Physiological regulation of glucose transporter (GLUT4) protein content in brown trout (Salmo trutta) skeletal muscle.

In brown trout, red and white skeletal muscle express the insulin-regulatable glucose transporter 4 (btGLUT4). 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 order to determine whether changes of btGLUT4 expression at the mRNA level are correlated with changes at the protein level, we performed in vivo experiments 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. In the present study we show that btGLUT4 protein content in red muscle decreases after fasting and increases after insulin administration. By contrast, btGLUT4 protein content in white muscle decreases after fasting but is not affected by insulin treatment. 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.