paralog that cannot bind to the outer mitochondrial membrane. Transcript levels of HKIb in gas gland were 52-fold and 57-fold more abundant than in heart and RBCs. It appears that high levels of GLUT1 protein and an unusual isoform of HKI are both critical for the high rates of glycolysis in gas gland cells.