INHIBITION OF THE HIGH-AFFINITY GLUCOSE-TRANSPORTER GLUT-1 AFFECTS THE SENSITIVITY TO GLUCOSE IN A HAMSTER-DERIVED PANCREATIC BETA-CELL LINE (HIT)

HIT is a hamster-derived beta-cell line which in contrast to normal be ta cells that only express the high K(m) GLUT-2 glucose transporter, a lso expresses the low K(m) glucose transporter GLUT 1. In HIT cells th e abnormal glucose transport mechanism is associated with a marked shi ft to the left of the glucose-induced insulin release dose-response cu rve. We have used this cell model to investigate whether changes in gl ucose transport affect the glucose-induced insulin release. HIT cells were first incubated with a concentration of cytochalasin B (0.4 mumol /l) that selectively inhibits the GLUT-1 but not the GLUT-2 transporte r. The consequences of blocking glucose phosphorylation and insulin re lease were studied. Exposure to 0.4 mumol/l cytochalasin B for 1 h cau sed a selective loss of the low K(m) transport: the calculated V(max) of GLUT 1 was reduced from 1726 +/- 98 to 184 +/- 14 pmol . mg protein -1 5 min-1 (mean +/- SEM, n = 6, p < 0.005), while no major difference in the high K(m) (GLUT-2) transport was observed. In cytochalasin B e xposed HIT cells the glucose phosphorylating activity (due to hexokina se and glucokinase) was unaffected. In these cells, however, the dose- response curve of glucose-induced insulin release was significantly sh ifted to the right: the 50 % of maximal response (increment over basel ine) was reached at an average glucose concentration of 2.9 +/- 0.2 mm ol/l (vs 0.6 +/- 0.01 mmol/l in control HIT cells mean +/- SE, n = 5, p < 0.05) and the maximal effect was reached at 11.0 mmol/l glucose (v s 2.8 mmol/l in control HIT cells p < 0.005). These results are consis tent with the hypothesis that the affinity of the glucose transport sy stem may contribute to determination of the glucose threshold concentr ation that triggers insulin secretion.