USE OF HEXOSE-TRANSPORT MUTANTS TO EXAMINE THE EXPRESSION AND PROPERTIES OF THE RAT MYOBLAST GLUT-1 TRANSPORT PROCESS

Rat L6 myoblasts were recently shown to possess the GLUT 1, 3 and 4 tr ansporters, and not the GLUT 2 isoform [1]. This investigation examine d the expression and properties of the GLUT 1 isoform. GLUT 1 transcri pt level was significantly reduced in cells grown at high densities an d during myogenic differentiation. A comparison of the GLUT 1 and 4 tr anscript levels in myogenesis-competent and impaired cells revealed an inverse relationship between these two isoforms. This relationship wa s confirmed by studies using two independent spontaneous GLUT 3(-) GLU T 4(-) mutants, M1 and M3. These mutants possessed very high level of the GLUT 1 isoform, but negligible amount of the GLUT 3 and 4 isoforms . GLUT 1 expression was also subject to positive regulation. Glucose s tarvation was found to increase not only the levels of the GLUT 1 tran script and transporter, but also the intrinsic activity of the GLUT 1 transporter. Studies with MZ and M3 mutants revealed that the GLUT 1 t ransporter was not functional in glucose-grown cells, even though it w as present at a very high level in the plasma membrane. This transport er became functional when cells were starved for glucose. The function al GLUT 1 transporter had an apparent K-m value of around 0.9 mM, and was sensitive to cytochalasin B, phloretin, phlorizin and pCMBS.