TRANSCRIPTS FOR THE HIGH AND LOW-AFFINITY HEXOSE TRANSPORTERS IN RAT MYOBLASTS

The present investigation examined the glucose transporter (GLUT) tran script levels in glucose-grown rat L6 myoblasts, which possessed a hig h (HAHT) and a low (LAHT) affinity hexose transport system. The most p rominent GLUT transcript detected in these cells was a GLUT 3-recogniz able 4.1-kilobase transcript. Mutants defective in HAHT contained only residual levels of the GLUT 3 transcript and transporter. Correspondi ng changes in GLUT 3 transcript levels and HAHT activity were observed in myotubes, confluent cultures, and myogenesis-impaired cells. The o ther prominent GLUT transcripts detected were the GLUT 4-recognizable 2.8- and 1.4-kilobase transcripts. The close correlation between the G LUT 4 isoform and LAHT activity was indicated by the absence of this i soform in LAHT- mutants, and their corresponding changes in myoblasts grown under different conditions. These findings suggested the HAHT an d LAHT transporters might be coded or regulated by the genes responsib le for the GLUT 3 and 4 transcripts, respectively. Although relatively high levels of the GLUT 1 transcript and transporter were detected in glucose-grown HAHT-LAHT- mutants, these cells did not exhibit any GLU T 1 transport activity. Since the GLUT 1 transporter was not functiona l, it was not likely responsible for the HAHT and LAHT activities obse rved in glucose-grown L6 myoblasts.