DEVELOPMENT AND REGULATION OF GLUCOSE-TRANSPORTER AND HEXOKINASE EXPRESSION IN RAT

The ontogenesis of the glucose transporters GLUT-1, GLUT-2, and GLUT-4 and the hexokinases HK-I, HK-II, and HK-IV (glucokinase) was studied in rat tissues. In brown adipose tissue, high levels of GLUT-4 and HK-II were observed during fetal life; both decreased at birth and then increased through out development. At birth, cold exposure increased GLUT-4 and HK-II expression in brown adipose tissue, whereas fasting decreased it. GLUT-1 and HK-I were present in fetal muscle, but GLUT-4 and HK-II were absent. The coordinate appearance of GLUT-4 and HK-II in skeletal muscle was concomitant with the acquisition of insulin sensitivity after weaning. In the heart, the glucose transporter isoform switched from GLUT-1 to GLUT-4 during the suckling period. The coordinate expression of GLUT-4 and HK-II in heart was observed after weaning. GLUT-2, detected in fetal liver, increased throughout development. GLUT-1 and HK-I were detectable in fetal liver, whereas glucokinase appeared after weaning. Consumption of a high-carbohydrate diet after weaning increased GLUT-4 and HK-II in muscle and GLUT-2 in liver, whereas consumption of a high-fat diet prevented these changes. These results showed that 1) GLUT-1 and HK-I are abundant in most fetal rat tissues, 2) GLUT-4 and HK-II expression is associated with the appearance of tissue insulin sensitivity, and 3) GLUT-2 is expressed early in liver, before the appearance of glucokinase.