cytochalasin-b and Hypoglycemia

The evidence suggests that glucose transport across the blood brain barrier (BBB) in the dog is normally not a rate-limiting step in cerebral metabolism; however, transport may become rate-limiting under conditions of extreme hypoglycemia or anoxia. Studies on the mechanism of glucose transport from blood to brain do not at this time permit us to distinguish between active transport and facilitated diffusion; however, a decrease in the rate of unidirectional transport during anoxia suggests that an energy-dependent process may be involved. In spite of this evidence, glucose transport across the BBB is similar to the facilitated diffusion of glucose into the red cell in terms of the structural requirements of the glucose molecule, the pattern of inhibition by phlorizin, phloretin and cytochalasin B, and the lack of sensitivity to Na+ or insulin.

Topics: Animals; Biological Transport, Active; Blood Glucose; Blood-Brain Barrier; Brain; Capillaries; Cytochalasin B; Dogs; Endothelium; Erythrocytes; Glucose; Humans; Hypoglycemia; Hypoxia; Kinetics; Methylglucosides; Models, Biological; Monosaccharides; Phloretin; Phlorhizin; Structure-Activity Relationship

The embryonic heart depends on glucose during early organogenesis. Glut-1 functions in constitutive glucose uptake in adult tissues and is the predominant glucose transporter in embryonic and fetal tissues. This study focuses on Glut-1 expression in the heart during normal organogenesis using immunohistochemistry for Glut-1 distribution, Western analysis for Glut-1 protein levels, and reverse transcriptase polymerase chain reaction for Glut-1 mRNA levels. The role of Glut in glucose uptake response to hypoglycemia in the embryonic heart is evaluated using the Glut inhibitor cytochalasin B. Cardiac Glut-1 expression is also evaluated after in vitro hypoglycemic exposure. Glut-1 levels are highest on gestational days 9-10, intermediate on gestational day 10.5, and lowest on gestational days 11.5-13.5 in the normal embryonic heart. Cardiac Glut-1 mRNA levels similarly decline between gestational days 9.5 and gd 13.5. Cytochalasin B produces a dose-dependent decrease in glucose uptake in hearts exposed to hypoglycemia for 30 min or 6 h, implicating Glut in this response. Glut-1 protein expression is unchanged after 2 or 6 h but increased after 12 and 24 h of hypoglycemia in the gestational day 9.5 heart. Thus, Glut-1 expression is prominent in the embryonic heart and is correlated with changes in cardiac glucose requirements during normal organogenesis. Glut activity increases in response to acute hypoglycemia and the expression of Glut-1 increases in response to prolonged hypoglycemia. These results support the importance of Glut-1 during normal cardiogenesis and in response to hypoglycemia in the embryonic heart.

Topics: Age Factors; Animals; Blotting, Western; Cytochalasin B; Deoxyglucose; Electrophoresis, Polyacrylamide Gel; Embryo, Mammalian; Female; Glucose; Glucose Transporter Type 1; Heart; Hypoglycemia; Immunohistochemistry; Mice; Mice, Inbred Strains; Monosaccharide Transport Proteins; Myocardium; Pregnancy; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; Time Factors; Tritium

Human skin fibroblasts from 'normal' subjects were found to possess at least two hexose transport systems. One system was responsible for the uptake of 2-deoxy-D-glucose (dGlc), D-glucose and D-galactose, whereas the other was responsible primarily for the uptake of 3-O-methyl-D-glucose (MeGlc). The transport of dGlc was the rate-limiting step in the uptake process; over 97% of the internalized dGlc was phosphorylated and the specific activity of hexokinase was several times higher than that for dGlc transport. The dGlc transport system was activated by glucose starvation, and was very sensitive to inhibition by cytochalasin B and energy uncouplers. Fibroblasts isolated from a patient with symptoms of hypoglycaemia were found to differ from their normal counterparts in the dGlc transport system. They exhibited a much higher transport affinity for dGlc, D-glucose and D-galactose, with no change in the respective transport capacity. Transport was not the rate-limiting step in dGlc uptake by these cells. Moreover, the patient's dGlc transport system was no longer sensitive to inhibition by cytochalasin B and energy uncouplers. This suggested that the intrinsic properties of the patient's dGlc transport system were altered. It should be noted that the patient's dGlc transport system could still be activated by glucose starvation. Despite the changes in the dGlc transport system, the MeGlc transport system in the patient's fibroblasts remained unaltered. The observed difference in the properties of the two hexose transport systems in the 'normal' and the patient's fibroblasts strongly suggests that the two transport systems may be coded or regulated by different genes. The present finding provides the first genetic evidence from naturally occurring fibroblasts indicating the presence of two different hexose transport systems.

Topics: 3-O-Methylglucose; Biological Transport; Cytochalasin B; Deoxyglucose; Fibroblasts; Galactose; Glucose; Hexoses; Humans; Hypoglycemia; Kinetics; Methylglucosides; Skin; Uncoupling Agents