GLUT-4 AND GLUT-1 GLUCOSE-TRANSPORTER EXPRESSION IS DIFFERENTIALLY REGULATED BY CONTRACTILE ACTIVITY IN SKELETAL-MUSCLE

Mammalian skeletal muscle expresses GLUT-4 and GLUT-1 glucose transporters. Here, we have investigated whether GLUT- 1 and GLUT-4 expression is regulated in muscle by contractile activity. GLUT-1 mRNA levels were high in skeletal muscle at days 16 and 17 of fetal life and decreased markedly by days 19 and 21. In contrast, GLUT-4 mRNA levels were clearly detectable at day 21 of fetal life, and they increased progressively during postnatal life. The timing data for GLUT-4 induction and GLUT- 1 repression suggest that these processes are related to skeletal muscle innervation. GLUT-4 mRNA decreased markedly in adult rat and rabbit tibialis anterior muscle after severage of peroneal nerve. In contrast, GLUT-1 mRNA levels showed a 9-fold increase in rat muscle 3 days after denervation. Direct stimulation of rabbit tibialis anterior muscle with extracellular electrodes protected GLUT-4 mRNA levels against the effect of denervation. This indicates that the repression of GLUT-4 mRNA associated with denervation is due, at least in part, to electrical activity. Increased contractile activity induced for 24 h by indirect electrical stimulation at low frequency caused a marked and specific increase in GLUT-1 mRNA levels in rabbit tibialis anterior muscle. Our results indicate that (a) innervation-dependent basal contractile activity regulates in an inverse manner the expression of GLUT-1 and GLUT-4 in skeletal muscle, and (b) enhanced contractile activity stimulates GLUT-1 expression in the absence of modifications to GLUT-4 expression. This suggests the existence of different factors which depend on contractile activity and which control GLUT-1 and GLUT-4 glucose transporter expression in skeletal muscle.