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 to examine the identity and properties of proteins involved in rat cardiac myoblast glucose transport processes. The parental clone, H9C2, possessed a high (HAHT) and a low (LAHT) affinity hexose transport process, and the GLUT 1, 3 and 4 isoforms. Mutant RCM was devoid of HAHT, the GLUT 3 transcript, and a 41 kDa protein recognizable by an anti-mouse GLUT 3 Ab. Mutant EZ-4 was impaired in the GLUT 3 and 4 isoforms, and in HAHT and LAHT. These studies demonstrated a close association of the GLUT 3 and 4 isoforms with the HAHT and LAHT processes, respectively. Both GLUT 3 and 4 isoforms were regulated in opposite ways during myogenesis, and both GLUT 3 mutants were impaired in myogenesis. Despite its normal GLUT 1 transcript level, mutant EZ-4 was devoid of an efficient carrier-mediated glucose transport process, thus suggesting that the GLUT 1 transporter was inoperative in rat cardiac myoblasts. Unlike the rat skeletal. L6 GLUT 1 isoform, expression of the rat cardiac GLUT 1 isoform was not affected by glucose starvation, and was not reduced in multinucleated myotubes. These studies demonstrated the usefulness of transport mutants in determining the identity, expression and property of GLUT isoforms and their association with specific transport processes.