HEPATIC CHANGES IN THE EXPRESSION OF GLUCOSE TRANSPORTER (GLUT) 1 AND 2 AND TWO INSULIN RECEPTOR MRNA IN RESPONSE TO FOOD INTAKE IN CHANNEL CATFISH

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Increased growth is an economically important trait and has been used as a criterion during genetic selection of channel catfish brood stock.  Genetic selection toward increased growth often leads to hyperphagia and increased peritoneal fat accumulation rather than muscle protein accretion.  We hypothesized that  selection towards increased growth alters the ability of peripheral tissues such as muscle and liver to sense, acquire, and metabolize nutrients.  We also hypothesized that genetic selection may alter the insulin responsiveness of the peripheral tissue in channel catfish.  To test this hypothesis, we investigated changes in expression of mRNA encoding two glucose transporters (GLUT1 and 2) and insulin receptor (IR) in channel catfish.  Two distinct IR (IRa and b) were identified in channel catfish.  Expression of GLUT1 and both IR mRNA were readily detectable in various peripheral tissues and brain, whereas the expression of GLUT2 mRNA was primarily restricted to the liver.  During a 4 week feeding study, catfish that were fasted for the duration of the study (Fasted) tended (p < 0.10)to have lower levels of GLUT2 mRNA in the liver compared to fish that were fed daily for the duration of the study (Control).  Expression of both IR mRNA was elevated in the liver of Fasted fish (p=0.06 for IRa and p<0.01 for IRb, respectively) compared to Control.  Fish that were fasted for the first 14 days and received food for the subsequent 14 days had similar expression of GLUT2 and IR mRNAs with Control fish that received food daily for 28 days.  In contrast, expression of GLUT1 mRNA in the liver was unaffected by prolonged fasting or refeeding (p>0.10).   Muscle expression of both IR mRNA was unaffected by food intake (p > 0.10).  The results of our study suggest that changes in expression of IR mRNA is affected by food intake in a tissue-specific manner.  Our results also showed that hepatic expression of GLUT 2 mRNA was more sensitive to changes in food intake compared to GLUT 1 mRNA expression.  The exact mechanism(s) by which fasting caused changes in the expression of GLUT 2 and IR mRNA in the liver is unclear.  Currently, we are investigating the relationship between expression of these genes and genetic selection toward growth in channel catfish.