Fate of plasma glucose in tissues of brown trout in vivo: effects of fasting and glucose loading

We report the fate of glucose, both as a source of energy and as a temporary store, in the tissues of brown trout (Salmo trutta) in control, fasted and glucose-loaded fish. Tissue glucose utilization (³H-2-deoxyglucose phosphorylation) and storage (conversion of ¹⁴C-glucose into glycogen, protein, and lipid) were measured in immature brown trout, and the oxidation rate was calculated as glucose utilization minus storage and ¹⁴C-ionic metabolites remaining in the tissue. The glucose utilization rate is tissue-specific, the highest values being found in spleen, kidney, hindgut, brain, and gill. All these tissues also showed a highly active glycolytic pathway. The lowest utilization indices were observed in white and red muscles, skin, stomach and caeca, which also presented the largest proportion of glucose converted into stores (mainly protein and glycogen). Fasting reduced the glucose disappearance rate by 24%, although there were no significant variations in glucose utilization indices or distribution profile. After a glucose load, plasma glucose and insulin levels rose and the rates of glucose utilization, storage, and oxidation also increased in all tissues (from 1.5- to 4-fold). The relative importance of each tissue in glucose disposal was similar to that in normoglycaemia. In liver, only glucose storage was measured reliably; the conversion of glucose to glycogen was higher than in other tissues, and rose markedly (35-fold) in glucose-loaded fish. In most tissues glucose flux into lipids, glycogen and protein increased. The distribution of glucose may not be a merely substrate-mediated process because fasting in glucose-loaded fish caused lower tissue glucose utilization, particularly in gut, red muscle and gills. Conversion of glucose to tissue stores was reduced, lipids being the most affected.