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1 Bioloxia Funcional e Ciencias da Saude/Facultade de Bioloxia, Universidade de Vigo, Vigo, Spain
2 Department of Biology, University of Ottawa, Ottawa, Canada
* To whom correspondence should be addressed. E-mail: jsoengas{at}uvigo.es.
The expression and activities of glucokinase (GK) and hexokinase (HK) were assessed in different tissues of rainbow trout (Oncorhynchus mykiss) under different feeding conditions (fed, fasted for 14 days and re-fed for 7 days). Two different HK-I cDNAs were identified with different tissue distributions. One transcript named heart or H-HK-I was observed in the four brain regions assessed, white muscle, kidney, and gills but not in liver or erythrocytes. A second transcript named liver or L-HK-I was found in all tissues surveyed. GK mRNA was identified only in liver and the four brain regions. GK expression was altered by feeding conditions especially in liver and hypothalamus where food deprivation decreased and re-feeding increased expression; changes in expression reflected activity changes and changes in tissue glycogen levels. In contrast, feeding conditions did not alter expression of either HK-I transcript but did alter tissue HK activities.The reduced phosphorylating capacity noted with food deprivation correlates primarily with changes in tissue HK whereas increased capacity as with re-feeding was associated with changes in GK; these changes fit with the different Km values of the GK and HK enzymes. These results provide evidence for the hypothalamus acting as a glucosensor in trout as hyperglycemia produced increased GK expression and activity as well as increased glycogen levels. Thus, even though trout utilize glucose poorly, none of the parameters tested here relate to this inability to utilize glucose and suggest that at least rainbow trout, if given an appropriate carbohydrate diet, could metabolically adjust to such diets.
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