USE OF TRANSPORT MUTANTS TO EXAMINE THE IDENTITY AND EXPRESSION OF GLUT ISOFORMS IN RAT CARDIAC MYOBLASTS

Two different spontaneous glucose transport (GLUT) mutants were used t o examine the identity and properties of proteins involved in rat card iac myoblast glucose transport processes. The parental clone, H9C2, po ssessed a high (HAHT) and a low (LAHT) affinity hexose transport proce ss, and the GLUT 1, 3 and 4 isoforms. Mutant RCM was devoid of HAHT, t he GLUT 3 transcript, and a 41 kDa protein recognizable by an anti-mou se GLUT 3 Ab. Mutant EZ-4 was impaired in the GLUT 3 and 4 isoforms, a nd in HAHT and LAHT. These studies demonstrated a close association of the GLUT 3 and 4 isoforms with the HAHT and LAHT processes, respectiv ely. Both GLUT 3 and 4 isoforms were regulated in opposite ways during myogenesis, and both GLUT 3(-) mutants were impaired in myogenesis. D espite its normal GLUT I transcript level, mutant EZ-4 was devoid of a n efficient carrier-mediated glucose transport process, thus suggestin g that the GLUT 1 transporter was inoperative in rat cardiac myoblasts . Unlike the rat skeletal L6 GLUT 1 isoform, expression of the rat car diac GLUT 1 isoform was not affected by glucose starvation, and was no t reduced in multinucleated myotubes. These studies demonstrated the u sefulness of transport mutants in determining the identity, expression and property of GLUT isoforms and their association with specific tra nsport processes.