Furthermore, both btGLUT4 and okGLUT4 have been shown to be regulated by insulin, providing the first evidence for the existence of an insulin-regulated glucose transporter in fish. By expressing okGLUT4 in Xenopus oocytes, we have recently shown that okGLUT4 is a functional glucose transporter with similar biochemical properties to mammalian GLUT4 but with a lower affinity for glucose( Capilla et al., 2004). In mammals, GLUT4 appears to be essential for the maintenance of glucose homeostasis because it mediates the action of insulin by enhancing the uptake of glucose by peripheral tissues in postprandial conditions( Watson and Pessin, 2006). Our results show a good correlation between the changes observed in btGLUT4 protein and the previously reported changes in mRNA levels in response to alterations in circulating insulin, indicating that the regulation of btGLUT4 in brown trout takes place predominantly in the red skeletal muscle. By contrast, btGLUT4 protein content in white muscle decreases after fasting but is not affected by insulin treatment. In the present study we show that btGLUT4 protein content in red muscle decreases after fasting and increases after insulin administration. In order to determine whether changes of btGLUT4 expression at the mRNA level are correlated with changes at the protein level, we performed in vivoexperiments to alter blood insulin concentrations and determined the abundance of btGLUT4 protein in trout red and white skeletal muscle by immunoblotting using an antibody to salmon GLUT4. We have previously shown that the mRNA expression of btGLUT4 in red muscle, but not white muscle, is altered under experimental conditions designed to cause changes in the plasma levels of insulin, such as fasting, insulin and arginine administration. In brown trout, red and white skeletal muscle express the insulin-regulatable glucose transporter 4 (btGLUT4).
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