STABILITY OF GLUT-1 AND GLUT-4 EXPRESSION IN PERFUSED RAT MUSCLE STIMULATED BY INSULIN AND EXERCISE

In vivo exercise and insulin may change the concentrations of GLUT-4 p rotein and mRNA in muscle. We studied in vitro whether adaptations in glucose transporter expression are initiated during a single prolonged period of contractions or during insulin stimulation. Rat hindquarter s were perfused at 7 mM glucose for 2 h with or without insulin (>20,0 00 mu U/ml) while the sciatic nerve of one leg was stimulated to produ ce repeated tetanic contractions. During electrical stimulation, contr action force decreased 93 +/- 1% (SE; n = 26) and muscle glycogen was markedly diminished (P < 0.05). Both contractions and insulin markedly increased glucose transport and uptake (P < 0.05). At the end of cont ractions, glycogen was higher in the presence of than in the absence o f insulin (24 +/- 4 vs. 14 +/- 3 mu mol/g for the soleus and 13 +/- 2 vs. 8 +/- 1 mu mol/g for the red gastrocnemius, respectively; P < 0.05 ). In nonstimulated muscle, glucose transporter mRNA and protein conce ntrations were higher in the soleus than in the white gastrocnemius (G LUT-4 mRNA 184 +/- 18 vs. 131 +/- 36 arbitrary units; GLUT-1 mRNA 173 +/- 29 vs. 114 +/- 26 arbitrary units GLUT-4 protein 0.96 +/- 0.09 vs. 0.46 +/- 0.03 arbitrary units GLUT-1 protein 0.41 +/- 0.08 vs. 0.19 /- 0.05 arbitrary units, respectively; P < 0.05). These concentrations were not changed by contractions or insulin. In conclusion, GLUT-1 an d GLUT-4 mRNA and protein levels are higher in slow-twitch oxidative t han in fast-twitch glycolytic fibers. In vitro neither a prolonged per iod of exhaustive contractions per se nor maximum insulin changes gluc ose transporter expression in muscle during stimulation. Finally, insu lin decreases net glycogen breakdown in contracting muscle.