FRUCTOSE TRANSPORT AND GLUT-5 PROTEIN IN HUMAN SARCOLEMMAL VESICLES

Sarcolemmal vesicles were produced from human skeletal muscle biopsy m aterial obtained at rest and immediately after maximal dynamic exercis e (100% maximal O-2 uptake) for analysis of fructose transport and hex ose transporter (GLUT-5) protein concentration. Human sarcolemmal vesi cles displayed a time-dependent uptake of D-fructose that displayed sa turable Michaelis-Menten type kinetics (maximal transport 477 +/- 37 p mol.min(-1).mg protein(-1); half-maximal concentration constant 8.3 +/ - 1.2 mM). At a hexose concentration of 5 mM, vesicle transport rate w as eight times faster for glucose than for fructose. Preincubation of human muscle vesicles with 35 mu M cytochalasin B before the uptake as say resulted in >95% inhibition in D-glucose uptake, whereas transport of D-fructose was unaffected. Sarcolemmal vesicles prepared from exer cised human muscle showed a significant increase (49%) in vesicle GLUT -4 content (P < 0.03, n = 10), which accounts for the increase in vesi cle glucose transport that we have recently reported [S. Kristiansen, M. Haroreaves, and E. A. Richter. Am. J. Physiol. 270 (Endocrinol. Met ab. 33): E197-E201, 1996]. In contrast, exercise did not increase the vesicle GLUT-5 protein content or induce changes in vesicle fructose t ransport activity. In conclusion, we propose that fructose transport i nto human skeletal muscle occurs via a mechanism distinct from that ut ilized by glucose on the basis of differences in sensitivity to cytoch alasin B and responsiveness to exercise. Furthermore, our findings sig nify that uptake of fructose in human skeletal muscle is mediated by t he GLUT-5 transporter.