The 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- and 170-fold higher, respectively, than that in heart and red blood cells (RBCs) as determined by the production of 3H2O from [2-3H]glucose. In the gas gland, essentially all of the glucose consumed was converted to lactate. Glucose uptake in the 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 the high efficacy of this treatment in heart and RBCs. Glucose transport is via glucose transporter 1 (GLUT1), which is localized to the glandular cells. GLUT1 western blot analysis from whole-tissue lysates displayed a band with a relative molecular mass of 52 kDa, consistent with the deduced amino acid sequence. Levels of 52 kDa GLUT1 in the gas gland were 2.3- and 33-fold higher, respectively, than those in heart and RBCs, respectively. Glucose phosphorylation is catalysed by hexokinase Ib (HKIb), a paralogue that cannot bind to the outer mitochondrial membrane. Transcript levels of HKIb in the gas gland were 52- and 57-fold more abundant, respectively, than those 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.
The authors declare no competing or financial interests.
K.A.C., C.E.S., J.R.H., R.L.G., H.P. and A.R. were responsible for the execution of the studies. K.A.C., C.E.S., J.R.H., R.L.G. and H.P. reviewed drafts of the manuscript. W.R.D. was responsible for experimental design, interpretation of the findings and drafting of the article.
This work was supported by the Natural Sciences and Engineering Research Council of Canada (W.R.D.); the Canadian Institutes of Health Research (FRN-123888; R.L.G. and H.P.); the Research and Development Corporation of Newfoundland and Labrador (W.R.D., R.L.G. and H.P.); and by an infrastructure grant from the Canada Foundation for Innovation (7411; R.L.G. and H.P.). W.R.D. holds the Canada Research Chair (Tier 1) in Marine Bioscience.
- Received April 11, 2016.
- Accepted June 28, 2016.
- © 2016. Published by The Company of Biologists Ltd