Effect of Insulin on GLUT4 Cell Surface Content and Turnover Rate in Human Skeletal Muscle as Measured by the Exofacial Bis-Mannose Photolabeling Technique

Insulin-stimulated glucose transport across the skeletal muscle cell membrane is a major regulatory step in postprandial glucose disposal. To estimate the total molar concentration of GLUT4 as well as the turnover rate of GLUT4 in human vastus lateralis muscles at the cell surface in the basal state and after insulin exposure, we have applied the sensitive exofacial bis-mannose photolabeling technique on in vitro incubated human skeletal muscle strips from healthy subjects. In addition, we have measured 3-O-methylglucose transport in other muscle strips prepared from the same surgically removed human skeletal muscle biopsies to compare glucose transport with cell surface level of GLUT4. Maximal in vitro insulin stimulation (2,400 pmol/l) resulted in a twofold increase compared with basal in both surface GLUT4 content (0.38 ± 0.05 vs. 0.19 ± 0.03 pmol/g wet muscle wt, P < 0.005) and 3-O-methylglucose transport (1.24 ± 0.13 vs. 0.63 ± 0.08 umol · ml⁻¹ · h⁻¹ P < 0.005). The insulin-induced increment in 3-O-methylglucose transport was strongly correlated with the insulin-induced increase in cell surface GLUT4 content (r² = 0.91; P < 0.005). The calculated turnover rate of human skeletal muscle GLUT4 amounted to ∼8 × 104 min⁻¹ at 35δC and was unaffected by insulin. In conclusion, maximal in vitro insulin stimulation of vastus lateralis muscle strips from healthy subjects resulted in a twofold rise in glucose transport as well as in cell surface content, whereas the turnover rate of GLUT4 was unaffected by insulin under the chosen experimental conditions.