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In most animal cells, glucose is transported across cell membranes by facilitated diffusion mediated by a family of glucose transporters (GLUTs). Several GLUT isoforms (GLUT1-5, 8-11) have been identified in mammals, with different tissue-specific expression patterns and biochemical properties. In particular, GLUT4 is expressed predominantly in muscle and adipose tissue and is responsible for the stimulation of glucose uptake in these two tissues by insulin. In mammals, insulin rapidly increases the number of GLUT4 molecules at the plasma membrane through its stimulation of the translocation of GLUT4 from intracellular storage sites in both muscle and adipose cells (Kahn 1996). In fish, insulin is also known to increase the in vivo uptake and utilization of glucose in skeletal muscle and to regulate the number of specific insulin receptors in muscle as well as in adipose tissue. Although the direct effects of insulin on glucose uptake by fish muscle or adipose cells are not clearly established, it is possible that insulin could exert its hypoglycemic effect by stimulating glucose uptake mediated by a